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Air compressors provide air and power for various tools, machinery and manufacturing processes in a wide range of industries. They may be used individually as a surface finishing tool, or they may provide efficient energy generation for many manufacturing processes and pneumatic power systems, including automotive repair equipment, surface finishing and pneumatic power tools.
Compressors come in a variety of styles, the differences lying in the power sources they use, their method of operation and their size. Industrial air compressors are available in three main configurations: reciprocating compressors, rotary screw compressors and centrifugal compressors. Reciprocating air compressors create air compression with pistons; these have a broad range of output capabilities and are fairly cost-effective. Rotary screw compressors are generally configured as regular, lubricated compressors, which may be designed as oilless air compressors, while centrifugal air compressors are always oil-free. Rotary screw and reciprocating compressors may be designed as portable air compressors or as central facility units. The oil-free compressor style is being adopted by another common design, the rotary air compressor. This air compressor comes in a variety of sizes, including the mini air compressor, which can be as small as ten pounds. Regardless of the type of air compressor a buyer is shopping for, they can buy it new or pay a slimmer price and get a used air compressor that is often in excellent shape and will last long enough to be worthwhile. Air compressors vary in size as well as the ways in which they are powered; they can be powered by electricity or a gas motor. Electric air compressors come equipped with a power cord, while 12 volt batteries, which are always a part of 12 volt air compressors, can be recharged in an electrical outlet or a car cigarette lighter, depending on the size. Gas air compressors operate by way of a gasoline run motor.
Automotive industries use air compressors extensively for tire inflation, parts cleaning and surface finishing; homes and commercial businesses use air compressors in a range of appliance and recreational product applications such as paintball gun canisters. Gas stations use compressors for gas pumps; airbrush paint applications also use air compressors in auto body shops, commercial and private airbrush art and home painting projects. Power tools such as jackhammers, jacklegs, needle scalers, tuggers/winches, air chisels, chipping hammers and rock drills use compressors or portable air compressors. Sandblasters also operate on compressed air. Other tools that use compressed air include nail guns, sanders, drills, staplers and spray guns. Industrial air compressors provide air for air purification systems, air lock systems, blast forges and temperature control systems. Another use for air compressors is filling the metal oxygen cylinders used by deep sea divers to swim and research. Air compressors are usually made from one of three basic metals: aluminum, steel and cast iron. However, when lightweight compressors are needed, such as a mini compressor or portable compressor, plastic is used in place of metal.
Air compressors are mechanical devices that compress air by pulling it in from the atmosphere and decreasing its volume while simultaneously increasing its pressure. Reciprocating air compressors use pistons to do this work, which move in and out of a cylindrical mouth piece that lets the air go in and out without altering the compression process and only releasing the air when it will be useful. Rotary screw compressors use two helical screws fitted into one another, rotating quickly in opposite directions, which pulls the air in small doses, compressing it while it cycles through the rotating screws. Both of these use oil to lubricate internal parts, and oil separators are often attached downstream to filter out contaminating lubricants, although some rotary screw compressors are oil-free. Using oilless air compressors is ideal for applications where a pure air stream is required, as downstream filter cleaning can add to maintenance costs and is not guaranteed to capture every drop of oil. Centrifugal air compressors are dynamic compressors that use a rotating impeller to create air velocity, which is pushed through a diffuser and converted into pressure. The compressed air is then stored in a holding tank or released into a pressure system, ready for work.
Compressors have two components: a compressing mechanism and a power source for the compressing mechanism. The energy for compression can be taken from a gas-powered motor, an electrical motor or a power takeoff. The various compressing mechanisms that do the actual work of compression are pistons, vanes and impellers. By storing and compressing the air, air compressors convert mechanical energy into pneumatic energy. Air compressor manufacturers often design products that can be driven by natural gas, which greatly reduces cost and energy consumption. Until recently, air compressors have mainly been used for surface finishing, cleaning and inflating applications - its use as a power generator has not been seriously explored until now. As automotive manufacturers search for alternatives to internal combustion engines, many manufacturers are beginning to experiment with the clean pneumatic power provided by air compressors. Because of air compressors' importance to an extensive variety of industrial operations, their widespread use will continue into the foreseeable future.
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Air Compressor Types
12 volt air compressors are machines that require 12 volts of power to reduce the volume of air in a tank in order to increase the pressure.
Axial compressors have flow in the axial direction by accelerating air tangentially with blades attached to the rotors. This increases the kinetic energy of the air and diffuses it through static vanes to increase its pressure.
Centrifugal compressors act on air with blades on a rotating impeller. The rotary motion of the air causes an outward velocity from the centrifugal force, and then a diffuser transforms this outward velocity into pressure.
Compressors are mechanisms used for compressing air to higher than atmospheric levels.
Diaphragm compressors achieve compression with the use of a flexing diaphragm that moves back and forth in a closed chamber; the design is an alteration of the reciprocating piston concept. The motion of the connecting rod under the diaphragm causes the flexing and only a short stroke is needed to generate similar pressure effects as those of a reciprocating piston compressor.
Double acting compressors use both sides of the piston to compress the air, both the forward and the back stroke.
Ejector compressors use a high-pressure jet stream. The drive of the stream is transferred to the low pressure of the air.
Electric air compressors are machines that use electrical power to pressurize air before releasing it in a high energy form.
Free piston compressors have an adjustable compression piston that moves along the length of a steel cylinder column. The guiding and compression pistons collide at the return stroke because the compressed air pushes back the compression piston in the last stage.
Gas air compressors are gas-fueled machines that reduce the volume of air in order to use the pressurized air for power.
Industrial air compressors are mechanical devices used for industrial purposes that provide air at higher than atmospheric pressure.
Labyrinth compressors are oil-free and work without piston rings. A series of labyrinths creates the seal between the cylinder wall and the piston.
Liquid ring compressors have only one moving part, the impeller shaft assembly. The service liquid rotating in its casing forms the liquid ring seal, and air enters through the suction port, moves between the impeller blades and is compressed before discharging.
Lobe compressors use two mating lobes on different shafts that rotate in opposite directions to capture incoming air and compress it against the casing. Lobe compressors supply very high flows at pressure ranges between non-positive displacement compressors and other types of positive displacement units.
Mini air compressors are machines that reduce the volume of air in order to pressurize it and convert mechanical energy into pneumatic power. The pressurized air can then be used for a variety of applications; due to the limited size of mini air compressors, however, the output is small and limited to 250 pounds per square inch (PSI).
Non-positive displacement compressors depend on motion to transfer energy from the compressor rotor to the air. Initial acceleration of the air produces a negative (suction) pressure at the inlet port, which draws air in.
Oilless air compressors provide air and power for various tools, machinery and manufacturing processes in industries that require clean air.
Portable air compressors are hand-held systems that do not require an electrical outlet.
Positive displacement compressors work by successively trapping a volume of air and reducing it, thereby increasing the pressure. The quantity of heat produced rises proportionally to the pressure rise, resulting in substantial temperature increases of the air and the compressor itself.
Reciprocating compressors move a piston to the top of a cylinder to create compression. These require either water or air cooled.
Rotary compressors reduce the volume of air by compressing it between intermeshing, counter-rotating components that force the air into a tank.
Screw compressors use two contra-rotating rotors that turn in a synchronous mesh. As air enters the sealed chamber, the rotors revolve, reducing the volume of trapped air and sending it compressed through the discharge port at the designated pressure level.
Swash plate compressors move pistons parallel to the crankshaft, either by a cam or by a plate mounted axially on the shaft and inclined to it.
Used air compressors are previously owned machines that reduce the volume of air in order to increase its pressure.
Vane compressors have an eccentrically mounted rotor that is the only moving part and rotates within the stator. As the rotor rotates, centrifugal force forces the vanes from their slots, forming compression cells, and this pumping action of the vanes sliding in and out moves the air from the inlet side of the compressor to the outlet side.
Air Compressor Terms
Aftercooling - The removal of heat when the compression process is complete.
Air Pressure Regulator - A component of an air compressor that allows the user to adjust the air pressure in the air line.
Backflow - A condition caused by a difference in pressure in which air will flow back into the distribution pipes rather than in the intended direction.
Casing - The element that houses the rotor and related internal components of an air compressor. This includes the integral inlet and discharge connections.
Collapse Pressure - The lowest amount of differential pressure something is able to withstand without deformation.
Compression/Pressure Ratio - The ratio of the absolute inlet pressure to the absolute outlet pressure. Compression/pressure ratio typically applies to a single stage of compression but could also apply to a full multistage compressor.
Cylinder - The piston compartment in an actuator or reciprocating compressor.
Discharge Piping - The piping between the aftercooler and the compressor and the air receiver and the cooler separator.
Drive - A flange-mounted belt drive, motor or direct coupling between the engine or motor and the compressor.
Full-Load - The operation of an air compressor at full speed, having a completely open inlet and discharge delivering upper limit airflow.
Guide Vane - An adjustable fixed part that directs the flow of air approaching the inlet of an impeller.
Impeller - The component of the rotating element of a dynamic compressor that gives energy to the flowing medium through centrifugal force. An impeller is comprised of blades that rotate with the shaft.
Intank Check Valve - A valve intended to stop air pressure and volume from slipping out of the compressor tank back into compressor heads when the compressor is not running.
Intercooler - Heat Exchangers that eliminate heat produced during compression between the stages of a compressor.
Load Factor - The ratio of the maximum rated compressor load to the average compressor load within a certain period.
Load/Unload Control - A method of control that permits the compressor to run either at no load or at complete load at the same time that the driver remains at a constant speed. Load/unload control is an attempt to match air delivery to the demand.
Maximum Pressure Rating - The highest-pressure level recommended for a compressor.
No Load - When an air compressor is running at full RPM and is wide open, but no air is sent because the inlet is either closed off or modified and will not allow inlet air to be trapped.
Noncooled Compressor Cylinders - Compressor cylinders on a reciprocating compressor that run at low compression ratios and undergo little temperature change. These are used mainly in oil and gas field applications.
Pressure Inlet - The total pressure (static plus velocity) at the inlet flange of the compressor.
Pressure Rise - The difference between the intake pressure and the discharge pressure
Pumping/Surge - The reversal of flow in a dynamic compressor. Pumping/surge takes place when the handled capacity is reduced to an insufficient pressure in order to maintain flow.
Rotor - A revolving element of a compressor. It consists of the impeller and shaft and may have shaft sleeves and a thrust balancing device.
Shaft - The part that the rotating elements are attached to and through which energy is transferred from the prime mover.
Shaft Sleeves - Mechanisms used to position the impeller or to shield the shaft.
Sole Plate - The pad the compressor is mounted on. This is implanted in concrete and usually metallic.
Stack Up - The interaction between the stages of a centrifugal compressor. In the design of a multi-stage compressor, every stage can only run at one point of its characteristic curve, and the determination of this point is done through the design conditions of temperature, flow and pressure.
Surge Limit - The capacity in a dynamic compressor under which the process becomes unsteady.
Thrust Balancing Device - Part of a revolving element that offsets the thrust of compressor impellers.
Selection and Application
Compressed Air – 1
a. How Can Air Generate Power?
b. Where is Compressed Air Used?
