ABSOLUTE PRESSURE is the arithmetic sum of gauge and atmospheric pressures. It must be used in all calculations involving the basic gas laws.
ABSOLUTE TEMPERATURE is the temperature of a body referred to the absolute zero, at which point the volume of an ideal gas theoretically becomes zero. On the Fahrenheit scale this is minus 459.67°F; on the Celsius scale it is minus 273.15°C. Engineering values of minus 460°F and minus 273°C are used herein.

AFTERCOOLING involves cooling of gas in a heat exchanger following the completion of compression to (1) reduce the temperature, and (2) to liquefy condensable vapors.

ALTITUDE is the elevation above sea level.

BAROMETRIC PRESSURE is the absolute atmospheric pressure existing at the surface of the earth. It is the weight of a unit column of air above the point of measurement. It varies with altitude and, at any given location, with moisture content and weather.

BRAKE HORSEPOWER is the actual horsepower output to the drive motor.

CAPACITY of any compressor is the quantity of gas actually delivered when operating between specified inlet and discharge pressures. For ejectors, capacity is measured in lb/hr. For all other compressor types, capacity is a volume measured at the conditions of pressure, temperature, gas composition, and moisture content existing at the compressor inlet flange.

CLEARANCE in a reciprocating compressor cylinder is that volume contained in one end of the cylinder which is not swept by the movement of the piston. It includes space between piston and head at the end of the compression stroke, space under the valves, etc., and is expressed as a percentage of the piston displacement per stroke. Clearance may be different for the two ends of a double-acting cylinder. An average is generally used.

COMPRESSIBILITY is the factor of a gas or a gas mixture that causes it to differ in volume from that of a perfect gas when each is under the same pressure and temperature conditions. Occasionally it is called deviation. It must be determined experimentally.

SUPER-COMPRESSIBILITY is a term used with various meanings, most frequently the same as compressibility, although this is not assured. A current ASME Power Test Code uses it as a ratio of gas densities rather than volumes. Therefore it is 1/Z in this case. Super-compressibility should never be used unless its meaning is clarified completely. Compressibility is much to be preferred and is used herein.

COMPRESSION EFFICIENCY is the ratio of the theoretical work requirement (using a stated process) to the actual work required to be done on the gas for compression and delivery. Expressed as a percentage, compression efficiency accounts for leakage and fluid friction losses, and thermodynamic variations from the theoretical process.

COMPRESSION RATIO is the ratio of the absolute discharge to the absolute intake pressure. It usually applies to a single stage of compression, but may be applied to a complete multistage compressor as well.

DENSITY is the weight of a given volume of gas, usually expressed in lb/cu ft at specific temperature and pressure.

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

DISCHARGE PRESSURE is the total gas pressure (static plus velocity) at the discharge flange of the compressor. Velocity pressure usually is considered only with dynamic compressors.
Note: Pressure may be expressed as gauge or absolute pressures. psiG plus atmospheric pressure equals psiA. Note that psiG does not define a pressure unless the barometric pressure (atmospheric) is also stated.

DISCHARGE TEMPERATURE is the temperature existing at the discharge flange of the compressor.
Note: In a multistage compressor, the various stages will have different discharge pressures and temperatures.

DISPLACEMENT applies only to positive-displacement compressors. It is the net volume swept by the moving parts in a unit of time, usually one minute.

DRY-BULB TEMPERATURE is the ambient gas temperature.

DRY GAS is any gas or gas mixture that contains no water vapor and/or in which all of the constituents are substantially above their respective saturated vapor pressures at the existing temperature. (See Wet Gas).
Note: In commercial compressor work, a gas may be considered dry (even though it contains water vapor) if its dewpoint is low at the inlet condition (say minus 50° to minus 60°F).

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

ENERGY of a substance is its capacity, either latent or apparent, to exert a force through assistance, that is to do work.

INTERNAL ENERGY is that energy which a substance possesses because of the motion and configuration of its atoms, molecules, and subatomic particles.

KINETIC ENERGY is the energy a substance possesses by virtue of its velocity of motion. It enters into dynamic and ejector compressor calculations, but seldom into positive displacement problems.

POTENTIAL ENERGY is the energy a substance possesses because of its elevation above the earth (or above some other chosen datum plane).

ENTHALPY (Heat Content) is the sum of the internal and external energies.

ENTROPY is a measure of the unavailability of energy in a substance.

FIXED COMPRESSION RATIO is the design (built-in) compression ratio for a rotary unit having this feature.

GAS HORSEPOWER (See Horsepower)

GAUGE PRESSURE is pressure as determined by most instruments and gauges. Barometric pressure must be allowed for to obtain the true or absolute pressure.

GRAVITY (See Specific Gravity).

HEAT is energy transferred because of a temperature difference. There is no transfer of mass.

HORSEPOWER is a unit of work equal to 33,000 foot-pounds per minute.

HORSEPOWER, THEORETICAL is the work theoretically required to compress and deliver a given gas quantity in accordance with a specified process.

HORSEPOWER, INDICATED is that obtained by indicator card analysis of compression or expansion of a cylinder of a reciprocating compressor. It is the same as gas horsepower.

HORSEPOWER, GAS is the actual work required to compress and deliver a given gas quantity, including all thermodynamic, leakage and fluid friction losses. It does not include mechanical losses.

