Resource
Library

RESOURCE LIBRARY

As the united voice of the compressed air industry, and serving as the unbiased authority on technical, educational, promotional, and other matters that affect the industry, CAGI has compiled a Resource Library to provide an extensive array of information to help educate on the fundamentals of compressed air through educational and technical resources.

EDUCATION RESOURCES
TECHNICAL RESOURCES

EDUCATION RESOURCES

Compressed Air & Gas Handbook

The Compressed Air and Gas Handbook is the authoritative reference source for general information about compressed air and for specific information about proper installation, use, and maintenance of compressors and pneumatic equipment. This illustrated volume contains engineering information from leading manufacturers and valuable reference data pertinent to compressed air systems. The following individual chapters are available as free downloadable PDF documents.

CAGI Handbook

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

Terms & Conditions: This publication is designed to provide information regarding the subject matter covered. CAGI cannot warrant the particular use of the information provided. The materials are not intended to substitute for competent engineering advice. The e-mail address you provide will only be used to send you updates on the CAGI handbook and other information that you may find of interest. At any time, you have the opportunity to opt-out to choose not to receive this information.

To purchase a hardbound copy of the 6th edition of the Compressed Air and Gas Handbook, click here and use the Contact Form.

Videos

Watch our educational videos below to learn more about compressed air and gas.

About CAGI

This video is an overview of the Compressed Air & Gas Institute. The video provides information on the CAGI objectives and the various resources that are available through the CAGI web site, including information on educational material, Performance Verification Program and its members.

Isentropic Efficiency Video

In an effort to eliminate the confusion created when trying to compare the specific power, which is an energy consumption metric, between two similar-sized compressors with slightly different rated discharge pressures, CAGI has introduced an efficiency rating value on its Compressor Data Sheets . . . Isentropic Efficiency. Isentropic efficiency incorporates operating pressure and greatly simplifies the process of comparing the efficiencies among several like-sized compressors each having slightly different full load operating pressures. For example, for the same horsepower compressors, one sheet may show 125 psig operating pressure and another may show 130 psig so comparing isentropic helps equalize the comparison.

Performance Verification Program

This video is an overview of the CAGI Performance Verification Program, a voluntary program that verifies product performance that participating manufacturers publish on the standard CAGI data sheets.

Compressed Air...Industry's Fourth Utility

Compressed Air: Industry's Fourth Utility is an introductory look at compressed air systems. The video serves as an overall guide to compressed air systems and introduces topics that will be explored in greater detail in the other installments in the series. Viewers receive an overview of three important considerations in understanding and planning compressed air systems: air compression, air treatment, and distribution.

Principles of Air Compression

Principles of Air Compression is a 14-minute video that explains, in plain language, the theory and principles involved in the compression of air. It shows how both positive displacement and dynamic type compressors operate and introduces the audience to key terms as PSIG, SCFM, relative humidity and dew point. The video will provide a solid understanding of the basics of compressed air.

Performance Under Pressure

Performance Under Pressure is a fast-moving 16-minute color animated video explaining the role of compressed air and gas in a variety of applications from residential to industrial.

How to Select an Air Compressor

How to Select an Air Compressor sponsored in part by the U. S. Department of Energy, the video provides excellent advice from industry experts and users about the process of selecting an air compressor, going beyond the compressor itself and encouraging buyers to look at their complete air systems. While price is an important consideration, purchasing a compressor on price alone could cost plenty. The number one expense in owning a compressor is the power cost involved in running one. This video discusses the total cost of a compressor and what must be considered before making the purchase decision. The video demonstrates that a thorough study of an air system may show that the buyer does not need another compressor but, rather, a more efficient system.

Air Treatment

Air Treatment provides an excellent overview of compressed air treatment. It introduces the viewer to the different ISO air quality classes and answers such questions as:

  • Why treat air?
  • Where do contaminants come from?
  • What are the negative consequences of contaminants in a compressed air system?
  • How much should air be treated?
  • What role does maintenance play in air system efficiency?

Media Connections

The following industrial magazines cover the subject of compressed air on a regular basis.

Calendar of Tradeshows

IWF Atlanta
August 23-26, 2022, Atlanta, GA
For more information.

Hannover Messe USA/IMTS
September 12-17, 2022, Chicago, IL
For more information.

Automechanika Frankfurt
September 13-17, 2022, Frankfurt, Germany
For more information.

International Manufacturing Technology Show
September 12-17, 2022, Chicago, IL
For more information.

