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compressors • C O M P R E S S O R S • compressors • C O M P R E S S O R S • compres-

                                                                                                             Finding the
                                                                                                                       right match
                                                                                                             To select the most energy-efficient capacity

                                                                                                             control system, you need to determine your

                                                                                                             compressor type and the plant’s load cycle

                                                                                                             Niff Ambrosino, General Manager,                        Compressor capacity control systems
                                                                                                             Scales Air Compressor Corp. of New Jersey,            regulate air output to match system demand
                                                                                                             West Paterson, New Jersey                             and are one of the most important factors in
                                                                                                             Compressed air systems hold one of the keys to        determining overall system efficiency. To
                                                                                                             greater productivity, efficiency and profitabil-      select the most energy-efficient capacity
                                                                                                             ity. This is the fifth in a series of articles that   control system, you need to determine the
                                                                                                             introduces some key points of Compressed Air          compressor type and the plant’s load cycle.
                                                                                                             Challenge training.
                                                                                                                   he greatest expense associated with             Start/stop
                                                                                                             T     compressed air is the cost of energy.
                                                                                                             Compressors operate most efficiently when
                                                                                                                                                                   The most basic form of compressor capacity
                                                                                                                                                                   control is start/stop. It requires compressing
                                                                                                             fully loaded. However, most compressed air            air and storing it in an air receiver. When pres-
                                                                                                             systems allow compressors to operate par-             sure reaches an upper pre-set limit (cut-out),
                                                                                                             tially loaded.                                        the air compressor turns off and no further air

                                                                                       56   J U LY 2 0 0 1                                               PLANT SERVICES                               
                                                                                                                                                                   mpressors • C O M P R E S S O R S • compressors • C O M P R E S S O R S • compressors • C O M P R E S S O R
    is compressed. The stored air supplies demand.                                         turning off when system pressure reaches the
        When pressure drops to a predetermined low                                         cut-out point (typically 10 percent above the
    setting (cut-in), the compressor starts and the                                        cut-in setting), the compressor motor continues
    Figure 1.

                                          Average kW vs average capacity with load/unload capacity control


        Percent kW input




                                 0   20                                  40                60               80                100   120
                                                                                   Percent capacity

                                                                   1 gal/cfm   3 gal/cfm        5 gal/cfm        10 gal/cfm

    cycle is repeated. The cut-out pressure is typi- to run while the compressor operates in an
    cally set 25 percent above the cut-in pressure. “unloaded” mode. When unloaded, compression
    This can be an efficient way to control a com- is interrupted and the compressor’s internal
    pressor, because power is used only while air is pressure drops.
    being compressed. However, using compressed                Typically, constant speed controls are found
    air at a pressure above the required minimum on reciprocating and rotary screw compressors,
    increases power consumption, thereby partially and on some centrifugal compressors. Unloaded
    offsetting any gains. Therefore, many systems input power requirements on reciprocating
    will, or should, use a pressure regulator to compressors is 8 to 12 percent of full-load
    reduce line pressures
    to the required mini- Figure 2.
    mum. Note that every
    2 psi increase in dis-
                                                   Rotary compressor performance with inlet valve modulation
    charge        pressure,          120.0

    requires an additional
    one percent in energy.
                                          Percent kW input power

       Start/stop controls            80.0

    are found on recipro-
    cating compressors
    ranging from less than            40.0

    one hp to 30 hp.                  20.0
    System storage capac-
    ity must be considered             0.0
                                           0      20            40              60              80            100       120
    to provide adequate                                                   Percent capacity

    time between motor                           Inlet modulation - no blowdown          Inlet modulation with blowdown

    starts. Started too fre-
    quently, an electric
    motor can fail prematurely.                            requirements. On rotary screw units, the
                                                           unloaded requirement is approximately 15 to 25
    Constant speed or load/unload                          percent of full load power. Constant speed or
    With constant speed, or load/unload, control, load/unload controls are used in applications
    system pressure is set at a high point (cut-out) where loads are continuous and a start/stop con-
    and a low point (cut-in). However, rather than trol would cause premature motor failure.                                                                      PLANT SERVICES                                    J U LY 2 0 0 1   57
compressors • C O M P R E S S O R S • compressors • C O M P R E S S O R S • compressors •

                                                                                                                      As with start/stop control, over-pressuriza-                                  inlet modulation control is used, the rotary
                                                                                                                   tion is required to maintain minimum plant                                       compressor will typically modulate (throttle
                                                                                                                   operating pressure. When running in the                                          back) to approximately 40 percent of full capac-
                                                                                                                   unloaded mode, power is still consumed, but the                                  ity before completely unloading (blowing
                                                                                                                   compressor is not delivering any useful work.                                    down). At this point, the timing relay is acti-
                                                                                                                      In the load/unload mode, adequate storage                                     vated, and assuming system pressure does not
                                                                                                                   is critical for part-load efficiency. Figure 1 (pg.                              fall below the cut-in point within the specified
                                                                                                                   57) illustrates the effect of receiver size on a                                 period of time, the compressor will turn off.
                                                                                                                   lubricated rotary screw compressor with load/                                    Overall rotary screw part-load efficiency will be
                                                                                                                   unload controls .                                                                directly affected by system storage capacity
                                                                                                                                                                                                    when operating in the load/unload mode.

                                                                                                                                                                                                    Modulating control
                                                                                                                     Rotary screw air-end                                                           Modulating (inlet throttling) control is found
                                                                                                                                                                                                    on rotary screw and centrifugal compressors.
                                                                                                                   compressor efficiency is a                                                       Modulation allows for stepless output control
                                                                                                                                                                                                    to meet varying demand. At less than full
                                                                                                                  function of rotor tip speed.                                                      capacity, the inlet valve throttles back, thereby
                                                                                                                                                                                                    matching system demand by restricting air-
                                                                                                                                                                                                    flow into the compressor. Because the inlet
                                                                                                                                                                                                    valve remains wide open (fully loaded) up to
                                                                                                                                                                                                    the modulation set point, the minimum allow-
                                                                                                                   Dual control                                                                     able system pressure and the control set point
                                                                                                                   Dual control allows for manual selection of                                      can be the same.
                                                                                                                   either start/stop or constant speed control. A                                      Rotary screw. On a rotary screw compressor,
                                                                                                                   typical application would have the compressor                                    modulation is an inefficient way to control capac-
                                                                                                                   operating at a constant speed during produc-                                     ity, because it affects the inlet pressure of air being
                                                                                                                   tion when demand for compressed air is contin-                                   compressed. Throttling the inlet valve creates a
                                                                                                                   uous and in the start-stop mode during low                                       vacuum at the rotor’s inlet. This reduces the
                                                                                                                   demand, non-production periods. Dual control                                     absolute pressure at the start of the compression
                                                                                                                   is found on reciprocating compressors and                                        cycle, thereby increasing the overall compression
                                                                                                                   smaller rotary screw units.                                                                                           Continued on pg. 60 a a

                                                                                                                                                   Figure 3.
                                                                                                                   Auto-dual control
                                                                                                                   Auto-dual control auto-
                                                                                                                   matically selects either                                         Centrifugal compressor percent power vs percent capacity
                                                                                                                   constant speed or start/                                          120
                                                                                                                   stop control. With autod-
                                                                                                                   ual control, a reciprocat-
                                                                                                                   ing compressor runs in a
                                                                                                                                                       Percent of full load power

                                                                                                                   constant speed mode.
                                                                                                                   When it unloads, a timing                                          80

                                                                                                                   relay is activated. If system
                                                                                                                   pressure doesn’t fall below                                        60

                                                                                                                   the cut-in set point within
                                                                                                                   a specified time period,                                           40

                                                                                                                   the compressor shuts off.
                                                                                                                   When system pressure
                                                                                                                   eventually falls to the cut-
                                                                                                                   in set point, the compres-
                                                                                                                   sor restarts and the control                                       0
                                                                                                                                                                                           0   20         40          60         80         100          120
                                                                                                                   process resumes. On a
                                                                                                                                                                                                           Percent capacity
                                                                                                                   rotary screw compressor
                                                                                                                   with load/unload control,                                                    Constant pressure, inlet guide vane modulation control
                                                                                                                                                                                                Constant pressure, blow-off valve modulation control
                                                                                                                   the control process is                                                       On-line/off-line control (average power)
                                                                                                                   identical to that for a
                                                                                                                   reciprocating unit. When

                                                                                            58   J U LY 2 0 0 1                                              PLANT SERVICES                                                        
compressors • C O M P R E S S O R S • compressors • C O M P R E S S O R S • compressors •
                                                                                                                  Figure 4.