Advantages of Air Power – 1-2
a. Air Power versus Electric Power
b. Air Power versus Hydraulic Power
Types of Air Compressors – 2-4
a. Reciprocating Type: 1. Single-Stage, 2. Two-Stage, 3. Rocking Piston Type, 4. Diaphragm Type
b. Rotary Type: 1. Rotary Sliding Vane Type, 2. Rotary Helical Screw Type, 3. Scroll Type
Types of drives: a. V-Belt Drives, b. Direct Drives, c. Gas Engine – Power Take-Off
Accessories – a. Air Receiver, b. Belt Guard, c. Diagnostic Controls, d. Intake Filter e. Manual and Magnetic Air Compressor Inst
The purpose of this is to help users understand compressed air as a power source and to provide technical guidance for selecting the right
air compressor for specific applications. The central focus is on packaged complete unit air compressors, most commonly used in sizes from 1/4 to 30 horsepower as measured according to standards for continuous duty compressors. Content has been provided by members of the Stationary Single/Double Acting Unit Type Compressor Section. Products within the scope of this section are most frequently used for general purpose industrial air supply, but they also find use in off-shore drilling, construction jobs, locomotives, ships, mining, and other specialized applications.
MEMBERS INCLUDE: Campbell Hausfeld, CompAir, Curtis-Toledo, Inc. DeVilbiss Air Power Company Gardner Denver, Inc. Ingersoll-Rand Company
Quincy Compressor Saylor
COMPRESSED AIR, HOW CAN AIR GENERATE POWER?
The normal state of air, barometric, is called atmospheric pressure. When air is compressed, it is under pressure greater than that of the atmosphere and it
characteristically attempts to return to its normal state. Since energy is required to compress the air, that energy is released as the air expands and
returns to atmospheric pressure. Our ancestors knew that compressed air could be used for power when they discovered that internal energy stored in compressed air is directly convertible to work. Air compressors were designed to compress air to higher pressures and harness that energy. Unlike other sources of power, no conversion from another form of energy such as heat is involved at the point of application. Compressed air, or pneumatic devices are therefore characterized by a high power-to-weight or power-to-volume ratio.
Not as fast as electricity, nor as slow as hydraulics, compressed air finds a broad field of applications for which its response and speed make it ideally
suited. Where there is an overlap, the choice often depends on cost and efficiency, and air is likely to hold the advantage.
Compressed air produces smooth translation with more uniform force, unlike equipment that involves translatory forces in a variable force field. It is a
utility that is generated in-house, so owners have more control over it than any other utility. In addition, air does not possess the potential shock hazard
of electricity or the potential fire hazard of oils. The advantages of air power will be discussed further in the proceeding pages.
WHERE IS COMPRESSED AIR USED?
Compressed air powers many different kinds of devices. It can be used to push a piston, as in a jackhammer; it can go through a small air turbine
to turn a shaft, as in a dental drill; or it can be expanded through a nozzle to produce a high-speed jet, as in a paint sprayer.
Compressed air provides torque and rotation power for pneumatic tools, such as drills, brushes, nut runners, riveting guns, and screwdrivers. Such tools are
generally powered by some form of rotary air motor such as the vane or lobe type, or by an air turbine. Equally common are devices producing lateral
motion and direct force, either steady or intermittent. Common examples are clamps, presses, and automatic feeds. Or, air pressure is used to accelerate a
mass, which then exerts an impact upon an anvil, as in paving breakers and pile drivers.
Common applications in industrial plants and on construction sites are air-powered nailers and staplers. In paint spraying and in air conveying,
the dynamic pressure of the air imparts motion.
ADVANTAGES OF AIR POWER
When there are a dozen or more forms of energy to choose from, what advantages does air power offer? Here, compressed air stacks up against two
of its competitors—electricity and hydraulics.
AIR POWER VERSUS ELECTRIC POWER
Cost: Air tools have fewer moving parts and are simpler in design, providing lower cost maintenance and operation than electric tools.
Flexibility: Air tools can be operated in areas where other power sources are unavailable, since engine-driven portable compressors are their
source of air power. Electric power requires a stationary source.
Safety: Air-powered equipment eliminates the dangers of electric shock and fire hazard. Air tools also run cooler than electric tools and have the
advantage of not being damaged from overload or stalling.
Weight: Air tools are lighter in weight than electric tools, allowing for a higher rate of production per man-hour with less worker fatigue.
AIR POWER VERSUS HYDRAULIC POWER Cost: An air system has fewer parts than a hydraulic system, lowering service and maintenance costs.
Also, the use of a single compressed air supply permits operation of many separate systems at once. Hydraulic systems require more complex
and costly controls. Flexibility: Compressed air systems offer simpler installation than hydraulics, particularly where tools are frequently interchanged. Compressed air systems also offer better adaptability for automation and flexibility for changing or expanding operations.
Maintenance: Air systems have less downtime than hydraulic systems because they have less complex controls. Less preventative maintenance is required
with air, whereas hydraulic fluids must be monitored and replaced periodically.
Safety: Hydraulic devices operating near open flame or high temperatures present fire hazards, unless fire-resistant fluids are used. Leakage in hydraulic systems can result in the presence of dangerous hydraulic fluids and even complete system shutdown. In contrast, compressed air devices operate with
lower system pressures, and accidental air leaks release no contaminants.
Weight: High ratio of power-to-weight in air tools contributes to a lower operator fatigue versus hydraulic tools.
TYPES OF COMPRESSORS
Air compressors in sizes from 1/4 to 30 horsepower include both reciprocating and rotary compressors, which compress air in different ways. Major types
of reciprocating compressors include reciprocating single acting, reciprocating double acting, reciprocating diaphragm, and reciprocating rocking piston
type. Major types of rotary air compressors include rotary sliding vane, rotary helical screw and rotary
scroll air compressors.
RECIPROCATING SINGLE ACTING COMPRESSORS
Reciprocating single acting compressors are generally of one-stage or two-stage design. Compressors can be of a lubricated, non-lubricated or oil-less design.
In the single-stage compressor, air is drawn in from the atmosphere and compressed to final pressure in a single stroke. The single-stage reciprocating
compressor is illustrated in Figure 1. Single-stage compressors are generally used for pressures of 70 psi (pounds per square inch) to 135 psi.
In the two-stage compressor, air is drawn in from the atmosphere and compressed to an intermediate pressure in the first stage. Most of the heat of
compression is removed as the compressed air then passes through the intercooler to the second stage, where it is compressed to final pressure. The
two-stage reciprocating compressor is illustrated in Figure 2. Single and two-stage reciprocating compressors are frequently used in auto and truck
repair shops, body shops, service businesses, and industrial plants. Although this type of compressor is usually oil lubricated, hospitals and laboratories
can purchase oil-less versions of the compressors.
2 AIR COMPRESSOR SELECTION AND APPLICATION
Intake Filter Valve Plate Multi-Fin Cylinder Automotive Piston with Compression Rings and Oil Control Rings Oil Sump ( Splash Lubrication
with or without Pressure Lubrication Flywheel Designed for Compressor Cooling Reciprocating Single Stage (Twin Cylinder)
Head Reciprocating Two Stage Head Unloader Connecting Rods Crankcase Oil Drain Fly Wheel Valve Plate
ROCKING PISTON TYPE
Rocking piston compressors are variations of
reciprocating piston type compressors (Fig. 4). This
type of compressor develops pressure through a
reciprocating action of a one-piece connecting rod
and piston. The piston head rocks as it reciprocates.
These compressors utilize non-metallic, low friction
rings and do not require lubrication. The rocking
piston type compressors are generally of smaller
size and lower pressure capability.
Diaphragm compressors (Figure 5) are a variation
of reciprocating compressors. The diaphragm compressor
develops pressure through a reciprocating
or oscillating action of a flexible disc actuated by
an eccentric. Since a sliding seal is not required
between moving parts, this design is not lubricated.
Diaphragm compressors are often selected when
no contamination is allowed in the output air line
or atmosphere, such as hospital and laboratory
applications. Diaphragm compressors are limited
in output and pressure, and they are used most for
ROTARY SLIDING VANE TYPE
The rotary sliding vane compressor consists of a
vane-type rotor mounted eccentrically in a housing
(Figure 6). As the rotor turns, the vanes slide out
against the housing. Air compression occurs when
the volume of the spaces between the sliding vanes
is reduced as the rotor turns in the eccentric cylinder.
Single or multi-stage versions are available. This
type of compressor may or may not be oil lubricated.
Oil-free rotary sliding vane compressors are
restricted to low-pressure applications because of
high operating temperatures and sealing difficulties.
Much higher pressures can be obtained with oil
Some of the advantages of rotary sliding vane
compressors are smooth and pulse-free air output,
compact size, low noise levels, and low vibration
AIR COMPRESSOR SELECTIONAND APPLICATION 3
Reciprocating Single Stage, Oilless
Rocking Piston Type
Motor Shaft Eccentric
Rotary Vane Type
ROTARY HELICAL SCREW TYPE
Rotary helical screw compressors (Figure 7) utilize
two intermeshing helical rotors in a twin-bore case.
In a single-stage design, the air inlet is usually located
at the top of the cylinder near the drive shaft end.
The discharge port is located at the bottom of the
opposite end of the cylinder. As the rotors unmesh
at the air inlet end of the cylinder, air is drawn into
the cavity between the main rotor lobes and the
secondary rotor grooves. As rotation continues, the
rotor tips pass the edges of the inlet ports, trapping
air in a cell formed by the rotor cavities and the
cylinder wall. Compression begins as further rotation
causes the main rotor lobes to roll into the secondary
rotor grooves, reducing the volume and raising cell
pressure. Oil is injected after cell closing to seal
clearances and remove heat of compression.
Compression continues until the rotor tips pass the
discharge porting and release of the compressed air
and oil mixture is obtained. Single or multi-stage
versions are available. This type of compressor can
be oil lubricated, water lubricated or oil-free. Some
advantages of the rotary helical screw compressors
are smooth and pulse-free air output, compact size,
high output volume, low vibrations, prolonged
service intervals, and long life.
ROTARY SCROLL TYPE
Air compression within a scroll is accomplished
by the interaction of a fixed and an orbiting helical
element that progressively compresses inlet air
(Figure 8). This process is continuously repeated,
resulting in the delivery of pulsation-free compressed
air. With fewer moving parts, reduced maintenance
becomes an operating advantage. Scroll compressors
can be of a lubricated or oil-free design.
TYPES OF CONTROLS
Controls are required for all compressors in order
to regulate their operation in accordance with
compressed air demand. Different controls should
be chosen for different types of compressor
applications and requirements.
For continuous operation, when all or most of the air
requirements are of a steady nature, constant speed
controls are required. Use constant speed controls
whenever the air requirement is 75 percent or more
of the free air delivery of the air compressor or when
motor starts per hour exceed motor manufacturer
recommendations. Constant speed controls include
load/unload control for all types and inlet valve
modulation for rotary compressors.
Start-stop controls are recommended for a compressor
when adequate air storage is provided
and air requirement is less than 75 percent of the
compressor free air delivery.
Dual controls allow for switching between
constant speed and start-stop operation by setting
a switch. With dual controls, the operator can select
a different type of control to suit his or her specific
air requirements each time the compressor is used.
Dual controls are helpful when a compressor is
used for a variety of applications.
Sequencing controls provide alternate operation of
each compressor at each operating cycle and dual
operation during peak demands. Sequencing controls
are ideal for operating a group of compressors at
peak efficiency levels.
4 AIR COMPRESSOR SELECTION AND APPLICATION
Rotary Helical Screw
Oilless or Lubricated
Cooling Fan with Integral
TYPES OF DRIVES
Most compressors are driven with electric motors,
internal combustion engines, or engine power takeoffs.
Three types of drives are commonly used with
these power sources.
V-Belt Drives are most commonly used with
electric motors and internal combustion engines.
V-Belt drives provide great flexibility in matching
compressor load to power source load and speed
at minimum cost. Belts must be properly shielded
Direct Drives provide compactness and minimum
drive maintenance. Compressors can be flangemounted
or direct-coupled to the power source.