International Tire Exhibition & Conference (ITEC)
September 13-15, 2022, Akron, OH
For more information.

Experience Power
October 3-6, 2022, Denver, CO
For more information.

CABP Expo
October 4-6, 2022, Atlanta, GA
For more information.

Energy Utility Environment Conference (EUEC)
October 5-7, 2022, Tucson, AZ
For more information.

WEFTEC
October 8-12, 2022, New Orleans, LA
For more information.

AFPM Summit
October 18-20, 2022, San Antonio, TX
For more information.

SupplySide West Conference
October 31 – November 4, 2022, Las Vegas, NV
For more information.

AAPEX
November 1-3, 2022, Las Vegas, NV
For more information.

SEMA Show
November 1-4, 2022, Las Vegas, NV
For more information.

Fabtech
November 8-10, 2022, Atlanta, GA
For more information.

International WorkBoat Show
November 30 – December 2, 2022, New Orleans, LA
For more information.

The Performance Racing Industry Trade Show
December 8-10, 2022, Indianapolis, IN
For more information.

AFFI-Con American Frozen Food
February 25-28, 2023, San Diego, CA
For more information.

SNAXPO23
March 19-21, 2023, Orlando, FL
For more information.

MCAA Annual Convention
March 26-30, 2023, Phoenix, AZ
For more information.

SupplySide East Conference
April 18-19, 2023, Secaucus, NJ
For more information.

International Powder & Bulk Solids Conference
April 25-27, 2023, Rosemont, IL
For more information.

AWFS – Association of Woodworking & Furnishings
July 25-28, 2023, Las Vegas, NV
For more information.

Pack Expo
September 11-13, 2023, Las Vegas, NV
For more information.

AeroDef Manufacturing
November 7-9, 2023, Long Beach, CA
For more information.

TECHNICAL RESOURCES

Glossary

Access the list of common terms used in the compressed air industry.

 

A:

Absolute Pressure - Total pressure measured from zero.

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 gas actually compressed and delivered to the discharge system at rated speed and under rated conditions. Also called Free Air Delivered (FAD).

Adiabatic Compression - See Compression, Adiabatic.

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.

Air Receiver - See Receiver.

Air Bearings - See Gas Bearings.

Aftercooler - A heat exchanger used for cooling air discharged from a compressor. Resulting condensate may be removed by a moisture separator following the aftercooler.

Atmospheric Pressure - The measured ambient pressure for a specific location and altitude.

Automatic Sequencer - A device which operates compressors in sequence according to a programmed schedule.

B:

Brake Horsepower (bhp) - See Horsepower, Brake.

C:

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 compressors

Check Valve - A valve which permits flow in only one direction.

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.

Compressibility – A measure of the instantaneous relative volume change of a gas as a response to a pressure change.

Compression, Adiabatic - Compression in which no heat is transferred to or from the gas during the compression process.

Compression Isentropic (or Isentropic Efficiency) - Isentropic efficiency is a ratio that indicates how the real energy consumption of an air compressor compares to that for an idealized compression process. The value is expressed as a percentage, the higher the number indicates that the machine is more efficient at converting electrical energy into compressed air potential energy. Isentropic compression is considered where the system is frictionless, and there is no transfer of heat. Such idealized processes are useful as a model and basis for real compression processes. Isentropic efficiency can be calculated and measured independently of compression technology. Efficiency is not measured directly but is derived from power consumption, pressure ratio, and delivered capacity.

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 = C.

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.

Critical Pressure - The limiting value of saturation pressure as the saturation temperature approaches the critical temperature.

Critical Temperature - The highest temperature at which well-defined liquid and vapor states exist. Sometimes it is defined as the highest temperature at which it is possible to liquify a gas by pressure alone.

Cubic Feet Per Minute (cfm) - Volumetric air flow rate.

Cfm, Actual cfm (acfm) – 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. Also known as free air delivered (fad).

Inlet cfm (icfm) - cfm flowing through the compressor inlet filter or inlet valve under rated conditions.

Standard cfm (scfm) - Flow of free air measured and converted to a standard set of reference conditions (14.5 psia, 68ºF, and 0% relative humidity).

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.

D:

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 absorbing moisture.

Desiccant - A material 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 is 100%.

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.

Diaphragm cooling - A method of removing heat from the flowing medium by circulation of a coolant in passages built into the diaphragm.

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

Digital Controls - See Logic Controls.