                                                                                                                                                                  Rotary compressor performance with variable displacement


                                                                                                                          Percent kW input power




                                                                                                                                                         0   20             40              60             80                100           120

                                                                                                                                                                                     Percent capacity

                                                                                                                                                                    Rotary compressor performance with variable displacement

                                                                                                                  s s    Continued from pg. 58                                               For example, running at 100 percent capacity,
                                                                                                                  ratio. Brake hp is dependent on the volume of air                          air pressure at the rotor inlet is 0 psig, the same
                                                                                                                  compressed and the compression ratio.                                      as ambient (assume 14.7 psia). With a 100 psig
                                                                                                                     As compression ratios increase, so does bhp.                            discharge pressure, the overall compression
                                                                                                                                                                                             ratio is 7.8:1. When system demand decreases,
                                                                                                                                                                                             the inlet valve throttles back, restricting flow.
                                                                                                                                                                                             This results in a lower absolute pressure at the
                                                                                                                                                                                             rotor inlet.
                                                                                                                                                                                                At 65 percent flow, hp requirements are
                                                                                                                                                                                             reduced. However, because inlet pressure at the
                                                                                                                                                                                             rotors is reduced to approximately 9.5 psia,
                                                                                                                                                                                             more work is required to raise inlet pressure to
                                                                                                                                                                                             its final discharge pressure of 100 psig (114.7
                                                                                                                                                                                             psia). As a result, the overall compression ratio
                                                                                                                                                                                             is now 12:1. Although less air is being com-
                                                                                                                                                                                             pressed, which reduces the power required, the
                                                                                                                                                                                             higher pressure ratio increases input power. As
                                                                                                                                                                                             a result, a modulating inlet controlled compres-
                                                                                                                                                                                             sor operating at only 65 percent of full-flow
                                                                                                                                                                                             capacity still requires 91 percent of full-load
                                                                                                                                                                                             power (Figure 2, pg. 57).
                                                                                                                                                                                                Centrifugal compressors. When applied to
                                                                                                                                                                                             centrifugal compressors, inlet modulation offers
                                                                                                                                                                                             efficient part-load control, especially if inlet
                                                                                                                                                                                             guide vanes are used to direct air in the same
                                                                                                                                                                                             direction as the impeller rotation. Required
                                                                                                                                                                                             power and output flow are reduced almost pro-
                                                                                                                                                                                                On a centrifugal compressor, however, inlet
                                                                                                                                                                                             modulation is limited by the percent that inlet
                                                                                                                                                                                             flow can be reduced before it goes into surge
                                                                                                                                                                                             (flow reversal). This is typically 70 to 80 percent
                                                                                                                                                                                             of full-flow capacity. Below that, modulating a
                                                                                                                                                                                             blow-off valve vents excess flow capacity to the

                                                                                                                  For FREE information, Circle 157
                                                                                                                                                                                             atmosphere. Known as a “constant pressure”
                                                                                                                                                                                                                                   Continued on pg. 62 a a

                                                                                            60   J U LY 2 0 0 1                                                       PLANT SERVICES                                        
compressors • C O M P R E S S O R S • compressors • C O M P R E S S O R S • compressors •                            s s   Continued from pg. 60

                                                                                                                     control, the compressor load remains constant           known as a turn valve, poppet valve or spiral
                                                                                                                     and no further reduction in power is realized.          valve—is found on rotary screw compressors at
                                                                                                                     Some centrifugals can operate in load/unload            50 hp or above. Similar to modulation, VDC
                                                                                                                     mode below the surge point. Typically, the              automatically regulates compressor output to
                                                                                                                                                                             match system demand. Unlike modulating con-
                                                                                                                                                                             trol, however, VDC reduces output by varying
                                                                                                                                                                             compressor displacement, which allows the
                                                                                                                                                                             inlet valve to remain wide open. This is accom-
                                                                                                                                                                             plished by opening windows and ports in the
                                                                                                                  On a rotary screw compressor,                              cylinder wall that runs along the length of the
                                                                                                                                                                             rotor to return the unneeded air to the inlet
                                                                                                                   modulation is an inefficient                              rotor before compression begins.
                                                                                                                                                                                Because inlet air is not restricted, pressure
                                                                                                                     way to control capacity.                                at the rotor inlet is always at the atmospheric
                                                                                                                                                                             level. Therefore, the compressio n r atio
                                                                                                                                                                             remains unchanged even during part-load
                                                                                                                                                                             operation. As we know, bhp is dependent on
                                                                                                                                                                             the volume of air compressed and the com-
                                                                                                                                                                             pression ratio. By maintaining a constant com-
                                                                                                                     unloaded hp is 20 percent of full-load power.           pression ratio and decreasing the volume to be
                                                                                                                     Running in the load/unload mode requires ade-           compressed, power requirements are reduced
                                                                                                                     quate storage to maximize part-load efficiency          almost proportionally.
                                                                                                                     (See Figure 3, pg. 58).                                     For example, at 65 percent flow, a variable
                                                                                                                                                                             displacement rotary screw compressor
                                                                                                                     Variable displacement control                           requires approximately 75 percent of full-load
                                                                                                                     Variable displacement control (VDC)—also                power. In many cases, VDC regulates output

                                                                                                                                                   For FREE information, Circle 111

                                                                                            62   J U LY 2 0 0 1                                            PLANT SERVICES                         
    from full capacity down to 40 percent of flow. Below that, the compressor
    operates in load/unload mode or by modulating the inlet valve as previ-
    ously described (Figure 4, pg. 60).

    Variable speed drive control
    A variable speed drive controls compressor speed to match output with
    demand. As system pressure rises above the set point, the drive reduces
    compressor speed by changing the frequency supplied to the motor. The
    efficiency loss across the drive is typically between two to four percent.
       Compressor displacement is directly proportional to speed. By controlling
    displacement via speed and not restricting inlet flow, the compression ratio
    remains constant. This results in relatively good part-load efficiencies.
       Rotary screw air-end compressor efficiency is a function of rotor tip
    speed. Operating the rotor at any tip speed other than the designed opti-
    mum speed increases specific power (kW/100 cfm) and reduces overall
    compressor efficiency. Adjusting compressor rpm and, therefore, rotor tip
    speed to match system demand results in less than optimum operating
    efficiency at most load settings.
       When running fully loaded, most VSD factory packages operate the air-end
    above optimum rotor tip speeds. This allows the air-end to operate at a more
    efficient rotor tip speed at part load; however, less than maximum air-end effi-
    ciency is realized at full load.
        Typically, VSD-controlled rotary screw compressors improve efficiency
    at reduced flows. The best energy savings come from applications in which
    most system demand is in the low- to mid-capacity range. VSD typically
    can regulate compressor output down to 40 percent of full load—and in
    some cases, as low as 20 percent. When operating below the VSD range,
    capacity is controlled in either the load/unload or start/stop modes
    (Figure 5).

    Multiple compressor systems
    When operating a facility with multiple compressors, an efficient overall con-
    trol scheme should focus on three items:
      • Shut off unneeded compressors.
      • Delay bringing additional compressors on line until absolutely needed.

    Collaborative effort
       The Compressed Air Challenge (CAC), a non-profit corporation, began in 1997 as a

       collaboration of public, non-profit and private organizations dedicated to increas-

       ing the understanding and improving the efficiency of compressed air systems

       within the U.S. industry.

          The purpose of the CAC is to provide a solutions-neutral environment for edu-

       cating both suppliers and users of industrial compressed air systems on the bene-

       fits of taking a “systems approach.”

          The CAC has developed two levels of training for plant engineers:

       “Fundamentals of Compressed Air Systems” and “Advanced Management of

       Compressed Air Systems.” Other educational products include publications, such

       as the Sourcebook. Other materials are under development. For more information,

       call 800-862-2086 or visit

                                                                                             For FREE information, Circle 117                                          PLANT SERVICES                                    J U LY 2 0 0 1   63
compressors • C O M P R E S S O R S • compressors • C O M P R E S S O R S • compressors •

                                                                                                                          • Make sure compressors are operating fully                                compressor’s capacity need be considered.
                                                                                                                            loaded or in the most energy-efficient par-                                 For example, let’s take a plant compressed air
                                                                                                                            tial-load control mode possible.                                         system using three 1,000-cfm rotary screw com-
                                                                                                                                                                                                     pressors. Average system demand is 2,400 cfm,
                                                                                                                                                                                                     with some 2,800 cfm peaks. Assume the full-load
                                                                                                                                                                                                     hp requirement is 220 bhp per compressor. With
                                                                                                                                                                                                     all units running in a modulating control mode,
                                                                                                                                                                                                     average power will be approximately 627 bhp or 95
                                                                                                                                                                                                     percent of full-load power, while delivering only

                                                                                                                  Ideal ly, in a multiple co mp r e s s o r
                                                                                                                                                                                                     80 percent of full-load capacity (Figure 2). By run-
                                                                                                                                                                                                     ning two units fully loaded (2,000 cfm) and the
                                                                                                                                                                                                     third one as the trim compressor (40 percent load,
                                                                                                                    syste m, only one unit should                                                    400 cfm), average power requirements still remain
                                                                                                                                                                                                     at approximately 616 bhp or 93 percent of full-load
                                                                                                                    o perate at les s than ful l load.                                               power (Figure 2). By running the trim compressor
                                                                                                                                                                                                     in the load/unload mode and installing 5,000 gal-
                                                                                                                                                                                                     lons of storage capacity (5 gal/cfm of trim com-
                                                                                                                                                                                                     pressor capacity), the new average power
                                                                                                                                                                                                     requirement drops to approximately 576 bhp or 87
                                                                                                                                                                                                     percent of full-load power (Figure 1).
                                                                                                                                                                                                        Using variable displacement control on the
                                                                                                                                                                                                     trim compressor, average power requirements
                                                                                                                          Ideally, in a multiple compressor system, only                             will be approximately 581 bhp or 88 percent of
                                                                                                                        one unit should operate at less than full load.                              full load (Figure 4). With variable displacement
                                                                                                                        This unit is referred to as the “trim” compres-                              control, the 5,000 gallons of storage are not
                                                                                                                        sor. Centrifugals, or rotary screw compressors                               necessary to maintain part-load efficiency.
                                                                                                                        with modulation controls, make good base-load                                Therefore, storage system size can be limited to
                                                                                                                        units. Reciprocating compressors and rotary                                  meet intermittent demand surges or other plant
                                                                                                                        screw packages with variable displacement or                                 requirements.

                                                                                                                         Figure 5.

