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This document is a compilation of information assembled
from many different sources.

These sources would include some of the following prominent and well respected
manufacturers, consultants and suppliers: Morel Consultants Group, Fluid Power
Associates, The Compressed Air and Gas Institute, Sullair Corporation, Kaeser
Corp., Atlas Copco, Ingersoll Rand, Gardner Denver, Quincy, domnick hunter
Filters Ltd., VanAir, Hankison Corporation, and VMAC.

We hope you find this material informative, practical, well organized, and
relevant to whatever job you may perform.

                              IMPORTANT NOTE!

             The depiction of any specific manufacturers’ equipment
             does not constitute an endorsement of their product over
             another manufacturers. The products show in this manual
             are chosen for their graphics and ease of explanation. All
             of the information in this manual is for education
             use only. It has been carefully compiled and we believe
             the information to be accurate. However, since any
             segment of the information may apply on one application
             and not a similar one due to differences in environment,
             personnel, operating conditions, materials and components
             and since errors can occur in circuits, tables and text, we
             assume no liability for the safe or satisfactory operation
             of any machine or installation designed from the
             information contained in this manual.

             We would like to thank the management and staff of VMAC
             for their cooperation in supplying material for this course
             manual and communicating their requirements.

                          Table of Contents:
Acknowledgement and Forward
Table of Contents
SECTION 1. Basic Compressed Air Theory
SECTION 2. Competitive Air Compressor Styles
A. Rotary Screw (Underdeck and Box-mount)
B. Underhood Reciprocating Compressors
C. Tow Behind Portable Compressors

SECTION 4. Air Treatment
   A. Airline Filters
   B. Coolers
   C. Air-cooled and Water-cooled

SECTION 5. Air Dryers
   A. Deliquescent Type
   B. Desiccant Type
   C. Non-regenerative (Single Tower)

SECTION 6. Air Receivers
SECTION 7. Air Tools & Air Line Accessories
SECTION 8. Tables and Charts
SECTION 9. Glossary of Terms

Section 2.   Basic Compressed Air Theory

Water Vapor in Air
John Dalton was the first to surmise that the total pressure (Pm) exerted by a mixture of
gases or vapors is the sum of the pressures of each gas if it were to occupy the same
volume by itself.

Elementary as it may seem, the concept of Dalton’s Law is often overlooked in
considering problems in humidity, because one forgets that the water in a gas is actually a
gas itself and must be treated in accordance with the gas laws.

Air must be considered a mixture of gasses -- oxygen, nitrogen, and water vapor
(Neglecting the minor constituents).

All discussions of humidity can then be reduced to discussion of water vapor pressure,
and all definitions encountered in humidity can be expressed in terms of vapor pressure.
The pressure which each gas component of a multiple constituent gas (such as air) exerts
is called its partial pressure.

At atmospheric pressure, the term used to express the amount of water vapor is relative
humidity, which varies as a function of the temperature. As a rule of thumb, the water-
holding capability of a gas doubles with every 20 o F. rise in temperature. Air, for
example, can hold about 21 grains of water per cubic foot at a temperature of 100 o F and
relative humidity of 100% at atmospheric (sea level) pressure. At 80 o F and the same
pressure, the same volume of air can only contain approximately 10 grains.

 Another way of expressing this is to say that if 1 cubic foot of air at 100 o F, contains 21
grains of water, it is (100%) saturated, and has a relative humidity of 100 %; if this
volume is cooled down to 80 o F, 11 grains would condense and 10 grains would remain
in vapor phase. These 10 grains of remaining moisture would still constitute a relative
humidity 100 % at the new temperature. Another factor that should be pointed out is that
up to pressures of 300 atmospheres, the volumetric water-holding capacity of most gasses
is independent of pressure.

For this presentation, when we refer to the term dewpoint, we shall be referring to the
pressure dewpoint only, and not the atmospheric dewpoint.

The Reciprocating (Piston Type) Compressor

The reciprocating type compressor falls under the category of positive-displacement
compressors. This means that the compressor confines successive volumes of air within a
closed space and increases the pressure by reducing the volume of the space.

A bicycle pump is a simple positive displacement compressor.

The piston, secured to a handle by a long rod, has a cup shaped seal face which flares
open on the downward stroke of the pump. Downward motion of the piston causes
sufficient initial pressure to open the cup and produces a tight seal between the seal and
the cylinder wall. Air is forced through a non-return valve into the object being inflated.
The up-stroke of the piston creates a partial vacuum inside the cylinder, permitting
atmospheric air to flow past the leather cup, and fill the chamber so the cycle may be

Reciprocating compressors utilize the same principal as the bicycle pump. Air is drawn
into a cylinder and compressed by a piston, which is driven by a connecting rod from a
crankshaft. The difference between the suction and discharge pressure represents the
work done on the air by the compressor.

The ‘pressure ratio’ of the compressor is the ratio of the absolute discharge pressure to
the absolute suction pressure. As the compressor forces the molecules closer together, it
also increases their velocity. This increase in molecular velocity causes an increase in
temperature. The size of the temperature increase will depend on:

1.     The type of gas being compressed
2.     The suction temperature
3.     The pressure ratio
4.     The cooling design and media
In a conventional single-stage, single-acting compressor, rotation of the compressor shaft
is converted into reciprocating motion by a crank and connecting rod. The compressor
input shaft is connected to the crankshaft. The crankshaft is then attached to the
connecting rod, which is connected to the crosshead.
The piston reciprocates in the cylinder as in the bicycle pump just
discussed. Instead of the leather cup in the cylinder, inlet valves, and
discharge valves are fitted to the cylinder itself. These compressor
valves are basically non-return valves, which permit air to flow in one
direction only.

Atmospheric air enters the cylinder via the intake filter, which
prevents dust and other foreign material from entering the
compressor, and then the air continues on through the inlet valves.
Downward movement of the piston creates a partial vacuum inside
the cylinder and the higher atmospheric air in the suction port opens
the inlet valves and fills the cylinder with atmospheric air. On the return stroke, when the
pressure in the cylinder exceeds the pressure of the discharge port spring, the discharge
valve opens, and air is discharged from the compressor at pressure.

In a double-acting compressor, the same action takes place on each side of the piston.
Compressor valves are practically always of the automatic, self-acting type, operated
by the differential pressure between the inside and the outside of the cylinder. Valves
are generally designed as complete assemblies fitted to the compressor cylinder at the
valve port.

Each valve plate covers a port in the seat, which is slightly smaller than the moving
valve plate. In a discharge valve, this port opens directly into the cylinder. Air being
compressed within the cylinder increases enough in pressure to overcome the
resistance of the springs plus the pressure of the air in the discharge port of the
cylinder, causing the valve to open and release air from inside the cylinder into the
discharge port.

At the end of the stroke, pressure on both sides of the moving valve plates equalizes and
the springs close the valves. Inlet valves operate similarly, except that they admit air into
the cylinder whenever the pressure within the cylinder is slightly below the atmospheric
or intake pressure.

Air that enters the cylinder may be compressed from the initial suction pressure to the
desired final discharge pressure in one continuous thermodynamic and mechanical step.
This is known as a single-stage compressor. On the other hand, it is sometimes necessary
to bring about part of the desired pressure in one cylinder, cool the air in a heat exchanger
(intercooler) outside the cylinder, and then uses a second cylinder or more to compress
the cooled air to the desired final discharge pressure. This is known as a multi-stage

Contamination of Atmospheric Air

It is important to note that the atmospheric air contains many types of contaminants.
These contaminants include moisture, dust and dirt particle smaller than the air intake
filter rating. This, combined with the lubricant carryover, all end up in the compressed
air. The compression of these solid contaminants only increases their density within the
compressed air. Therefore, the discharge air at the outlet of the compressor is far from
being clean, and must be filtered further before use in a pneumatic piece of equipment.

Noise Level

One thing that should be kept in mind when considering the installation of a reciprocating
air compressor is the noise level when the unit is operating. Sound arises from the
compressor location from numerous sources, and each source has its own sound level.
The total noise therefore consists of a large number of sound levels and an attempt should
be made to damp out the worst of these.

Roughly speaking, the noise levels can be divided into two groups, the low frequency
pulsating air intake sound and the higher frequency machine noises from the compressor,
motor, belts and cooling fans. The low frequency air intake sound can normally be
reduced by means of a specially designed silencer.

In all reciprocating installations, a pressure storage tank, (air receiver) must be used.
This receiver tank is used to store the compressed air, increase the cooling of the
compressed air, collect as much residual condensate as possible and to equalize the
pressure variations on the discharge piping.

Rotary Screw Compressors
(Lubricated type)

Rotary screw compressors are also positive displacement compressors with a built-in
pressure ratio. As there are no inlet and discharge valves, and no unbalanced forces, as
in the reciprocating type compressor, the rotary screw compressor can operate at high
inlet shaft speeds. This results in high flow rates with a compact compressor package by
comparison to the recip type.
The compressor element consists of a pair of helical screws, the male having convex
threads sometimes referred to as lobes and the female rotor has concave flutes.

The number of male lobes or female flutes varies by manufacturer and application.

The rotor speed ratio is inversely proportional to the lobe-groove ratio. These rotors
mesh inside a one-piece, dual bore cylinder (housing), which provides the air inlet
passages, oil injection points, compression zone and discharge ports.

The male rotor usually drives the female rotor, without metal to metal contact, through
the oil film developed between the rotors, although female rotor drive is possible in some

The drive method can be direct drive, gear drive or belt drive. The drive types can be a
gas or diesel engine, a hydraulic motor or an electric motor.

Each rotor is supported by rolling element bearings located near the ends of the rotor
body. The bearings at one end, usually the discharge end take the rotor axial thrust, carry
radial loads, and provide for the small axial running clearances necessary. Bearings at
the opposite end are floating bearings, which allow for unequal thermal expansion of the
rotor and cylinder.                                                                         11
Section 3.          Competitive Air Compressor Styles
A. Rotary Screw (Under-deck and Box-mount and Utility)
B. Underhood Reciprocating Compressors
C. Tow Behind Portable Air Compressors

The under-deck compressor was designed as an alternative to the tow-behind
compressors for all of the same reasons as the development of the underhood design by

The leader in this field is VanAir out of New Buffalo, Michigan. They started as a
subsiduary of Sullair Corporation.

These compressors are very much the same as a tow-behind in their componants and
layout, other than that they use the vehicle engine as the power source.

This is an example of a “side-mount” PTO and a “split-shaft” PTO.
PTO stands for “Power Take Off “
That is the means of connecting the power supply (the truck’s engine) to the compressor.

The type of installation chosen depends on the type of vehicle and the available space.
Another method of mounting and driving this type of equipment is the “Above-deck”
version. The above-deck or Box-mount compressor is very much like the Utility Portable
compressor, only instead of an integral engine, it has a hydraulic drive unit. This is to take
advantage of vehicles that are already equipped with extended hydraulic systems. It allows
the compressor to be mounted anywhere on the vehicle.