Discharge Pressure - Gas pressure produced at the discharge port of a compressor.

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.

Droop - The drop in pressure at the outlet of a pressure regulator when a demand for air occurs.

Dynamic Type Compressors - Compressors in which air or gas is compressed by the mechanical action of rotating impellers imparting velocity and pressure to a continuously flowing medium. (Can be centrifugal or axial design)

E:

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 piston displacement.

Exhauster - A term sometimes applied to a compressor in which the inlet pressure is less than atmospheric pressure.

Expanders - Turbines or engines in which a gas expands, doing work, and undergoing a drop in temperature. Use of the term usually implies that the drop in temperature is the principal objective. The orifice in a refrigeration system also performs this function, but the expander performs it more nearly isentropically, and thus is more effective in cryogenic systems.

F:

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).

Fluidics - The general subject of instruments and controls dependent upon low rate of flow of air or gas at low pressure as the operating medium. These usually have no moving parts.

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.

G:

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.

Gas bearings - Load carrying machine elements permitting some degree of motion in which the lubricant is air or some other gas.

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.

Guide vane - A stationary element that may be adjustable and which directs the flowing medium approaching the inlet of an impeller.

H:

Head, Adiabatic - The energy, in foot pounds, required to compress adiabatically to deliver one pound of a given gas from one pressure level to another.

Head, Polytropic - The energy, in foot pounds, required to compress polytropically to deliver one pound of a given gas from one pressure level to another.

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.

Hysteresis - The time lag in responding to a demand for air from a pressure regulator.

I:

Impeller - The part of the rotating element of a dynamic compressor which imparts energy to the flowing medium by means of centrifugal force. It consists of a number of blades which rotate with the shaft.

Indicated Power - Power as calculated from compressor-indicator diagrams.

Indicator card - A pressure - volume diagram for a compressor or engine cylinder, produced by direct measurement by a device called an indicator.

Inducer - A curved inlet section of an impeller.

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 the compressor.

Isentropic compression - See Compression, Isentropic.

Isothermal compression - See Compression, Isothermal.

L:

Leak - An unintended loss of compressed air to ambient conditions.

Liquid piston compressor - A compressor in which a vaned rotor revolves in an elliptical stator, with the spaces between the rotor and stator sealed by a ring of liquid rotating with the impeller.

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.

M:

Modulating Control - System which adapts to varying demand by throttling the compressor inlet proportionally to the demand.

Multi - casing compressor - Two or more compressors, each with a separate casing, driven by a single driver, forming a single unit.

Multi - stage axial compressor - A dynamic compressor having two or more rows of rotating elements operating in series on a single rotor and in a single casing.

Multi - stage centrifugal compressor - A dynamic compressor having two or more impellers operating in series in a single casing.

Multi - stage compressors - Compressors having two or more stages operating in series.

P:

Perfect Intercooling - The condition when the temperature of air leaving the intercooler equals the 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, Polytropic.

Polytropic head - See Head, Polytropic.

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, Critical - See Critical Pressure.

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.

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.

R:

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.

Reynold number - A dimensionless flow parameter (h < D/:), in which h is a significant dimension, often a diameter, < is the fluid velocity, D is the mass density, and : is the dynamic viscosity, all in consistent units.

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.

S:

Seals - Devices used to separate and minimize leakage between areas of unequal pressure.

Sequence - The order in which compressors are brought online.

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 mounted.

Specific gravity - The ratio of the specific weight of air or gas to that of dry air at the same pressure and temperature.

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 (rpm) 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 the unit.

Supercompressibility - See Compressibility.

Surge - A phenomenon in centrifugal compressors where a reduced flow rate results in a flow reversal and unstable operation.

Surge limit - The capacity in a dynamic compressor below which operation becomes unstable.

T:

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, Critical - See Critical 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 same velocity.

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 of pressures.

Torque - A torsional moment or couple. This term typically refers to the driving couple of a machine or motor.

Total Package Input Power - The total electrical power input to a compressor, including drive motor, belt losses, cooling fan motors, VSD or other controls, etc.

U:

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.

V:

Vacuum pumps - Compressors which operate with an intake pressure below atmospheric pressure and which discharge to atmospheric pressure or slightly higher.

Valves - Devices with passages for directing flow into alternate paths or to prevent flow.

Volute - A stationary, spiral shaped passage which converts velocity head to pressure in a flowing stream of air or gas.

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.