Couplings must be properly shielded for safety.
Lower horsepower compressors also are built as
integral assemblies with electric motors.
Engine Drives, gasoline or diesel engine, or power
takeoff drives, are used primarily for portability
reasons. A gearbox, V-Belt, or direct drive is used to
transmit power from the source to the compressor.
AIR COMPRESSOR PACKAGED UNITS
Air compressor packaged units are fully assembled
air compressor systems, complete with air compressor,
electric motor, V-belt drive, air receiver, and
automatic controls. Optional equipment includes
aftercoolers, automatic moisture drain, low oil safety
control, electric starter, and pressure reducing valve.
Air compressor units come with a variety of configurations:
gasoline or diesel engines, optional direct
drive, optional separate mounted air receivers,
The most common type of packaged unit compressor
configuration is the tank-mounted single acting,
single- or two-stage reciprocating design. Models are
offered in the range of 1/4 through 30 horsepower.
Electric motors or gas engines drive the compressors.
Typical examples are shown in Figures 9 through
Most compressors available in this horsepower range
are air cooled. Installation is convenient because the
unit requires only a connection to electrical power
and a connection to the compressed air system.
AIR COMPRESSOR SELECTIONAND APPLICATION 5
Figure 9 Base Plate Mounted Package
Figure 10 Tank Mounted Simplex Packages
Tank Mounted Duplex Package
Figure 12 Gasoline Engine Drive Package
AIR COMPRESSOR PERFORMANCE
The volume of compressed air delivered by an air
compressor at its discharge pressure, normally
is stated in terms of prevailing atmospheric inlet
conditions (acfm). The corresponding flow rate in
Standard cubic feet per minute (scfm) will depend
upon both the Standard used and the prevailing
atmospheric inlet conditions.
Varying flow rates for more than one discharge
pressure simply reflect the reduction in compressor
volumetric efficiency that occurs with increased
system pressure (psig). For this reason, the maximum
operating pressure of a compressor should
be chosen carefully.
Displacement is the volume of the first stage cylinder(
s) of a compressor multiplied by the revolutions
of the compressor in one minute. Because displacement
does not take into account inefficiencies related
to heat and clearance volume, it is useful only as a
general reference value within the industry.
Standard accessories are available to help ensure
reliable and trouble-free compressor operation.
Some special purpose devices also are available
to meet unusual requirements. Below is a list of
commonly used accessories.
A receiver tank is used as a storage reservoir for
compressed air. It permits the compressor not to
operate in a continuous run cycle. In addition, the
receiver allows the compressed air an opportunity
A belt guard protects against contact with belts
from both sides of the drive and is a mandatory
feature for all V-belt driven compressor units
where flywheel, motor pulley, and belts are used.
Protective devices designed to shut down a compressor
in the event of malfunction. Devices may
include high air temperature shut down, low oil
level shut down and low oil pressure shut down,
preventative maintenance shut down, etc.
The intake filter eliminates foreign particulate
matter from the air at the intake suction of the air
compressor system. Dry (with consumable replacement
element) or oil bath types are available.
MANUAL AND MAGNETIC STARTERS
Manual and magnetic starters provide thermal overload
protection for motors and are recommended
for integral horsepower and all three-phase motors.
Local electrical codes should be checked before
purchasing a starter.
6 AIR COMPRESSOR SELECTION AND APPLICATION
Low Oil Level Switch…
protects unit from operating
in low oil level conditions
cools discharge air allowing
moisture to condense in tank.
Automatic Drain Valve…
ensures removal of water
protects motor from electrical
overload (required on 5
through 30 HP units).
Duplex Tank Mounted Compressor
with Alternator Panel
AIR COMPRESSOR INSTALLATION
The air compressor location should be as close as
possible to the point where the compressed air is to
be used. It is also important to locate the compressor
in a dry, clean, cool, and well-ventilated area. Keep it
away from dirt, vapor, and volatile fumes that may
clog the intake filter and valves. If a dry, clean space
is unavailable, a remote air intake is recommended.
The flywheel side of the unit should be placed
toward the wall and protected with a totally enclosed
belt guard, but in no case should the flywheel be
closer than 12 inches to the wall. Allow space on all
sides for air circulation and for ease of maintenance.
Make sure that the unit is mounted level, on a
solid foundation, so that there is no strain on the
supporting feet or base. Solid shims may be used to
level the unit. In bolting or lagging down the unit,
be careful not to over-tighten and impose strain.
MOTOR OVERLOAD PROTECTION
All compressor motors should be equipped with
overload protection to prevent motor damage. Some
motors are furnished with built-in thermal overload
protection. Larger motors should be used in conjunction
with starters, which include thermal overload
units. Such units ensure against motor damage due
to low voltage or undue load imposed on the motor.
Care should be taken to determine the proper
thermal protection or heater element. The user
should consider the following variables: the load to
be carried, the starting current, the running current,
and ambient temperature. Remember to recheck
electric current characteristics against nameplate
characteristics before connecting wiring.
Fuses and circuit breakers are for circuit protection only
and are not to be considered motor protection devices.
Consult your local power company regarding proper fuse
or circuit breaker size.
AIR COMPRESSOR SYSTEM
A compressed air supply within a manufacturing
plant or an automotive collision and body shop
often consists of one compressor that can meet the
overall air requirements. Makes sense, right? But
consider an alternative: multiple smaller horsepower
compressors positioned at strategic points throughout
the plant or shop. These compressors would
feed into a common air line or into individual lines
serving one or more points of use.
In the central system, the compressor is of a size to
supply total compressed air requirements, at least
in the beginning. This option has the advantage of
one compressor, one point of maintenance, and one
electric power connection. The potential disadvantage
is the requirement of more piping, which causes
the system to be costly to install and more costly to
In the alternate system, the plant or shop starts
with a single small compressor installation. Then, as
expansion takes place, instead of replacing the single
unit with a larger capacity single unit, another unit
of the same size is installed.
Initial cost is less in the smaller multiple units than
in the larger central unit. Maintenance cost is less,
and cost of operation is also less, since each unit
operates independently of the others. This is the
optimum compressor installation—one that has
the lowest installation, maintenance, and operating
costs, and also the flexibility to meet changing
requirements of a shop or plant. Hence, many
plants have started to follow the trend towards
smaller multiple compressor units.
Further advantages of multiple units are: one
standby compressor can serve a number of departments;
units are complete and ready for electric
and air piping connections; no special foundation is
required; units are usually air cooled, thus saving
on water and installation cost; and units are easily
moved from place to place. In addition, smaller
units can meet a plant’s special, occasional, or
part-time requirements, with notable savings in
cost of operation.
AIR COMPRESSOR SELECTIONAND APPLICATION 7
Compressed air system designs vary dependent
upon application and installation requirements.
As you have seen, the compressed air system’s design
begins with the proper selection of the compressor.
Selection must address compressor’s type and size
based upon the application. The compressor must be
sized to support all compressed air requirements.
Determine your flow (cfm) and pressure (psi) during
the busiest time of the business day.
Other important considerations include the physical
location of compressor, including space limitations,
with appropriate access to compressor and accessories
for proper operation, periodic inspections
and routine maintenance.
Know the electrical service controlling the compressor.
The compressor should be installed on a separate
electrical circuit, which should be protected with a
properly sized breaker and disconnect. The breaker
should be sized for twice full load amps.
Since voltage drops over distance, electrical wiring
that runs longer than 50’ will require use of larger
wire size to avoid voltage drop. Always use a
licensed electrician when configuring your electrical
If your application requires an uninterrupted air
supply, a back-up air compressor is recommended.
This will ensure compressed air is available during
scheduled maintenance and repair. With a second
compressor, sequencing controls can be supplied,
allowing the compressors to operate in a ‘lead-lag’
mode of operation providing additional compressed
air during peak demand periods.
Sequencing controls can be supplied to balance
the run time between the two compressors.
Incorporating hour meters will allow scheduling
of periodic maintenance.
Air treatment is the next concern in
designing a compressed air system.
A by-product of the air compression
process is oil and water. The clean up
of compressed air begins with cooling.
An air receiver allows the initial cooling
of compressed air as it exits the
compressor. Cooling allows oil and
water vapor to condense into a liquid
state where these contaminants can be
drained from the system. Disposal of
liquid condensate must comply with
local, state, and federal requirements.
Compressed air treatment products
are designed to remove oil and water
in a liquid or gaseous state from the
compressed air stream.
Hot, saturated compressed air from the compressor’s
discharge is routed through an aftercooler. A
cooling medium (ambient air or water) is passed
across piping, conveying compressed air towards
its intended use. Cooling of compressed air allows
gross moisture in a vapor state to condense into a
liquid. Liquid is separated from the compressed
air stream and mechanically removed from the
compressed air system.
The aftercooler’s performance is based upon its
ability to cool the compressed air stream to a lower
temperature. The aftercooler can be supplied as a
stand-alone unit or be supplied with the compressor.
All compressed air treatment components should
be installed with bypass valving. This allows an
individual component to be taken off line for
maintenance or repair without interrupting the
compressed air supply.
Secondly, once compressed air is cooled, further
drying can be accomplished through the use of a
compressed air dryer. There are many types of
dryers. Dryers can be typically grouped into two
8 AIR COMPRESSOR SELECTION AND APPLICATION
Tank Mounted Duplex Package with Control Panel
major categories: refrigerant or desiccant. The
design, performance and cost of a dryer will
depend upon the application.
With a desiccant dryer, water vapor is removed
through absorption and adsorption processes. In the
event compressed air lines are exposed to temperatures
below 32°F (or 0°C), the use of a desiccant dryer
is required to eliminate the hazard of a compressed
air line freezing.
Refrigerant type air dryers are the most economical
compressed air dryers in terms of initial purchase
price, cost of installation and operation. Within a
refrigerant air dryer, compressed air is cooled,
water vapor is condensed into liquid water where
it is mechanically separated and drained from the
compressed air system. Refrigerant air dryers are
supplied with automatic condensate drains.
NOTE: An aftercooler and/or dryer can be supplied
within a stand-alone air compressor package eliminating
the additional field expense of installation
(piping and wiring).
A properly sized dryer will prevent liquid water
within a compressed air system. All dryers are rated
for inlet conditions of 100°F, 100% relative humidity,
and 100 psi. Increasing inlet pressure and lowering
inlet temperature will improve dryer efficiency.
Once liquid condensate has been removed from the
compressed air stream through the effective use of
an after cooler and dryer, a compressed air filter is
recommended for removal of solid particulates,
aerosol mists and gaseous vapors.
Acompressed air filter is designed with a replaceable
element that allows contaminants to impinge upon
the elements surface area. As the element becomes
wetted, filtration efficiency actually improves. As
liquids, aerosols and particulates randomly collide
on small diameter fibers, the filtration process
coalesced invisible contamination into larger droplets
that gravitate to the base of the filter housing.
Lastly, liquids are drained from the filter through a
drain valve. Compressed air filters are designed for
specific applications. Aproperly sized and positioned
compressed air filter eliminates contaminants from
passing downstream. An electric drain provides a
reliable alternative to float-type, gravity-feed drains
that corrode and clog over time. Electric drains can
be viewed as a low-cost alternative to manually
draining the individual components of a compressed
air system. Operation of all drains should be checked
regularly to avoid costly loss of compressed air.
The compressed air piping system should be
designed to deliver compressed air to the pneumatic
application at the appropriate flow and pressure. The
air distribution system should incorporate a leak-free
piping system sized to minimize air pressure drop
from its supply—the compressor and compressed
air treatment components—to the point of use.
