Absolute Pressure -
Total pressure measured from zero.
- 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
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).
- 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
Air Bearings - See
Aftercooler - A heat
exchanger used for cooling air discharged from a compressor.
Resulting condensate may be removed by a moisture separator
following the aftercooler.
- the temperature of the surrounding environment;
technically, the temperature of the air surrounding a power supply
or cooling medium; abbreviated [ABM]
- The measured ambient pressure for a specific location
- A device which operates compressors in sequence
according to a programmed schedule.
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 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
factor expressing the deviation of a gas from the laws of
thermodynamics. (See also Super compressibility)
- Compression in which no heat is transferred to or from
the gas during the compression process.
- Compression is which the temperature of the gas remains
- Compression in which the relationship between the
pressure and the volume is expressed by the equation PVn is a
Compression Ratio -
The ratio of the absolute discharge pressure to the absolute inlet
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.
- 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 liquefy a gas by pressure
Cubic Feet Per Minute (cfm)
- Volumetric air flow rate.
cfm, free Air - cfm
of air delivered to a certain point at a certain condition,
converted back to ambient conditions.
Actual cfm (acfm) -
Flow rate of air at a certain point at a certain condition at that
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, 68°F, and 0% relative
- 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.
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%. The temperature to which air must be cooled, at a
given pressure and water-vapor content, for it to reach saturation;
the temperature at which dew begins to form.
Demand - Flow of air
at specific conditions required at a point or by the overall
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
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 -
Air pressure produced at a particular point in the system under
- The temperature at the discharge flange of the
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
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
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:
- Ratio of theoretical power to power actually imparted to
the air or gas delivered by the compressor.
- Ratio of the theoretical work (as calculated on an
isothermal basis) to the actual work transferred to a gas during
- Ratio of power imparted to the air or gas to brake
- Ratio of the polytropic compression energy transferred
to the gas, to the actual energy transferred to the gas.
- 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 principle 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.
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
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.
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.
Head, Adiabatic -
The energy, in foot pounds, required to compress adiabatically to
deliver one pound of a given gas from one pressure level to
Head, Polytropic -
The energy, in foot pounds, required to compress polytropically to
deliver one pound of a given gas from one pressure level to
Horsepower, Brake -
Horsepower delivered to the output shaft of a motor or engine, or
the horsepower required at the compressor shaft to perform
- The horsepower calculated from compressor indicator
diagrams. The term applies only to displacement type
Horsepower, Theoretical or
Ideal - The horsepower required to isothermally compress
the air or gas delivered by the compressor at specified
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
Hysteresis - The
time lag in responding to a demand for air from a pressure
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.
- 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.
- See Compression, Isentropic.
- See Compression, Isothermal.
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.
- Control method that allows the compressor to run at
full-load or at no load while the driver remains at a constant
Modulating Control -
System which adapts to varying demand by throttling the compressor
inlet proportionally to the demand.
- 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
compressor - A dynamic compressor having two or more
impellers operating in series in a single casing.
- Compressors having two or more stages operating in
- The condition when the temperature of air leaving the
intercooler equals the air at the compressor intake.
Performance curve -
Usually a plot of discharge pressure versus inlet capacity and
shaft horsepower versus inlet capacity.
- 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.
- See Compression, Polytropic.
Polytropic head -
See Head, Polytropic.
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.
(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).
- 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.
- 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
Pressure, Intake -
The absolute total pressure at the inlet connection of a
Pressure Range -
Difference between minimum and maximum pressures for an air
compressor. Also called cut in-cut out or load-no load pressure
Pressure ratio - See
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
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
Receiver - A vessel
or tank used for storage of gas under pressure. In a large
compressed air system there may be primary and secondary
- 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.
Seals - Devices used
to separate and minimize leakage between areas of unequal
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
Specific Power - A
measure of air compressor efficiency, usually in the form of
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.
- 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
- 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.
- See Critical Temperature.
- The total temperature at the discharge connection of the
Temperature, Inlet -
The total temperature at the inlet connection of the
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.
- 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.
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 slightly
Valves - Devices
with passages for directing flow into alternate paths or to prevent
Volute - A
stationary, spiral shaped passage which converts velocity head to
pressure in a flowing stream of air or gas.
- Compressors cooled by water circulated through jackets
surrounding cylinders or casings and/or heat exchangers between and
SOURCE: Compressed Air & Gas