                                                                                                                                                                     Variable speed lubricant injected rotary screw compressor package

                                                                                                                            Percent kW input power





                                                                                                                                                           0   20              40                   60                  80                    100             120
                                                                                                                                                                                              Percent capacity

                                                                                                                                                                    %kW input vs % capacity          With unloading           With stopping

                                                                                                                        VSD controls are good choices to operate as trim                               If you understand how your compressor’s
                                                                                                                        units. Rotary screw compressors with on-                                     capacity controls work and affect part-load
                                                                                                                        line/off-line controls also run efficiently as trim                          power, you can implement an energy-efficient
                                                                                                                        units, provided adequate system storage is avail-                            control strategy, thereby lowering your plant’s
                                                                                                                                                                                                     overall energy costs. p
                                                                                                                                                                                                                 For more information, Circle 557
                                                                                                                        able. When determining the required ratio of
                                                                                                                        system volume to system capacity, only the trim

                                                                                            64   J U LY 2 0 0 1                                                           PLANT SERVICES                                                  
compressors • C O M P R E S S O R S   • compressors • C O M P R E S S O R S   • compressors
    Compressed air systems hold one of the keys to greater productivity, efficiency and profitability.
    This is the sixth in a series of articles that introduces some key points of Compressed Air Challenge training

    By Paul Shaw

                 roper compressed air storage is one           receiver should never be operated beyond its

    P            of the most important components
                 of a well designed compressed air
                 system, but it’s usually the most
                 overlooked. Call them what you
    want—tanks, air receivers, storage receivers,
    bottles or vessels—they do the same thing: store
    compressed air for future use. Storage can help
                                                               maximum allowable working pressure or without
                                                               a properly sized and installed safety valve.

                                                               Primary storage volume requirements
                                                               Traditionally, air receivers were sized for recip-
                                                               rocating air compressors. The general guideline
                                                               was that for every cfm of air produced required
    minimize system pressure loss, keep unneeded               one gallon of storage. Today, the suggested mini-
    compressors from starting, enhance compressor              mum size is between three and five gallons of
    part-load performance, separate moisture,                  storage per cfm produced. Size requirements
    reduce pulsations from reciprocating compres-              can also vary by the type of compressor, com-
    sors, and stabilize and balance air pressure.              pressor capacity control, the quantity and dura-
    When re-engineering a compressed air system or             tion of transient demand events and the interval
    buying a new air receiver, size generously.                between them. When determining the volume of
       For example, a beverage producer was losing             system storage per cfm required for multiple
    pressure at different times during the day, which          compressor systems, only the capacity of the
    disrupted production and caused another com-               trim compressor needs to be considered for
    pressor to come online. The initial response to            reducing part-load energy requirements.
    this problem was to investigate purchasing a               Further consideration must be given to ample
    larger air compressor.                                     storage to prevent another compressor from
       While working with its compressed air system            coming online to meet intermittent require-
    specialist, the company decided to monitor the             ments, as well as the time required to bring an
    system to determine exactly what was happening.            additional compressor on line should the size
    This revealed a demand event of approximately              and duration of a demand event require it.
    150 cfm that occurred several times a day and
    lasted for only a few minutes. The study also              Collaborative effort
    determined that without these intermittent
    demands, the current air compressor was suffi-
    ciently sized and operated the system efficiently.
    The final solution was to install additional storage.
                                                                 T     he Compressed Air Challenge (CAC), a non-profit corporation, began in 1997
                                                                       as a collaboration of public, non-profit and private organizations dedicated to
                                                                 increasing the understanding and improving the efficiency of compressed air sys-
    This was cost effective, and thanks to energy sav-           tems within the U.S. industry.
    ings, provided a payback of less than two years.
                                                                     The purpose of the CAC is to provide a solutions-neutral environment for edu-
                                                                 cating both suppliers and users of industrial compressed air systems on the bene-
    Proper air receiver design
                                                                 fits of taking a “systems approach.”
    Air receivers should be built to the American
    Society of Mechanical Engineers (ASME) code,                     The CAC has developed two levels of training for plant engineers:
    which governs design, material, fabrication and              “Fundamentals of Compressed Air Systems” and “Advanced Management of
    quality control. An ASME coded tank has a “U”                Compressed Air Systems.” Other educational products include publications, such
    stamp on it, is registered with the National                 as the Sourcebook. Other materials are under development. For more information,
    Board of Boiler Pressure Vessel Inspectors, and              call 800-862-2086 or visit
    should comply with state and local codes. An air                                         PLANT SERVICES                                                  SEPTEMBER 2001         47
• compressor
                                             Figure 1.

                                                                              Average kW versus average capacity with load/unload capacity control

• compressors • C O M P R E S S O R S

                                                Percent kW input





                                                                         0            20                 40                60                  80                    100                 120
                                                                                                                    Percent capacity

                                                                                             1 gal/cfm         3 gal/cfm           5 gal/cfm            10 gal/cfm

                                                                               The following graph illustrates the effect of     Pressure differential and
                                                                             receiver size on a lubricated rotary screw com-     the effects on storage
                                                                             pressor with load/unload controls (Figure 1).       For storage to work there must be a pressure dif-
                                                                                                                                 ferential and an allowable pressure band. Without
                                                                                                                                 an allowable pressure differential, there is no
                                                                                                                                 storage. The usable pressure differential and the
                                                                                                                                 air receiver size determine the available storage.
                                                                                                                                    A receiver has a given volume, nor mally
                                                                                                                                 measured in gallons, except in large sizes, where
                                                                                                                                 they are measured in cubic feet. There are 7.48
                                                                                                                                 gallons in a cubic foot.
                                                                                                                                    The amount of free air in the receiver depends
compressors • C O M P R E S S O R S

                                                                                                                                 upon the pressure. At sea level, the atmospheric
                                                                                                                                 pressure is 14.7 psia (0 psig). If air in the receiver has
                                                                                                                                 been compressed to 100 psig, the absolute pressure
                                                                                                                                 is 114.7 psia (100 + 14.7). A 1,000 gallon receiver at
                                                                                                                                 100 psig will hold the equivalent of: (1,000 *
                                                                                                                                 114.7)/(7.48 * 14.7) = 1,043 cu.ft. of free air.
                                                                                                                                    The amount of usable compressed air would
                                                                                                                                 depend on the pressure differential. For example:
                                                                                                                                 with the 1000 gallon air receiver and an allowable
                                                                                                                                 pressure differential of 10 psi (100 to 90 psig) the
                                                                                                                                 available compressed air in storage would be: 1000
                                                                                                                                 gal. x (100 - 90)/(7.48 * 14.7) = 91 cu. ft.
                                                                                                                                    The time required for the pressure to fall
                                                                                                                                 from 100 to 90 psig is proportional to the rate of
                                                                                                                                 demand (presuming no supply during the
                                                                                                                                 demand event):
                                                                                                                                    T = V *(p1- p2)/ (C * pa)
                                                                                                                                    Where T = time in minutes
                                                                                                                                    V = receiver volume in ft.3
                                                                                                                                           (in this case 1000/7.48 = 134 ft.3
                                                                                                                                    p1 = Initial pressure - psig (in this case 100)
                                                                                                                                    p2 = Final pressure - psig (in this case 90)

                                        48   SEPTEMBER 2001                                                   PLANT SERVICES                                 
       C = air demand - cfm of free air
       pa = atmospheric pressure - psia
            (in this case 14.7)

        For a demand rate of 200 cfm of free air, the time would be:
        134 * (100-90)/(200 *14.7) = 0.456 minutes
        Similarly, the pressure would take one minute to fall from 100 psig to 78 psig.
        Be careful not to raise compressor operating pressure to increase storage.
    Every 2 psi increase in operating pressure consumes one percent more in
    energy, and may also overload the motor. For example, the increase in energy
    cost for raising a 100-hp (110 bhp) air compressor’s operating pressure 10 psi
    is five percent or 5.5 bhp. For a 24/7 operation having a $0.05 per kWh energy
    rate, this is $1,900 per year in additional energy costs. As you can see, purchas-
    ing additional storage is cost effective and can provide a rapid payback.
        Pressure/flow controllers or pressure regulators help capture available stor-
    age and run the system more efficiently by providing a normal pressure band
    at the compressors and primary receivers, and by maintaining a stable, reduced
    system pressure.

    Primary storage location
    A constant source of debate is whether to locate the primary air receiver before
    or after the dryer. For a reciprocating compressor, a receiver placed after the
    compressor provides a pulsation dampening that benefits both the compressor
    and the system.
        For all types of systems, locating the storage before the dryer reduces the
    moisture load by separating condensate, providing some temperature reduc-
    tion from radiant cooling and reducing the likelihood of liquid slugging if the
    aftercooler moisture separator trap of the compressor should fail. However,
    the downside is that as the receiver fills with saturated air, a large demand
    event could overload the dryer and cause moisture to be carried downstream.
        Locating the air receiver after the dryer provides the dry air needed to
    accommodate large demand events without creating overloading. However,
    other problems can arise, such as the loss of the pressure band because of pres-
    sure drops through the dryer and filters, and compressor signal reaction. What
    is the optimum? Place storage before and after the dryer, using the wet receiver
    as the control point. This allows you to reap the benefits of both locations.

    Secondary storage
    Secondary storage can provide a source of compressed air for a single operation,
    reduce pressure drops or help balance the system. In a stamping operation, for
    example, secondary storage provides for the sudden burst of air required
    for proper press operation. Without secondary storage, there may be a
    pressure drop at the clutch requiring larger line sizes or higher pressure, or
    sometimes both.
       Storing air for a large pulse load may require needle and check valves. A
    check valve eliminates the possibility of storage being drawn off by events else-
    where in the system. Meanwhile, a needle valve controls the rate of refilling, so
    that a large demand event becomes a small, steady draw on the compressed air
       Figure 2 (see pg. 50) shows a dust-collector system. In this case, the pulsing
    of the dust collector created a pressure drop and supply problems in the sys-
    tem header. Metering with a needle valve eliminated the pressure drop in the
    header. The needle valve is set so that the secondary air receiver re-fills at a
    slow rate, just in time before the next pulse. This turned a high intermittent
    demand into a steady, small demand.