Minimizing the number of 90-degree elbows will
maximize delivered air pressure. It is estimated each
elbow equates to 25’ of additional compressed air
piping. Pipe diameter should not be less than the
discharge port of your compressor. If multiple
compressors are being utilized, pipe diameter should
equal the sum of each compressor’s discharge. Avoid
straight runs that dead-end. The most efficient design
incorporates a “LOOP” that minimizes pressure drop
at any one work station.
Different materials can be used for compressed air
headers; materials include steel, black iron, stainless
or anodized aluminum. It is critical that the material
AIR COMPRESSOR SELECTIONAND APPLICATION 9
being installed has a pressure and temperature
rating with an appropriate safety factor to support
the compressed air pressure requirement. Do not
under-size pipe. The cost difference between one
pipe diameter and the next larger size is minimal.
The larger the pipe diameter, the lower the pressure
loss will be due to friction. A larger diameter pipe
allows for additional compressed air during peak
use periods and positions the system for future
expansion. The compressed air velocity in the main
distribution header should not exceed 30 ft/sec.
A compressed air drop leg, also referred to as a
feeder line, begins with a TEE assembly that directs
the compressed air in a vertical path. This unique
flow pattern will guard against liquid or particulate
contamination passing to a pneumatic process.
Each compressed air drop should include a TEE
directing compressed air supply to its specified use.
The base of the drop leg incorporates a drain valve.
Each drop leg might include an FRL (point-of-use
filter, regulator and lubricator).
The point-of-use filter is designed to trap any particulate
matter that may have been generated in the
distribution header. The regulator is designed to provide
controlled, consistent air pressure as required
for specific pneumatic equipment or application.
The lubricator ensures that the pneumatic device
receives required lubrication to maintain operating
performance, reduce wear and prolong service life. It
is important to understand that lubricating oil carried
over from the air compressor has gone through the
compression process where it has been exposed to
heat, water vapor and particulate matter. Oxidation
has allowed this oil to become tacky and corrosive.
The entire compressed air treatment process and
the FRL eliminates the possibility of contamination
entering pneumatic equipment and processes.
Absolute Pressure – Total pressure measured from
zero. Gauge pressure plus atmospheric pressure.
For example, at sea level, the gauge pressure in
pounds per square inch (psi) plus 14.7 gives the
absolute pressure in pounds per square inch (psi).
Absolute Temperature – See Temperature, Absolute.
Absorption – The chemical process by which a
hygroscopic desiccant, having a high affinity with
water, melts and becomes a liquid by absorbing
the condensed moisture.
Actual Capacity – Quantity of air or gas actually
compressed and delivered to the discharge system
at rated speed and under rated conditions. It is
usually expressed in cubic feet per minute (acfm)
at compressor inlet conditions. Also called Free Air
Adiabatic Compression – See Compression,
Adsorption – The process by which a desiccant
with a highly porous surface attracts and removes
the moisture from compressed air. The desiccant is
capable of being regenerated.
Aftercooler – A heat exchanger used for cooling air
discharged from a compressor. Resulting condensate
may be removed by a moisture separator following
Air Receiver – See Receiver.
ASME National Board (U Type) – an air tank made,
tested, inspected, and registered to meet standards
of ASME. A certificate is supplied with each tank
to indicate compliance and show register number.
The ASME certificate is required by law in many
cities and states to pass safety codes. It assures that
(1) code-approved materials are used, (2) the steel
plate is without defects and is of specified thickness,
(3) proper welding techniques are employed by
experienced operators, (4) openings and support are
the correct size, and (5) the tank has passed rigid
tests. ASME tanks must be used where OSHA
compliance is required.
ASME Standard (UM Type) – an air tank made and
tested in accordance with the American Society of
Mechanical Engineers standards. ASME certificate
of compliance is furnished with each tank.
Atmospheric Pressure – The measured ambient
pressure for a specific location and altitude.
Automatic Sequencer – Adevice which operates
compressors in sequence according to a programmed
Booster Compressor – Machine for compressing air
or gas from an initial pressure that is above atmospheric
pressure to an even higher pressure.
Brake Horsepower (bhp) – See Horsepower, Brake.
Capacity – The amount of air flow delivered under
specific conditions, usually expressed in cubic feet
per minute (cfm).
Capacity, Actual – The actual volume flow rate of air
or gas compressed and delivered from a compressor
running at its rated operating conditions of speed,
pressures, and temperatures. Actual capacity is generally
expressed in actual cubic feet per minute (acfm)
at conditions prevailing at the compressor inlet.
Capacity Gauge –A gauge that measures air flow
as a percentage of capacity, used in rotary screw
10 AIR COMPRESSOR SELECTION AND APPLICATION
AIR COMPRESSOR SELECTIONAND APPLICATION 11
CFM, Free Air – Cubic feet per minute of air delivered
to a certain point at a certain condition, converted
back to ambient conditions.
CFM, Standard – Flow of free air measured and
converted to a standard set of conditions of pressure,
temperature and relative humidity.
Check Valve – A valve which permits flow in only
Clearance – The maximum cylinder volume on the
working side of the piston minus the displacement
volume per stroke. Normally it is expressed as a
percentage of the displacement volume.
Clearance Pocket – An auxiliary volume that may
be opened to the clearance space, to increase the
clearance, usually temporarily, to reduce the volumetric
efficiency of a reciprocating compressor.
Compressed Air – Air from atmosphere which has
been reduced in volume, raising its pressure. It then
is capable of performing work when it is released
and allowed to expand to its normal free state as it
passes through a pneumatic tool or other device.
Compression, Adiabatic – Compression in which
no heat is transferred to or from the gas during the
Compression, Isothermal – Compression is which
the temperature of the gas remains constant.
Compression, polytropic – Compression in which
the relationship between the pressure and the volume
is expressed by the equation PVn is a constant.
Compression Ratio – The ratio of the absolute
discharge pressure to the absolute inlet pressure.
Constant Speed Control – A system in which the
compressor is run continuously and matches air
supply to air demand by varying compressor load.
Cut-In/Cut-Out Pressure – Respectively, the minimum
and maximum discharge pressures at which
the compressor will switch from unload to load
operation (cut in) or from load to unload (cut out).
Cycle – The series of steps that a compressor with
unloading performs; 1) fully loaded, 2) modulating
(for compressors with modulating control),
3) unloaded, 4) idle.
Cycle Time – Amount of time for a compressor to
complete one cycle.
Degree of Intercooling – The difference in air or gas
temperature between the outlet of the intercooler
and the inlet of the compressor.
Deliquescent – Melting and becoming a liquid by
Desiccant – Amaterial having a large proportion of
surface pores, capable of attracting and removing
water vapor from the air.
Dew Point – The temperature at which moisture in
the air will begin to condense if the air is cooled at
constant pressure. At this point the relative humidity
Demand – Flow of air at specific conditions
required at a point or by the overall facility.
Diaphragm – A stationary element between the
stages of a multi-stage 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.
Discharge Pressure – Air pressure produced at a particular
point in the system under specific conditions.
Discharge Temperature – The temperature at the
discharge flange of the compressor.
Displacement – The volume swept out by the piston
or rotor(s) per unit of time, normally expressed in
cubic feet per minute.
Efficiency – Any reference to efficiency must be
accompanied by a qualifying statement which
identifies the efficiency under consideration, as in
the following definitions of efficiency:
Efficiency, Compression – Ratio of theoretical
power to power actually imparted to the air or gas
delivered by the compressor.
Efficiency, Isothermal – Ratio of the theoretical work
(as calculated on a isothermal basis) to the actual
work transferred to a gas during compression.
Efficiency, Mechanical – Ratio of power imparted
to the air or gas to brake horsepower (bhp).
Efficiency, Polytropic – Ratio of the polytropic
compression energy transferred to the gas, to the
actual energy transferred to the gas.
Efficiency, Volumetric – Ratio of actual capacity to
Exhauster – A term sometimes applied to a compressor
in which the inlet pressure is less than
Filters – Devices for separating and removing particulate
matter, moisture or entrained lubricant from air.
Flange connection – The means of connecting a
compressor inlet or discharge connection to piping
by means of bolted rims (flanges).
Free Air – Air at atmospheric conditions at any
specified location, unaffected by the compressor.
Full-Load – Air compressor operation at full speed
with a fully open inlet and discharge delivering
maximum air flow.
Gas – One of the three basic phases of matter. While
air is a gas, in pneumatics the term gas normally is
applied to gases other than air.
12 AIR COMPRESSOR SELECTION AND APPLICATION
Gauge Pressure – The pressure determined by most
instruments and gauges, usually expressed in psig.
Barometric pressure must be considered to obtain
true or absolute pressure.
Horsepower, Brake – Horsepower delivered to the
output shaft of a motor or engine, or the horsepower
required at the compressor shaft to perform work.
Horsepower, indicated – The horsepower calculated
from compressor indicator diagrams. The term
applies only to displacement type compressors.
Horsepower, Theoretical or Ideal. – The horsepower
required to isothermally compress the air or gas
delivered by the compressor at specified conditions.
Humidity, Relative – The relative humidity of a
gas (or air) vapor mixture is the ratio of the partial
pressure of the vapor to the vapor saturation
pressure at the dry bulb temperature of the mixture.
Humidity, Specific – The weight of water vapor in
an air vapor mixture per pound of dry air.
Indicated Power – Power as calculated from
Indicator card – A pressure-volume diagram for a
compressor or engine cylinder, produced by direct
measurement by a device called an indicator.
Inlet Pressure – The actual pressure at the inlet
flange of the compressor.
Intercooling – The removal of heat from air or gas
between compressor stages.
Intercooling, degree of – The difference in air or gas
temperatures between the inlet of the compressor
and the outlet of the intercooler.
Intercooling, perfect – When the temperature of
the air or gas leaving the intercooler is equal to the
temperature of the air or gas entering the inlet of
Isentropic compression – See Compression,
Isothermal compression – See Compression,
Leak – An unintended loss of compressed air to
Load Factor – Ratio of average compressor load to
the maximum rated compressor load over a given
period of time.
Load Time – Time period from when a compressor
loads until it unloads.
Load/Unload Control – Control method that allows
the compressor to run at full-load or at no load
while the driver remains at a constant speed.
Modulating Control – System which adapts to
varying demand by throttling the compressor inlet
proportionally to the demand.
Multi-stage compressors – Compressors having
two or more stages operating in series.
Perfect Intercooling –The condition when the
temperature of air leaving the intercooler equals
the temperature of air at the compressor intake.
Performance curve – Usually a plot of discharge
pressure versus inlet capacity and shaft horsepower
versus inlet capacity.
Piston Displacement – The volume swept by the
piston; for multistage compressors, the piston
displacement of the first stage is the overall piston
displacement of the entire unit.
Pneumatic Tools – Tools that operate by air pressure.
Polytropic compression – See Compression,
Positive displacement compressors – Compressors
in which successive volumes of air or gas are
confined within a closed space and the space
mechanically reduced, resulting in compression.
These may be reciprocating or rotating.
Power, theoretical (polytropic) – The mechanical
power required to compress polytropically and to
deliver, through the specified range of pressures,
the gas delivered by the compressor.
Pressure – Force per unit area, measured in pounds
per square inch (psi).
Pressure, absolute – The total pressure measured
from absolute zero (i.e. from an absolute vacuum).
Pressure Dew Point – For a given pressure, the
temperature at which water will begin to condense
out of air.
