Chapter 7 Auxiliaries

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					Chapter 7: Auxiliaries



I
     n this chapter, we will cover a broad range of        cumulation of dirt or dust can decrease the
     auxiliary equipment, including fans, boiler           efficiency of the fan by slowly changing its
     feedwater pumps, motors and turbines, elec-           configuration and adding weight to the fan.
tronic motor drives, materials handling equipment,         This inspection should be done at least twice
and boiler sootblowers. We will also discuss cogen-        yearly, more frequently if the fans are lo-
eration and the sale of excess electricity.                cated in a high-dust environment (pulveriz-
                                                           ers, conveyor belts, etc.)
Fans                                                    2. Fan bearings should be checked for wear and
                                                           adequate lubrication. Clearance between the
There are three types of induced draft (ID) and            fan and its housing should also be checked.
forced draft (FD) fans most commonly used in
power plants; the backward-curved, also known
as the airfoil; the straight blade and the radial       Boiler Feedwater Pumps
tip. The most efficient is the airfoil; it has a 90%
efficiency. Other advantages of the airfoil in-         Boiler feedwater pumps function as the delivery
clude: very stable operation, low noise level and       system to the boiler, providing water under pres-
the suitability for high-speed service.                 sure to the boiler. They do this by taking suction
     Straight blade and radial tip fans are more sub-   from a deaerating feedwater heater and pump-
ject to erosive conditions. As an example, straight     ing the feedwater into the boilers through high
blade fans are used mainly for pulverizer exhaust       pressure feedwater heaters. There are numerous
and radial tip fans for applications downstream         methods available to control feedwater volume
from a low efficiency particulate removal system.       and provide sufficient suction head to the
     The desired air or gas flow and its pressure       pumps.
in a fan are determined by the employment of                 One way to control volume is to use a vari-
one of the following:                                   able speed drive motor with the boiler feed
                                                        pump. Another way is to use an electric motor
    Inlet damper control                                running at a constant speed, coupled to a vari-
    Inlet vane control                                  able speed hydraulic drive. Where the expense of
    Variable speed control                              a variable speed drive is not justified or desir-
                                                        able, volume can be controlled by a throttling
     Dampers or vanes are used when the fan is cou-     valve installed between the pump and the boiler.
pled to a single or two-speed motor. Fan manufac-       A decision depends on whether the capital ex-
turers usually favor vanes over dampers as they feel    pense of a variable speed drive is offset by a sav-
they are slightly more efficient due to the pre-swirl   ings in electrical power. A feedwater control valve
effect that vanes impart to the air or gas flow. A      requires a large drop in pressure for proper op-
variable-speed drive, however, is the most efficient    eration; therefore the pump must have added
control. It provides only the power necessary to        head capacity.
overcome system resistance at a given condition. It          In this type of operation, the condensate in the
is particularly effective when operating conditions     deaerator will be saturated, requiring the deaerator
call for frequent low load periods. The diagram be-     to be elevated as far as possible above the boiler
low illustrates the amount of power necessary at        feedwater pump to provide sufficient net positive
various speeds for a hypothetical centrifugal fan and   suction head (NPSH) for the pump. This elevation
system resistance.                                      may require additional structural and piping costs
                                                        which can be offset by the installation of booster
Fan Maintenance Checkpoints                             pumps that provide the additional suction head for
                                                        the boiler feedwater pumps.
1. Fans and their housings need to be checked                A minimum of two feedwater pumps are usually
   periodically for dirt and dust buildup. An ac-       installed, depending on the need to maintain ca-