This is an example of a Utility Compressor. This is a Hydraulic Motor driven above-deck
This compressor has it’s own engine.        PTO compressor.

There are a number of disadvantages to the “hydraulic drive” style of compressor. The
most notable would be the noise level. These units typically do not have the sound
attenuation that is available on some portable compressors. This is an important
consideration in most municipalities. Another would be the potential for hydraulic leaks
in environmentally sensitive areas. There is also the need for another piece of
mechanical equipment (the hydraulic drive unit) which requires maintainance.
Like the Utility style compressor, the hydraulic drive PTO takes up valuable space on the
truck deck.

The utility style is nothing more than a portable air compressor without the wheeled

Another compeditor to the VMAC underhood is the Stellar Underhood reciprocating
(piston type) compressor. Usually gas driven and tank mounted in the box of the vehicle.

These units are low capacity and very noisy. They require substantially more
maintenance than a screw compressor and are being less and less acceptable to end-users. 14
The unit in the middle is about 60 CFM and the one on the right is about 35 CFM.

The most common compressor which the VMAC compressor is designed to replace is the
“Tow-Behind” deisel or gas engine driven portable air compressor. Here are jus a few
examples of towable compressors in some of the sizes that compete.

All of these compressors operate in virtually the same fashion. Even though they are, by
far, the most common form of portable air compressors due to their capacities (75 CFM
to 1600 CFM), there are still some serious issues to examine when choosing them as a
source of portable air.

These are some of their drawbacks:

   1.   Higher initial purchase price.
   2.   Higher maintenance costs.
   3.   Another engine to maintain.
   4.   Another (possibly different from the vehicle) fuel to have on hand.
   5.   Utilizes the only hitch on the vehicle.
   6.   Some sites are inaccesable to tow-behinds.
   7.   Sometimes these units are unstable when being towed.
   8.   More capital equipment in inventory.
   9.   A unit which is substantially larger in capacity than is required for the job.

Section 4.               VMAC UNDERHOOD Air Compressors
 VMAC System (Main Components) Operational Overview
  •   The Control Inlet Valve
      Allows the air into the compressor element. Remains closed when system is
      working but air is not being consumed.

  •   Pressure Regulator
      Designed to set system pressure between 150 - 175 PSI.

  •   Pressure Transducer
      Used on electronic engines to control the constant delivery of air at the required

  •   Gear box
      Allows for the required ratio between crank pulley and rotors to produce up to 70
      CFM of compressed air.

  •   Clutch
      A 12- volt clutch that free wheels (not engaged) unless energized.

  •   Air Filter
      Prevents the ingestion of dust & dirt into the rotors and bearings of the

  •   Control Box
      Allows for the on/off operation of the system. When turning the system on, a 12-
      volt signal is sent to engage the clutch and to power up the throttle control. An
      hour-meter counts the running time of the compressor.

  •   Mounting Bracket
      Allows for the compressor to be mounted & aligned with the crank pulley.
      Features an auto tensioner and idlers (which eliminates the requirement for belt
      tightening) and a belt routing that eliminates side loading on the engine.

  •   Receiver Tank (Serves three purposes):
         •   a) Lubricant Reservoir - Holds approximately 5 liters (1.32 gal US) of
         •   b) Air/Oil Separator - Provides a turbulent path so that lubricant is
             removed from compressed air by re-direction of the air flow.
         •   c) Three gallon Air Reservoir

  •   Throttle Control

  •   Electronic - works with the Pressure Transducer to maintain engine speed for
      required CFM.
   •   Mechanical - follows the pressure drop in the Tank and adjusts the engine speed
       for the required CFM.

Air Flow

System Operation
Turning the switch on the control box energizes (engages) the compressor clutch
allowing for the turning of the gear box input shaft and thus the rotors.
As the rotors (C) begin to turn, air is drawn into the compressor through the inlet filter as
a slight vacuum is created (A). This filter is typically a 5-10 micron, corrugated dry
paper particulate filter.
(It is very important that this filter be well maintained, as it prevents contaminants
from entering the compressor and contributing to bearing failure, which is one of
the most frequent causes of failure in a rotary screw).

There will be a momentary increase in engine speed (4 to 6 seconds) until the system is
brought to the desired pressure which is set by the pressure regulator valve.

The air then travels through the inlet valve (B), which is a spring loaded poppet type
valve. The slight vacuum that was created is enough to overcome the spring tension of
the poppet valve. Air enters the rotor assembly (C) and is drawn into the cavity between
the male rotor lobes and the female rotor flutes. As rotation continues, the rotor lobes
pass the edges of the inlet ports, sealing and trapping the air in a cell formed by the rotor
cavities and the cylinder wall.

Further rotation causes the male rotor lobe to roll into the female rotor flute, reducing the
volume of the female rotor flute and thus raising the cell pressure.

An atomized lubricant is injected at a point (N). This is at a point after the rotors have
been sealed off from the inlet but before the full compression cycle has been completed.

This lubricant has three tasks to perform:                                                      17
   1. Sealing the clearances between the rotors and between the rotors and the casing.
   2. Absorbing and carrying away the heat of compression.
   3. Lubricating the rotors and the bearings.

Compression continues until the rotor lobes pass the leading edge of the discharge port in
the end plate of the compressor housing and releases the compressed air and lubricant
mixture into the air (D) discharge line.
When this pressure is reached, the control inlet valve will close and the system will now
sit and churn oil until there is approximately a 25 pound pressure drop sensed from the air
storage section of the receiver. At this time the air system will start to follow the demand
automatically either through the pressure transducer or by comparing pressure as
explained in the mechanical throttle section.

Air/Lubricant Flow
Once the compressed air and lubricant mixture has passed into the discharge line, it
enters the air-oil separator vessel, where the majority of the lubricant is removed by
centrifugal force and changes in pressure as the air is re-directed while entering the
vessel. The majority of the now liquefied lubricant falls to the bottom of the reservoir.
The air continues on to an intake pipe where it is directed to the inlet of a ‘coalescing’
type separator filter.

As the air passes through the separator filter, from the inside of the filter media to the
outside, virtually all of the remaining lubricant is removed from the air and falls to a quiet
collection area at the bottom of the separator chamber. The virtually lubricant free air
then leaves the separator housing through a discharge orifice.

Lubricant carry-over from a rotary screw compressor is not acceptable as a lubricant for
most air-using devices down-stream of the compressor, so every effort must be made to
reduce the carry-over volume. Once the air has passed from the separator, it must be
cooled again and then filtered even more before being used in an air tool. The aftercooler
can be an air-to-air type.

As this additional cooling condenses a large amount of moisture and some residual
lubricant from the compressed air, this moisture must be separated and drained away.
The compressed air can then be used in most applications.

Lubricant Flow
The lubricant starts out in the reservoir. There is a sight glass in the end plate of the
separator chamber to verify the lubricant level. As the compressor starts to create
pressure and flow, the lubricant is forced, by air pressure in the reservoir, through a return
pipe and through a lubricant filter then to the lubricant cooler and finally to the
compressor oil-injection port. This lubricant is injected as an atomized mist directly into
the compressor cylinder between the male and female rotors. At the point of injection,
the lubricant is at a higher pressure than the trapped volume of air in the rotor cavity. As
one of the purposes of this lubricant is to carry away the heat of compression, the            18
lubricant, upon leaving the compressor, is very hot and must be cooled again once it has
been separated and filtered before being re-injected again.

The lubricant collected in the area at the bottom of the separator chamber is returned to
the inlet of the compressor via the scavenge line. If the scavenge line were to get
plugged, the lubricant level would rise to a point where the out-going air would ‘pick up’
the collected lubricant and carry it on downstream. This situation would eventually either
evacuate the lubricant reservoir or the compressor would shut down on high temperature
as it was starved of lubricant. The scavenge line entrance is protected by a filter screen.

Lubricant Cooler
The lubricant constantly flows through the counter-flow cooler. This cooler uses the
vehicle coolant to increase the temperature of the compressor lubricant during
compressor start up. After a few cycles, the compressor lubricant surpasses the
temperature of the vehicle coolant. At this point the vehicle coolant starts to act as a true
cooling agent and draws heat out of the compressor lubricant.

Once the lubricant has been cooled, the lubricant flows to the injection port in the
compressor housing, where it is injected to start the compression cycle again.

Section 5.                  Air Treatment
Airline Filters
Sources of Contamination

Contamination of compressed air results from several sources. The air in industrial areas
is very significant, with the average metropolitan environment containing in the order of
4 million dirt particles per cubic foot, as well as water vapor, which condenses out after
compression and cooling.

Next, the compressor contributes its share of the pollution consisting of wear particles
and, if oil-lubricated, carbonized compressor lubricant.

Most contamination in a compressed air system can be removed simply by filtration. It is
important however to select the correct type of filtration to obtain reliable results.

Mechanical separation of particles relies on three mechanisms of filtration:

1.    Direct interception
2.    Inertial impact
2.    Diffusion or Brownian movement

                                           The first method, direct interception, affects
                                           the larger particles in a gas stream, which are
                                           literally sieved out. These present no real
                                           problem other than clogging filter material.

Inertial impact occurs when a particle
traveling in a gas stream is deflected
around the first, second or even third
fiber in the filter material, but is
eventually unable to negotiate the
torturous path between the fibers and
cannot change direction as quickly as
the gas stream. It therefore collides
with a fiber and remains attached to it.

                                            The third method, diffusion or Brownian
                                            movement, affects the very fine particles
                                            which are subject to inter-molecular and
                                            electrostatic forces which cause them to         20
spiral in the gas stream, thus increasing their effective diameter.

There is a critical particle size as a result of particles falling between the inertial
impaction and diffusion mechanisms. This critical size is 0.3 micron and it is very
relevant when considering removing fine particles, either liquid or solid. This particle
size is typically used in filter efficiency and integrity tests.

 If a filter tested on this particle size proves to be 100 % efficient, then it is quite safe to
state that this filter is capable of removing any particle above this size.

Research has shown that every cubic meter of air in industrial areas contains 140 million
dirt particles. In heavy industrial areas the problem is even worse. Furthermore, up to
80 % of this contamination is smaller than 2 micron, allowing it to pass straight through
the compressor intake filters, which range between 5 -10 microns. Compress this air to
100 psig, and you have a staggering 980 million dirt particles per cubic meter, not to
mention oil, operating dirt, scale, rust and condensed moisture.

Dirt and other contamination can have a serious effect on the wear and efficiency of
pneumatic machinery. It will increase maintenance and downtime, and can be a
determining factor in the early replacement of the pneumatic equipment.

The coalescing type filter, as depicted in these photos, is typical of a low capacity
coalescing type filter. The flow pattern is the same as that of larger filters, however the
filter media sometimes has an anti re-entrainment barrier on the outside of the filter
cartridge to prevent liquid contaminants from getting picked up by the outgoing, clean
The air enters through the inlet port and travels down through the inner core of the filter,
passes through the filter media and leaves through the discharge port. Moisture and oil
collect in the bottom of the housing and are expelled by a condensate drain valve, and
other solid contaminants collect in the filter element.