Pressure, discharge – The pressure at the discharge
connection of a compressor. (In the case of compressor
packages, this should be at the discharge
connection of the package)
Pressure Drop – Loss of pressure in a compressed air
system or component due to friction or restriction.
Pressure, intake – The absolute total pressure at the
inlet connection of a compressor.
Pressure Range – Difference between minimum and
maximum pressures for an air compressor. Also
called cut in-cut out or load-no load pressure range.
Pressure ratio – See Compression Ratio.
Pressure rise – The difference between discharge
pressure and intake pressure.
Pressure, static – The pressure measured in a flowing
stream in such a manner that the velocity of the
stream has no effect on the measurement.
AIR COMPRESSOR SELECTIONAND APPLICATION 13
Pressure, total – The pressure that would be produced
by stopping a moving stream of liquid or gas.
It is the pressure measured by an impact tube.
Pressure, velocity – The total pressure minus the
static pressure in an air or gas stream.
Rated Capacity – Volume rate of air flow at rated
pressure at a specific point.
Rated Pressure – The operating pressure at which
compressor performance is measured.
Required Capacity – Cubic feet per minute (cfm) of
air required at the inlet to the distribution system.
Receiver – A vessel or tank used for storage of gas
under pressure. In a large compressed air system
there may be primary and secondary receivers.
Reciprocating compressor – Compressor in which
the compressing element is a piston having a
reciprocating motion in a cylinder.
Relative Humidity – The ratio of the partial pressure
of a vapor to the vapor saturation pressure at the dry
bulb temperature of a mixture.
Rotor – The rotating element of a compressor.
In a dynamic compressor, it is composed of the
impeller(s) and shaft, and may include shaft
sleeves and a thrust balancing device.
Seals – Devices used to separate and minimize
leakage between areas of unequal pressure.
Sequence – The order in which compressors are
Shaft – The part by which energy is transmitted
from the prime mover through the elements mounted
on it, to the air or gas being compressed.
Sole plate – A pad, usually metallic and embedded
in concrete, on which the compressor and driver are
Specific gravity – The ratio of the specific weight
of air or gas to that of dry air at the same pressure
Specific Humidity – The weight of water vapor in
an air-vapor mixture per pound of dry air.
Specific Power – A measure of air compressor
efficiency, usually in the form of bhp/100 acfm.
Specific Weight –Weight of air or gas per unit volume.
Speed – The speed of a compressor refers to the
number of revolutions per minute of the compressor
drive shaft or rotor shaft.
Stages – A series of steps in the compression of air
or a gas.
Standard Air – The Compressed Air & Gas Institute
and PNEUROP have adopted the definition used in
ISO standards. This is air at 14.5 psia (1 bar); 68 °F
(20° C) and dry (0% relative humidity).
Start/Stop Control – A system in which air supply is
matched to demand by the starting and stopping of
Temperature, Absolute – The temperature of
air or gas measured from absolute zero. It is the
Fahrenheit temperature plus 459.6 and is known as
the Rankine temperature. In the metric system, the
absolute temperature is the Centigrade temperature
plus 273 and is known as the Kelvin temperature.
Temperature, Discharge – The total temperature at
the discharge connection of the compressor.
Temperature, Inlet – The total temperature at the
inlet connection of the compressor.
Temperature Rise Ratio – The ratio of the computed
isentropic temperature rise to the measured total
temperature rise during compression. For a perfect
gas, this is equal to the ratio of the isentropic
enthalpy rise to the actual enthalpy rise.
Temperature, Static – The actual temperature of a
moving gas stream. It is the temperature indicated
by a thermometer moving in the stream and at the
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.
Theoretical Power – The power required to compress
a gas isothermally through a specified range
Torque – A torsional moment or couple. This term
typically refers to the driving couple of a machine
Total Package Input Power – The total electrical
power input to a compressor, including drive motor,
cooling fan, motors, controls, etc.
Unit type compressors – Compressors of 30 bhp
or less, generally combined with all components
required for operation.
Unload – (No load) Compressor operation in which
no air is delivered due to the intake being closed or
modified not to allow inlet air to be trapped.
Vacuum pumps – Compressors which operate
with an intake pressure below atmospheric pressure
and which discharge to atmospheric pressure or
Valves – Devices with passages for directing flow
into alternate paths or to prevent flow.
Water cooled compressor – Compressors cooled
by water circulated through jackets surrounding
cylinders or casings and/or heat exchangers
between and after stages.
14 AIR COMPRESSOR SELECTION AND APPLICATION
Miscellaneous Consumption Consumption Consumption
Portable (cfm) (cfm) (cfm)
Tools 15% Use 25% Use 35% Use
FACTOR FACTOR FACTOR
Drill, 1/16” to 3/8” 4.0 6.0 9.0
Drill, 3/8” to 5/8” 5.0 9.0 12.0
Screwdriver # 2 to
# 6 Screw 2.0 3.0 4.0
Screwdriver # 5 to
5/16” Screw 4.0 6.0 8.0
Trapper, to 3/8” 4.0 6.0 8.0
Nutsetters, to 3/8” 4.0 6.0 8.0
Nutsetters, to 9/16” 8.0 13.0 18.0
Nutsetters, to 3/4” 9.0 15.0 21.0
Impact Wrench, 1/4” 2.0 4.0 5.0
Impact Wrench, 3/8” 3.0 5.0 7.0
Impact Wrench, 1/2” 5.0 8.0 11.0
Impact Wrench, 5/8” 5.0 8.0 11.0
Impact Wrench, 3/4” 5.0 9.0 12.0
Impact Wrench, 1” 7.0 11.0 16.0
Impact Wrench, 1 1/4” 8.0 14.0 19.0
Die Grinder, Small 2.0 4.0 5.0
Die Grinder, Medium 4.0 6.0 8.0
Horizontal Grinder, 2” 5.0 8.0 11.0
Horizontal Grinder, 4” 9.0 15.0 21.0
Horizontal Grinder, 6” 11.0 18.0 25.0
Horizontal Grinder, 8” 12.0 20.0 28.0
Vertical Grinders and
Sanders, 5” Pad 5.0 9.0 12.0
Vertical Grinders and
Sanders, 7” Pad 11.0 18.0 25.0
Vertical Grinders and
Sanders, 9” Pad 12.0 20.0 28.0
HOW TO SELECT AN AIR COMPRESSOR
After listing all the air operated devices to be supplied by the air
compressor, determine, from chart, the pressure range and volume
of air required by each device. The air compressor must maintain
a minimum pressure at least equal to the highest of these pressure
ranges. For example, if the highest pressure range required by
any one device in a given group is 120 psi-150 psi, a compressor
cutting in at not less than 120 psi and cutting out at 150 psi should
Check electrical characteristics before ordering compressor.
AIR CONSUMPTION CHART FOR INDUSTRIAL TYPE TOOLS,
Cubic Feet Per Minute Required to Operate Various Pneumatic Equipment at Pressure Range 70-90 psig
Miscellaneous Consumption Consumption Consumption
Portable (cfm) (cfm) (cfm)
Tools 15% Use 25% Use 35% Use
FACTOR FACTOR FACTOR
Burring Tool, Small 2.0 4.0 5.0
Burring Tool, Large 4.0 6.0 8.0
Rammers, Small 4.0 3.0 9.0
Rammers, Medium 5.0 9.0 12.0
Rammers, Large 6.0 10.0 14.0
Backfill Tamper 4.0 6.0 9.0
Compression Riveter .2 cu. ft. per cycle
Air Motor, 1 Horsepower 5.0 9.0 12.0
Air Motor, 2 Horsepower 11.0 18.0 25.0
Air Motor, 3 Horsepower 14.0 24.0 33.0
Air Motor Hoist, 1000 # 1. cu. ft. per foot of lift
Air Motor Hoist, 2000 # 1. cu. ft. per foot of lift
Paint Spray Gun (Production) 3.0 5.0 7.0
Scaling Hammer 2.0 3.0 4.0
Chipping Hammer 5.0 8.0 11.0
Riveting Hammer (Heavy) 5.0 8.0 11.0
Riveting Hammer (Light) 2.0 4.0 5.0
Circular, 8” 7.0 11.0 16.0
Circular, 12” 10.0 16.0 24.0
Chain, Lightweight 4.0 7.0 10.0
Chain, Heavy Duty 13.0 22.0 31.0
Always check with tool manufacturers for actual air consumption of
tools being used. The above is based on averages and should not be
considered accurate for any particular make of tool.
Above tools are rated based upon typical “on-load” performance
For other use factors adjust the cfm air consumption on a proportional
basis. (Example: 30 seconds on; 30 seconds off use 50% as use factor).
AIR COMPRESSOR SELECTIONAND APPLICATION 15
Air Pressure Portable Tools cfm
in psi Per Unit
70-100 **Air Filter Cleaner 3.0
70-100 **Body Polisher 20.0
70-100 **Body Sander (Orbital) 10.0
70-100 Brake Tester 4.0
70-100 **Carbon Remover 3.0
90-100 Dusting Gun (Blow Gun) 2.5
70-100 Panel Cutter 4.0
70-90 **Drill, 1/16” to 3/8” 4.0
70-90 **Impact Wrench 3/8” sq. dr. 3.0
70-90 **Impact Wrench 1/2” sq. dr. 4.0
70-90 **Impact Wrench 5/8” sq. dr. 5.0
70-90 **Impact Wrench 3/4” sq. dr. 8.0
70-90 **Impact Wrench 1” sq. dr. 12.0
70-90 **Die Grinder 5.0
90-100 **Vertical Disc Sanders 20.0
90-100 **Filing and Sawing Machine, (Small) 3.0
90-100 **Filing and Sawing Machine, (Large) 5.0
90-100 **Burring Tool 5.0
125-150 Rim Stripper 6.0
125-150 Tire Changer 1.0
125-150 Tire Inflation Line 2.0
125-150 Tire Spreader 1.0
125-150 **Vacuum Cleaner 7.0
AIR CONSUMPTION CHART FOR AUTOMOTIVE SERVICE SHOPS.
Cubic Feet Per Minute Required to Operate Various Pneumatic Equipment, for average service shop usage factor.
Air Pressure Portable Tools cfm
in psi Per Unit
90-100 **Air Hammer 4.0
90-100 Tire Hammer 12.0
125-150 Bead Breaker 12.0
90-100 Spring Oiler 4.0
90-100 **Engine Cleaner 5.0
90-100 **Paint Spray Gun (production) 8.0
90-100 **Paint Spray Gun (touch up) 4.0
90-100 **Paint Spray Gun (undercoat) 19.0
120-150 **Grease Gun 3.0
145-175 Car Lift* (air powered hydraulic) 6.0
125-150 Floor Jacks (air powered hydraulic) 6.0
120-150 Pneumatic Garage Door 3.0
90-100 Radiator Tester 1.0
90-100 Spark Plug Cleaner 5.0
90-100 Spark Plug Tester .5
70-100 Transmission and Differential Flusher 3.0
70-100 **Fender Hammer 9.0
70-100 **Car Washer 9.0
70-100 **6” Medium Duty Sander 40.0
* This is for 8,000 lbs. capacity. Add .65 cfm for each 1,000 lbs. capacity
over 8,000 lbs.
**These devices are rated based upon typical “on-load” performance
Always check with tool manufacturers for actual consumption of tools
being used. The above is based on averages and should not be considered
accurate for any particular make of tool.