Energy Efficiency Handbook                                                                                35
pacity in the event of pump failure. This also allows     perform. These motors represent another exces-
pump deactivation to perform normal mainte-               sive energy consumption area. Some surveys
nance, such as replacing the bearing seals, packing       have shown as many as 60% of all motors in in-
or repairing the electric motor driving the pump.         dustrial facilities to be oversized.
     Recirculation of the feedwater back to the
deaerator is necessary when a single speed motor          Electronic Motor Drives
is used and flow is at or below the pump’s mini-
mum flow. Steam turbines are often used to drive          A promising area for saving energy and reducing
feedwater pumps because the speed of the tur-             maintenance is the marrying of energy-efficient
bine and pump can be varied like a variable               electric motors to the latest generation of elec-
speed motor. An automatic recirculation control           tronic motor drives. These drives can make mo-
valve or a modulated control valve controlled by          tors more productive by controlling starting,
the operation’s distributed control system can            stopping, speed regulation, reversing and even
perform this function. Pumps should be in a re-           positioning.
circulation mode as little as possible to maximize             The new drives offer protective features for
energy savings.                                           the motor. They can control high inrush current
                                                          and its subsequent voltage sag on the power dis-
                                                          tribution grid. Other features can limit current
Motors and Turbines                                       and shut down overloads that can allow a motor
                                                          to self-destruct. Electronic motor drives can also
Electric induction motors or steam-driven tur-            minimize motor burnout, a potential fire hazard.
bines can be used to drive fans, pumps or other                Most importantly, these new drives save en-
industrial process machinery. Picking one or the          ergy. For example, varying a pump’s speed
other depends on the availability of steam versus         rather than throttling the output flow can save
the cost of electricity. If high pressure steam is        25-40 percent of the electricity consumed. Put-
available and there is a need for low pressure            ting a motor on standby rather than allowing it
steam, a turbine can serve as a pressure reduc-           to continue running when it’s not needed can
tion station while driving the fan or pump. If            also save energy. A typical motor can consume
there is no need for low pressure steam, a con-           10-20 times its cost per year; the payback period
densing steam turbine could be used, although             for installing drives is often under two years.
turbines of this size are not efficient. If electricity        It is estimated that less than 3 percent of all
is being generated on-site, it is usually more effi-      installed AC motors have electronic drives. Other
cient to use electric motors.                             estimates indicate that at least a quarter of all AC
    Motors are available today that are known as          motors could benefit from the use of drives.
“premium efficiency.” They are more expensive,            Other factors that make the use of electronic
but the efficiency is more than offset by the cost.       drives attractive include declining prices, better
With electricity costs typically at $0.065/kwh and        accuracy and reliability, the discovery of new ap-
higher, the payback analysis justifies the extra          plications and increased user acceptance.
expense. Here are some typical motor efficien-                 Another advantage of electronic drives is
cies available today from “premium efficiency”            their easy setup and adjustment. These functions
motor manufacturers:                                      are done through an operator keypad or inter-
                                                          face device. Since the settings are entered digi-
         HP Size                    Efficiency            tally, they are very accurate and not subject to
                                                          drifting. Older analog drives were set by electro-
         5-10                       85%
                                                          mechanical potentiometers, similar to older radio
         15-40                      90%                   controls. These were subject to dust and dirt con-
         50-150                     94%                   tamination and often requiring frequent calibra-
         200 or greater             95%                   tion and readjustment.
                                                               Today’s electronic drives have a much larger
                                                          tolerance of line voltage fluctuation, thanks to
     Motors should be checked on a regular basis          the integration of an electronic technique known
for excessive vibration, bearing wear and ade-            as pulse width modulation. The older power-
quate lubrication in accordance with manufactur-          switching devices, silicone-controlled rectifiers,
er’s recommendation.                                      had tolerances as low as 5-10 percent; with the
     It is common to find motors installed that           newer systems, tolerances start at 10 percent and
are oversized for the task they are required to           can go up to as high as 30 percent.


