Compressed Air Definitions and Terminology
Absolute Pressure:
The sum of the gauge pressure and the atmospheric pressure (PSIA). (Example: 50 PSIG equals 50 plus atmospheric pressure, 14.7 at sea level, so 50 PSIG is the same as 64.7 PSIA at sea level
ACFM:
Actual Cubic Feet Per Minute. Refers to a volume of air (one cubic foot) at ambient conditions, no matter what those conditions are. Changes in pressure, temperature, and relative humidity do not change these ratings. Therefore, ACFM is a measure of volume, regardless of weight. (see corrections for altitude chart)
After-Coolers:
Heat exchangers for cooling air or gas discharged from compressors. They provide the most effective means of removing moisture from compressed air and gases.
Atmospheric Pressure:
The pressure of the atmosphere at any location that will decrease as the elevation above sea level increases. Sea level atmospheric pressure is 14.7 PSAI, or 29.92 inches of mercury absolute. At 2500 feet, the pressure is 13.41 PSIA, or 27.32 inches of mercury.
Atmosphere:
A pressure measurement obtained by dividing absolute pressure by atmospheric pressure. (Example: 114.7 PSIA/14.7 PSIA equals 7.8 atmospheres.)
Auto Drains
Auto Drains discharge water and oil from collection points within the air system.
Booster Compressors:
Machines designed for compressing air or other gases from an initial pressure, which is above atmospheric pressure, to a higher pressure.
Capacity
Capacity of a compressor is the actual volume rate of flow of gas compressed and delivered at conditions of total temperature, total pressure, and composition prevailing at the compressor inlet.
Capacity (Actual)
Quantity of gas actually compressed and delivered to the discharge system at rated speed of the machine and under rated pressure conditions. Actual capacity is usually expressed in cubic feet per minute (cfm) at first stage inlet gas conditions.
Check Valve
A valve that permits flow in one direction only.
Coalescing Filter:
Removes small particles, dirt, and oil from the compressed air.
Compression Ratio:
Ratio of absolute discharge pressure to the absolute intake pressure (CR).
Compressors:
Machines designed for compressing air or other gases from an initial intake pressure to a higher discharge pressure.
Constant Speed Control
Unit runs continuously but matches air supply to demand by "loading" or "unloading" the compressor.
Displacement
Displacement of a compressor is the volume swept out per unit of time; it is usually expressed in cubic feet per minute
Desiccant Air Dryers
A device that will remove moisture for dew-point requirements to -40 or a -100 degrees F.
Double Acting Compressors:
Machines in which compression takes place on both strokes per revolution in each compressing element.
Double Stage Compressors:
Machines in which compression from initial to final pressure is completed in two stages.
Dryer:
An integral part of the compressed air system that removes the heat of the compressed air and also removes the moisture from the air due to the heat loss.
Dynamic:
Machines where air or gas is compressed by the dynamic action of rotating vanes or impellers imparting velocity and pressure to the air or other gases.
Efficiency, volumetric
Ration of actual capacity to displacement, stated as a percentage.
Filters
Are devices for separating and removing dust and dirt from air before it enters a compressor.
Free air
Free air is defined as air at atmospheric conditions at any specific location. Because the altitude, barometer, and temperature may vary at different localities and at different times, it follows that this term does not mean air under identical or standard conditions
Gas
While from a physical point of view a gas is one of the three basic phases of matter, and thus air is a gas, a special meaning is assigned in pneumatics practice. The term gas refers to any gas other than air.
Gauge Pressure:
The pressure on the gauge measured above atmospheric pressure (PSIG); that is, that measured on a given gauge, not effected by altitude or atmospheric pressure.
Horsepower, brake
Brake horsepower is the horsepower input to the compressor shaft, or more generally to any driven machine shaft.
Horsepower, theoretical, or ideal
The ideal, or theoretical horsepower of a compressor is defiled as the horsepower required to compress adiabatically the air or gas delivered by the compressor through he specified range of pressures.
Humidity, specific
Specific humidity is the weight of water vapor in an air vapor mixture per pound of dry air
Humidity, relative
The relative humidity of a gas (or air) vapor mixture is the ration of the partial pressure of the vapor to the vapor saturation pressure at the dry bulb temperature of the mixture.
Ideal Compression Ratio:
In two-stage units, the square root of the total compression ratio is used to size stage compression ratio and balances the load between stages (ICR).
Inter-Coolers
These are heat exchangers for removing the heat of compression between stages of a compressor. They usually condense and remove a considerable amount of moisture as well.
Intercooling
This is the removal of heat from the air or gas between stages or stage group.
Liquid piston
A liquid piston compressor is a rotary compressor in which a vaned rotor revolves in an elliptical casing, with the rotor spaces sealed by a ring of liquid rotating with it inside the casing.
Load factor
This factor is the ration of the average compressor load during a given period of time to the maximum rated load of the compressor. It applies also to air tools, where it is the product of the work factor times the time factor.
Logic Controls
These are control devices the operation of which may be reduced to binary operation such as an on-off, 0-1, or open-closed. They are also referred to as digital controls.
Mechanical Ratio:
Multi-Stage only. The ratio of the displacements of the low pressure and high pressure stages (MR).