This type of a filter frequently has a built-in, float type auto drain trap and the some
manufacturers have a moisture level sight glass mounted on the side. This is to provide
visual assurance that the moisture drain is functioning. This unit is not shown with a
differential pressure gauge which is usually attached to the top of the filter.

The purpose of this type of gauge is to show the condition of the filter media by
registering the pressure difference between the inlet side of the filter and the outlet side.

The manufacturers’ recommendations should be followed in respect to their suggested
maximum allowable pressure drop before change out of the filter cartridge. A high
pressure drop across filters must be compensated for with higher initial system pressure.
Running with a dirty filter could be costly, from a fuel efficiency standpoint.

It is important to note that, as the airflow of the coalescing type filter is from the inside
out. The particulate type filter, used for solid dirt and dust removal, is most often from
the outside-in, as this configuration takes advantage of the larger surface area of pleated
filter media, giving the filter element a longer life as it has more area to catch the dirt and
dust particles before a high differential pressure is detected. If the media is not a pleated
type media, the particulate filter may have an inside out flow as well.
The compressor inlet filter is a dry, pleated particulate filter, with a 5-10 micron rating.

Typical grades and specifications of filtration would be:

    Types of Filters              Particulate           Oil Removal           Extra Fine Oil
                                                        Coalescing          Removal Coalescing
                             General Shop Air        Instrument Air        Food Industry
Liquid Removal                 100% of Water           99.99=% of Oil        99.99=% of Oil
Max. Liquid Loading            2000 ppm w/w            1000 ppm w/w           100 ppm w/w
Solid Particulate Removal         1 micron               0.01 microns         0.01 microns
Oil Carry-over                   1 ppm w/w              0.01 ppm w/w         0.001 ppm w/w
Pressure Drop                 Dry: 1psi Wet:2 psi    Dry: 1psi Wet:3 psi     Dry: 2psi Wet:6 psi

Air-cooled versions

An aftercooler is a heat exchanger used to cool compressed air. Reduction of the
compressed air temperature will cause moisture and oil droplets to precipitate out of the
air. These contaminants are collected and drained off with a moisture separation device
and drain trap.

                          This is a typical float type drain trap

The aftercooler should be located as close as possible to the compressor outlet.
A heat transfer will occur between two bodies of different temperature until temperature
equilibrium is reached. This transfer of heat can take place in three different ways.
Generally these take place simultaneously.

1.    Conduction
2.    Convection
3.    Radiation

The air-cooled aftercooler looks very much like a car radiator, and acts like one as well.
However, rather than water filling the interior, the hot compressed air enters the bottom
of the air-cooled aftercooler and travels within a tube system, discharging through the
upper discharge port into a moisture separator. The tubes have fins, or metal plates
between them to increase their surface area, and dissipate the heat more effectively.

As heat from the compressed air transfers to the cooler atmospheric air, some of the heat
of compression is removed from the compressed air and carried away.

Section 6.                Air Dryers
The Deliquescent Dryer

                                                          Absorption Type
                                                          Incoming air enters the
                                                          vessel near the base and
                                                          passes through the
                                                          mechanical separation
                                                          section. Due to expansion,
                                                          the free liquids and solids
                                                          drop to the bottom of the
                                                          To some degree, this
                                                          constitutes a pre-drying of
                                                          the air.

                                                          The air then enters the
                                                          desiccant bed of
                                                          deliquescent materials such
                                                          as water-soluble salts or
                                                          shotted urea. The
                                                          hygroscopic chemicals
                                                          condense water vapor as
                                                          they deliquesce or dissolve.
                                                          Absorption occurs until the
                                                          tablets are completely
                                                          consumed, at which time,
                                                          they must be replaced.

Dew point suppression is subject to the following:

1.    Age and Composition of the deliquescent material.
2.    Compression of the tablets.
3.    The vessel configuration.
4.    The compressed air inlet temperature.
5.    The compressed air velocity. (Based on pressure)


1.    Lowest initial cost of all dryer types.
2.    No electrical hook-up.
3.    No moving parts.
4.    Simple operation.

1.    Dewpoint suppression is between 20o-30o F (On average).
2.    Deliquescent material must be added to or replaced as it absorbs and melts.
3.    Downtime to replace deliquescent material.
4.    Ecological problem of disposing of the dissolved deliquescent material.
5.    Cost of replacement deliquescent and of disposal of dissolved deliquescent
6.    Carryover of corrosive deliquescent materials into the downstream air using
7.    Parts of the deliquescent materials can solidify in the bed, causing channels for
      the air to by-pass most of the drying material. This reduces the dryer’s

                              The Desiccant Dryer
                              The dryer contains two highly efficient polypropylene
                              coalescer elements at the inlet and outlet ports. Also, the
                              dryer includes one desiccant bag specifically designed to
                              absorb water, oil and condensation from the compressed air

                              Desiccant Dryer Specifications:
                              Height:             19 ½”
                              Inside Diameter:    4”
                              Inlet and Outlet:   1” NPT
                              Flow Capacity:      70 CFM @ 100 PSIG
                              Desiccant Dryer Specifications cont’d:

                              Desiccant:              Activated Alumina
                              Special V-Band stainless steel clamps permit easy removal
                              of the cover for desiccant cartridge replacement.
                              Multiple layers filter 25 microns.
Polypropylene inlet and outlet coalescer filter elements available in sizes of .1, 1 and 5
Maximum working pressure: 150 PSIG
Pressure drop: 3 PSIG @ 100 PSIG
Dewpoint: As low as -40
Two air pressure gauges (top and bottom)

Section 7.                  Air Receivers
All applications could benefit greatly by the installation of air receiver tanks. It is
generally accepted that no receiver is too large for an application and no number of
receivers are too many, and the installation of a vertical or horizontal receiver tank
usually depends on physical space available.

The receiver serves many important functions. It damps pulsations from the discharge
line of a reciprocating compressor, resulting in essentially steady pressure in the system.
It serves as a reservoir to take care of sudden or unusually heavy demands in excess of
the compressor capacity. It prevents too frequent cycling of the compressor. In addition,
it serves to precipitate some of the moisture that may be present in the air as it comes
from the compressor or that may be carried over from the aftercooler.

The minimum receiver capacity for certain applications may be calculated, but
experience and judgment are important at this point. Receivers are also used to meet
heavy, short time demands of certain equipment, and the manufacturers of this equipment
could supply the information on the air requirements in such cases.

The storage of quantities of moisture in receiver tanks leads to the formation of rust and
scale on the inside of the tank, which can become loose and get carries down stream in
the outgoing air. This rust and scale can cause problems of blockage in air using
components and premature blockage of filters.
The reason for large volumes of liquids in the air receiver is that the tank is made of steel,
and the wall temperature of the vessel is the same as the ambient temperature.

If the compressor is supplied with an air-cooled aftercooler, there is always a difference
between the inlet and discharge air temperature (the approach temperature). As the
cooling is not 100 percent efficient, the air discharging from the aftercooler will always
be higher than the ambient air being used to cool the hot compressed air.

As soon as air leaving the aftercooler enters the receiver tank it comes in contact with the
cooler steel wall of the tank, which is usually at ambient temperature. At this point,
moisture starts to condense out of the compressed air as the air chills. If the air is stored
in the tank for enough time, the temperature of the air in the tank will be the same as the
ambient, and no more moisture will condense out.


High inlet air temperatures increase the likelihood of the compressor overheating,
particularly with air-cooled units, where the cooling oil temperature rises in proportion to
the inlet air temperature. High intake temperatures also reduce the efficiency of a
compressor, as the capacity of the unit is reduced in direct proportion to its absolute
intake temperature.

In the winter months, you can expect a significant change in the viscosity of the
compressor lubricant which could lead to compressor overheating at start-up. There is
also a possibility of blockage of the outgoing airflow due to ice build-up in the main
compressed air distribution line.

Section 8.               Air Tools & Air Line Accessories
These are typical air tools which a VMAC Underhood compressor might power.

Pavement Breaker          Backfill Tamper                     Chipping Hammer

      Rock Drill           Scaling Hammer/Needle Scaler               Utility Drill

                 This is a typical Air Tool Air Consumption Chart

Tool                          Class Weight Application
Light Chipping                                   For light chipping and overhead work, also fine detail
                      18       10#   12 lbs.
Hammer                                           and restoration work
Chipping                              16-19      For chipping in horizontal and overhead applications.
                    26-33      15#
Hammer                                 lbs.      Also used in industrial applications
                                      30-33      For cutting and driving large rivets and heavy duty
Rivet Buster        44-50      30#
                                       lbs.      demolition work.
Light Paving
                      37       30#   35.5 lbs. For breaking light concrete and other light jobs.
Light Paving
                      48       40#   39 lbs. For concrete bridge deck and general demo work.
Medium Paving                                    For concrete road breaking and other general demo
                      49       60#   69.5 lbs.
Breaker                                          work.
Heavy Paving                                     For difficult, heavy demo work breaking tough,
                      62       90#   92 lbs.
Breaker                                          reinforced concrete.
                                                 For excavation of clay and hardpan. Also for light
Light Digger         33.4      20#   24.7 lbs
                                                 demolition work in horizontal position.
                                                 For excavation of clay and hardpan. Also for light
Medium Digger         37       30#   33.8 lbs.
                                                 demolition work in horizontal position.
                                                 For compacting backfill in ditches and trenches. Also
Backfill Tamper       32       35#   40.5 lbs.
                                                 used around foundations and poles.
                                                 For construction and maintenance, setting anchors,
Hammer Drill          21       9#     9 lbs.
                                                 and drilling holes in concrete and bricks
                                                 For construction and maintenance, setting anchors,
Light Rock Drill      53       30#   34 lbs.
                                                 and drilling holes in concrete and bricks
Light Rock Drill      80       40#   45.5 lbs. Drill for depths up to 6 feet 1 1/2" diameter
Medium Rock
                     123       50#   48.5 lbs. Drill for depths up to 10 feet 1 3/4" diameter
Heavy Rock
                      95       55#   56 lbs. Drill for depths up to 20 feet 2" diameter
3/8 " Drill           4
1/2" Drill            4
High Speed
Tire Buffer           4
1/4" Ratchet          3
3/8" Ratchet          4
1/2" Impact           4
3/4" Impact          7.5
1" Impact             12

The purpose of the Filter Regulator Lubricators are as follows:

   A. The air leaving the aftercooler still contains dirt, compressor lubricant
      contamination and water, which is potentially damaging to the air operated
      equipment (i.e. tools). The inline filter will reduce this contaminant to a much
      more acceptable level.
   B. The air pressure required by virtually all air tools is 90-100 PSIG (max.). As the
      pressure leaving the compressor is much higher than 100 PSIG, the pressure level
      must be reduced to the tool manufacturers level or a serious reduction of the life
      expectancy of the tool can be expected. The rule of thumb is that the life will be
      decreased by ½ for every 20 PSIG above the manufacturers recommended
      maximum pressure.
   C. Almost all tools require lubrication to function properly. Some tools have built in
      oil reservoirs, but most do not. The oil carryover from the compressor is not tool
      oil and is detrimental to the tools correct operation. The tool oil must arrive at the
      tool in an atomized condition to be properly ingested by the tool. This is
      accomplished with the use of a lubricator as the last component of the FRL. One
      consideration when using a lubricator is the distance the air/oil mist must travel
      between the FRL and the tool. Excessive distances will cause the oil mist to come
      out of suspension and flow as a liquid. This is not the optimum way to lubricate a
      tool.                                                                                  30
Absolute efficiency

    An arbitrary filter efficiency rating.