       The following formula is used to size a secondary receiver:

                                                                                          For FREE information, Circle 109                                      PLANT SERVICES                                SEPTEMBER 2001      49
• compressor
                                                                  V = (T * (C - R) * Pa)/p1 - p2                       pressure air compressor, usually a reciprocating
                                                                  Where:                                               unit, supplies air at a higher pressure to a smaller
                                                                  V = receiver volume, ft3                             air receiver.
                                                                  T = time allowed for pressure drop of 10 psi            For example, a plant had three 3,000 CFM
                                                               to occur (10 minutes)                                   centrifugal air compressors. Two supplied the
                                                                  C = intermittent demand, cfm (100 cfm)               plant; the third was left on standby. A few times
                                                                  R = rate of flow through the needle valve            a year, a compressor would shut down unex-
                                                                  Pa = absolute atmospheric pressure,                  pectedly. This caused the operating pressure to
• compressors • C O M P R E S S O R S

                                                                       psia (14.7 psia)                                fall below the required 80 psig, resulting in
                                                                  p1 = initial receiver pressure, psig (100 psig)      machine shutdowns that crippled plant
                                                                  p2 = final receiver pressure, psig (90 psig)         processes for hours.
                                                                                                                          One solution was to install excess primary air
                                                                  Proper use of secondary storage can provide          storage to cover an unexpected loss of an air
                                                               many advantages and benefits and should be con-         compressor. Another was building in a high-
                                                               sidered in an overall compressed air system             pressure, off-line storage and metering it into
                                                               strategy.                                               the system.
                                                                                                                          It takes approximately one minute for the
                                                               High pressure off-line storage                          3,000 cfm standby centrifugal compressor to
                                                               In some compressed air systems, there can be a          start producing air. For example, the start up
                                                               very large demand event that happens infre-             time for a compressor with a 10 psi differential
                                                               quently. In others, large air compressors can often     would require a 32,987 gallon air receiver (1
                                                               take several minutes before going online. With          min.* 3,000 cfm * 14.7 psia/10 psid = 4410 cf *
                                                               both cases, sometimes an air compressor is left on      7.48 = 32,987 gallons).
                                                               to provide coverage. While both conditions                 Because the manufacturer had neither the
                                                               would benefit from using a very large air               room nor the capital for this size air receiver, it
                                                               receiver, the payback doesn't always make it            elected to use high-pressure, off-line storage. A
                                                               economically viable.                                    small, high-pressure air compressor capable
                                                                  A more cost-effective solution would be using        of providing 300 psig was installed, as well as
                                                               high-pressure, off-line storage. Here a small, high-    a 1,500 gallon air receiver and the required
                                                                                                                                                     Continued on pg. 52 a a

                                             Figure 2.   Local storage

                                                                Regulator 60 psi (typical)
                                                                                            Needle valve
compressors • C O M P R E S S O R S

                                                                                                                                 Solenoids pulse
                                                                                                                                .1 to .25 seconds
                                                                                                     Check valve
                                                          From air

                                                                                                                               4- x 6-in-manifold

                                                                              Must be sized for
                                                                         peak rate of flow -2 in.

                                                              Must be within 25 ft. of
                                                          manifold for .1 second pulse

                                                                          30 to 60 gallon

                                                                          Drain                        Service valve

                                        50   SEPTEMBER 2001                                       PLANT SERVICES                              
                                        s s    Continued from pg. 50

                                        accessories to reduce the pressure to 80 psig:
                                           1,500 gallon/7.48. * (300 psig - 80 psig) /
                                           14.7 psia = 3,001 cu. ft.
                                           So far, the system has worked as designed and shutdown problems have
                                        been eliminated. High-pressure, off-line storage can be another cost effec-
                                        tive solution for a well-designed compressed air system.
                                           A complete understanding of your compressed air system and its end
                                        uses is essential to implementing storage solutions. Adequate primary and
                                        secondary storage can make systems easier to control, reduce maintenance
                                        costs and help compressors operate more efficiently. p

                                        Paul Shaw is the sales and engineering manager, Scales Air Compressor Corporation,
                                        Meriden,Connecticut. He can be reached at 203-630-5555, ext. 118.

                                          Storage formulas
                                             There are 7.48 gallons to the cubic foot.
                                             Convert gallons to cubic feet:          Gallons

                                             Cubic feet of free air required to raise a receiver from some
                                             pressure greater than 0 gage to a final higher pressure:
                                             Receiver volume in cu. ft. x (final psig - initial psig)
                                                                               Atm. pressure

                                             *Usable cubic feet of stored air:
                                             Receiver volume (ft3) x (final psig - required operating psig)
                                                                                  Atm. pressure

                                             Fill time for an air receiver:
                                              Receiver volume in cu. ft. x (final psig - initial psig)
                                                      Atm. pressure x compressor cfm

                                             If the demand in cfm is known, the size of an air receiver
                                             can be calculated as follows:
                                             V = T x C x Pa
                                                     P1 - P2

                                               WHERE              :
                                                V = Receiver volume, in cubic feet
                                                T = Time allowed (minutes) for pressure drop to occur
                                                C = Air demand, cfm of free air
                                                Pa= Absolute atmospheric pressure, psia
                                                P1 = Initial receiver pressure, psig
                                                P2 = Final receiver pressure, psig

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52    SEPTEMBER 2001                                PLANT SERVICES                                             

              Air compressor heat recovery
                     is a hot topic
                                       Done wisely, it’ll yield a cool bundle of cash

             By William Scales, P.E.                 transfer, an amount of energy from        air pressure and raises its tempera-

                       ompressed air is an expen-    the compressed air that is equiva-        ture, energy is available for trans-
                       sive utility and managing     lent to the amount of energy that         fer. Heat must be removed to
                       the air system reduces        the electric motor placed into the        maintain the compressor toler-
             costs. However, it may be possible      compressed air. This may appear to        ances and clearances, and the com-
             to reduce your plant’s total energy     be a paradox, but it confirms the         pressed air cooled to make it
             consumption by recovering the           first law of thermodynamics and           suitable for the intended use. The
             heat air compressors produce.           the principle of the conservation         compressed air still contains suffi-
             The electric motor—or other             of energy, which states that energy       cient energy to do useful work after
             prime mover—puts energy into            can neither be created nor                the heat has been removed. The air
             the air stream through the com-         destroyed; it can only change form.       compressor increased the pressure
             pression process. Heat and work         The air entering the compressor at        and proportionately reduced the
             are two different methods of trans-     atmospheric pressure has a base           volume. The energy of the original
             ferring energy.                         level of energy content. After the        entering air is now at an elevated
                It is possible to extract, by heat   compression process increases the         pressure, ready to do work down-


                                                                                           WF     Wall fan - this fan near ceiling - on in winter

                                                                                           L      Outside louver - open in summer
                                       C                                                   D      Doors open in winter, closed in summer

                                                                                           RF     Roof fan - on in summer

                           T                                                               C      Compressors
                                                                                           T      Thermostat controls fans and louver

                                                                                Figure 1. General arrangement of a compressor room.

Plant Services                                                                                    May 2003 | 45

        stream in the compressed air sys-     One of the better methods for       ever, the availability of the heat
        tem. The heat energy is usually     improving the overall efficiency of   and the opportunity to recover
        rejected to the atmosphere or the   any compressed air system is to       and use it are two different mat-
        cooling water.                      recover the rejected heat. How-       ters. Depending on the compres-
                                                                                  sor type, radiant heat losses and
                                                                                  cooling method, it’s possible to
                                                                                  recover fifty percent to ninety per-
                                                                                  cent of the total energy input in
                                                                                  the form of heat. The most com-
                                                                                  mon uses for the recovered energy
                                                                                  include process heating, supple-
                                                                                  mental space heating, heating
                                                                                  water or preheating boiler make-
                                                                                  up water. The applications are lim-
                                                                                  ited only by the imagination and
                                                                                  possible opportunities, even in
                                                                                  northern or colder environments.

                                                                                  How much
                                                                                  heat is available?
                                                                                  Motor power can be quantified as
                                                                                  kilowatts or BTU per hour. One
                                                                                  horsepower is the equivalent of
                                                                                  2,545 BTU per hour. Although
                                                                                  most rotary screw and reciprocat-
                                                                                  ing air compressors are sold in
                                                                                  nominal horsepower sizes, they
                                                                                  generally can operate at loads 10
                                                                                  percent above the motor name-
                                                                                  plate rating to achieve rated com-
                                                                                  pressor discharge pressure and full
                                                                                  capacity output. The horsepower at
                                                                                  the compressor shaft, also referred
                                                                                  to as brake horsepower (bhp), can
                                                                                  be 10 percent above motor name-
                                                                                  plate horsepower and consume
                                                                                  most of the motor’s 1.15 service fac-
                                                                                  tor safely. Therefore, a 100-horse-
                                                                                  power compressor (110 bhp)
                                                                                  converts electricity into almost
                                                                                  280,000 BTU per hour at full load.
                                                                                  In addition, a motor having an
                                                                                  assumed efficiency of 93 percent
                                                                                  dissipates an additional 19,600
                                                                                  BTU per hour.
                                                                                     The heat balance differs by