Moisture Separators:
Devices for collecting and removing moisture precipitated from the air and gas during the process of cooling.
Multi-casing Compressor
When a single motor or turbine drives two or more compressors, each with a separate casing,, the combined unit is called a multi-casing compressor.
Multi-Stage Compressors or Compound Compressors
A machine in which compression from initial to final pressure is completed in two or more distinct steps or stages.
Oil/Water Separator
A device that separates the remaining compressor oil from condensate.
Particulate Filter:
Removes small particles and dirt from the compressed air.
Performance Curve
This curve is a plot of expected operating characteristics, e.g., discharge pressure vs. inlet capacity, shaft horsepower vs. inlet capacity.
Piston Displacement
Net volume actually displaced by the compressor piston or rotor at rated machine speed, generally expressed in cubic feet per minute (usually cfm) For multi-stage compressors, the piston or rotor displacement of the first stage only is commonly stated as that of the entire machine.
Positive Displacement:
A machine where successive volumes of air or gas are confined within a closed space and pressure is increased as the volume of the closed space is decreased
Pressure (PSI)
Pounds-Per-Square-Inch- a rating of Air Pressure in the system. (PSIG) "Gauge" Gauge pressure shows amount of air pressure above ambient
Pressure Discharge
Discharge pressure is the absolute total pressure at the discharge flange of a compressor
Pressure, intake
Intake pressure is the absolute total pressure at the inlet flange of a compressor.
Pressure rise
This is the difference between the discharge pressure and the intake pressure
Pressure static
Static pressure is the pressure measured in a flowing stream (liquid or gas) in such a manner that no effect on the measurement is produced by the velocity of the stream.
Rotary Screw Compressors:
Machines that use male and female helical rotors to smoothly compress air.
Receivers
Receivers are tanks used for the storage of air discharged from compressors. They serve also to damp discharge line pulsation's.
Reciprocating Compressors:
Machines that use a reciprocating motion (piston) to compress air.
Refrigerated Air Dryer
A device that will remove moisture for dew-point requirements no lower than 33-degrees F.
Rotor
The rotor is the rotating element of a machine and, in the case of a compressor, is composed of the impeller (or impellers) and shaft, and may include shaft sleeves and a thrust balancing device.
Rotary Compressors
These are machines in which compression is effected by the positive action of rotating elements. (Capable of 100% Duty)
Rotary Screw
Machines that use male and female helical rotors to smoothly compress air
Rotary, Sliding Vane
Such compressors are machines in which axial vanes slide radially in an eccentrically mounted rotor.
Rotary, two-impeller positive displacement
These are machines in which two mating lobed impellers revolve within a cylinder or casing and are prevented from making contact with each other by timing gears mounted outside the cylinder.
Rotary, liquid-piston compressors
These are machines in which water or other liquids are used, usually in a single rotating element, to displace the air or gas handled.
Seals
Seals are devices used between rotating and stationary parts to separate, and minimize leakage between, areas of unequal pressures.
Single-Acting Compressors:
Machines in which compression takes place on one stroke per revolution in each compressing element.
Single-Stage Compressors:
Machines in which compression from initial to final pressure is complete in a single step or stage.
Standard Air:
Measured at 68 degrees F, 14.7 PSIA, and 36% relative humidity (.075lb/ft3 density). This agrees with the Compressed Air and Gas Institute (CAGI). The gas industries uses 60 degrees F, 14.7 PSIA, and dry as conditions for standard air (SCFM).
SCFM:
Standard Cubic Feet Per Minute. At "standard conditions" (see above) will one standard cubic foot of air actually occupy one cubic foot of volume. Another way to express one standard cubic foot of air is .075 of a pound of air. A standard cubic foot varies in volume as it deviates from standard conditions, but it always weighs .075 of a pound. Therefore, SCFM is a measure of weight, regardless of volume. (see corrections for altitude chart)
Temperature rise ratio
This is the ration of the computed isentropic temperature rise to the measured total temperature rise during compression. For a perfect gas, this is equal to the ration of the isentropic enthalpy rise to the actual enthalpy rise.
Torque
Torque is a torsional moment or couple. It usually refers to the driving couple of a machine or motor.
Turbine
A turbine is a prime mover in which a stream of fluid, such as water, steam or gas, provides the impulse to drive a bladed rotor.
Two-Stage Compressors
These are machines in which air or gas is compressed from initial pressure to an intermediate pressure in one or more cylinders or casing
Water-Cooled Compressors
These are machines cooled by water circulated through jackets surrounding the cylinders or casings.
Vacuum Pumps:
Machines for compressing air or other gases from an initial pressure that is below atmospheric pressure to a final pressuring that is near atmospheric
Vapor Filters
Are filters designed for the removal of vapors and odors, which still may be present, after the air has passed through the coalescing and/or particulate filters. The activated carbon element attracts residual vapors and binds them to the surface of the activated carbon grain molecules.
Volumetric Efficiency:
The ratio of the actual inlet capacity (ACFM) to the first stage displacement expressed as a percentage. Volumetric efficiency varies by clearance or blow-by, heating losses, valve losses, and specific gravity of gas being compressed.