36                                                                              Council of Industrial Boiler Owners
Materials Handling Equipment                          vacuum on the piping system, using a steam ex-
                                                      hauster or motor-driven blower. Water exhaus-
Solid fuel-fired power plants need systems to re-     ters have been used for flyash conveying from
ceive, store and deliver fuel and to collect, store   large pulverized coal fired units.
and remove ash. This can include furnaces de-             Systems with capacities in excess of 15 or so
signed to burn coal, wood, food processing wastes     tons- per-hour tend to use vacuum blowers where
and municipal and industrial processed refuse.        smaller systems have historically used steam ex-
The size and complexity of these systems vary         hausters. Water pollution and efficiency prob-
widely but all of them require energy to operate.     lems with steam exhausters have caused many of
                                                      these systems to be converted to motor-driven
Fuel Handling Systems                                 blowers, a move that almost always reduces
                                                      power consumption.
Pneumatic systems are available for certain re-           If the decision is made to retain the vacuum
quirements but most fuel-handing systems are          system, the maintenance of the piping is essential
mechanical, using belt or drag conveyors. They        for efficient operation. An active leak detection
are very efficient on the basis of tons per kW.       program is essential as leaks will cause loss of
Energy-saving options for conveyors are few;          conveying capacity, increase running time and
however, some of the equipment installed as part      maintenance of the pipe, fittings and air cleaning
of the conveyor system might deserve a second         equipment as well as consuming more energy.
look.                                                 See Chapter 11, Compressed Air Systems and Diesel
     For example, a major power consumer in a         Engine Power Cogeneration, for more discussion on
coal handling system can be a crusher, used to        leak detection. If steam exhausters are used, it is
size coal. Power can be saved, in some cases, by      important to maintain the steam nozzles and
changing to a different crusher or installing a by-   venturi sections to maintain efficiency.
pass which separates properly sized particles             Pneumatic ash systems are most often run in-
from the crusher. Some installations require that     termittently, pulling ash on a batch basis. Power
the crusher need only be used intermittently. A       consumption can be reduced by maximizing the
change in the coal supply could cause a crusher       conveying rate of the system. If the system is
to be taken out of service. Dusty areas of the sys-   manually operated, the operator should feed the
tem containing electrical equipment need to be        intakes at the highest obtainable rate. System
monitored and kept clean to minimize power            controls should be optimized to minimize the
draw.                                                 non-conveying cycle times.
     Fugitive dust collection systems on coal con-        In the case of continuous pull systems, often
veyors should be maintained in the same way as        found on dust hoppers in the gas cleaning train,
other dust collectors. These usually have bag fil-    controls should be set to avoid pulling on empty
ters and the cleaning cycles should be adjusted to    hoppers.
minimize draft loss and reduce fan power con-             Larger industrial coal-burning installations
sumption.                                             may find a conversion to a mechanical system
     Pneumatic fuel-handling systems of both the      practical. Power consumption is far less but this
lean and dense phase kinds are used by a num-         would have to be weighed against the capital cost
ber of plants. They usually consume more power        of a new system.
than mechanical types on a kW per ton basis.              Ash systems serving biomass-fired boilers
Good maintenance procedures can conserve              generally use mechanical systems. Finally, hy-
power in these systems. In some cases the system      draulic ash systems are rarely used because of
can be redesigned to eliminate changes in direc-      high power consumption and water usage. These
tion, significantly reducing line loss. Longer term   systems can be converted to mechanical or pneu-
solutions might include replacing the pneumatic       matic to solve probable water cleanup problems
system altogether.                                    and potentially reduce power use depending on
                                                      the type of replacement system selected.
Ash Handling Systems
Although ash handling systems move lower ton-         Boiler Sootblowers
nage of material than fuel handling systems, they
often consume far more power than necessary           Efficient heat transfer is one of the major con-
because of their design and mode of operation.        tributors to an energy-efficient boiler system.
The most common types move ash by inducing a          One of the most important boiler auxiliary op-


Energy Efficiency Handbook                                                                             37
                                                         Generation and cogeneration operation and
                                                                    benefits comparison
                                                        Figure 7-2. Industrial steam-electric cogeneration, system
                                                        thermal efficiency 80 to 90%.