                  For FREE information, Circle 314

46 | May 2003                                                                     Plant Services
                                    Compressors                                              OEM quality
         compressor type. Presently, the
         most common compressor found
                                                  requires an additional manufactur-
                                                  ing line, so management is consid-          You need
         in manufacturing plants is the           ering the addition of a fourth

         lubricant-injected, rotar y screw        50-hp or 100-hp compressor.
         unit supplied as a packaged com-             At a consultant’s recommenda-
         pressor, a fact that makes it easier     tion, an inexpensive flow meter
         to recover the heat. In these com-       determined the milling machine
         pressors, approximately eighty           and robot production line
         percent of the heat is rejected in       required approximately 95 cfm
         the lubricant cooler. Most of the        (almost 25 hp), 55 cfm for the
         remaining heat is rejected in the        process and 40 cfm for the parts
         aftercooler, with a small percent-       dr ying. The parts dr yer on the
         age rejected in the form of heat         new milling machine process was
         radiated from the compressor             replaced with a two-kW blower.
         housing and lubricant separator              However, the existing compres-
         receiver. In a two-stage lubricant-      sors didn’t have any spare capac-
         free rotar y screw compressor,           ity. In the event of a malfunction,       When you need quality parts
         almost all the rejected heat is          some production process would           for industrial pumps and other
         divided evenly between the after-        be interrupted. Because no more         rotating equipment your choice
         cooler and intercooler. In two-          compressor room space is avail-         should be ProCast.
         stage, water-cooled reciprocating        able, the alternative is a 25-hp air-   ProCast is a subsidiary of
         compressors, the intercooler and         cooled, rotary screw compressor         ITT Industries, the largest pump
         aftercooler each may each reject         installed at the drying end of the      company in the world. It has the
         40 percent of the heat, while the        new milling machine line.               resources and technical expertise to
         cylinders account for 20 percent.            Recovering the heat from the        manufacture NEW replacement parts
         A centrifugal compressor may             air-cooled compressor and direct-       that meet or exceed OEM standards.
         have each intercooler and after-         ing it at the parts to be dried elim-   And, we can do it worldwide.
         cooler share almost equally in the       inates 40 cfm of consumption.           ProCast provides technical engineer-
         heat load. Given these facts, con-       The excess capacity from the 25-        ing assistance, raw castings, and
         sider the possibilities of heat          hp compressor can feed back into        finished machine parts or total
         recover y from an engine-driven          the main plant air system. This         equipment repair. Standard lead times
                                                                                          are four weeks for cast parts and two
         air compressor.                          solution, combined with other
                                                                                          weeks for turned parts, with
                                                  measures, reduces compressed air
                                                                                          expedited overnight delivery for
         Some heat                                consumption to the point that one
                                                                                          emergency breakdowns.
         recovery projects                        50-hp compressor can be shut off.
         Consider a rather small compres-         The solution negates the need to        To learn more, contact ProCast at
         sor room with three 50-hp, air-          purchase a larger compressor. In        1-800-543-6263, or visit
         cooled compressors, each of which        addition, a simple heat recovery
         is operating at full capacity to main-   system installed in the main com-       where you can order your
         tain the required air pressure. One      pressor room is used for supple-        replacement parts on line 24/7.
         production line requires com-            mental factor y space heating in
         pressed air for a special milling        winter. Both projects have short
         machine and an associated robot,         payback periods.                        ProCast
         while the final stage uses com-              For many years, hot air from the
         pressed air for cleaning and drying      compressor discharge prior to the
         parts. Increased production              aftercooler had been used to reheat

                                                                                          For FREE information, Circle 320

Plant Services                                                                    May 2003 | 47
 For Our Readers...…

Circle the number on the reader service card for

products in which you have an interest and you will
receive the latest catalogs or literature from manu-
facturers advertising in Plant Services.                The initial cost of a                   compressor system. The initial

500   Air Filters & Dryers
                                                                                                cost of a good ventilation system is
511   Anti-Slip Coatings                                  good ventilation                      usually far less than the cost of a
498   Bearings
508   Belts
                                                                                                hot operating environment. It’s
497   Boilers/Boiler Controls
                                                        system is usually far                   advisable to consult with HVAC
513   Bunker, Silo Clean out                                                                    experts to ensure proper ventila-
496   Cleaning Equipment & Services                    less than the cost of
495   Compressors
                                                                                                tion and to evaluate possible heat
494   Computerized Maintenance                                                                  recovery opportunities. However,
      Management Sys.                                      a hot operating
493   Contract Maintenance/Contract Services
                                                                                                remember to keep installations as
492   Cooling Towers                                         environment.                       simple as possible.
491   Doors, Seals, Shelters
490   Drives,Variable Speed
                                                                                                   When considering heat recovery
489   Dust/Fume Collectors & Separators                                                         for supplemental space heating,
488   Electrical Service
502   Environmental Compliance Instrumentation
                                                                                                the expense of properly sized duct-
487   Fans & Blowers                                   the compressed air after initial cool-   work and booster fans can be sig-
486   Filtration Equipment
485   Floors, Decks, Grating
                                                       ing or drying. This adds additional      nificant, but the payback also can
484   Forklift Trucks                                  energy to the air. However, care         be substantial. The compressor
483   Hazardous Waste Treatment
                                                       must be taken to ensure the end          ventilating system design should
482   Heat Exchangers, Condensers & Coolers            uses can accept the elevated tem-        consider its effect on the rest of the
481   Hoists & Cranes                                  perature. Also, all piping should be     plant’s ventilating system, includ-
480   Hose,Tubing & Fittings
479   Instruments & Controls
                                                       insulated to reduce heat lost            ing heating, air conditioning and
478   Insulation Materials                             through radiation and, more              possible negative pressures. Bal-
477   Lighting
476   Loading Docks
                                                       importantly, to protect personnel        ance savings against installation
475   Lubricants & Greases                             who might touch exposed pipes.           cost and evaluate payback using life
474   Maintenance Tools & Replacement Parts
473   Maintenance Storage
                                                       Air from the discharge of non-           cycle cost analysis before proceed-
472   Modular Buildings/Partitions                     lubricated rotary screw compres-         ing with any project.
471   Motor Controls/Starters
470   Motors, Electric
                                                       sors or from centrifugal compres-
509   Motor Testing                                    sors, prior to the aftercooler, can be   Case studies
507   Multi-meters/Oscilloscopes
501   Oil/Water Separators
                                                       used to regenerate the desiccant in      A company that packaged pharma-
469   PCB Handling                                     heat-of-compression regenerative         ceutical products operated two
468   Paints & Coatings
                                                       dryers.                                  100-hp, air-cooled, rotar y screw
466   Pipe Fittings & Couplings                           Compressed air from lubricant-        compressors at full capacity for
504   Power Monitoring Systems                         free rotary screw compressors or         8,000 hours per year. Occasionally,
457   Power Transmission Equipment
465   Predictive Maintenance Systems
                                                       from upstream of the aftercooler         when a special process was in oper-
464   Programmable Controllers                         on centrifugal compressors can be        ation, a third compressor was nec-
463   Pumps
462   Pneumatic Equipment
                                                       used to regenerate desiccant in          essary; for this example, only the
503   Rental Equipment                                 heat-of-compression regenerative         two compressors will be assumed to
506   Roller Chain & Sprockets
461   Roofs
                                                       dryers.                                  be running. The brake horsepower
460   Safety Equipment & Systems                                                                (bhp- measured at the compressor
512   Safety Screens, Sheilds & Curtains
459   Seals, Gaskets & Packing Materials
                                                       Supplemental space                       shaft) is 110, and the motor effi-
510   Shredders & Compactors                           heating and ventilation                  ciency is 0.93; the fan motor has an
458   Steam Traps & Condensate Handling
499   Training Programs
                                                       Improper ventilation in compres-         efficiency of 0.90 and requires 5
456   Truck Restraints                                 sor rooms can result in elevated         bhp. The conversion factor is 2,545
455   Uninterruptible Power Sources                    operating temperatures that              BTU per hour per bhp.
505   Vacuuming Equipment
454   Valves                                           reduce equipment life, generally         Energy input = (110/0.93 + 5/0.9) x
453   Water Pollution Control Equipment &              increase maintenance and repair          2,545 x 2 = 630,320 BTU per hr., of
452   Water Treatment Chemicals/Equipment
                                                       cost and yield an unreliable air         which only 80 percent, or 504,000

48 | May 2003                                                                                    Plant Services

         BTU/hr., is recoverable.                fans may be necessar y, and the
            For natural gas costing $0.50        cost of their operation should be
         per therm and a heating season of       considered. Size the louvers for
         4,000 hours, the annual saving is:      minimum restriction and main-
         504,000 BTU/hr. x $0.50/100,000         tain an air velocity between 10
         BTU x 4,000 hr./yr. = $10,080/yr.       ft./sec. and 15 ft./sec. Follow the
            Many applications require            compressor and dryer manufac-
         deducting the energy cost of oper-      turer’s recommendation for maxi-
         ating additional ventilating or         mum static pressure.
         booster fans from the savings. Fig-        Introduce cooling air at a rela-
         ure 1 shows the general arrange-        tively low point, but not low enough
         ment of the compressors within          to pick up dirt or dust. Dusty envi-
         the plant and warehouse area.           ronments require generously sized
         Plant personnel were able to add        air panel filters. Keep the filters,
         an outside louver and wall fan          louvers and cooler surfaces clean
         without needing any ductwork. In        for best compressor performance
         winter, plant air is heated and dis-    and reduced maintenance costs.
         charged to the warehouse. During           Direct the cooling air toward
         warmer months, the heated air is        the compressor, dr yer fan inlet
         directed outside.                       and through the cooling surfaces
            Another manufacturer has mul-        across the compressor and dryer.
         tiple 350-hp and 150-hp, water-         Exhaust warm air from the room
         cooled, rotary screw compressors in     at a level above the compressor or
         two plant areas. The original com-      other air-cooled equipment. In
         pressor area in the boiler room has     multiple compressor installations,
         no additional space available. The      ensure that warm air from one
         second area in the middle of the        compressor or dryer isn’t directed
         plant has no outside walls. A plant     towards the fan inlet of another.
         expansion required an additional           In most rotary screw compres-
         350-hp compressor and a larger          sors, the air leaving the coolers can
         cooling tower. The compressor ven-      be 20° F to 40° F warmer than the
         dor proposed an air-cooled com-         inlet air; therefore, recirculated
         pressor. The heat recovered in          plant air can be used for supple-
         winter was sufficient to heat the       mental heating. However, recov-
         entire section of the plant in which    ered heat won’t warm sub-freezing
         the compressor was located. In          outside air to a temperature that
         summer, heat was ducted through         will reduce heating fuel costs.
         the roof. The initial estimate of          Water-cooled compressors are
         annual fuel savings was $28,000.        also a potential source of heat
                                                 recovery. The warm outlet water
         Recovery tips                           can be directed through either a
         Louvers and ductwork create             liquid-to-liquid heat exchanger to
         restrictions to the flow of ventilat-   heat a fluid or a water-to-air unit to
         ing air and may impede the neces-       warm the loading dock or manu-
         sary fan airflow to effect proper       facturing area. These heat recov-
         cooling. In some cases booster          er y systems are available as