16 AIR COMPRESSOR SELECTION AND APPLICATION
Compressor Pressures Air Consumption in Cubic Feet Per Minute Horsepower of
per square inch of Total Equipment Compressor Required
Cut In Cut Out Average Use* Continuous Operation** Two-Stage One-Stage
80 100 Up to - 6.6 Up to - 1.9 1/2
80 100 6.7 - 10.5 2.0 - 3.0 3/4
80 100 10.6 - 13.6 3.1 - 3.9 1
80 100 Up to - 14.7 Up to - 4.2 1
80 100 13.7 - 20.3 4.0 - 5.8 1 1/2
80 100 14.8 - 22.4 4.3 - 6.4 1 1/2
80 100 20.4 - 26.6 5.9 - 7.6 2
80 100 22.5 -30.4 6.5 - 8.7 2
80 100 26.7 - 32.5 7.7 - 10.2 3
80 100 30.5 - 46.2 8.8 - 13.2 3
80 100 32.6 - 38.0 10.3 - 18.0 5
80 100 46.3 - 60.0 13.3 - 20.0 5
80 100 60.1 - 73.0 20.1 - 29.2 7 1/2
80 100 73.1 - 100.0 29.3 - 40.0 10
80 100 100.1 - 150.0 40.1 - 60.0 15
80 100 150.1 - 200.0 60.1 - 80.0 20
80 100 201.0 - 250.0 80.1 - 100.0 25
120 150 Up to - 3.8 Up to - 1.1 1/2
120 150 3.9 - 7.3 1.2 - 2.1 3/4
120 150 7.4 - 10.1 2.2 - 2.9 1
120 150 Up to - 12.6 Up to - 3.6 1
120 150 10.2 - 15.0 3.0 - 4.3 1 1/2
120 150 12.7 - 20.0 3.7 - 5.7 1 1/2
120 150 15.1 - 20.0 4.4 - 5.7 2
120 150 20.1 - 25.9 5.8 - 7.4 2
120 150 26.0 - 39.2 7.5 - 11.2 3
120 150 39.3 - 51.9 11.3 - 17.3 5
120 150 52.0 - 67.5 17.4 - 27.0 7 1/2
120 150 67.6 - 92.5 27.1 - 37.0 10
120 150 92.5 - 140.0 37.1 - 57.0 15
120 150 140.1 - 190.0 57.1 - 77.0 20
120 150 190.1 - 240.0 77.1 - 97.0 25
145 175 Up to - 11.9 Up to - 3.4 1***
145 175 12.0 - 18.5 3.5 - 5.3 1 1/2
145 175 18.6 - 24.2 5.4 - 6.9 2
145 175 24.3 - 36.4 7.0 - 10.4 3
145 175 36.5 - 51.0 10.5 -17.0 5*
145 175 51.1 - 66.0 17.1 - 26.4 7 1/2
145 175 66.1 - 88.2 26.5 - 35.3 10
145 175 88.3 - 135.0 35.3 - 55.0 15
145 175 135.1 - 185.0 55.1 - 75.0 20
145 175 185.1 - 235.0 75.1 - 95.0 25
* These figures are not to be
regarded as the capacity of the
compressor in free air output, but
instead, are the combined free air
consumption of all the tools in
the establishment, as well as
tools anticipated for future added
equipment. (A factor has been
introduced to take into account
intermittent operation of tools
likely to be in use simultaneously
in the average garage or industrial
plant. (See Example 1 on page
number 17 for the use of the f
igures given in this column.)
**These figures are to be employed
when the nature of the device
is such that normal operation
requires a continuous supply of
compressed air. Therefore, no
factor for intermittent operation
has been used, and the figures
given represent the compressor
capacity in free air output. (See
Example 2 on page number 17
for the use of the figures given
in this column.)
**Do not recommend a compressor
of less than 1 1/2 H.P. if the
pneumatic equipment includes a
lift of 8,000 lbs. capacity.
COMPRESSOR SELECTOR CHART
AIR COMPRESSOR SELECTIONAND APPLICATION 17
It is required to supply a compressor to operate the equipment
listed below such as might be found in an average service station.
Add the cfm required by all the devices.
2 – Car Lifts @ 6.0 cfm - 12.0 cfm 145 to 175 psi
2 – Grease Guns @ 3.0 cfm - 6.0 cfm 120 to 150 psi
1 – Spring Oiler @ 4.0 cfm - 4.0 cfm 90 to 100 psi
1 – Spark Plug Cleaner @ 5.0 cfm - 5.0 cfm 90 to 100 psi
2 – Tire Inflators @ 2.0 cfm - 4.0 cfm 125 to 150 psi
1 – Dusting Gun @ 2.5 cfm - 2.5 cfm 90 to 100 psi
1 – Trans. and Diff. Flusher @ 3.0 cfm - 3.0 cfm 70 to 100 psi
Total 25.5 cfm - 36.5 cfm
On this page, under the column “Average Use”, and opposite
the pressure range required (145 psi to 175 psi), find the line
indicating 36.5 cfm or more. The compressor required will be a
3 HP, 2-stage unit.
A compressor is needed to operate the following equipment, all
of which is to be in operation continuously, or nearly so. Total of
cfm required for all the devices and the pressure ranges.
1 – Fender Hammer @ 9.0 cfm 70 to 100 psi
1 – Paint Spray Gun (Production Type) @ 8.0 cfm 90 to 100 psi
1 – Body Polisher @ 20 cfm 70 to 100 psi
1 – Touch-Up Spray Gun @ 3.5 cfm 90 to 100 psi
1 – Vacuum Cleaner @ 7.0 cfm 125 to 150 psi
Total 47.5 cfm
On this page, under the column “Continuous Operation”, and
opposite the pressure range required (120 psi - 150 psi), find the
line indicating 47.5 cfm or more. The compressor needed will be
a 15 HP, 2-stage unit.
In the case of an industrial plant where some of the pneumatic
equipment will be operated under “Average Use” and part will be
in operation continuously, total the cfm required, as well as the
pressure ranges, of each group, as follows:
Below, under column “Average Use”, select a unit having delivery
of 12.5 cfm at 145-175 psi as that pressure range required to operate
the equipment shown. It will be a 2 HP, 2-stage unit.
Below, under column “Continuous Operation”, select a unit having
a delivery of 10.75 cfm at 80-100 psi as that pressure range required
to operate the equipment shown. This unit will be a 3 HP, 2-stage
To supply one compressor rather than two, for the above equipment,
total the HP, which in this case would be 5 HP operating at
a pressure range of 145 to 175 psi.
1 - Car Lift @ 6.0 cfm 145 to 175 psi
5 - Dusting Guns @ 2.5 cfm 90 to 100 psi
1 - Panel Cutter @ 4.0 cfm 70 to 100 psi
Total 12.5 cfm
1 - Paint Spray Gun @ 7.00 cfm 70 to 90 psi
1 - Impact Wrench @ 3.75 cfm 70 to 90 psi
Total 10.75 cfm
Note: Pressure regulators must be used to regulate to the allowable
maximum pressure of the devices.
SELECTING THE PROPER AIR COMPRESSOR
TO USE WITH AN AIR CYLINDER
Air cylinders use compressed air to produce force or motion. The
compressed air is directed into a cylinder chamber and it forces a
piston to move in a linear direction. The distance the piston travels
is called the length of stroke. Apiston rod attached to the piston
exerts a force in pounds to produce work or motion to a mechanism
at a rate of so many strokes per minute.
In commercial and industrial uses, a piece of equipment using an air
cylinder of a given diameter will be rated as to force (thrust load) in
pounds, length of stroke and the number of strokes per minute, and
you should obtain this information from your supplier.
Using the thrust load and cylinder diameter figures, make your
choice of a single or two stage air compressor and determine the
pressure needed from chart “A”.
Determine the cfm of free air needed by the air cylinder from chart
“B” by using the factor shown opposite your cylinder diameter
and pressure requirement (see example for explanation of how to
determine factors not shown). Multiply this factor by the number of
inches of stroke and the number of strokes per minute to determine
the cfm requirement.
From selector charts, determine your air compressor selection.
18 AIR COMPRESSOR SELECTION AND APPLICATION
CHART A – CYLINDER DIAMETER REQUIRED TO DEVELOP POWER TO OVERCOME THE LOAD INDICATED:
Thrust Load Pressure in Cylinder – psi
in Pounds 70 80 90 100 110 120 125 130 140 150 160 170 175 180 190 200
500 3 1/8 2 7/8 2 3/4 2 1/2 2 1/2 2 3/8 2 3/8 2 1/4 2 1/4 2 1/8 2 2 2 2 1 7/8 1 7/8
1000 4 3/8 4 3 7/8 3 5/8 3 1/2 3 3/8 3 1/4 3 1/4 3 1/8 3 2 7/8 2 3/4 2 3/4 2 3/4 2 5/8 2 5/8
1500 5 1/4 5 4 5/8 4 3/8 4 1/4 4 4 3 7/8 3 3/4 3 5/8 3 1/2 3 3/8 3 3/8 3 3/8 3 1/4 3 1/8
2000 6 1/8 5 3/4 5 3/8 5 1/8 4 7/8 4 5/8 4 5/8 4 1/2 4 3/8 4 1/8 4 3 7/8 3 7/8 3 7/8 3 3/4 3 5/8
2500 6 7/8 6 3/8 6 5 3/4 5 1/2 5 1/8 5 1/8 5 4 7/8 4 5/8 4 1/2 4 3/8 4 3/8 4 1/4 4 1/8 4
3000 7 1/2 7 6 5/8 6 1/4 6 5 3/4 5 5/8 5 1/2 5 1/4 5 1/8 5 4 3/4 4 3/4 4 5/8 4 1/2 4 3/8
CHART B – CUBIC FEET OF AIR REQUIRED FOR SINGLE ACTING AIR CYLINDER*:
*To obtain CFM required; multiply factor above by 2 if cylinder is double acting: then multiply by number of inches of stroke;
then multiply by number of strokes per minute.
Piston Dia. (in.) 90 psi 125 psi
1 3/4 .0102 .0131
1 7/8 .0115 .0149
2 .0133 .0172
2 1/8 .0150 .0194
2 1/4 .0168 .0217
2 3/8 .0187 .0242
2 1/2 .0207 .0268
2 5/8 .0228 .0296
2 3/4 .0250 .0324
2 7/8 .0275 .0355
3 .0299 .0386
3 1/8 .0323 .0418
Piston Dia. (in.) 90 psi 125 psi
3 1/4 .0350 .0454
3 3/8 .0378 .0489
3 1/2 .0405 .0524
3 5/8 .0434 .0562
3 3/4 .0467 .0605
3 7/8 .0496 .0642
4 .0530 .0685
4 1/8 .0564 .0730
4 1/4 .0599 .0775
4 3/8 .0635 .0822
4 1/2 .0672 .0870
4 5/8 .0708 .0915
Piston Dia. (in.) 90 psi 125 psi
4 3/4 .0748 .0970
4 7/8 .0789 .1020
5 .0832 .1076
5 1/8 .0872 .1127
5 1/4 .0913 .1180
5 3/8 .0957 .1237
5 1/2 .1004 .1299
5 5/8 .1050 .1361
5 3/4 .1096 .1416
5 7/8 .1146 .1482
6 .1200 .1550
6 1/8 .1250 .1623
Piston Dia. (in.) 90 psi 125 psi
6 1/4 .1300 .1681
6 3/8 .1346 .1742
6 1/2 .1402 .1813
6 5/8 .1460 .1888
6 3/4 .1510 .1955
6 7/8 .1570 .2060
7 .1630 .2105
7 1/8 .1684 .2181
7 1/4 .1746 .2257
7 3/8 .1802 .2332
7 1/2 .1870 .2419
A 2 1/4 dia. cylinder, double acting, with an 8” stroke is required to clamp a casting during machining. A pressure of 100 psi will be
required and it is expected that 16 castings will be clamped every minute. To determine cfm required, multiply factor opposite 2 1/4 dia.
cylinder in 90 psi column by 2 for double acting (2 x .0168), then multiply this by 8 for 8” stroke (2 x .0168 x 8), then multiply this by
strokes per minute (2 x .0168 x 8 x 16). The result is 4.3 cfm of free air required at 90 psi. This same calculation is repeated using the
factor in the 125 psi column.