Reynold number - A dimensionless flow parameter (h < D/:), in which h is a significant dimension, often a diameter, < is the fluid velocity, D is the mass density, and : is the dynamic viscosity, all in consistent units.

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.

S:

Seals - Devices used to separate and minimize leakage between areas of unequal pressure.

Sequence - The order in which compressors are brought online.

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 mounted.

Specific gravity - The ratio of the specific weight of air or gas to that of dry air at the same pressure and temperature, normally at standard conditions of 14.5 psia, 68°F, 0% relative humidity.

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 (rpm) 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 the unit.

Supercompressibility – Thermodynamic term indicating conditions of pressure and temperature when a gas departs from the perfect gas laws.

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, Polytropic.

Polytropic head - See Head, Polytropic.

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, Critical - See Critical Pressure.

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.

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.

R:

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.

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 the compressor.

Isentropic Efficiency (or Compression Isentropic) - Isentropic efficiency is a ratio that indicates how the real energy consumption of an air compressor compares to that for an idealized compression process. The value is expressed as a percentage, the higher the number indicates that the machine is more efficient at converting electrical energy into compressed air potential energy. Isentropic compression is considered where the system is frictionless, and there is no transfer of heat. Such idealized processes are useful as a model and basis for real compression processes. Isentropic efficiency can be calculated and measured independently of compression technology. Efficiency is not measured directly but is derived from power consumption, pressure ratio, and delivered capacity.

Isothermal compression - See Compression, Isothermal.

L:

Leak - An unintended loss of compressed air to ambient conditions.

Liquid piston compressor - A compressor in which a vaned rotor revolves in an elliptical stator, with the spaces between the rotor and stator sealed by a ring of liquid rotating with the impeller.

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.

M:

Modulating Control - System which adapts to varying demand by throttling the compressor inlet proportionally to the demand.

Multi - casing compressor - Two or more compressors, each with a separate casing, driven by a single driver, forming a single unit.

Multi - stage axial compressor - A dynamic compressor having two or more rows of rotating elements operating in series on a single rotor and in a single casing.

Multi - stage centrifugal compressor - A dynamic compressor having two or more impellers operating in series in a single casing.

Multi - stage compressors - Compressors having two or more stages operating in series.

P:

Perfect Intercooling - The condition when the temperature of air leaving the intercooler equals the temperature of air at the compressor intake.

W:

Water - cooled compressor - Compressors cooled by water circulated through jackets surrounding cylinders or casings and/or heat exchangers between and after stages.

Myth Busters

There are some common misunderstandings about compressed air and gas as a power source. Let's clear those up. Below are general myths, compressor myths and air treatment myths.

General Myths

Myth:
Compressed air is not a desirable source of power.

Fact:
When properly applied, compressed air can be the best choice. It is a safe, clean source of power that is easy to use and maintain.

Myth:
Compressed air is very expensive.

Fact:
Like the other power sources (electric, battery, natural gas, steam, and hydraulic), compressed air can be costly. Compressed air cost can be minimized through proper application, installation, and maintenance.

Myth:
Compressed air is dirty.

Fact:
This is generally not true. Poor system design or lack of basic maintenance can result in contaminated air at point of use.

Compressors Myths

Myth:
It is safe to clean dirt & debris from a work area using compressed air.

Fact:
No. This is an unsafe practice. Cleaning a work area using compressed air could result in serious injury as a result of particulate striking or entering the eyes, ears or skin. A safer alternative is to use a vacuum, mop, or broom to remove dirt and debris.

Myth:
Variable speed drive is always the best solution for the most efficient part-load performance.

Fact:
Not necessarily. Inherent to their design, variable speed drive compressors have an optimum tip speed range where they operate most efficiently. If the compressor load requirements fall outside of this range, other control options may provide the most efficient solution. The compressor provider will need to consider the numerous application requirements in order to recommend the most efficient compressed air solution for the job.

Myth:
Variable speed drive increases oil carryover.

Fact:
Operating at reduced speeds should have no detrimental impact on oil carryover. The reduced airflow will lower the velocity through the separator media, which will improve its efficiency. There are two primary contributors to an increase in carryover, lower pressure, and higher temperature. Neither of these issues is caused by the addition of a VSD.

Myth:
Using a variable speed drive compressor eliminates the need for a flow controller.

Fact:
Every application is different, but there are clearly systems that would benefit from the combined actions of a flow controller and a VSD part-load compressor to deliver a stable and constant system pressure. This is especially true in systems that experience highly variable demands.