Absolute filtration rating

    The diameter of the largest hard spherical particle that will pass through a filter under specified test
    conditions. This is an indication of the largest opening in the filter elements.

Absolute micron rating

    All (not just 98%) particles larger than the stated micron size have been removed from the fluid being

Absolute pressure

    The total pressure measured from absolute zero ( i.e., from an absolute vacuum).

Absolute temperature

    The temperature of a body referred to the absolute zero degrees, at which point the volume of an ideal
    gas theoretically becomes zero. (Fahrenheit scale is minus 459.67°F / Celsius scale is minus

Absolute viscosity (Dynamic)

    Is the force in newton required to move a fluid layer of one square meter area and a thickness of one
    meter with a velocity of one meter per second.


    A method to trap liquids or gases by causing them to penetrate into the absorbent material.

Absorbent filter

    A filter medium that holds contaminant by mechanical means.


    The quality, state or degree of being acid. In oils, acidity denotes the presence of acid-type constituents
    whose concentration is usually defined in terms of neutralisation number.


    Actual cubic feet per minute.

Activated alumina

    An adsorption type desiccant.

Actual capacity

    Quantity of gas actually compressed and delivered to the discharge system at rated speed of the
    machine and under rated pressure conditions.


      A method causing a liquid or gas to condense on the surface only of an adsorbing material.

Adsorbent filter

      A filter medium primarily intended to hold soluble and insoluble contaminants on its surface by
      molecular adhesion.

Adsorptive filtration

      The attraction to, and retention of particles in, a filter medium by electrostatic forces, or by molecular
      attraction between the particles and the medium.


      To combine or charge with gas.


      A suspension of fine solid or liquid particles in a gas.


      Heat exchangers for cooling air or gas discharge from compressors. Designed to reduce the
      temperature and liquefy condensate vapours. Both air cooled and water cooled units are available.


      The removal of heat from a gas after compression is completed.


      A colourless, odourless, tasteless gas. A mixture of individual gases. The gaseous mixture surrounding
      the earth. Standard density of dry air free of carbon dioxide (0°C, 101,325 kPa) is equal to 1,292 8 g/L.
      Standard conditions for air in spectroscopy are 101,325 kPa, 15°C, 0,03 % CO2, dry.

Air amplifier

      A component on a compressed air line designed to increase in magnitude the flow by releasing small
      amounts of compressed air at high velocity through an internal, ring shaped nozzle. This column of air
      released through the front creates a vacuum behind, thus pulling ambient air through the rear and
      pushing ambient air in front.

Air borne

      Supported or transported by air.

Air bubble technique

      When compressed air is forced through a submerged perforated hose or pipe. Some applications
      include; ice prevention, reduction of salt intrusion, underwater basting, pneumatic breakwaters and
      general mixing and agitation

Air cooled compressor

      A compressor cooled by atmospheric air circulated around the cylinders or casing.

Air cylinder

    A component made up of a cylinder barrel, end covers, a piston rod, a steel or stainless steel piston. A
    device which induces action or motion with compressed air being the medium through which the
    power is transmitted.

Air dryer

    A device for drying compressed air by means of condensation obtained by over-compression or
    cooling, absorption, adsorption or a combination of the above methods.

Air leak

    A crack or hole that accidentally admits a gas or lets it escape.

Air motor

    Is a compact, low mass unit giving smooth, non-vibrating power. Several types include vane, piston,
    percussion and turbine type motors

Air nozzle

    A projecting aperture at the end of a tube, pipe etc. serving as an outlet for compressed air. Reduces the
    demand on the compressor by generating the highest thrust and volume for the lowest possible air

Air pressure

    The total gas pressure (static plus velocity).

Air receiver

    A receptacle that serves to store compressed air for heavy demands in excess of compressor capacity.


    The elevation of a compressor above sea level.

Alternating current (AC)

    An electrical current that periodically reverses its direction. Standard in US and Canada is 60 cycles
    per second. Europe and other countries is 50 cycles per second.


    Undisturbed environmental surroundings, particularly to air and temperature.

Ampere (AMP)

    A unit of electrical current or rate of flow of electrons through a conductor. One volt across one ohm of
    resistance causes a current flow of one ampere. One ampere is equal to 6.25 x 1018 electrons per
    second passing a set point in a circuit.

Ancillary equipment

    Components subordinate to the compressor.

Anti re-entrainment sock

    The outer layer on a coalescing filter that prevents separated liquids from re-entering the air stream.
    Also known as anti re-entrainment barrier.                                                                   43

      American Society of Mechanical Engineers


      American Society for Testing Materials

Atmosphere (atm)

      The standard atmosphere is defined as the pressure exerted by a column of mercury 760 mm high with
      a density of 13,595 g/cm³ at the standard acceleration due to gravity of 9,806 65 m/s². The 760th part
      of this pressure unit is the torr. The technical atmosphere (at) denotes the pressure of a force of 1 kg
      acting on an area of 1 cm².

Atmospheres absolute (ATA)

      It is the weight of the column of air existing above the earth's surface at 45° Lat. and sea level. Is
      equivalent to 14.696 psiA or 1.0333 kg/sq cm. Equals atmospheres gauge plus 1.

Atmospheric dew point

      Is the temperature at which water vapour begins to condense at atmospheric pressure. Is the same as
      dew point, but is related to atmospheric air only.

Atmospheric pressure

      Weight of the earth's atmosphere over a unit area of the earth's surface, measured with a mercury
      barometer at sea level. which corresponds to the pressure required to lift a column of mercury 760 mm.


      The use of a sound barrier separating the sound from the receiver.

Back pressure

      Resistance to air flow; usually stated in inches H2O or PSI.


      A unit of pressure equal to 0.99 atmospheres or 14,233 psi.


      Bar gauge (similar to the acronym "psig")

Barometric pressure

      Is the absolute atmospheric pressure existing at any given point in the atmosphere. It is the weight of a
      unit column of gas directly above the point of measurement. It varies with altitude, moisture and
      weather conditions.


      The difference in pressure between the opening pressure and re-close pressure of a valve. May be
      expressed in percentage of set pressure or "psig".

      The portion of a safety/relief valve that surrounds the spring. The spring housing.

Brake horse power

      The maximum rate at which an engine can do work as measured by the resistance of an applied brake.
      Expressed in horsepower.


      A hand held pneumatic tool. Designed for light demolition work, digging, making holes etc.


      British standard pipe parallel.


      British standard pipe tapered.


      British thermal unit. The amount of heat required to raise the temperature of one pound of water one
      degree Fahrenheit under set conditions of temperature and pressure.

Buna N

      A synthetic rubber frequently used for vessel and liquid filter element gasket.

Burst pressure

      Maximum pressure a vessel, or air filter housing will withstand without bursting.

By pass

      Condition that exists when the air, gas, or fluid normally passing through an element is being shunted
      around the element.

By pass valve
An automatic or manual valve that causes the flow to be by passed.


      The pressure screw cover and/or lever housing on a valve. May be screwed, bolted. packed, or plain


      Capacity of a compressor is the full rated volume of flow of gas compressed and delivered at certain
      set conditions.

Capacity filtration

      The amount of air that a filter can handle. Expressed in CFM.

Carbon dioxide

    A heavy colourless gas that does not support combustion but is formed by the combustion and
    decomposition of organic substances. Found in some ambient air conditions.

Carbon monoxide

    A colourless odourless very poisonous gas formed by the incomplete burning of carbon. Found in
    some ambient air conditions.

Carbon residue

    The carbon left after evaporating oil under controlled conditions.


    The pressure containing stationary element that encloses the rotor and associated internal components
    of a compressor, including integral inlet and discharge connections.


    °C The international temperature scale where water freezes at 0 (degrees) and boils at 100 (degrees).
    Also known as the centigrade scale.

Centrifugal compressor

    A dynamic compressor. A machine in which air or gas is compressed by the mechanical action of
    rotating vanes or impellers imparting velocity and pressure to the air or gas. In a centrifugal
    compressor, flow is in a radial direction. Air enters the compressor through the machine mounted inlet
    control valve and flows to the first stage where the impeller imparts velocity energy to the air. The air
    then proceeds through a diffuser section which converts the velocity energy to pressure energy. A
    multistage centrifugal compressor is a machine having two or more of these stages.


    Cubic feet per minute. An airflow measurement of volume.

Chipping hammer

    A hand held pneumatic tool. Designed to chip masonry, plaster, concrete etc.

Check valve

    A valve that permits flow in one direction only.


    A chemical element that is a heavy strong smelling greenish yellow imitating gas used as a bleach,
    oxidising agent and disinfectant. Found in some ambient air conditions.


    A filter element which, when loaded, can be restored by a suitable process, to an acceptable percentage
    of its original dirt capacity.

Clean room

    A facility or enclosure in which air content and other conditions (such as temperature, humidity, and
    pressure) are controlled and maintained at a specific level by special facilities and operating processes
    and by trained personnel.
Clean pressure drop

    The pressure loss across the filter element determined under steady state flow conditions using a clean
    test fluid across a clean filter element.


    The maximum cylinder volume on a working side of the piston, minus the piston displacement volume
    per stroke. It is usually expressed as a percentage of the displace volume.

Clearance pocket

    An auxiliary volume that may be opened to the clearance space for increasing the clearance, usually
    temporarily, to reduce the volumetric efficiency of the compressor.


    A device for mounting cylinders.


    An inward structural failure of a filter element which can occur due to abnormally high pressure drop
    (differential pressure) or resistance to flow.

Collapse pressure

    The minimum differential pressure that an element is designed to withstand without permanent


    Compressed natural gas, primarily methane.

Closed loop system

    A system in which distilled water, antifreeze, and/or corrosion inhibitors are circulated through a
    collector and storage tank in a closed loop. Heat picked up from the collector by the circulating fluid is
    transferred to the storage tank through the closed loop or other heat exchangers.

Coalescing filter

    A filter unit that combines three principles to filter out oil aerosols: 1) Direct interception - A sieving
    action, 2) Inertial impaction - Collision with filter media fibres, 3) Diffusion -Particles travel in a spiral
    motion, presenting an effective frontal area thus capturing particles within the filter medium.


    The A.S.M.E. Boiler and Pressure Vessel Code.

Composition of air

    A colourless, odourless, tasteless gas. A mixture of individual gases. The gaseous mixture surrounding
    the earth.