The result is (2 x .0217 x 8 x 16) 5.56 cfm required at 125 psi. Since the cfm at 100 psi is required and it is known that 100 psi is about 1/3
the way from 90 psi to 125 psi, it can be estimated that the cfm required at 100 psi will be about 1/3 the difference of that required at 90
and 125 psi.
(5.56 – 4.3)
= .42. The approximate cfm required at 100 psi will then be 4.3 plus this difference (4.3 + .42) or 4.72 cfm.
AIR COMPRESSOR SELECTIONAND APPLICATION 19
AIR FLOW CHART
Another industrial use for compressed air is using a blast of compressed
air, released at the proper moment, to blow away small
parts from a punch after forming and blanking.
An automatic valve allows air to flow from a properly positioned
and aimed nozzle against the work pieces. The pressure employed
and the diameter of the passage through the nozzle determine the
volume of free air which will flow through the nozzle.
The chart below indicates the rate of flow (volume) per minute,
through various sizes of orifices at definite pressures.
Flow is expressed in cubic feet per minute (cfm), and is assumed to
take place from a receiver or other vessel, in which air is contained
under pressure, into the atmosphere at sea level. Temperature of
air in receiver is assumed at 60 deg. F. This table is only correct for
orifices with narrow edges; flow through even a short length of
pipe would be less than that given below.
Gage Pres. in Flow of Free Air (cfm) Through Orifices of Various Diameters
Receiver (lbs.) 1/64” 1/32” 3/64” 1/16” 3/32” 1/8” 3/16” 1/4”
1 .027 .107 .242 .430 .97 1.72 3.86 6.85
2 .038 .153 .342 .607 1.36 2.43 5.42 9.74
3 .046 .188 .471 .750 1.68 2.98 6.71 11.9
5 .059 .242 .545 .965 2.18 3.86 8.71 15.4
10 .084 .342 .77 1.36 3.08 5.45 12.3 21.8
15 .103 .418 .94 1.67 3.75 6.65 15.0 26.7
20 .119 .485 1.07 1.93 4.25 7.7 17.1 30.8
25 .133 .54 1.21 2.16 4.75 8.6 19.4 34.5
30 .156 .632 1.40 2.52 5.6 10. 22.5 40.0
35 .173 .71 1.56 2.80 6.2 11.2 25.0 44.7
40 .19 .77 1.71 3.07 6.8 12.3 27.5 49.1
45 .208 .843 1.9 3.36 7.6 13.4 30.3 53.8
50 .225 .914 2.05 3.64 8.2 14.5 32.8 58.2
60 .26 1.05 2.35 4.2 9.4 16.8 37.5 67
70 .295 1.19 2.68 4.76 10.7 19.0 43.0 76
80 .33 1.33 2.97 5.32 11.9 21.2 47.5 85
90 .364 1.47 3.28 5.87 13.1 23.5 52.5 94
100 .40 1.61 3.66 6.45 14.5 25.8 58.3 103
110 .43 1.76 3.95 7.00 15.7 28.0 63 112
120 .47 1.90 4.27 7.58 17.0 30.2 68 121
130 .50 2.04 4.57 8.13 18.2 32.4 73 130
140 .54 2.17 4.87 8.68 19.5 34.5 78 138
150 .57 2.33 5.20 9.20 20.7 36.7 83 147
175 .66 2.65 5.94 10.6 23.8 42.1 95 169
200 .76 3.07 6.90 12.2 27.5 48.7 110 195
The capacity of an air compressor cannot be checked accurately by use of this table and a narrow edge orifice. Specialized equipment is
necessary to check compressor capacity.
Example: An air ejector is being used on a punch press. It is connected to an air line with pressure at 120-150 psi. It has a nozzle orifice 3/32
in. in diameter, and, through use of a stop watch, it delivers compressed air for a total of 30 seconds out of each one minute of operation.
Reference to the chart indicates at 150 psi a 3/32 in. diameter orifice will allow 20.7 cfm to flow through the nozzle in one minute.
However, air flow intakes place only for 30 seconds out of each 60 seconds or 30/60 of the time, therefore, only 1/2 of 20.7 or 10.35 cfm
will flow for each elapsed minute.
From page 17, under the column “Continuous Operation” and opposite the pressure range 120-150 psi, select the air compressor, which
will be a 3 HP, 2-stage unit.
20 AIR COMPRESSOR SELECTION AND APPLICATION
1. Comp. R.P.M =
motor pulley dia. x motor r.p.m.
comp. pulley dia.
2. Motor Pulley p.d. =
comp. pulley dia. x comp. r.p.m.
3. Comp. Pulley p.d. =
motor pulley dia. x motor r.p.m.
4. Motor R.P.M. =
comp. pulley dia. x comp. r.p.m.
motor pulley p.d.
5. Free Air = piston displacement x volumetric eff. (%)
6. Required Piston Displacement =
7. Piston Displacement In Cu. Ft. Min.* =
Cyc. bore in In. x Cyl. bore x stroke in In. x r.p.m.
8. Cu. Ft. Compressed Air =
cu. ft. free air x 14.7
(p.s.i.g. + 14.7)
9. Cu. Ft. Free Air =
cu. ft. compressed air x (p.s.i.g. + 14.7)
10. Cu. Ft. Free Air Req’d. To Raise Rec. From 0 Gage
To Final Pressure =
vol. of rec. in cu. ft. x p.s.i.g.
(atmospheric pressure) p.s.i.a.
11. Cu. Ft. Free Air Req’d. To Raise Rec. From Some Press.
Greater Than 0 Gage To A Final Higher Pressure =
vol. of rec x
(final p.s.i.g. – initial p.s.i.g.)
in cu. ft. (atmospheric pressure) p.s.i.a.
12. Piston Speed In Ft. Per Min. =
2 x stroke (in inches) x r.p.m.
13. Gallons =
14. Cu. Ft. = gallons x .134
15. Total Force in lbs. of Air Cylinder =
Area of the Cylinder
P.S.I.G. of air
Dia. in sq. inches press. used
16. C.F.M. of Free Air req’d to operate = Vol. of Cyl. x Cycles x
(Gage Press p.s.i.g. + 14.7
Air Cylinder (Single Acting) in cu. ft. Per Min. 14.7
17. Pump Up Time (Min) =
V (tank size in gal.) x (final tank press. – initial tank press.)
7.48 x atmos. press. (p.s.i.a.) x pump delivery (c.f.m.)
*Piston displacement for multi-stage compressors – only the low pressure cylinders are considered.
AIR COMPRESSOR SELECTIONAND APPLICATION 21
Compressed Air and Gas Institute
Cleveland, Ohio 44115
URL: www.cagi.org 7/2002
G11 Air Compressor 64cfm / 30 litres / sec FAD
Product Code: G11TM Floor Mounted Pricing: $9,323 +GST
Product Code: G11TMFF Tank Mounted with inbuilt dryer Pricing: $11,707 +GST
Description: Atlas Copco 11kw rotary screw air compressor, tank mounted - from 44.9 cfm - FAD. AS1210 Approval on vessel. Delivery nationwide arranged (not included in this price, from $185 + GST depending on location). Nationally backed and serviced. Extended warranty available - Yes.
Original Manufacturer: Atlas Copco. Current Stock available: YES on FF
Atlas Copco’s oil-injected rotary screw GXe compressors are the powerful and reliable industrial screw compressors for small and medium sized industries. The GXe compressors are available in various versions (floor mounted, tank mounted, tank mounted with integrated dryer) to provide flexibility. Built from high-quality components and materials, they provide a reliable source of high-quality air in temperatures up to 46°C/115°F.
Features & benefits
Technical data / Technical Specifications
Capacity FAD (l/s) 12.9 - 60.2 l/s
Installed motor power 7.5 - 22 kW
Working pressure 7.5 - 13 bar(e)
Reliability - The GXe / G / GA series is designed, manufactured and tested in accordance with ISO 9001, ISO 14001 and ISO 1217. The screw
compressor technology allows 100% continuous duty cycle and the reinforced frame eliminates resonance. GXe compressors are built for a long lifetime of reliable operation.
Reduced energy costs - Our GXe compressors offer the low energy consumption and high efficiency of a rotary screw compressor. Compared to piston compressors that suffer from increased energy consumption over time, these screw compressors always provide high efficiency.
Plug and play installation - In addition to boasting a minimum footprint, the GXe series discharges cooling air from the top, allowing placement against the wall or in a corner. The tank mounted GXe with built-in dryer reduces space requirements even further, making it ideal if you have limited space at your facility
Easy maintenance - Service points are grouped together and are accessible through the removable panel. The spin-on oil separator and filter are designed to be easy to maintain.
Silent operation - Atlas Copco supplies GXe compressors with full sound enclosures which reduce the sound levels. The rotary screw technology minimizes vibration, while optimized cooling air flow enhances quiet operation.
Integrated air treatment - The tank-mounted GXe 2-11 FF compressors are available with an advanced built-in refrigerant air dryer.
By cooling the compressed air and removing water before it can enter your compressed air network, it prevents rust in your compressed air piping and avoids damage to your air tools.
Get a reliable compressed air source for your factory with our easy to understand infomation on rotary screw air compressors.
What about the pros and cons of the rotary screw compressor, what to look for when buying one and common breakdowns that might occur during operation? The rotary screw compressor uses two rotors (helical screws) to compress the air. There’s a ‘female’ rotor and a ‘male’ rotor. The rotors are of different shape, but fit each other exactly. When the rotors start turning, air will get sucked in on one side and gets ‘trapped’ between the rotors. Since the rotors are continuously turning, the air gets pushed to the other end of the rotors (the ‘pressure side’) and new fresh air gets sucked in.
Because this is a continuous process, this kind of compressor doesn’t make a lot of noise; it runs quiet and smooth.
Here’s more information about screw compressor elements.
Compressor element (oil-free type). NB: G, GXe are NOT Oil free and are Oil Injected Screws (OIS)
Compared to piston-type reciprocating compressors, the screw compressor is much more expensive. But it will run 24/7 and 365 days a year without any problems. The capacity (liters of air per second) of screw compressors are generally much bigger compared to piston-type compressors.
When you need a lot of air in your workshop or factory, this type of compressor is usually the best choice!
Oil-free or lubricated
The rotary screw compressor is available in an oil-injected and oil-free versions. The basic principle is the same (the rotors ‘push’ the air to one side), but they are quite different machine. The big difference is the design of the compressor elements, the part where the actual compression takes place. The oil-injected version needs oil to operate properly; the oil-free version doesn’t need oil.
Because of this, the rotors used in oil-free screw compressors are of superior quality with very little space in between them. They do not touch each other though; otherwise they would wear-down too quickly. For this reason, they are a lot more expensive.
Oil-injected models are by far the most common of screw-type compressors. Oil-free models are used for specific special applications. I’ve mostly seen them on big factories like oil/gas or chemical refineries, big food factories or other places where the compressed air must be 100% oil-free (otherwise it could contaminate the food, product or chemical process).
Oil-injected rotary screw compressors.