                                                                                          For FREE information, Circle 308

Plant Services                                                                 May 2003 | 49

                                                                                        packaged units and include the
                                                                                        pumps and controls for simplified
                                                                                        installation and economical oper-
                                                                                        ation. Motor losses, however, are
                                                                                        not easily recovered, but this heat
                                                                                        will be rejected to the area in
                                                                                        which the compressor is located.

                                                                                        Using imagination
                                                                                        In one case, a manufacturer used
                                                                                        natural gas to heat wax used in its
                                                                                        end product. The plant engineer
                                                                                        designed a system of coils that
                                                                                        used the heat from the lubricant
                                                                                        of a nearby water-cooled rotar y
  Ingersoll-Rand SSR                                        AMSOIL Sirocco™ Synthetic   screw compressor to help melt the
                                Sullair Sullube 32®
     Ultra-Coolant®                                             Compressor Oil
                                                                                        wax and substantially reduce the
This photo compares Ingersoll-Rand SSR Ultra-Coolant® and Sullair Sullube 32® with
                                                                                        plant’s gas consumption.
AMSOIL Sirocco™ Synthetic Compressor Oil. Only Sirocco™ readily separates from water.
                                                                                           In a food processing plant, the
Advanced Technology Finally, a cost-effective synthetic rotary                          jacket water from an engine-
screw compressor oil that replaces Ingersoll-Rand SSR Ultra-                            driven compressor liquefied
Coolant® and Sullair Sullube 32® without the need for flushing.                         chocolate and sugar. Passing the
AMSOIL Sirocco™ Synthetic Compressor Oil lasts just as long                             hot lubricant from the air com-
(8000+ hours) and is also compatible with other compressor oils made                    pressor through multiple heat
from synthetic PAO, ester, or petroleum. Minimize your inventory,                       exchangers could have achieved
prevent mistakes, and simplify your compressor maintenance by                           similar results.
taking advantage of AMSOIL Sirocco™ Synthetic Compressor Oil.                              The opportunities are excel-
Environmentally Friendly AMSOIL Sirocco        is biodegradable
                                                            ™                           lent. Use your imagination to
and does a better job of separating from water when compared                            recover air compressor heat and
to Ingersoll-Rand SSR Ultra Coolant® or Sullair Sullube 32®.                            reap the benefits of reduced
These improvements ease condensate disposal, presenting fewer                           energy consumption and lower
environmental concerns.                                                                 operating costs. p
Fairly Priced With all these benefits, you’d expect to pay more.
Sirocco™ Synthetic Compressor Oil is priced over $1,000 less per                        William Scales, P.E., is chief executive
drum when compared to Ingersoll-Rand SSR Ultra Coolant® and                             officer of Scales Air Compressor Corp.
Sullair Sullube 32®.                                                                    He can be reached at 516-248-9096.
Act Now! Your wait for high-performance compressor oil at reasonable
prices is over. Contact AMSOIL today.                                                   Figures: Scales Air Compressor Corp.

or visit our website at                                                                     READER INTEREST REVIEW                                                                            Please circle the appropriate
Click on our Industrial                                                                   number on the reader service
Products link.                                                                            card to indicate the level of
                                                                                          interest in this article.

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      Plant Services Magazine 2003, for problem solving and innovation!
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 50 | May 2003                                                                             Plant Services

              ost compressed air systems are relatively mod-       • Is the current compressed air quality satisfactory?
              est at first, but grow and develop as production,     • Does this project require professional help?
              inappropriate uses and leaks increase over time.
              Even assuming that air leaks and inappropriate       Compressor size
uses have been investigated and reduced, adding production            e current average and peak compressed air flow rates, in
equipment raises demand for compressed air. Some equipment         cubic feet per minute (cfm), should have been established
may need a different operating pressure, and the required air       before considering any proposed additions. e rated output
quality may change. ese modifications represent potential           of the existing compressor(s) also should have been estab-
problems for the plant engineer who wonders:                       lished. e specifications for the proposed equipment that
• What is the required volumetric flow rate and the pressure        needs additional compressed air should state the required
  of compressed air, and what size of air compressor should        flow rate, pressure and air quality. is information provides
  be added?                                                        the new total flow rate. Deducting the rated output of the
• What type of compressor and control system offers greatest        existing compressor(s) gives the additional air flow required
  reliability and lowest life-cycle operating costs?               from a new compressor.
• Where should the compressor be located?                                is calculation ignores potential leakage and increased
• Is sufficient power, ventilation and cooling capacity available?   inappropriate compressed air use. Also, it doesn’t account
• What type of system capacity controls would be best?             for differences in the frequency of operation of each piece of
• Is primary compressed air storage sufficient?                      production machinery. Individual consumption peaks might
• Is distribution piping adequate?                                 not occur simultaneously. Nevertheless, you’ll need to deter-
• Is secondary compressed air storage sufficient?                    mine the average and peak flow rates (Table 1).

O                                             .PLANTSERVICES.                                                    41

   Don’t add a fudge factor to the       the compressor that combines the         compressor. Conventional wisdom
required air f low when calculating      best full-load and part-load economy     says that three 50%-capacity com-
the rated capacity for the additional    to minimize the operating cost over      pressors are better than two 100%
air compressor(s) because it could       the full range of the compressed air     compressors, because this provides
result in the new unit(s) operating      requirements.                            more f lexibility without sacrificing
at less than full capacity most of the      Another important consideration is    system reliability, particularly during
time, robbing efficiency. Should you     standby capacity in case of compres-     periods of reduced consumption. It’s
decide to oversize the additional        sor malfunction or needed repair. This   also beneficial to operate the smallest
compressor by 20% or more, select        may require at least one additional      total compressor horsepower, particu-
                                                                                  larly for periods of reduced capacity
                                                                                  requirements, such as a second- or
                                                                                  third-shift operation. These factors,
                                                                                  combined with reliable compressor
                                                                                  service, are keys to maintaining energy
                                                                                  and production efficiencies and profit-
                                                                                  able outcomes.

                                                                                  Compressor type
                                                                                  Each t ype of compressor has its
                                                                                  advantages, disadvantages and pre-
                                                                                  ferred range of capacity and pressure.
                                                                                  Table 2 provides a simple method
                                                                                  for comparing different compressor
                                                                                  types. Life-cycle cost analysis always
                                                                                  is recommended, and should include
                                                                                  specified maintenance.

                                                                                    Don’t add a fudge
                                                                                  factor to the required
                                                                                       air flow when
                                                                                      calculating the
                                                                                    rated capacity for
                                                                                    the additional air
                                                                                     Compressor cooling is a major con-
                                                                                  sideration. If water-cooled, the impor-
                                                                                  tant issues include availability and
                                                                                  quality of cooling water, disposal or
                                                                                  recirculation, possible treatment and
                                                                                  overall cost. If compressors are air-
                                                                                  cooled, adequate room ventilation is
                                                                                  essential. Heat recovery also is a poten-
                                                                                  tial opportunity.
                                                                                     Compressor size and type deter-
                                                                                  mines the electrical power require-
                                                                                  ments. Additional ancillary equip-
                                                                                  ment may require a different voltage
                                                                                  and current. Consider, too, the
                                                                                  availability of the required electri-
                                                                                  cal supply and its support equip-
                                                                                  ment. Ensure that proper circuit
                                                                                  protection is provided for the added
                                                                                  electrical load.

42                                                                           October 2005

              Table 1. Basic demand worksheet
  End user     Minimum         Average     Peak flow   Cycle time
  identity     flow (cfm)     flow (cfm)     (cfm)     (seconds)

Note: In some cases, the minimum flow rate may be very low or
zero (cycle time - off) until an intermittent operation (demand
event) occurs, when there is a large demand (peak flow rate) for a
time (cycle time – on). The combination of these determines the
average rate of flow. End users having a constant demand should
be tabulated by the average flow rate. Peak flow events may require
additional primary storage and secondary storage.

Location, location, location
Many plants have a compressor room and, in some cases, that
room is shared with other equipment. Several factors must
be considered if a compressor is to be added. These include
the need for a foundation, space for maintenance activities,
space for drying and filtration equipment, room ventilation
to handle the added heat release and the sound level.
   It might make sense to install the new compressor in a dif-
ferent location, perhaps closer to the point of greatest demand
or at the application requiring the highest pressure.