    A factor expressing the deviation of gas from the laws of hydraulics.

Compressibility factor Z

    Is the ratio of the actual volume of the gas to the volume determined according to the perfect gas law.          47

    To reduce the volume of, by or as if by pressure.

Compressed air

    Air under pressure greater than that of the atmosphere.

Compression efficiency

    Is the ratio of the theoretical work requirement to the actual work required to be performed on the gas
    for compression and delivery.

Compression Isothermal

    Is a compression in which the temperature of a gas remains constant.

Compression ratio

    The ratio of the absolute discharge pressure to the absolute inlet pressure.


    A machine that compresses air, gases.


    A product of condensation.


    A device that changes a vapour into a liquid. Accomplished by exposing a tube containing vapour to
    air or by passing the tube through a water jacket.


    The transfer of heat energy through a material (solid, liquid, or gas) by the motion of adjacent atoms
    and molecules without gross displacement of the particles.

Constant speed control

    The unit that runs continuously and matches air supply to demand, by loading and unloading the


    Foreign matter carried in the air, gas or fluid to be filtered out. Includes air borne dirt, metallic
    particles produced by wear of moving parts of the air compressor, rust from metal pipelines.

Contaminant capacity

    the weight of a specified artificial contaminant that must be added to the influent to produce a given
    differential pressure across a filter at specified conditions. Used as an indication of relative service life.

Contaminant failure

    any loss of performance due to the presence of contamination. Two basic types of contamination
    failure are: Perceptible -- gradual loss of efficiency or performance, and Catastrophic -- dramatic,
    unexpected failure.

Control valve

    A valve that controls the flow in air lines.


    Is a means of transferring heat through mass flow. Also the transfer of heat within a fluid by
    movements within the fluid.


    Fluid cooling agent.


    To subject petroleum oil to heat for breaking down into lighter products.

Critical pressure

    Is the saturation pressure at the critical temperature. It is the highest vapour pressure that the liquid can

Critical temperature

    The highest temperature at which well-defined liquid and vapour states exist.


    Canadian Standards Association


    Approach temperature. Usually the difference between cooling water temperature in to compressed air
    temperature out of an inter-cooler or after-cooler. Sometimes used to define oil cooler efficiency
    (cooling water temperature in to oil temperature out)

Cubic feet per minute (CFM)

    CFM. An airflow measurement of volume.

cu m/sec

    A volume. Cubic meters per second.

Cut in cut out pressure

    The settings on a pressure switch used to either load or unload the air compressor on a constant speed
    application, or start or stop the compressor on a start/stop application. The cut out pressure is also
    known as the maximum pressure, or the point at which there is no air being delivered. The cut in
    pressure is referred to as the minimum pressure, or the pressure that the system is allowed to fall to
    before air volume is required.


    A single complete operation consisting of progressive phases starting and ending at the neutral

The piston chamber in a compressor or actuator.)

     Direct current. A continuous, one directional flow of electricity.

Degree Celsius (°C)

     An absolute temperature scale. ((°F - 32)x 5/9).
Degree Fahrenheit (°F)

     An absolute temperature scale. ((°C x 9/5) + 32).

Degrees Kelvin (°K)

     An absolute temperature scale. The Kelvin unit of thermodynamic temperature, is the fraction 1/273,16
     of the thermodynamic temperature of the triple point of water. The triple point of water is the
     equilibrium temperature (0,01 °C or 273,16 K) between pure ice, air free water and water vapour.

Degree Rankine (°R)

     An absolute temperature scale. (°F + 459,67).

Degree Réaumur (°Ré)

     An absolute temperature scale. ((°F - 32) x 4/9).

Degree of saturation

     Is the ratio of weight of vapour existing in a given space to the weight that would be present if the
     space were saturated at the space temperature.


     A solid absorption agent used in deliquescent type dryers.

Delta P

     Describes the pressure drop through a component and is the difference in pressure between two points.

Delta T

     A term indicating a temperature relationship between two temperatures or temperature variation
     between two points.


     Flow of air under specific conditions required at a particular point.


     The ability of a fluid that is insoluble in water to separate from water with which it may be mixed in
     the form of an emulsion.


     Is the weight of a given volume of gas, usually expressed in lb / cu ft at SPT condition.

Depth filter

    A filter medium that retains contaminants primarily within tortuous passages.


    An adsorption type material used in compressed air dryers. Industry standards are activated alumina,
    silica gel and molecular sieves.

Design pressure

    The maximum continuous operating pressure as designed by the manufacturer.


    Opposite of absorption or adsorption. In filtration, it relates to the downstream release of particles
    previously retained by the filter.

Dew point

    Of a gas is the temperature at which the vapour in a space (at a given pressure) will start to condense
    (form dew). Dew point of a gas mixture is the temperature at which the highest boiling point
    constituent will start to condense.


    A stationary element between stages of a multistage centrifugal compressor. It may
    include guide vanes for directing the flowing medium to the impeller of the
    succeeding stage. in conjunction with an adjacent diaphragm, it forms the diffuser
    surrounding the impeller.
Diaphragm compressor

    Is a positive displacement reciprocating compressor using a flexible membrane or diaphragm in place
    of a piston.

Differential pressure

    The difference in pressure between any two points of a system or component.

Differential pressure indicator

    an indicator which signals the difference in pressure between any two points of a system or a


    A stationary passage surrounding an impeller, in which velocity pressure imparted to the flow medium
    by the impeller is converted into static pressure.

Direct current

    DC. A continuous, one directional flow of electricity.

Directional control valve

    A valve to control the flow of air in a certain direction.

Dirt holding capacity

    The quantity of contaminant a filter element can trap and hold before the maximum allowable back
    pressure or delta P level is reached.


    The movable seating surface in a valve.

Discharge piping

    Is the piping between the compressor and the aftercooler, the aftercooler separator and the air receiver.

Discharge pressure

    Is the total gas pressure (static plus velocity) at the discharge port of the compressor. Velocity pressure
    is considered only with dynamic compressors.

Discharge temperature

    Is the temperature existing at the discharge port of the compressor.

Displacement compressor

    A machine where a static pressure rise is obtained by allowing successive volumes of gas to be
    aspirated into and exhausted out of a closed space by means of the displacement of a moving member.

Displacement of a compressor

    The volume displaced by the compressing element of the first stage per unit of time.

Disposable filter

    A filter element intended to be discarded and replaced after one service cycle.


    Dioctylphalate aerosol (Efficiency Test Material).

Double acting compressor

    A positive displacement type compressor.


    The portion of the flow stream which has already passed through the system or the portion of the
    system located after a filter or separator/filter.


    Occurs when a valve does not close completely after popping and remains partly open until the
    pressure is further reduced.

Drain valve

    A device designed to remove surplus liquid from the compressed air system. Manual units range from
    petcock to a ball, gate or globe valve. Mechanical types consist of ball float. Electrical drains include
    solenoid type that is energised by a timer signal, or electric motor driven units. Also pneumatically
    activated drains.


    A coupling between the compressor and the engine or motor. The three types of drives most common
    are; flange mounted motor, V belt drive or direct coupling.


    Is a pipe coming from the top of the airline to feed air to an outlet for tools or air operated devices, so
    that condensation does not easily flow into the tool.

Dry bulb temperature

    Is the ambient gas temperature as indicated by a standard thermometer.

Dry gas

    Is any gas or gas mixture that contains no water vapour and/or in which all of the constituents are
    substantially above their respective saturated vapour pressures at the existing temperature.

Dry unit (oil free)

    Is one in which there is no liquid injection and/or liquid circulation for evaporative cooling or sealing.

Dynamic losses

    Friction against duct walls, internal friction in the air mass and direction variations will cause a speed
    reduction and are therefore called dynamic losses.

Dynamic type compressors

    Machines in which air or gas is compressed by the mechanical action of rotating vanes or impellers
    imparting velocity and pressure to the flowing medium. (Raise the pressure of the air by converting the
    energy from the velocity of the air to pressure.)

Dynamic viscosity (Dynamic)

    Is the force in newton required to move a fluid layer of one square meter area and a thickness of one
    meter with a velocity of one meter per second.


    This term refers to the hardness or softness of gaskets.

Dust cake

    A layer of dust built up on an air filter.

Dust holding capacity

    The amount of atmospheric dust which a filter will capture.

Effective area

    The area (in sq. inches) of the filter element that is exposed to the flow of air or fluid for effective


    Ability of a filter to remove particle matter from an air stream. Measured by comparing concentrate of
    material upstream and downstream of the filter. Typical particulate sizes range from .3 micron to 50

Efficiency compression

    Is the ratio of the theoretical work requirement to the actual work required to be performed on the gas
    for compression and delivery.

Efficiency mechanical

    Is the ratio of the thermodynamic work requirement in the cylinder to actual brake horsepower

Efficiency volumetric

    Is the ratio of actual capacity to piston displacement, stated as a percentage.


    The medium or material that does the actual filtering or separating. May be paper, wire mesh, special
    cellulose, inorganic plastic, or a combination.


    The ability of a non-water-soluble fluid to form an emulsion with water.


    Additive that promotes the formation of a stable mixture, or emulsion, of oil and water. Common
    emulsifiers are: metallic soaps, certain animal and vegetable oils, and various polar compounds.


    Intimate mixture of oil and water, generally of a milky or cloudy appearance. Emulsions may be of two
    types: oil-in water (where water is the continuous phase) and water-in-oil (where water is the
    discontinuous phase).

End cap

    A ported or closed cover for the end of a filter element.

Energy storage

    The ability to convert energy into other forms, such as heat or chemical reaction, so that it can be
    retrieved for later use. Also the development, design, construction and operation of devices for storing
    energy until needed. Technology includes devices such as compressed gas.


    Is the sum of the internal and external energies.


    Is a measure of the unavailability of energy in a substance.

Environmental contaminant

    all material and energy present in and around an operating system, such as dust, air moisture,
    chemicals, and thermal energy.


    The escape of water molecules from a liquid to the gas phase at the surface of a body of water.


    Turbines or engines in which gas expands, does work, and undergoes a drop in temperature.


    Free air delivery. Air at the atmospheric conditions of the site and unaffected by the compressor. Flow
    is measured at the discharge valve of the compressor, after the aftercooler, the water separator and built
    in check valve. Capacity and power consumption are corrected to ISO 1217 standard reference
    conditions: Ambient temperature = °20C, Ambient pressure = 1 bar(a), Relative humidity = 0%,
    Cooling water/air = 20°C, Effective working pressure at discharge valve = 7 bar(a).


    A structural failure of the filter medium due to flexing caused by cyclic differential pressure.


    An analytical method of assessing machine health by quantifying and examining ferrous wear particles
    suspended in the lubricant or hydraulic fluid.


    A device that removes solid contaminants, such as dirt or metal particles, from a liquid or gas (air is a
    gas), or that separates one liquid from another, or a liquid from a gas. The term filter describes the
    complete unit ... housing, filter element, internal by pass.