How do they work? As its name implies: there’s oil injected in the compressor element (where the actual compression takes place), during the compression of the air. This oil is later removed by the oil separator, so we end up with clean compressed air. Of course there’s a lot more to it, learn how an oil-injected rotary screw compressor works. Although 99,9% of the oil stays inside the compressor, there is always a little oil that passes through the separator and leaves the compressor with the compressed air. This is called “oil carryover”.
Oil-injected screw compressors therefore don’t produce oil-free air and they can’t be used in places where oil-free air is needed.
But for most factories, workshops and machinery, the small oil-carryover of the compressor is not a problem. In fact, it helps to prevent again rust (inside the compressed air system) and helps the machines run smoothly.
High volume of air, steady flow.
Lower energy cost (compared to piston-type compressors of the same size)
Suitable for continuous operation (24/7)
Expensive compared to piston compressors.
Not suitable for long stand-stills.
Oil-free rotary screw compressors
The basic workings are the same as the oil-injected screw compressor, only this time, there’s no oil, only air! Because there’s no oil injected during compression, the compression is usually done in two stages. Because if we would compress the air in one go from 1 to 7 bars, it would get really, really hot.
Stage one compresses the air to a few bars (say 2,5 bars). The air will be very hot at this time, so it flows through an inter-cooler first before entering the second stage. Stage two will compress the air further from 2,5 bar to the end-level, mostly 7 bar.
Normally the two stages will be built on 1 gearbox, with one electro motor driving them at the same time.
Here’s more info on how an oil-free rotary screw compressor works.
If you need 100% oil-free air and lots of it, the oil-free rotary screw compressor is the way to go. Of course, it comes with a bigger price-tag, but if you really need 100% oil free air, there’s not really a choice.
100% oil-free air
More expensive than oil-injected type
Servicing/repairing more difficult, and more expensive than oil-injected type
More noise than oil-injected compressors
Rotary screw air compressors are the most widely used air compressor models in the industrial marketplace. Rotary screw compressors provide continuous compressed air for precision tasks and they are extremely efficient and remarkably quiet. Screw compressors utilize some of the most advanced technology available in the compressor industry and have a very low energy consumption. These machines are built to run 24 hours a day, 7 days a week, for many years!
There are many different types of rotary screw compressors. The classifications vary based on stages, cooling method, drive type, among others.
The most widely used type is the single stage, oil-injected rotary screw compressor. The main difference is that oil-lubricated compressors inject oil into the compression chamber to serve a few different purposes.
Oil-free compressors can be used in specific applications where the tiny amounts of excess oil in the air would contaminate the product or process.
Oil-injected models are by far the most common and for the purpose of this article we will focus on this type.
The power (and name) behind these remarkable machines comes from two counter-rotating screws which are housed in a chamber, formally known as an airend. Outside air first travels through a filter to catch any harmful particles and debris that could cause damage. Once filtered, the air goes through an inlet valve and into the space between the interlocking screws. As the rotors turn, the air moves along and travels to the other end of the compression chamber. The area in which the air is contained gets increasingly smaller as the air moves along, and the smaller space increases the pressure. This process is one smooth continuous motion, and therefore produces minimal pulsing or surging which can occur with piston compressors. The result is a high-volume, steady stream of compressed air.
Did you know?
Rotary screw technology is the same technology utilized by superchargers in high-performance engines!
In an oil-injected rotary screw air compressor, the oil serves five key purposes: it cleans, cools, lubricates, seals, and protects. Therefore, what is produced by the airend is a compressed air/oil mixture. This mixture flows into a separator tank where the two are, well… separated! Primarily, a mechanical separation takes place using a filter. Further, oil can be removed using centrifugal force or a rapid change in direction. In the case of centrifugal force, rapid spinning allows the heavier oil particles to drop to the bottom while the lighter air spins around on top. Simultaneously, the separated elements leave the tank.
The air leaves the tank and travels through a minimum-pressure valve, and (in some cases) a cooler. The minimum-pressure check valve is dual-purpose and ensures there is sufficient air pressure for proper operation while also working to keep the air pressure from coming back into the system so, when necessary, the motor can start without a load.
The compressed air can still be very hot and might need to go through a cooler to bring it down to 12-20 degrees Fahrenheit above ambient temperature. This cooling process causes water vapor to condense inside the cooler, and therefore, in most compressors there is a water separation stage before the air leaves the package. After these last stages, compressed air finally leaves the compressor ready for treatment, regulation, storage, and to the customer’s processes.
Meanwhile, as the oil was collected from the tank earlier, a line transports the oil back to the compression chamber to be used again. But first, it must be filtered and possibly cooled. A thermostatic valve determines if the oil is too hot and, if necessary, sends the oil through an oil cooler before being filtered. Once filtered, the oil is ready to begin the journey over again.
Check out this cool animation (at bottom of page) from Atlas Copco detailing the process in an oil-injected rotary screw compressor:
Rotary screw compressors are the workhorses behind a majority of manufacturers worldwide. If you see a big building and they make stuff there, there's a good chance there is a rotary screw air compressor powering their manufacturing process.
There is a good reason for this. An industrial rotary screw compressor has a 100% duty cycle. It can run 24/7 without a break, and in fact it usually works better and lasts longer when it's used that way. A piston compressor normally works better when it can take a break - it likes a intermittent duty cycle. However, the rotary can go all out, all day without stopping - it doesn't like starting and stopping constantly.
Another reason is that when sized correctly, rotary screws can be some of the most energy efficient compressors on the market. The keys are correct sizing, proper air system design, and intelligent compressor control. You can throw the most efficient compressor in the world in an air system, but if the system and control scheme are poorly designed, the compressor won't be efficient.
Let's talk about how they compress air.
A typical rotary screw air compressor has two interlocking helical rotors contained in a housing. Air comes in through a valve, typically called the inlet valve and is taken into the space between the rotors. As the screws turn, they reduce the volume of the air, thus increasing the pressure.
There are rotary screw air compressors with just one screw, as well. However, they're not very popular when it comes to compressing air. You'll see them more in refrigeration applications. Their principle of operation is beyond the scope of this blog, but if you're interested, you can read more here. For the rest of this, it can be assumed that we are talking about compressors with more than one screw.
The assembly that includes the rotors and the housing they're in is called an "air end" or airend. This is actually the correct terminology for all rotary compressors, whether they be rotary vane, scroll, screw or lobe - the part that compresses the air is called the airend.
Rotary screw compressors can either be oil-flooded or "oil-free." Oil-free is in quotation marks because oil-free compressors don't provide oil-free air (there's oil in the air around us). However the difference is that with oil-free rotaries there is no oil in the compression chamber.
In an oil lubricated rotary screw compressor the male rotor is driven by the motor or engine, and the female rotor is driven by the male rotor, or actually by the thin film of oil that's between them. The oil also seals the compression chamber and acts as a coolant.
In an oil-free rotary screw compressor a set of gears controls the timing between the male and female rotor. There is no oil to seal the chamber, so without multiple stages you cannot achieve as high as a pressure as you can with an oil-lubricated one. Additionally there's no cooling oil, so they run hotter, and that decreases the efficiency. Because of this oil-free rotary screw compressors are usually limited to special applications. There are some oil-free compressors that use water as a coolant, but those are rare.
There is so much more to a rotary screw compressor than the airend. Let's take a look at the typical oil-lubricated rotary screw:
The airend doesn't just compress air; it compresses an air/oil mixture. That mixture then flows into a tank called the separator tank or sump. The oil is separated out of the air by centrifugal force - as the air spins around in the tank, the oil drops out because the oil particles are heavier than the air particles. Usually there are baffles in the tank that assist with this. There is also a separator element that takes out nearly all the remaining oil - all but a few parts per million (usually 3 ppm).
From there the oil and air take two separate paths. The air then goes out through a cooler and then out to your application. The oil will either go back into the airend or through an oil cooler. There is usually a thermostatic valve that directs the oil one way or the other, based on the temperature of the oil. You don't want the compressor to run too hot or too cold. If you run too hot you'll fry the oil, decrease efficiency and burn out other components. If you run too cold, you'll never get hot enough to boil off the liquid water that dropped out of the air when it was compressed. Too much liquid water in the oil will cause airend failure.
Usually there is a minimum pressure valve or a minimum pressure check valve which doesn't let the air out into the air system, until there is minimum pressure for the compressor to lubricate itself. There is an oil filter that filters out contaminants in the oil. There is also an air filter to keep large contaminants from getting in. Another common component is a blow-down valve (or unloading valve) that blows the excess pressure in the sump to idle pressure when the compressor is idling.
An oil-free rotary has different components. Normally there are two airends and the air is cooled with an intercooler between them. Typically the gears for both airends are housed in the gearbox and that gear box is lubricated. An oil seal and positive pressure are used to keep the oil from the gearbox out of the airend. There's no separator tank, oil cooler, or thermal valve, but the other components are usually there.
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Atlas Copco is a global leader in the design, manufacture, installation and service of innovative compressor technologies for a diverse range of industries.
From manufacturing to construction, mining to pharmaceuticals, our compressors work hard to increase efficiency, reduce energy consumption and decrease cost of ownership for compressed air users across the globe.
Gardner Denver has been in business since 1859, with over 100 years of experience in manufacturing air compressors but Atlas Copco has been in business far longer than this. Today we (CPS Hire, Atlas Copco's largest distributor in Australia) provide compressors to meet a wide range of customer requirements – from small, stand-alone compressors, to complete compressed air systems including compressor sequencers, remote monitoring, air treatment and other ancillary equipment.
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Atlas Copco Rotary Screw (Oil Injected) 7kw, 11kw & 15kw
The most common range of machines sold. Atlas Copco make and sell more rotary screw compressors than any other company in Australia. Why is this? Made for Australian conditions, made to last & out perform all others. For the most energy efficient, robust compressors money can buy, look no further.
Always consider total cost of ownership - Energy, Maintenance, Reliability - That is why we sell more
Email: Jason@CPScompressors.com.au for more info or to request site visit
G7 Air Compressor 44.9 cfm / 21.2 litres / sec FAD
Product Code: G7 Floor Mount Pricing: $8,050 +GST
Description: Atlas Copco 7kw rotary screw air compressor, floor mounted - from 44.9 cfm - FAD. Delivery nationwide arranged (not included in this price, from $175 + GST depending on location). Nationally backed and serviced. Extended warranty available - Yes.
Match to Stand Alone Dryer - FXe4 or FXe5 Here
Original Manufacturer: Atlas Copco. Current Stock available: YES
G15 Air Compressor 64cfm / 30 litres / sec FAD
Product Code: G11TM Floor Mounted Pricing: $POA +GST
Product Code: G11TMFF Tank Mounted with inbuilt dryer Pricing: $POA +GST
email for more info on price: firstname.lastname@example.org
Description: Atlas Copco 11kw rotary screw air compressor, tank mounted - from 44.9 cfm - FAD. AS1210 Approval on vessel. Delivery nationwide arranged (not included in this price, from $185 + GST depending on location). Nationally backed and serviced. Extended warranty available - Yes.
Original Manufacturer: Atlas Copco. Current Stock available: YES on FF
G7 Air Compressor 44.9 cfm / 21.2 litres / sec FAD
Product Code: G7TM Tank Mounted Pricing: $8,545 +GST
Product Code: G7TMFF Tank Mounted with inbuilt dryer Pricing: $10,245 +GST
Description: Atlas Copco 7kw rotary screw air compressor, tank mounted - from 44.9 cfm - FAD. AS1210 Approval on vessel. Delivery nationwide arranged (not included in this price, from $175 + GST depending on location). Nationally backed and serviced. Extended warranty available - Yes.
Original Manufacturer: Atlas Copco. Current Stock available: YES on FF