HORSEPOWER, PEAK is the maximum power required by a given compressor when operating at a (1) constant discharge pressure with variable intake pressure, or (2) constant intake pressure with variable discharge pressure.

HUMIDITY, in normal usage, has to do with moisture (water vapor) in the atmosphere. There are two engineering terms involved:

HUMIDITY, RELATIVE is the ratio of the actual partial vapor pressure in an air-vapor mixture to the saturated vapor pressure at the existing dry-bulb temperature mixture.

HUMIDITY, SPECIFIC is the ratio of the weight of water vapor in an air-vapor mixture to the weight of dry air. It is usually expressed as pounds of vapor per pound of dry air.

IDEAL GAS follows the perfect gas laws without deviation. Practically, there are no ideal gases, but it is the basis from which calculations are made and corrections applied.


INLET PRESSURE is the total pressure (static plus velocity) at the inlet flange of the compressor. Velocity pressure is usually considered only with dynamic compressors. (See note under Discharge Pressure).

INLET TEMPERATURE is the temperature at the inlet flange of the compressor.
Note: In a multistage compressor, the various stages may have different inlet temperatures.

INTERCOOLING involves the cooling of gas between stages of compression (1) to reduce the temperature, (2) to reduce the volume to be compressed in the succeeding stage, (3) to liquefy condensable vapors, and (4) to save power.

MACH NUMBER is the ratio of the actual velocity at a given point to the velocity of sound in the same gas at the conditions existing at this point. These are known as local conditions.

MECHANICAL EFFICIENCY is the ratio, expressed in percent, of the Thermodynamic Work Requirement in the cylinder to the actual shaft horsepower.

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

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

PERFECT GAS (See Ideal Gas).

PISTON DISPLACEMENT of a reciprocating compressor cylinder is the net volume displaced by the piston at rated machine speed, generally expressed in cfm. For single-acting cylinders it is the displacement of the compressing end only. For double-acting cylinders it is the total of both ends. For multistage compressors, the displacement of the first stage only is commonly stated as that of the entire machine.

POLYTROPIC HEAD is an expression used for dynamic compressors to denote the foot-pounds of work required per pound of gas.

PRESSURE is force per unit area. (See Absolute, Discharge, Gauge, Inlet, Saturated Vapor, Vapor).

A PROCESS occurs whenever the system undergoes either a change in state or an energy transfer at a steady state (See State).

A REVERSIBLE PROCESS is an ideal process that may be stopped and made to retrace its steps and restore to the systems or surroundings all work and heat previously removed. It is frictionless.

An IRREVERSIBLE PROCESS is one in which a portion of the original system energy is dissipated and cannot be returned to the system through its own operation. The system and/or surroundings cannot be returned to their original state.

An ADIABATIC PROCESS is one during which there is no change in temperature.

An ISENTROPIC PROCESS is one wherein the entropy remains constant.

An ISOTHERMAL PROCESS is one in which there is no change in temperature.

A POLYTROPIC PROCESS is one in which changes in gas characteristics during compression are considered.

PSYCHROMETRY deals with the properties of air-water vapor mixtures in the atmosphere.

RATIO OF SPECIFIC HEATS is the ratio of the specific heat at constant pressure to the specific heat constant volume. It may vary considerably with pressure and temperature.

SATURATION occurs when the vapor is at the dewpoint or saturation temperature corresponding to its partial pressure. A gas is never saturated with a vapor. The space occupied jointly by the gas and vapor may be saturated, however.

DEGREE OF SATURATION is the ratio of the weight of vapor existing in a given space to the weight that would be present if the space were saturated at the space temperature.

SATURATED AIR-VAPOR MIXTURE is one in which the space occupied by the mixture is saturated with water vapor at the mixture temperature.

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

SATURATION PRESSURE is another term for Saturated Vapor Pressure.

SATURATION TEMPERATURE is the temperature corresponding to a given saturated vapor pressure for a given vapor.

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

SLIP RPM is the speed required of a rotary compressor to maintain a given discharge pressure, supplying leakage only (zero actual output). The factor must be established by experiment.

SPECIFIC GRAVITY is the ratio of the density of a given gas to the density of dry air, both measured at the same specified conditions of temperature and pressure, usually 14.696 psiA and 60°F. It should also take into account any compressibility deviations from a perfect gas.

SPECIFIC HEAT (Heat Capacity) is the rate of change in enthalpy with temperature. It is commonly measured at constant pressure or at constant volume. The values are different and are known as cp and cv respectively.


SPECIFIC VOLUME is the volume of a given weight of gas, usually expressed as cu ft/lb at specific conditions.

STANDARD PRESSURE AND TEMPERATURE (SPT) as used on this web site is 14.696 psiA and 60°F unless specifically stated otherwise.

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

SUCTION PRESSURE is absolute static prevailing at the suction of the ejector.

SUPER-COMPRESSIBILITY (See Compressibility).

TEMPERATURE is the property of a substance that gauges the potential or driving force for the flow of heat. (See also Inlet, Discharge, and Absolute Temperature).


A THERMAL COMPRESSOR is an ejector used to compress waste or exhaust steam or any other gas through a moderate range of compression above atmospheric pressure.

VAPOR PRESSURE is the pressure exerted by a vapor confined within a given space. The vapor may be the sole occupant of the space, or may be associated with other gases.

VOLUME (See Specific Volume).

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

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

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

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