Filter breather

    A filtering unit for vented enclosures installed to prevent dirt and foreign matter from entering the
    enclosure. Also prevents oil loss by retaining oil droplets and draining the oil back to the sump.

Filter coalescing

    A filter unit that combines three principles to filter out oil aerosols: 1) Direct interception - A sieving
    action, 2) Inertial impaction - Collision with filter media fibres, 3) Diffusion -Particles travel in a spiral
    motion, presenting an effective frontal area thus capturing particles within the filter medium.

Filter efficiency

    The ability of a filter to remove specified test contaminants under specified test conditions from a
    specific test fluid, air, gas or liquid. Expressed as a percentage of the quantity of test contaminant
    introduced into the inlet of the filter test system.

Filter element

    The porous device which perform the actual process of filtration.

Filter head

     An end closure for the filter case or bowl that contains one or more ports.

Filter housing

     Something that covers or protects the filter assembly.

Filter inline

     Inlet and outlet connections are located at the same level on opposite sides of the filter or other device
     installed on the pipeline.

Filter life test

     A type of filter capacity test in which a clogging contaminant is added to the influent of a filter, under
     specified test conditions, to produce a given rise in pressure drop across the filter or until a specified
     reduction of flow is reached. Filter life may be expressed as test time required to reach terminal
     conditions at a specified contaminant addition rate.

Filter medium

     The porous material contained inside the filter to (a) separate the contaminants from the incoming air,
     gas, or liquid. (b) separate the liquid from the gas. Filter medium is usually made of paper, wire mesh,
     special cellulose, or a combination.

Filter separator

     Filtering unit that separates solids and liquid droplets from gas (air). Widely used in removing oil from
     a gas or air.


     The physical or mechanical process of separating insoluble particulate matter from a fluid, such as air
     or liquid, by passing the fluid through a filter medium that will not allow the particulate to pass
     through it.

First law of thermodynamics

     The amount of work done on or by a system is equal to the amount of energy transferred to or from the


     A bolted rim used for attachment to another object.

Flash point

     Is the lowest temperature to which oil must be heated under standardised test conditions to drive off
     sufficient inflammable vapour to flash when brought into contact with a flame. Flash points of
     petroleum based lubricants increase with increasing pressure.

Flexible mounting

     Vibration isolation mount. Provides reductions in vibration transmission.


     The volume of a substance passing a point per unit time (e.g., meters per second, gallons per hour,
Flow control valve

    A valve that controls the flow of air that passes through the valve. Used often for retardation or timing
    circuits, but especially for regulating the piston speed in cylinders.

Flow diagram

    A schematic flow sheet showing all controls involved with the system.

Flow meter

    An instrument for measuring the amount of air flow of a compressor. Measured in CFM.

Flow rate

    The rate (in litres or gallons per minute, cubic meters or cubic feet per second, or other quantity per
    time unit) Air related flows are usually expressed in CFM, SCFM, ACFM, ICFM


    A circulation process designed to remove contamination.

Forced draft fan

    A fan that generates (by pushing) a flow of ambient air over the exterior of the finned pipes to dissipate
    the sensible heat.


    Accumulation of foreign matter, such as mud or debris, in a cooler, pipe, or valve. In a cooler, H2O ?P
    and ?T will be seen to increase, as well as CTD.

Free air (FAD)

    Free air delivery. Air at the atmospheric conditions of the site and unaffected by the compressor. Flow
    is measured at the discharge valve of the compressor, after the aftercooler, the water separator and built
    in check valve. Capacity and power consumption are corrected to ISO 1217 standard reference
    conditions: Ambient temperature = °20C, Ambient pressure = 1 bar(a), Relative humidity = 0%,
    Cooling water/air = 20°C, Effective working pressure at discharge valve = 7 bar(a).


    Surface resistance to relative motion, which slows down movement and causes heat.


    Filter, Regulator and lubricator sometimes combined in one unit

Frost point

    Is the unique temperature to which the air (or any gas) must be cooled in order that it shall be saturated
    with respect to ice.

Full load

    Achieved when the air compressor is running at full RPM with a fully opened inlet and discharge,
    delivering the maximum volume at the rated pressure.


      A device attached to a safety or safety relief valve that prevents it from opening at the set pressure.


      A form of wear in which seizing or tearing of the gear or bearing surface occurs.


      A unit of volume. A US gallon has 231 cubic inches or 3,785 litres.


      A fluid (as hydrogen or air) that tends to expand indefinitely. Is one of three basic phases of matter.

Gas laws

      The behaviour of perfect gases, or mixtures thereof, follows a set of laws. Boyle' law, Charle's law,
      Amonton's law, Dalton's law, Amagat's law, Avogadro's law, Poisson's law.


      An instrument for measuring, testing, or registering.

Gauge pressure

      Is pressure as determined by most instruments and gauges.

Glycol dehydration

      A method of drying a gas.


      A motor controller that regulates the air flow through an end plate into the motor to control the speed
      and power. A speed regulation device built into or attached externally.


      Gallons per minute.

Heat exchanger

      Is used to cool compressed air or gas. Designed to reduce the temperature and liquefy condensate

Heatless dryer

      Heatless reactivated dryer. By means of expanding cold dry air to near atmospheric pressure inside the
      regeneration tower, the dryer air picks up moisture from the saturated desiccant bed and is then purged
      to atmosphere.

Horsepower (HP)

      Is a unit of work equal to 33,000 foot pounds per minute, 550 foot pounds per second, or 746 Watts.       58
Horsepower brake (BHP)

       The horsepower input to the compressor shaft, or more generally to any driven machine shaft.

Horsepower indicated

       The horsepower calculated from compressor-indicator diagrams. Applied only to displacement type

Horsepower theoretical

       The horsepower required to compress adiabatically the air or gas delivered by the compressor through
       the specified range of pressures.


       The moisture content of air.

Humidity specific

       The weight of water vapour in the air vapour mixture per pound of dry air.

Humidity relative

       The relative humidity of a gas (or air) vapour mixture is the ratio of the partial pressure of the vapour
       to the vapour saturation pressure at the dry bulb temperature of the mixture.


       Chemicals containing carbon and hydrogen.

Hydrogen chlorofluorocarbons (HCFC)

       Chemical species slated to replace CFCs in the near future.

Hygroscopic cells
Are any material with an affinity for moisture. These analyzers use sensing elements that contain moisture
adsorbing material. A change in the moisture content of the element is detected by an electric network and
is used as a measurement of dew point.


       A measurement. Inside diameter.


       Incapable of being mixed without separation phases. Water and petroleum oil are immiscible under
       most conditions, although they can be made miscible with the addition of an emulsifier.

Inches of water

       A measurement of vacuum or pressure that is used to measure the airflow restriction.

Induced draft

       An air flow caused by a fan that draws air through the heat exchanger core in a uniform pattern to
       dissipate the sensible heat.
Inertia forces

      When reciprocating compressors run, the moving parts such as pistons, rods, crossheads, connecting
      rods are repeatedly accelerated and retarded. These velocity changes set up pulsating inertia forces.
      The forces are of the first and second order. The first order forces have the same frequency as the
      compressor shaft speed and the second order forces have a frequency twice the shaft speed.


      The fluid entering a component.

Ingested contaminants

      Environmental contaminant that ingress’s due to the action of the system or machine.

Inlet pressure

      Is the total pressure (static plus velocity) at the inlet flange of the compressor.

Inlet temperature

      Is the temperature at the inlet flange of the compressor.
Inlet throttle

      A compressor control mechanism designed to control performance output of the compressor to the
      demands of the plant process.

Inline filter

      A filter assembly in which the inlet, outlet and filter element axes are in a straight line.


      Insoluble material suspended in the lubricating oil. This material may come from contamination or oil

Intake filter

      A device for separating solids or suspended particles in the air before they enter the air intake of the

Intake filter silencer

      A device for separating solids or suspended particles in the air before they enter the air intake of the
      compressor and reduce intake noise as on reciprocating compressors through a silencing chamber in
      the filter housing.


      Heat exchangers for removing the heat of compression between stages of a compressor.


      The removal of heat from the air or gas between stages.


      International Organisation for Standardisation.
Jack hammer

    A hand held rock drill worked by compressed air.


    A measurement. The international unit of energy. One joule is equal to one WATT - second or 0.737
    foot pounds.

Joule Thompson effect

    When a perfect gas flows through a throttling with constant inlet and outlet pressures, the temperature
    of the gas is the same before and after it flows through the throttling. There is a temperature drop,
    however, during the passage of gas through the throttling device itself. At this point internal energy is
    transformed into kinetic energy with an accompanying temperature drop. However, for real gases there
    is a sustained change in temperature, even though the energy content of the gas remains constant. This
    is called the Joule Thompson effect.


    Japanese tapered pipe thread (Female).


    Japanese tapered pipe thread (Male).

Kelvin (K)

    The Kelvin unit of thermodynamic temperature, is the fraction 1/273,16 of the thermodynamic
    temperature of the triple point of water. The triple point of water is the equilibrium temperature (o,01
    °C or 273,16 K) between pure ice, air free water and water vapour.


    Kilograms per square centimetre absolute.

Kickout type clutch

    A type of clutch which has a spring loaded jaw or detented rolling element to control torque in an
    assembly tool.


    A thousand.
Kilobar (kb)

    A unit of 1,000 bars of pressure.

Kilowatt (kW)

    A unit of power equal to 1,000 watts.

Kilowatt hour (kWh)

    A unit of work, being the work done in one hour at the rate of 1,000 watts.                                 61
Kinematic viscosity

       Is the dynamic viscosity divided by the density.

Kinetic energy

       Is the energy a substance possesses by virtue of its motion or velocity. Used primarily in calculations
       for dynamic and ejector type compressors.


       Kilopascal; a metric measure of pressure based on force per unit area. (1 kPa = 4.01 inches of water).


       Kilowatt - A unit of power equal to 1,000 watts.


       Kilowatt hour - A unit of work, being the work done in one hour at the rate of 1,000 watts.


       A deposit resulting from the oxidation and polymerisation of fuels and lubricants when exposed to
       high temperatures. Similar to, but harder, than varnish.




       A volume. Pounds per hour.

Leak air

       A crack or hole that accidentally admits a gas or lets it escape.


       The distance between the seat and disc seating surfaces when a valve is open.


       Filter parts that provide protection and support for the filter media.


       Electrical measurement. The out pit of one or several electric machines. Load also denotes the power
       carried by a particular circuit.

Load factor

       Ratio of the average compressor load during a given period of time to the maximum rated load of the


       The cylinder type section of the male rotor in a rotary screw compressor.                                 62

    A material (such as oil) used between moving parts of machinery to make the surfaces slippery and
    reduce friction.


    An instrument designed to add lubrication into the compressed air line.


    Maximum allowable working pressure. This data is found on the pressure vessel nameplate and is the
    maximum pressure at which the lowest set safety valve must be set (stamped).

Maximum operating pressure

    The highest operating pressure the system or component is designed to withstand.

Mechanical efficiency

    The ratio of the indicated power to shaft input.


