Compressed Air Terminology
and Definitions
Compressed Air Terminology
and DefinitionsA | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
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
Absolute Temperature:
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.
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.
Air-cooled compressors:
These are machines cooled by atmospheric air circulated around the cylinders
or casings.
Altitude:
The elevation above sea level.
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.
Barometric Pressure:
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.
Base plate:
A metallic structure on which a compressor or other machine is mounted.
Boolean logic:
The logic of binary systems, such as control systems in which all operations
my be reduced to on/off, open closed, or some similar dichotomous basis.
Booster Compressors:
Machines designed for compressing air or other gases from an initial pressure,
which is above atmospheric pressure, to a higher pressure.
Brake Horsepower:
The actual horsepower output to the drive motor.
C
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.
Clearance:
When referring to 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.
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 the compressor
Coalescing Filter:
Removes small particles, dirt, and oil from the compressed air.
Compressibility:
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.
Compression, adiabatic. This type of compression is effected when no heat is transferred to or from the gas during the compression process
Compression, isothermal. isothermal compression is a compression in which the temperature of the gas remains constant. For perfect gases, it is represented by the equation PV is a constant, if the process is reversible,
Compression, polytropic. Compression in which the relationship between the pre~ sum and the volume is expressed by the equation Pv is a constant.
Compression Efficiency:
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:
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.
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.
It is sometimes defined as the highest temperature at which it is possible to
liquify a gas by pressure alone.
D
Density:
The weight of a given volume of gas, usually expressed in lb/cu ft at
specific temperature and pressure.
Desiccant Air Dryers:
A device that will remove moisture for dew-point requirements to -40 or a -100
degrees F.
Dewpoint:
The temperature at which the vapour 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.
Diaphragm:
A stationary element between the stages of a multistage 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 routing:
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
Discharge Pressure:
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:
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:
Displacement of a compressor is the volume swept out per unit of time; it is
usually expressed in cubic feet per minute
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.
Dry-Bulb Temperature:
The ambient gas temperature.
Dry Gas:
Any gas or gas mixture that contains no water vapour and/or in which all of
the constituents are substantially above their respective saturated vapour
pressures at the existing temperature. (See Wet Gas).
Note: In commercial compressor
work, a gas may be considered dry (even though it contains water vapour) if its
dewpoint is low at the inlet condition (say minus 50° to minus 60°F).
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.
E
Efficiency:
Any reference to efficiency of a dynamic type compressor must be accompanied
by a qualifying statement which identifies the efficiency under consideration,
as in the following definitions.
Efficiency, compression. Ratio of calculated isentropic work requirement to actual thermodynamic work requirement within the cylinder, the Inner as determined from the cylinder indicator card.
Efficiency, isothermal. Ratio of the work calculated on an isothermal basis to the actual work transferred to the gas during compression.
Efficiency, mechanical. Ratio of thermodynamic work requirement in the cylinder (a shown by die indicator card) to actual brake horsepower requirement.
Efficiency, polytropic. Ratio of the polytropic compression energy transferred to the gas no the actual energy transferred to the gas.
Efficiency, volumetric. Ration of actual capacity to displacement, stated as a percentage.
Enthalpy:
(Heat Content) The sum of the internal and external energies.
Entropy:
A measure of the unavailability of energy in a substance.
F
Filters:
Are devices for separating and removing dust and dirt from air before it enters
a compressor.
Flange connection:
The flange connection (inlet or discharge) is a means of connecting the
casing to the inlet or discharge piping by means of bolted rims (flanges).
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.
G
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.
H
Head, adiabatic. The energy in
four pounds required to compress adiabatically and 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 and to transfer one pound of a given gas from one pressure level to another.
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 vapour in an air vapour mixture per pound
of dry air
Humidity, relative:
The relative humidity of a gas (or air) vapour mixture is the ration of the
partial pressure of the vapour to the vapour saturation pressure at the dry bulb
temperature of the mixture.
I
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).
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.
Impeller:
The part of the totaling element of a dynamic compressor that impacts energy
to
the flowing medium by means of centrifugal force. It consists of a number
of blades mounted so as to rotate with the shaft.
Inlet Pressure:
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:
The temperature at the inlet flange of the compressor.
Note: In a multistage compressor,
the various stages may have different inlet temperatures.
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.
Intercooling degree of. The difference in air or gas temperatures between the inlet of the compressor and the outlet of the intercooler.
Intercooling, perfect. Perfect intercooling exists when the temperature of the air leaving the intercoolers equals the temperature of the air at the compressor intake.
Isentropic cornpression:
An adiabatic compression with an increase in entropy; a reversible‑adiabatic
compression.
L
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.
M
Mechanical Efficiency:
The ratio, expressed in percent, of the Thermodynamic Work Requirement in
the cylinder to the actual shaft horsepower.
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.
Normal Air:
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.
O
Oil/Water Separator:
A device that separates the remaining compressor oil from condensate.
P
Particulate Filter:
Removes small particles and dirt from the compressed air.
Perfect Intercooling:
Is obtained when the gas is cooled to first stage inlet temperature
following each stage of compression.
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.
Polytropic Head:
An expression used for dynamic compressors to denote the foot-pounds of work
required per pound of gas.
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.
R
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.
S
Saturation:
Occurs when the vapour is at the dewpoint or saturation temperature
corresponding to its partial pressure. A gas is never saturated with a vapour.
The space occupied jointly by the gas and vapour may be saturated, however.
Saturation (degree of):
The ratio of the weight of vapour existing in a given space to the weight
that would be present if the space were saturated at the space temperature.
Saturated Air-Vapour Mixture:
Is one in which the space occupied by the mixture is saturated with water
vapour at the mixture temperature.
Saturated Vapour Pressure:
The pressure existing at a given temperature in a closed vessel containing a
liquid and the vapour from that liquid after equilibrium conditions have been
reached. It is dependent only on temperature and must be determined
experimentally.
Saturation Pressure:
Another term for Saturated Vapour Pressure.
Saturation Temperature:
The temperature corresponding to a given saturated vapour pressure for a
given vapour.
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)
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.
Slip:
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:
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
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) 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 Humidity: (See Humidity).
Specific Volume:
The volume of a given weight of gas, usually expressed as cu ft/lb at
specific conditions.
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).
Standard Pressure and
Temperature (SPT):
Generally is 14.696 psiA and 60°F unless specifically stated otherwise.
State:
(of a system or part thereof) Its condition at an instant of time as
described or measured by its properties.
Suction Pressure:
Absolute static prevailing at the suction of the ejector.
Super-Compressibility:
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.
T
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.
V
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
Vapour Filters:
Are filters designed for the removal of vapours and odours, which still may be
present, after the air has passed through the coalescing and/or particulate
filters. The activated carbon element attracts residual vapours and binds them to
the surface of the activated carbon grain molecules.
Vapour Pressure:
The pressure exerted by a vapour confined within a given space. The vapour
may be the sole occupant of the space, or may be associated with other gases.
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.
W
Water-Cooled Compressors:
These are machines cooled by water circulated through jackets surrounding the
cylinders or casings.
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:
Any gas or gas mixture in which one or more of the constituents is at its
saturated vapour pressure. The constituent at saturation pressure may or may not
be water vapour.
Work:
Energy in transition and is defined in units of Force times Distance. Work
cannot be done unless there is movement.
| Trident Compressed Air Ltd. 4009 County Road 46 Maidstone (Windsor) Ontario N0R 1K0 |
Phone: (519) 737-9905 |