Capacity controls
There are several types of capacity control for individual com-
pressors, sequencing controls for multiple compressors and
pressure and flow controls for compressed air systems. The
correct selection of each determines system efficiency over
the anticipated operating ranges.
   Two rules for achieving optimum efficiency are (1) only
the number of compressors needed to maintain the required
system pressure should be in operation at any given time, and
(2) all but one, a trim compressor, should be running at full
capacity and pressure. The trim compressor should have an
efficient capacity-control mode. If it’s a reciprocating com-
pressor, this could be unloading in a series of capacity steps.

     Standby capacity in case of
  compressor malfunction or needed
        repair may require an
       additional compressor.
For a rotary compressor, variable-speed control or variable
displacement is most efficient.

Storage and piping
The size and location of the primary air receiver affects the

October 2005                                    43

                                                               efficiency of your capacity control. Efficient system control
                          Electric Motor                       may require changes in air storage volume, but adding receiver
                              Predictive                       volume won’t compensate for insufficient compressor capacity
                                                               or inadequate distribution piping.
                    Maintenance made                              Many compressed air distribution systems originate at
                                  Simple                       a primary air receiver in the compressor room where dis-
                                                               tribution piping moves the air throughout the plant. As
This is the bolt that rusted & broke causing
                                                               production increases, another one or more buildings might
severe problems in your manufacturing
process.                                                       be erected, but the supply of compressed air still passes
                                                               through the original distribution piping. If the piping isn’t
That overheated the motor causing                              adequate for the increased demand, the result can be exces-
                                                               sive pressure losses and increased energy consumption.
unplanned motor failure.

That cost millions in downtime, annual raises                  The size and location of the primary
& your job.                                                    air receiver affects the efficiency of
Could this been prevented?
                                                                      your capacity control.
                                                                 Intermittent high-volume demand can cause severe
Certainly!                                                     dynamic pressure fluctuations in the entire compressed air
                                                               system that can upset manufacturing processes. Many fluc-
Baker Instrument Company offers electric                       tuations can be softened with an appropriately sized and
motor testing solutions that help you                          located secondary air receiver that can provide enough air to
manage your predictive maintenance
                                                               satisfy the intermittent demand without compromising the
program with ease. These motor circuit
                                                               pressure in the main system.
                                                               Air quality
                                                               Each piece of production equipment requires compressed
                                                               air at a given flow rate, pressure and air quality. These con-
                                                               siderations may vary significantly. A cardinal rule is to avoid
before they                                                    drying and filtering compressed air any more than is needed
develop.                                                       for the specific application. Going overboard can result in
Create a                                                       increased pressure losses and energy consumption. Consider
strong defense                                                 the idea of satisfying the major compressed air requirements
against motor
failure with a                                                   A typical compressed air system
e static and dynamic program -
empowering maintenance professionals with
tools to prove the reliability of all in-process

           Baker Instrument Company
           800/752-8272  970/282/1200                           Figure 1. Compressor efficiency is a function of pipe
                                      geometry, leaks, operating pressure and other factors.

      44                                                                           October 2005

                                           Table 2. Simple matrix for comparing compressor types1
                                                                             Compressor type
          characteristic2         Two-stage, double-
                                                          Lubricant-injected screw       Lubricant-free screw              Centrifugal
                                  acting reciprocating
         Size and weight                   3                         1                             2                            2
       Compact size and
                                           3                         1                            1-2                           1-2
       complete package
      Can be located close
                                           4                        2-3                           2-3                           3
        to points of use
        Maintenance cost                   3                         2                             2                            1
    Foundation requirements                4                         1                             1                            1-2
                                          1-2                       1-4                           1-3                          1-3
       capacity efficiency3
    Lubricant-free air delivery
                                          4/1                        2                             1                            1
         - lube/lube-free
       Lubricant carryover
                                          4/1                        3                             1                            1
         - lube/lube-free
     Lubricant changes or
                                          4/1                        3                             1                            1
    makeup - lube/lube-free
            First cost,
                                           4                         1                             2                            2
      including installation
       Full-load operating
                                        15 to 16                  16 to 19                      18 to 22                     16 to 20
        cost, kW/100 cfm4
 These evaluations are general in nature and might not cover specific features of a given compressor type or manufacturer. They’re intended
to provide a general guide for comparing compressors. It’s important to evaluate each point in any comparison of quoted equipment. Other
factors to be considered include relative size and cost, warranty and service.
  Each compressor type is rated from 1 to 4. Key: 1 = very good; 2 = good; 3 = fair; 4 = poor.
  Refer to the section on compressor controls. It’s important to compare kW/100 cfm at all reduced capacities.
  Operating costs are based on full capacity at a discharge pressure of 100 psig; a full-load motor efficiency of 92% and 0.746 kW/bhp.

centrally and supplementing these requirements locally,                   helpful. An alternative approach is hiring an independent
where needed.                                                             compressed air consultant to provide a product-neutral opin-
  Many industrial applications can be served well with a                  ion or solution. p
pressure dewpoint of 35°F to 38°F, which can be achieved
with a refrigerated dryer. Standard regenerative desiccant                Bill Scales, P.E. owns Scales Air Compressor Corp. in Carle Place,
dryers can drop the pressure dewpoint to –40°F, and more                  N.Y. Contact him at and (516) 248-9096
specialized dryers can bring it down to –100°F. Dry the air               ext. 611. David M. McCulloch owns Mac Consulting Services in
only to the requirements of the end users or to meet local                Shalimar, Fla. Contact him at and (850)
ambient conditions.                                                       651-4540.

                                                                          The Compressed Air Challenge (CAC) is a national collabora-
     Each piece of production                                             tive of public and private organizations dedicated to increasing
equipment requires compressed air                                         the understanding and improving efficiency of compressed air
   at a given flow rate, pressure                                         systems within U.S. industry. The CAC offers Fundamentals of
           and air quality.                                               Compressed Air Systems and Advanced Management of Com-
                                                                          pressed Air Systems training, and in cooperation with the U.S.
                                                                          Department of Energy, the Compressed Air Systems Sourcebook
   Improving your air quality also requires filters to remove             for Industry as well as the Qualified AIRMaster+ Specialist train-
particulates and might require coalescing and adsorption fil-             ing. CAC has built a reputation for being a reliable resource for
ters to remove liquids and other contaminants. These added                cost-effective solutions and unbiased information, including the
filters will result in increased pressure losses and maintenance          recent publication Best Practices for Compressed Air Systems, a
requirements.                                                             comprehensive and detailed reference for plant personnel. For
                                                                          more information about CAC training and publications call (301)
Outside help                                                              751-0115 or visit The authors
In most cases, seeking professional help is a good idea. Equip-           of this article, David McCulloch and Bill Scales were also the
ment distributors with good local service capabilities can be             authors of Best Practices for Compressed Air Systems.

October 2005                                                                                                    45
     Knowing operating
     characteristics and physical
     limitations will help you
     select the best machine
     for your plant
     By Niff Ambrosino

             ost industrial air compressors are
             supplied as self-contained packages
             that include drive motor, inlet filter, me-
     chanical and electrical controls and various optional      ating principle for positive-displacement compressors
     accessories. Depending on the type and size of air         is either reciprocating or rotary.
     compressor, the package might be mounted on an air
     receiver. Air compressors are classified either as posi-   Reciprocating type
     tive-displacement or dynamic.                              A reciprocating compressor uses a moving piston in
                                                                an enclosed cylinder. In a single-acting design, com-
     Positive-displacement compressors                          pression takes place on just one side of the piston and
     Air is drawn into an enclosed chamber where the            produces air on only one stroke per revolution. Dou-
     volume is reduced by mechanical means, causing the         ble-acting compressors develop compression on both
         pressure within to rise and forcing the air into       sides of the piston and produce air on two strokes per
                   the system. A simple example of a pos-       crankshaft revolution. This results in almost twice
                           itive-displacement compressor        the capacity of a single-acting design of identical bore
                                 is the hand pump for in-       and stroke. In either case, the compressor might be
                                     flating tires. The oper-   air or water-cooled, lubricated or non-lubricated.

34                                                                         January 2008

   Single-stage reciprocating com-           Rotary compressors                          spinning rotors, which then force the
pressors have one or more cylinders          The lubricated rotary-screw air com-        air into the decreasing inter-lobe cav-
connected in parallel to compress air        pressor is the most widely used design      ity until it reaches the discharge port
from atmospheric pressure to the final       for industrial applications. It’s charac-   at the opposite end of the rotor. Oil
discharge pressure in one step. Most         terized by low vibration, a simple in-      injected into the rotor housing lubri-
single-stage compressors are designed        stallation with minimal maintenance         cates the moving parts, removes heat
for a maximum discharge pressure of          in broad ranges of capacity and pres-       and seals the clearances to prevent
100 psig.                                    sure. A rotary-screw air end consists       back slippage of the compressed air.
   Multi-stage reciprocating compres-
sors, on the other hand, have two or
more cylinders connected in series.                       A two-stage double-acting type
Each stage adds some degree of com-                 reciprocating compressor is the most energy
pression. For example, in a two-stage                         efficient air compressor.
unit, air is compressed from atmo-
spheric pressure to an intermediate
pressure in the first stage, cooled by       of two close-clearance helical rotors       The air/lubricant mixture discharges
an intercooler and raised to the dis-        turning in synchronous mesh. The            into the lubricant reservoir, which
charge pressure in the second cylinder.      male rotor has four helical lobes and       also serves as a separator that relies
Multi-stage reciprocating compressors        the female rotor has five or six mating     on directional and velocity changes.
are more efficient, run cooler and have      grooves. In a lubricated rotary-screw       A coalescing-type filter reduces the
longer life than single- stage compres-      compressor, the male rotor drives the       final lubricant-to-air concentration to
sors, all because the intercooler(s) re-     female rotor.                               3 ppm to 5 ppm.
move the heat of compression. While            Ambient air is drawn through a               Operating at too low a system pres-
not typical for plant air, some special-     suction port into a space between the       sure (65 psig to 75 psig) increases the
application, two-stage compressors
can deliver 250 psig or more.
   Single-acting reciprocating compres-
sors are commonly air-cooled, have one
or two stages and available to 150 hp.
However, in most industrial applica-
tions, the maximum size is generally 30
hp. A measure of operating efficiency
is called specific power and is the kW
input to produce 100 cfm, or kW/100
cfm. For a single-stage, single-acting
compressor, the specific power is ap-
proximately 24 kW/100 cfm at 100 psig.
Typical specific power for a two-stage,
single-acting compressor at 100 psig is
19 kW/100 cfm to 21 kW/100 cfm.
   Double-acting compressors are gen-
erally water-cooled and range in size
from 25 hp (single-stage) to 500 hp.
Common sizes for two-stage indus-
trial applications range between 75 hp
and 250 hp. A two-stage double-acting
reciprocating compressor is the most
energy-efficient air compressor. Typi-
cal specific power at 100 psig is approxi-
mately 15 kW/100 cfm to 16 kW/100
cfm. Double-acting compressors have a
higher initial price, more expensive in-
stallation and higher maintenance costs
than other types of compressors.