    A mat of fibres that provide a barrier to particles entrained in the flow of air or gas.

Media migration

    Carry over of particles or contaminant from the filter or other filter materials into the clean side of the
    air flow.


    Is singular for median and is the material...paper, wire, cellulose or a combination... from which the
    filter element is made and which does the actual filtration or separation.

Mesh size

    Mesh is the number of openings in a square inch of screen or sieve. It is equal to the square of the
    number of strands of metal or plastic screening per lineal inch. Standard US mesh screen # 16 equates
    to a 1.19 millimetre particle diameter, mesh size #40 is 0.42 millimetres.


    Micrometer or one millionth of a meter; micron is sometimes represented in filtration by the Greek
    letter µ (um). A micron is 0.000039". Contaminant particles are measured by micron size and count.

Micron rating

    A measurement applied to filters or filter media to indicate the particle size at which suspended solids
    above that size will be removed.

Mineral oil

    The most commonly used lubricating oil for compressors.


    Millions of cubic feet per 24 hour.                                                                           63
Moisture separator

    A unit designed to separate condensate from the compressed air stream.

Moisture trap

    A device designed to enable accumulated liquids to be held for draining in a compressed air system.

Molecular sieves

    A solid adsorbent used for drying compressed air.

Molecular theory

    All matter consists of molecules which are in constant motion, but which are held together by
    molecular forces. In a solid the molecules are closely packed and arranged in such a pattern that the
    influence of the molecular forces is very strong. This gives the solid its consistency and form.
    Molecular motion consists largely of oscillations around points of equilibrium. In a liquid the
    molecules are about as close as in a solid, but they are not arranged in a lattice and the cohesive forces
    are weaker. The molecules are more mobile in relation to each other, whereby the characteristic liquid
    phase develops; the liquid accommodated itself to the walls of the containing vessel, and its free
    surface aligns itself horizontally in response to the force of gravity. In a gas, however, the molecules
    are farther apart, and they move freely about each other since the molecular forces are not as strong. A
    gas therefore expands through space and mixes with other gasses present. The total volume of the
    molecules in a gas is very small in relation to the volume of the gas. A gas can therefore be
    compressed into a small part of its original volume.

Multistage compressor

    A machine employing two or more stages to arrive at the final required pressure.

Natural frequency

    A projecting aperture at the end of a tube, pipe etc. serving as an outlet for compressed air. Reduces the
    demand on the compressor by generating the highest thrust and volume for the lowest possible air


    National Board of Boiler and Pressure Vessel Inspectors.


    National Electrical Code

Needle gauge

    A measuring tool used to check air pressure at the tool by inserting a hypodermic needle into the
    rubber hose.

Needle scaler

    A hand held pneumatic chipping tool which the blows are transmitted through a group of floating
    needles or metal rods.

Negative pressure

    A pressure below that of the existing atmospheric pressure taken as a zero reference.                        64

    National Electrical Manufacturers Association.


    National Fluid Power Association.


    Unit of free air flow rate. Normal cubic meter per hour.

No load

    The air compressor continues to run, usually at full RPM, but no air is delivered because the inlet is
    either closed off or modified, not allowing inlet air to be trapped.

Nominal efficiency

    An arbitrary filter efficiency rating.

Nominal filter rating

    Filter rating indicating the approximate particle size for which the majority will not pass through a
    given filter. It is generally interpreted as meaning that 85% of the particles of size equal to the nominal
    filter rating will be retained by the filter.

Nominal micron rating

    A term that means that 98% of all particles larger than a stated micron size have been removed from
    the product being filtered through the filter element.


    Are those constituents in the suction gas that cannot be condensed to a liquid with the cooling medium

Non-lubricated compressor

    A compressor designed to compress air or gas without contaminating the flow with lubricating oil.
    Piston rings and packing are usually made of TFE-based materials or carbon or other synthetic material
    that operate without lubrication.

Normal air

    Is the term used for average atmospheric air at sea level in a temperature zone where it contains some
    moisture. It is defined in the ASME Test Code For Displacement Compressors as being at 14.696
    psiA, 68 °F, 36% RH and weighing 0.075 lb/cu ft. The k value is 1.395.


    A projecting aperture at the end of a tube, pipe etc. serving as an outlet for compressed air. Reduces the
    demand on the compressor by generating the highest thrust and volume for the lowest possible air

Nozzle (valves)

    The stationary seating surface, the inlet of a valve.


     National Pipe Thread standard. A description of a specific pipe thread.


     National Pipe Thread tapered. A description of a specific pipe thread.


     A measurement. Outside diameter.

Oil aerosol

     A suspension of fine solid or liquid particles in a gas.

Oil free compressor

     A positive displacement air compressor which has no oil injected into the compression chamber for
     lubrication, cooling or sealing.

Oil system

     Consists of a vessel which is a combination of oil-sump and oil separator, an oil cooler and an oil filter.

Operating pressure

     The gauge pressure at which a pressure vessel is maintained in normal operation.


     An opening such as a hole or vent. An opening through which air can pass, or a restricted opening
     placed in a pipe line to provide a means of controlling or measuring flow.


     Occupational Safety and Health Administration.

Overpressure accumulation

     The permitted increase in pressure developed after the valve has opened. Usually expressed in
     percentage, ie.; 3% accumulation (A.S.M.E. Code, Section I). Flows through safety/relief valves are
     officially determined at these overpressure conditions.


     Measuring point for compressor acceptance test. Pressure at the compressor inlet flange.


     Measuring point for compressor acceptance test. Pressure at the compressor outlet flange.


     Measuring point for compressor acceptance test. Pressure before an orifice or nozzle, downstream of
     compressor discharge.                                                                                         66

     Measuring point for compressor acceptance test. Distribution pressure (headers).


     Measuring point for compressor acceptance test. Pressure at the point of use.

Particle density

     An important parameter in establishing an entrained particle's potential to impinge on control surfaces
     and cause erosion.

Particulate type filter

     A device designed to remove solids, such as dirt, scale, rust and other contaminants from the air

Paving breaker

     A hand held pneumatic tool. Designed for light demolition work, digging, making holes etc.


     Pressure dew point temperature (°C).

Perfect inter-cooling

     Is obtained when the gas is cooled to first stage inlet temperature following each stage of compression.

Performance curve

     A plot of expected operating characteristics (e.g., discharge pressure versus inlet capacity, shaft
     horsepower versus inlet capacity).


     The relationship of flow per unit area to differential pressure across a filter medium.


     Measure of alkalinity or acidity in water and water containing fluids. Can be used to determine the
     corrosion inhibiting characteristic in water based fluids. Typically, pH > 8.0 is required to inhibit
     corrosion of iron and ferrous alloys in water based fluids.


     The smaller of two mating or meshing gears; can be either the driving or the driven gear.

Piston displacement

     Net volume actually displaced by the compressor piston at rated machine speed, generally expressed in
     cubic feet per minute (usually CFM). For multistage compressors, the piston displacement of the first
     stage only is commonly stated as that of the entire machine.

Pleated filter

     A filter element whose medium consists of a series of uniform folds and has the geometric form of a
     cylinder, cone, disc, plate ... Synonymous with "convoluted" and "corrugated".

Pneumatic                                                                                                       67
    Of, relating to, or using air. Moved by air pressure. Filled with compressed air.


    Engineering science pertaining to gaseous pressure and flow.

PNEUROP oxidation test (POT)

    Is a standardised method to determine the carbon formation characteristics of compressor lubricants.

Point of use

    A single outlet or limited number of outlets in a building used to connect tools or equipment to the air


    A small channel or opening in a filter medium which allows passage of gas.

Positive displacement compressors

    Compressors in which successive volumes of air or gas are confined within a closed space, and
    compressed. They may be either reciprocating or rotating. (Trap air and then squeeze it to the desired

Pounds per square inch

    PSI - Pounds per square inch.

Pour point

    Is the temperature at which oil begins to flow under prescribed conditions.


    A measurement. Parts per billion


    A measurement. Parts per million


    Force per unit area, usually expressed in pounds per square inch (PSI) or BAR.

Pressure absolute

    The total pressure measured from absolute zero ( i.e., from an absolute vacuum).

Pressure back

    The pressure encountered on the return side of a system.

Pressure cracking

    The pressure at which a pressure operated valve begins to pass a gas.

Pressure critical

    Is the saturation pressure at the critical temperature.
Pressure dew point
      Is the temperature at which moisture begins to condense in a compressed air system.

Pressure discharge

      Is the total gas pressure (static plus velocity) at the discharge port of the compressor. Velocity pressure
      is considered only with dynamic compressors.

Pressure drop

      Resistance to flow. Defined as the difference in pressure upstream and downstream.

Pressure gauge

      A device that indicates pressure differential above or below atmospheric pressure.

Pressure inlet

      Is the total pressure (static plus velocity) at the inlet flange of the compressor.

Pressure rated

      The qualified operating pressure which is recommended for a component or a system by the

Pressure regulating valve

      A valve which enables pressure to be reduced, or kept constant at a desired level.

Pressure rise

      The difference between the discharge pressure and the intake pressure.

Pressure static

      The pressure measured in a flowing stream (liquid or gas) in such a manner that no effect on the
      measurement is produced by the velocity of the stream.

Pressure system

      The pressure which overcomes the total resistances in a system. It includes all losses as well as useful

Pressure total

      The pressure that would be produced by stopping a moving stream of liquid or gas.
Pressure velocity

      The total pressure minus the static pressure in an air or gas stream.

Preventive maintenance

      Also known as PM, maintenance performed according to a fixed schedule involving the routine repair
      and replacement of machine parts and components.


      Pounds per square inch.

PSIA                                                                                                                69
      Pounds per square inch, absolute.


      Pounds per square inch, differential.


      Pounds per square inch, gauge. Pressure indicated by a pressure gauge.


      Has to do with the properties of air-water vapour mixtures in the atmosphere.



Purge air

      The portion of dry, full line pressure, compressed air taken from the drying side tower of a dual tower
      desiccant dryer system. Expanded to a very low pressure and passed across the wet desiccant to strip
      the moisture in the desiccant of the regenerating tower. In the case of an external blower type dryer, the
      purge air is atmospheric air compressed by a blower and heated by an external heater to strip moisture
      off a wet desiccant bed.

      Measuring point for compressor acceptance test. Amount of condensate collected in the intercooler.


      Measuring point for compressor acceptance test. Amount of condensate collected in the aftercooler.


      Measuring point for compressor acceptance test. Amount of condensate collected in the air receiver.

Quick coupler

      A coupling device which consists of a spring loaded shutoff valve and a positive locking mechanism. It
      is used to connect tools, hoses and other accessories. Also known as Quick Disconnect.


      Tanks used for the storage of air discharged from compressors. They serve also to damp discharge line

Reciprocating compressors

      Machines in which the compressing element is a piston having a reciprocating motion in a cylinder.

Reduced pressure

      Ratio of the pressure of a gas to its critical pressure, in like units.