Myth:
All “synthetic” compressor fluids are the same.

Fact:
No, they are not the same. When looking at air compressor fluids, the term “synthetic” is often used to describe the base stock. Since “synthetic” simply means that the fluid is not a hydrocarbon base stock, it encompasses many different base stocks which offer dramatically different performance. The real question is what is the synthetic base stock, Polyglycol (PAG), Diester, POE? Once you determine the base stock you can evaluate its advantages and disadvantages.

Myth:
All Polyglycol (PAG) blended compressor fluids are the same.

Fact:
Polyglycol compressor fluids are generally blended with one of two types of “Esters,” “Polyolesters” or “Diesters.” These two differ drastically in their performance in a rotary screw air compressor. Polyolesters (POE): POEs handle heat very well and run varnish and sludge free. They are best known as jet engine lubricants. They are expensive and are used in applications where performance is critical, like in a rotary screw air compressor. Diesters: Diesters are low cost and have the potential to cause sludge. Sludge can become very costly to a company’s profitability, reducing compressor efficiency, and becoming very expensive to remove from a compressor system once it forms. Diesters also cause swelling in gaskets, seals, and hose liners. This results in problems with leaks, swelling of O-rings and deterioration of shaft seals. BEWARE, many generic Polyglycol/Ester blends turn out to be a Diester base with a minimum amount of Polyglycol. These generic fluids will have all the issues associated with a Diester fluid.

Myth:
Generic replacement parts and compressor lubricants are the same as manufacturer proprietary parts and lubricants.

Fact:
Maintenance kits and manufacturer proprietary replacement parts and lubricants offer the best overall performance. They are designed to maintain unit efficiency and reliability. Generic parts increase liability, decrease performance, and may void manufacturer warranties. Considering form, fit, and functions; generic, will-fit parts often fall short of achieving the function of a factory, OEM part.

Air Treatment Myths

Myth:
Filter elements should only be changed when differential pressure is high.

Fact:
You install compressed air filtration to improve air quality. DP gauges/indicators are blockage indicators not air quality indicators. To ensure your compressed air quality, filter elements should be changed annually in line with manufacturer's instructions.

Myth:
Coalescing filters are ONLY for oil removal.

Fact:
Coalescing filters have an even higher capture rate with solid contaminants than with liquids.

Myth:
Oil contamination is not present in atmospheric air.

Fact:
Atmospheric air typically contains between 0.05mg/m3 and 0.5mg/m3 of oil vapor from sources such as car exhausts and industrial processes. As oil free compressors use large quantities of atmospheric air and atmospheric air contains oil vapor which can cool and condense in the compressed air systems, the use of oil free compressors does not guarantee oil free air.

Myth:
Liquid oil and oil aerosol are the only contaminants present in a compressed air system.

Fact:
Generally, the following ten contaminants can be found in a typical compressed air system and these need to be removed or reduced for the system to run efficiently.

Water vapor
Liquid water
Water aerosols
Liquid oil
Oil vapor
Oil aerosols
Rust
Pipescale
Atmospheric dirt
Microorganisms

Regardless of whether the compressor design is oil injected or oil free, air treatment equipment is required to eliminate the above listed contaminants.

Myth:
Compressed air contamination is a compressor issue.

Fact:
In a typical compressed air system, compressed contamination comes from the following four different sources:

Source 1 - Atmospheric Air
Air compressors draw in huge amounts of atmospheric air, which continuously fills the system with contaminants such as water vapor, micro-organisms, atmospheric dirt & oil vapor.

Source 2 - The Air Compressor
In addition to the contaminants drawn in through the compressor intake, the compressor also adds additional wear particulates from its operation. Additionally, oil lubricated compressors carry over liquid oil, oil aerosols and oil vapor from the compression process. Once through the compression stage, the aftercooler will also condense water vapor, introducing it into the compressed air in both a liquid and aerosol form.

Source 3 - Compressed Air Storage Devices
Air receivers rust internally and over time this rust can dislodge from the interior walls of the receiver and contaminate the air stream. Receivers also allow compressed air to cool, allowing water vapor to condense into liquid water that can contaminate the compressed air system.

Source 4 - Compressed Air Distribution Piping
The piping system that distributes the compressed air around the facility can be a source of large amounts of contamination. Rust, scale, corrosion, and microbiological growth from inside the piping system creates contamination.