January 2008                                                                                  35

                 velocity across the separator, which leads to greater lubri-
                 cant carryover. To prevent carryover at startup and when
                 system pressure is too low, a minimum-pressure device is
                 used to maintain internal compressor pressure above the
                 manufacturer’s minimum.
                    The lubricant separated from the air stream circulates
                 through a cooler and filter before being injected back into
                 the air end. The temperature of the lubricant at the injection
                 port needs to be high enough to prevent condensation from
                 forming in the lubricant. Air-cooled units use a thermo-
                 static valve to maintain an injection temperature of 150°F to
                 170°F. Water-cooled packages use a water-flow-regulating
                 valve or thermostatic valve, or both.

                          Variable-displacement or
                       variable-speed capacity control
                      can improve part-load efficiency.

                    To remove condensate, rotary-screw packages use an after-
                 cooler to reduce the discharge air temperature and a moisture
                 separator with an automatic drain. Most industrial applications
                 use air-cooled heat exchangers for the lubricant and compressed
                 air. Water-cooled models use shell-and-tube exchangers.
                    Single-stage lubricated rotary-screw compressors are
                 available in sizes from 5 hp to 600 hp and produce between
                 35 psig and 210 psig. Typical 100-psig specific power at full
                 load is approximately 18 kW/100 cfm to 19 kW/100 cfm.
                 Variable-displacement or variable-speed capacity control
                 can improve part-load efficiency.
                    Two-stage compressors are available for operation up to 500
                 psig. They generally achieve better energy efficiency when used
                 as base-load compressors. At 100 psig, specific power is ap-
                 proximately 16 kW/100 cfm to 18 kW/100 cfm. Variable-speed
                 control can also make them suitable as trim compressors.
                    Non-lubricated rotary-screw air ends are similar to the
                 lubricated variety, except for the lack of lubricant injection.

                    More resources at

                    Efficiency and loading – “Load ‘em up”
                    Air dryers 101 – “There’s more than one way to dry the air”
                    Unit cost of air – “How much is too much?”
                    Heat of compression calcs – “What does Mother Nature say
                    about cooling hot air?”
                    Capacity control – “Finding the right match”
                    Speed-regulated drives – “Drive down the cost of compressed
                    Performance determinants – “Applying the pressure”
                    Dryer economics – “Don’t get hung out to dry”
                    Oil-free air – “Keep it clean”

                    For more, search using the keywords
                    cfm, compressor and efficiency.

36                                        January 2008

 TAble 1: CompArison of Compressor Types (100 hp To 500 hp)
 Category (**)              Two-stage double          Lubricant-injected    Lubricant-injected   Lubricant-free screw   Centrifugal
                           acting reciprocating      screw (single-stage)    screw (two-stage
 Size and weight                     3                        1                     2                     2                 2
 Complete package                    3                        1                     1                    1-2                1-2
 Can be located close
                                     4                       2-3                   2-3                   2-3                3
 to points of use
 Maintenance costs                   3                        1                     1                     1                 1
                                     4                        1                     1                     1                 1-2
 Reduced capacity
                                    1-2                      1-4                   1-4                   1-3               1-3
 Lubricant free
                                    4/1                       2                     2                     1                 1
 air – lube**/lube free
 Lubricant carry-over
                                    4/1                       3                     3                     1                 1
 - lube/lube free
 Lubricant changes
 or make-up                         4/1                       3                     3                     1                 1
 - lube/lube free
 Equipment cost                      4                        1                     2                     2                 2
 Installation cost                   4                        1                     1                     1                 2
 Full Load Operating
                                   15-16                    18-19                 16-18                 18-22             15-20
 kW/100 cfm ***
 Key: 1 = Very good; 2 = Good; 3 = Fair; 4 = Poor.

These evaluations are very general and might
not cover specific features of a given compres-
sor type or manufacturer. They are intended
to provide a general guide in how to compare
compressors. It’s important to evaluate each
point in any comparison of quoted equip-
ment. Other factors to be considered include
Warranty and Service requirements.
* It’s important to compare kW/100 cfm
  ratios at reduced capacity. Rotary com-
  pressors having variable displacement
  or variable-speed drive capacity controls
  might provide substantial energy savings
  when operating at reduced capacities.
** Any compressor requires appropriate
   downstream air quality treatment.
*** Specific power kW/100 cfm based upon
    full capacity at a discharge pressure of
    100 psig and a full-load motor efficien-
    cy of 92%. Higher motor efficiencies are
Source: “Best Practices for Compressed
Air Systems,” a Compressed Air Challenge
Publication (McCulloch and Scales 2007)
Niff Ambrosino is general manager at Scales
Industrial Technologies, West Paterson, N.J.
Contact him at and (973)
890-1010, ext. 230. This article is based on
information in the Compressed Air Challenge
publication “Best Practices for Compressed Air
Systems” manual (2007, David McCulloch and
William Scales, P.E.).
From “Best Practices for Compressed Air Sys-
tems”, a Compressed Air Challenge Publication
(McCulloch and Scales 2007)

January 2008                                                                                           37

     Two types are available - dry and wa-        spirally towards the center. Most
     ter-injected.                                scroll manufacturers limit maximum
        In the dry type, the male rotor doesn’t   discharge pressure to 115 psig.
     drive the female rotor. Instead, tim-
     ing gears, which are separated from          Dynamic compressors
     the compression chamber by lubricant         A dynamic compressor uses high-
     seals and air seals, maintain proper rotor   speed impellers to impart velocity to
     clearances. The operating temperatures       the air. A centrifugal compressor is
     are around 350°F to 400°F because            similar to a centrifugal pump, with
     there’s no fluid injection to help remove    ambient air entering at the center of
     the heat of compression. Single-stage        a high-speed impeller that accelerates
     models can reach 50 psig. Most manu-         the air radially. This velocity head is
     facturers use a two-stage design with an     translated to pressure head at the dis-
     interstage cooler to produce pressures       charge volutes or diffusers.
     of 100 psig to 150 psig. Typical specific       The number of stages and impeller
     power at 100 psig is approximately 18        blade configuration determines the
     kW/100 cfm to 22 kW/100 cfm.                 operating pressure and flow rate. Most
        The water-injected, non-lubricated ro-    industrial centrifugal air compressors
     tary-screw compressor can produce 100        use a multi-stage design. Backward-
     psig, and more in a single stage because     leaning impeller blades can achieve
     the water injected into the compression      higher discharge pressures, but some-
     chamber seals clearances and removes         what lower flow. Radial impeller
     the heat of compression. The water is        blades might achieve greater flow, but
     removed by conventional means, cooled        at lower pressures. Your air demand
     and recirculated. Automatic controls         and pressure variation limitations de-
     maintain the water level and quality.        termine the best configuration.
                                                     Compressor manufacturers have
     Other types                                  controls to avoid the phenomenon of
     The sliding-vane rotary compressor uses      surge, where air flow reverses inside
     a rotor with metallic or non-metallic        the compressor. As the compressor
     vanes that slide in and out of the rotor     discharge pressure increases, flow
     in an offset housing. As the rotor spins,    decreases. Eventually, the discharge
     the vanes are forced outward against the     pressure can’t overcome the system
     cylinder walls. During part of the revo-     pressure and the air flow reverses, go-
     lution, the contained volume between         ing from discharge to inlet. Most ca-
     vanes decreases, and pressure rises as it    pacity controls automatically avoid this
     nears the discharge port.                    condition at low demand. Typically
        The rotary-scroll compressor is a rel-    centrifugal compressor sizes start at
     atively new design in sizes from frac-       approximately 150 hp and the specific
     tional to 7.5 hp. Because of its small       power at 100 psig is approximately 15
     footprint, multiple compressors and          kW/100 cfm to 20 kW/100 cfm.
     drives can be mounted on a common
     base to provide higher capacities. One       Which do you need?
     of two identical intermeshing spirals        When comparing compressor types
     or scrolls is stationary and the other       for an application, it’s important to
     oscillates in relation to the station-       consider the relevant factors and spe-
     ary scroll. (Editor’s note: go to www.       cific requirements (Table 1). For ex- to           ample, if the compressor is expected
     learn how the scrolls interact.) The         to operate fully loaded, efficiency at
     spirals are mounted with 180° phase          reduced capacity might be less impor-
     displacement to form air pockets with        tant. However, if you anticipate wide
     variable volumes. As the moving scroll       swings in air demand, then reduced-
     orbits within the fixed scroll, the air      capacity efficiency might be a more
     pockets diminish in size as they scroll      important factor.

38                                            January 2008

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