Reduced temperature                                                                                                70
    Is the ratio in absolute units of the actual gas temperature to the critical temperature.


    The process of desiccants being regenerated by water being driven off the desiccant. Hot air or gas
    passes through the desiccant bed, heating it. As a result, the partial pressure of the water vapour
    becomes higher than that in the regenerating air. Water is therefore released from the desiccant and
    carried away with the air stream (purge air) until a new state of equilibrium is reached.


    An automatic or manual device designed to control pressure, flow or temperature.


    Heat exchangers for raising the temperature of compressed air to increase its volume.

Relative humidity

    The ratio of the actual water-vapour partial pressure to its saturation pressure at the same temperature.
    (considered only with atmospheric air).

Relative vapour pressure

    The ratio of the vapour pressure to the saturated vapour pressure at the temperature considered.

Reynold's number

    A dimensionless flow parameter, ( Jnr/m), in which J is a significant dimension, often a diameter, n is
    the fluid velocity, r is mass density, and m is dynamic viscosity, all in consistent units.

Rock drill

    A pneumatic percussive drill designed as a boring tool.

Rotary compressors

    Machines in which compression is effected by the positive action of rotating elements.

Rotary sliding vane compressors

    Machines in which axial vanes slide radially in an eccentrically mounted rotor.


    The rotating element of a machine and, in the case of a compressor, is composed of the impeller
    (impellers) and shaft, and may include shaft sleeves and a thrust balancing device.


    Society of Automotive Engineers.

Safety valve

    A device that limits fluid (liquid and gaseous) pressures by discharging some of the pressurised liquid
    or gas.

Safety relief valve                                                                                             71
    An automatic pressure relieving device actuated by the static pressure upstream of the device, which
    opens in proportion to the increase in pressure over the opening pressure.

Saturated air vapour mixture

    Is one in which the space occupied by the mixture is saturated with water vapour at
    the mixture temperature.
Saturated vapour pressure

    Is the pressure existing at a given temperature in a closed vessel containing a liquid and the vapour
    from that liquid after equilibrium conditions have been reached. It is dependent only on temperature
    and must be determined experimentally.


    Occurs when the vapour is at the dew point or saturation temperature corresponding to its partial
    pressure. A gas in never saturated with a vapour. However, the space occupied jointly by the gas and
    vapour may be saturated.

Saturation pressure

    Is another term for saturated vapour pressure.


    A coating or precipitate deposited on surfaces such as water pipes, steam boilers that are in contact
    with hard water. Water that contains carbonates or bicarbonates of calcium or magnesium are likely to
    cause scale when heated.


    Standard cubic feet per minute., SCFM or scfm. Flow of free air measured at some reference point and
    converted to a standard set of reference conditions (e.g., 14.4 psia, 80° F, and 60% relative humidity.)
    Scfm means cfm at standard conditions. However, standards vary and some care is necessary. In the
    United states, the usual standard is 14.696 psiA and 60°F. Some chemical engineers will use one ATA
    and 0°C, but usually will be specific about the reference point. Europeans normally use one ATA and
    0°C. It is not the same to all people, therefore it is best that the reference pressure and temperature be
    definitely specified.

Screw compressor

    Is a positive displacement rotary compressor.

Sea level

    This is the average level of the ocean over the entire earth., tidal fluctuation is taken into account when
    determining sea level.


    Devices used between rotating and stationary parts to separate, and minimised leakage between, areas
    of unequal pressures.


    The stationary seating surface, the inlet of a valve.

Second law of thermodynamics
       Heat cannot, of itself, pass from a colder to a hotter body.

Set pressure

       The gauge pressure at which a safety valve visibly and audibly opens or at setting which a relief valve
       discharges an unbroken stream of liquid.


       The part of the rotating element on which the rotating parts are mounted and by means of which energy
       is transmitted from the prime mover.

Shaft input

       The power required at the compressor drive shaft. Losses in external transmissions such as gears and
       belt drives are not included.

Single acting

       The piston only compresses air with its stroke in one direction.

Single stage compressors

       Machines in which air or gas is compressed in each cylinder or casing from initial intake pressure to
       final discharge pressure.

Silica gel

       A desiccant most commonly used in heat regenerative type dryers.


       Is the internal leakage within a rotary compressor. It represents gas at least partially compressed but
       not delivered. It is experimentally determined and expressed in CFM to be deducted from the
       displacement to obtain capacity.

Slip RPM

       Is the speed required of a rotary compressor to maintain a given discharge pressure, supplying leakage
       only (zero actual output). It is an experience factor.


       A soft, black, mayonnaise-like deposit which is typically an emulsion of oil, water, and oil insolubles.


       An air operated device for hoisting or pulling. Similar to a winch.


       Specific Power Consumption.

Specific energy requirement

       The shaft input per unit of compressor capacity.

Specific fuel consumption

       The ratio of fuel consumption to compressor capacity.
Specific gravity

    This property is the ratio of the specific weight of air or gas to that of dry air at the same pressure and

Specific heat

    The quantity of heat required to raise the temperature of a unit weight of a substance by one degree.

Specific volume

    Is the volume of a given weight of gas, usually expressed as cu ft/lb as SPT conditions.

Specific weight

    This property of a gas is the weight of air or gas per unit volume. Unless specified, it refers in
    compressor practice to the weight per unit volume at conditions of total temperature, total pressure and
    composition which prevail at the compressor intake.


    The number of revolutions per minute of the compressor shaft.


    The rod connecting the disc to the lever on a valve.


    Steps in the compression of a gas, In reciprocating compressors, each stage usually requires a separate
    cylinder, in dynamic compressors, each requires a separate rotor disc.

Standard air

    Air at a temperature of 68 °F, a pressure of 14.70 psia and a relative humidity of 36 per cent (0.0750
    density) (as per A.S.M.E. however in the gas industry the temperature of standard air is usually given
    as 60 °F. (Unless specifically stated otherwise)


    Of a system (or part thereof) is its condition at an instant of time as described or measured by its


    The rod connecting the disc to the lever on a valve.


    A device used to separate air solids from the stream of air that might become a source of trouble.
    Adulterants caught in the strainer are blown out through an orifice fitted with a valve or plug. The
    strainer should be opened periodically for a thorough cleaning.

Suction pressure

    This is the pressure found on the suction side of a refrigeration system.

Supercompressibility factor

    This is a factor expressing deviation of a gas from perfect gas laws.
Surface filtration

     Filtration that occurs at the surface layer (as opposed to within the body depth) of the filter, and is
     accomplished by passing the material to be filtered over a grating, screen, sieve or membrane fabric
     with micro sized holes. The size of the holes in the filter determines what materials will pass through
     and which materials will be retained by the filter.

Synthetic lubricant

     A lubricating oil made with synthetic base stocks.

     Measuring point for compressor acceptance test. Temperature at the compressor inlet flange.


     Measuring point for compressor acceptance test. Temperature before an orifice or nozzle, downstream
     of compressor discharge.


     Is the property of a substance that gauges the potential or driving force for the flow of heat.

Temperature absolute

     The temperature of a body referred to the absolute zero, at which point the volume of an ideal gas
     theoretically becomes zero. (Fahrenheit scale is minus 459.67°F / Celsius scale is minus 273.15°C).

Temperature discharge

     Is the temperature existing at the discharge port of the compressor.

Temperature inlet

     Is the temperature at the inlet flange of the compressor.

Temperature intake

     The total temperature at the intake flange of the compressor.

Temperature rise ratio

     Is the ratio of the computed isentropic temperature rise to the measured total temperature rise during

Temperature static

     The actual temperature of a moving gas stream. It is the temperature indicated by a thermometer
     moving in the stream with the same velocity as the stream.

Temperature total

     The temperature which would be measured at the stagnation point if a gas stream were stopped, with
     adiabatic compression from the flow condition to the stagnation pressure.

Thermodynamics first law of

     The amount of work done on or by a system is equal to the amount of energy transferred to or from the
     system.                                                                                               75
Thermodynamics second law of

     Heat cannot, of itself, pass from a colder to a hotter body.


     Torsional moment or couple. It usually refers to the driving couple of a machine or motor.


     An air operated device for hoisting or pulling. Similar to a slusher or winch

Two stage compressor

     Machines in which air or gas is compressed from initial pressure to an intermediate pressure in one or
     more cylinders or casings.


     Underwriters Lab.


     Thread - Unified national coarse.


     Thread - Unified national fine.

Unit type compressors

     Compressors of 20 HP or less, generally combined with all the components required to put the into


     The air compressor continues to run, usually at full RPM, but no air is delivered because the inlet is
     either closed off or modified, not allowing inlet air to be trapped.

Unloaded horsepower

     The power that is consumed to overcome the frictional losses when operating in an unloaded condition.

Utilisation factor

     The ratio in percentage of the time that the equipment is in operation to the total working time.

Vacuum pumps

     Compressors that operate with an intake pressure below atmospheric and discharge pressure usually
     atmospheric or slightly higher.

    Devices with passages for directing flow into alternate paths.


    Fine separated particles floating in the air and clouding it. A substance in the gaseous state.

Vapour pressure

    Is the pressure exerted by a vapour confined within a given space. The vapour may be the sole
    occupant of the space, or may be associated with other gases.


    The oxidation of conventional hydrocarbon lubricants when they reach the end of their useful life and
    begin to breakdown. Can cause operating temperature increase, increase brake horsepower and plugs
    separator, can destroy air end.

V belt drive

    A drive arrangement for power transmission to compressors.


    Is a measure of resistance to deformation, or reluctance to be squeezed out a bearing. Indicates the
    internal friction of a fluid. Viscosity in normal lubricants is reduced as temperature increases.

Viscosity index (VI)

    Is a measure of the rate of change of viscosity with temperature. Oils with high VI have low viscosity

Volumetric efficiency

    The ratio and percent of the actual delivered capacity (measured at inlet temperature, pressure and gas
    composition) to the piston displacement.


    The control ring in a valve which surrounds the seat, used to control pre open and blowdown.

Wet bulb temperature

    Is used in psychrometry and is the temperature recorded by a thermometer whose bulb has been
    covered with a wetted wick and whirled on a sling psychrometer. Taken with the dry bulb, it permits
    determination of relative humidity of the atmosphere.

Wet gas

    Is any gas or gas mixture in which one or more of the constituents is at its saturated vapour pressure.
    The constituent at saturation pressure may or may not be water vapour.

Wet helical lobe unit

    Is one which 1) handles a small constant flow of liquid with the gas; 2) utilises evaporative (injection)
    cooling; or 3) circulates a liquid for sealing and/or cooling. The last may or may not be evaporative

   An air operated device for hoisting or pulling. Similar to a slusher or tugger


   Is energy in transition and is defined in units of Force times Distance. Work cannot be done unless
   there is movement.

Working pressure

   The normal working pressure for an air motor (6 bar).


   An inert gaseous element.


   The portion of a safety/relief valve that surrounds the spring. The spring housing.


   The temperature of pure melting ice under standard atmospheric pressure on the Centigrade and
   Réaumur scales.


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