                                                        blowing frequency depends on slag buildup, but
Figure 7-1. Static pressure rise and power versus air   a normal range would be 4-8 hours.
flow.                                                       Superheater, reheater and economizer sec-
                                                        tions of the boiler are cleaned with long, retract-
                                                        able lances which are most effective at cleaning
erations is the on-line, in-service fireside cleaning   radiant and convective heat surfaces. Effective
of heat absorbing surfaces. This operation per-         cleaning radius, using a helical blowing pattern
forms two important functions; it assures proper        from two nozzles, is 4 to 9 feet.
heat transfer and also prevents sections of the
boiler from becoming plugged. Plugged sections          Sootblower Operation
can restrict gas flow and cause load limitations.
     Sootblowing systems are required on coal           Since deposits in the radiant and convection sec-
and oil-fired furnaces. Because oil has a low ash       tions of the boiler can vary from hard slag to a
content and the residue is a thin, water-soluble        dry powdery coating, the blowing sequence is not
substance, it’s removal is done by water-washing        set by a hard and fast rule. Sequence and fre-
the furnace walls during the annual shutdown.           quency, instead, must be adjusted during initial
For this reason, furnace-wall sootblowers are not       operation by starting with an assumed sequence
required on oil-fired units.                            and frequency of perhaps one complete cycle per
     In the superheater and reheater sections of        8-hour shift. The operator should observe foul-
oil-fired units, there is ash buildup on the tubing     ing patterns either through observation doors
surfaces. High-vanadium content oils, containing        (during operation) or by gas-side inspection dur-
additives to combat high-temperature corrosion,         ing shutdowns. Necessary adjustments can then
are especially prone to this. But when solid-           be made. Once the pattern is established it can
powder additives are employed, ash deposits in          be implemented and run automatically.
the high-gas-temperature areas increase mark-
edly. Fortunately, these deposits crumble and
pulverize easily and are readily removed with           Cogeneration
sootblowers.
     Coal-fired units require large numbers of          Cogeneration combines the production of electri-
permanently installed sootblowing equipment.            cal and thermal energy for eating and process
Factors such as ash-fusion temperatures and the         use from a single act of combustion. It uses less
percentage of ash in the coal determine just what       total fuel than needed to produce the two forms
sootblowing coverage is required.                       separately. Cogeneration represents a major step
     Superheated steam or compressed air is the         towards maximum energy efficiency and also
medium used to remove deposits, employing a             contributes to reduced pollution.
short, single-nozzle retractable blower and clean-          For example, in most cases, a company gen-
ing a surface with a five-foot radius. The effective    erates its own steam and purchases electric power


38                                                                               Council of Industrial Boiler Owners
 Generation and cogeneration operation and
            benefit comparison
Figure 7-3. Industrial combination turbine combined cycle
cogeneration, system thermal efficiency 80 to 90%.


from the local utility. The typical industrial
steam generation steam-electric cycle has a ther-
mal efficiency of 75-85 percent, whereas the util-
ity cycle has a thermal efficiency of only about 35
percent. The net thermal efficiency of this sys-
tem, depending on the relative amount of steam
and electricity required could be in the range of
66 to 80 percent.
     In a typical industrial steam turbine cogen-
eration cycle, however, high pressure steam first
produces electric power and then is used for
process needs. Because such a facility can attain
an overall efficiency of up to 88 percent the sav-          Figure 7-4. Generation and cogeneration operation and
ings in fuel use can be as high as 15-20 percent.           benefits comparisons. Above: Separate steam and elec-
Fuel used can be gas, oil, coal, wood, municipal            tric cogeneration. Below: Combined industrial steam and
solid waste or industrial wastes.                           electric cogeneration.
     In a gas turbine combined-cycle cogeneration
facility, after generating electricity, the hot ex-
haust from the gas combustion turbine-generator             temperature processes to generate electricity by
is used to make steam. The steam can be used                generating steam in a waste heat (heat recovery)
for electric power, process needs or space heat-            boiler.
ing. Overall thermal efficiencies could be as high
as 90 percent. This compares to a maximum fuel
efficiency to produce the steam and power sepa-             Sale of Excess Electricity
rately of 58 percent when a combustion turbine is
used. Fuel is usually limited to natural gas or oil.        A further economic consideration for Cogenera-
     Cogeneration plants are either topping or              tion is the ability to sell excess power generated
bottoming systems. A topping system has the                 to the local utility. Federal regulations, intended
steam producing electric power first and all or a           to encourage cogeneration, compel the utility to
part of the exhausted thermal energy is then                purchase a cogenerator’s excess power at a price
used in industrial processes or for space heating           determined by the utilities (called “avoided cost”)
or cooling. Bottoming systems use the waste heat            or cost of power displaced by the cogenerator’s
from industrial processes or other high-                    output.




Energy Efficiency Handbook                                                                                      39