Myth:
Static oil water separators are not suitable for synthetic lubricants / PAG's. This is evident with cloudy outlet water.

Fact:
Oil water separators are designed to reduce oil in water levels to acceptable limits. Some lubricants such as synthetics/PAG's also contain detergents and additives to extend the life of the compressor. Oil water separators are not designed to remove detergents and additives. Oil in water content cannot be accurately determined from visual inspection and lab analysis should be used. Lab analysis on cloudy outlet water is the only way of accurately testing oil in water content and will show it is within acceptable limits.

Myth:
Any dryer (refrigerated or desiccant) can be installed outdoors.

Fact:
All standard dryers are designed for indoor installation. However, many are often installed outside, with or without being installed under a roof. Outside installation under a roof is acceptable provided there is protection from freezing, snow, blowing rain, and the roof height is sufficient to prevent hot air recirculation. Outside installation of standard dryers with no roof is not recommended.

In the case where outside installation without a roof is a necessity, request an outdoor modification package to include freeze protection, UV protection, and NEMA 4 components throughout. A NEMA 4 electrical enclosure alone is not sufficient for an outdoor installation without a roof.

Myth:
Most dryers do not provide dryness levels to the same levels quoted in sales literature.

Fact:
Dryer installations are often plagued with a variety of mistakes which impact the level of compressed air dryness which they provide. Classic mistakes include, undersizing the equipment by disregarding the effect of dryer inlet temperature and cooling medium temperature, insufficient clearance around the dryer that causes problematic recirculation of hot air, failing to remove the condensed water from the air stream, and selecting the wrong dryer technology. When properly sized, installed, and maintained a dryer should perform exactly as quoted in its sales literature.

Myth:
Compressed air filters also dry compressed air.

Fact:
Compressed air filters are capable of removing bulk liquid water and some water aerosol from compressed air. Compressed air filters are not capable of reducing the level of moisture vapor in compressed air or reducing the pressure dew point of compressed air.

Calculation Tools

Want to know how much moisture is in compressed air? Or the cost of pressure drop? These questions and other application questions can be answered with these easy-to-use calculators. If you have questions about these tools or need more information, please contact a CAGI member company.

Conversion Calculation

Where:
Ps = Standard pressure, psia (CAGI & ISO use 14.5 psia)
Pa = Atmospheric pressure, psia
ppm = Partial pressure of moisture at atmospheric temperature
RH = Relative humidity
Ta = Atmospheric Temperature, °F
Ts = Standard Temperature, °F (CAGI and ISO use 68 °F)

To calculate the cost of compressed air, use the following formula:

Cost ($/year) = motor bhp x .746 x hours of operation (per year) x electric rate ($/kwh) / motor efficiency

Publications

As a service to its members and others interested in the industry, the Institute publishes and maintains reference materials and publications of interest to the industry.

Reference and Resource Materials

The following reference and resource materials are available for free download:

Ten Questions to Ask Before Buying That Piston Operated Air Compressor

Tech Sheet 101: Plastic Pipe Institute Recommendation Regarding the Use of Thermoplastic Piping for the Transport of Compressed Air or Other Compressed Gases

CAGI BL 300-2020, Performance Test Code for Electric Driven Low Pressure Air Compressor Packages

Tech Sheet RC101: Industrial Compressors and the UL 1450 Standard

Proposition 65: Implications for Companies in the Compressed Air & Gas Industry

Guidelines for Selecting a Compressed Air Service Provider

Publications

To purchase the following publications, click here to download order form and submit to the CAGI office. To order online, click here. Link to: https://idv2thomas.gli.us.com/

The Compressed Air and Gas Handbook is the authoritative reference source for general information about compressed air and for specific information about proper installation, use, and maintenance of compressors and pneumatic equipment. This illustrated volume contains engineering information from leading manufacturers and valuable reference data pertinent to compressed air systems.

ANSI/CAGI B19.1, Safety Standard for Compressor Systems (Supersedes ASME B19.1-1995). This Standard classifies and makes available general information on practices, specific requirements, and recommendations covering safety for air compressors, their drives, and auxiliaries. 2010

ANSI/CAGI B186.1, Safety Code for Portable Air Tools. This code promotes the safe design, and use of air tools by prescribing safety requirements to guard against injury to workers, by providing guidance to tool manufacturers, owners, employers, supervisors, and others concerned with, or responsible for, worker safety, and by assisting in the promulgation of appropriate safety directives and safety training programs. 2020 (2528)