Compressed Air Glossary

A 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

Adiabatic Process (see process)

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 0F: on the Celsius scale it is minus 273.15 0C.

Aftercooling involves cooling of gas in a heat exchanger flowing the completion of compression to reduce the temperature and to liquefy condensable vapors.

Altitude is elevation of compressor 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.

Basic Slip (see Slip)

Brake Horsepower (see Horsepower)

Breaking Pressure is the pressure of either the motive fluid or of the ejector gas discharge, which causes an ejector to become unstable. Note that there are two different breaking pressures – one of the motive fluid and one of the ejector discharge.

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 tow ends of a double-acting cylinder. An average generally is used.

Compressibility is that property 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 experimentally determined.

Compressibility Factor is the ratio of the actual volume of the gas to the volume determined according to the perfect gas law.

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.

Compression Efficiency is the ratio of the theoretical work required to be done on the gas for compression and delivery. Expressed as percentage, compression efficiency accounts for leakage and fluid friction losses and thermodynamic variations from the theoretical process.

Compression Ration is the ratio of the absolute discharge to the absolute intake pressure. T usually applies to a single stage of compression but may be applied to a complete multistage compressor as well.

Corrosive Gas is one that attacks normal materials of construction. Water vapor when mixed with most gases does not make the corrosive within the sense of above definition. In other gases CO 2 for example, it makes them corrosive.

Critical Temperature is the highest temperature at which a gas can be liquefied.

Critical Pressure is the saturation pressure at the critical temperature. It is the highest vapor pressure that the liquid can exert.

Dead-End Pressure is the suction pressure attained by an ejector or positive displacement vacuum pump at zero capacity with the suction absolutely blanked off.

Degrees Kelvin – an absolute temperature scale.

Degree Rankine – an absolute temperature scale.

Degree of Saturation (see saturation)

Design (Built in) Compression Ratio in a rotary compressor refers to the compression ratio that has been attained when the fixed discharge port is uncovered. A helical-lobe compressor (and most other rotary units) can gave an operating ratio somewhat higher or lower than the design ratio, with little change in efficiency.

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

Discharge pressure is the total pressure (static plus velocity) at the discharge flange of the compressor. Velocity pressure usually is considered only with dynamic compressors.

Discharge temperature is the temperature existing at the discharge flange of the compressor.

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 og the constituents are substantially above their respective saturated vapor pressures at the existing temperature.

Dry Unit is one in which there is no liquid injection and/or liquid circulation foe evaporative cooling or sealing.

Energy of a substance is its capacity either latent of apparent to exert a force through a distance, that is to do work.

External energy is the energy, which represented by the product of pressure and volume. It may be regarded as the energy a substance possesses by virtue of the space it occupies.

Internal energy is the 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 because of its motion or velocity. 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.

Enthalpy is the sum of the internal and external energies

Entrainment Ratios are used with ejector to convert weight of gas and /or water vapor handled to or from equivalent air. They are based on extensive tests.

Entropy is a measure of unavailability of energy in a substance.

Equivalent air is an ejector term – the calculated lb/hr of air at 70 0 F 14.969 psiA and containing normal atmospheric moisture that is equivalent to but not necessarily equal to, the weight rate of the gas handled by the ejector at suction conditions.

Equivalent gas is a mixture that is considered to have similar properties to a pure gas, although these properties must be calculated from the properties of components.

Evaporative Cooling takes place when liquid is injected to the gas stream before or during compression. As compression takes place, the gas temperature rises and some or all of the liquid is evaporated, the latent heat of liquid vaporization being removed from the gas, lowering its temperature.

Fixed Compression Ratio is the design compression ration 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 of absolute pressure.

Gravity (see Specific Gravity)

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

Heat Capacity (see specific heat)

Horsepower is a unit of work equal to 33,00 foot-pounds per minute.

Theoretical Horsepower is the work theoretically required to compress and deliver a given gas quantity in accordance with a specified process.

Indicated Horsepower is that obtained by indicator card analysis of compression or expansion in a cylinder of reciprocating compressor. It is the same as gas horsepower.

Gas Horsepower 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.

Brake Horsepower is the total power input required including gas horsepower plus all friction losses.

Peak Horsepower is the maximum power required by a given compressor when operating at a constant discharge pressure with variable intake pressure or constant intake pressure with variable discharge pressure.

Humidity in normal use has to do with moisture in the atmosphere. There are two engineering terms involved.

Relative Humidity is the ratio of the actual partial vapor pressure in an air-vapor mixture to the saturated vapor pressure at the existing dry-bulb mixture temperature, usually expressed in percent.

Specific Humidity is the ratio of weight of water vapor in an air-vapor mixture to the weight of fry air. It is usually expressed as pounds of vapor per pound of air.

An Ideal Gas follows the perfect gas laws without deviation. Practically there is no such thing but it is the basis from which calculations are made and corrections applied.

Indicated Horsepower (see horsepower)

Inert Gas is the one that does not enter into known chemical reactions. To the engineer the term usually means a gas that does not supply any of the needs of combustion.

Inlet Pressure is the total pressure (static plus velocity) at the inlet flange of the compressors.

Inlet temperature is the temperature at the inlet flange of the compressor.

Intercooling involves the cooling of gas between stages of compression to reduce the temperature, to reduce the volume to be compressed in the succeeding stage, to liquefy condensable vapors, and to save power.

Internal Energy (see Energy)

Isentropic Process (see Process)

Isothermal Process (see Process)

Kinetic Energy (see Energy)

Mach Number is the ratio of the actual gas 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.

Maximum Discharge Pressure, as applied to ejectors is the maximum absolute static recovery pressure against which the ejector will operate with stability.

Mechanical Efficiency is the ratio, expressed in percent of the Indicated Horsepower to the actual shaft horsepower.

Mole is a weight of gas in pounds numerically equal to the molecular weight of the gas or to the pseude molecular weight of a gas mixture.

Molar Heat Capacity or molar specific heat is the heat in Btu’s required to raise the temperature of one mole of gas 1 0 F.

Noncodensables are those constituents in the suction gas that cannot be condensed to a liquid with the cooling medium available.

Noncorrosive gas is one that does not attack normal materials of constructions.

Normal Air is the term used for average atmospheric air at sea level in a temperature zone where it contains some moisture. It is defined in the ASME Test Code for Displacement Compressor as being at 14.696 psiA, 68 0 F, 36% RH and weighing 0.075 lb/cu ft. The k value is 1.395.

Partial Pressure of a constituent in the mixture is the absolute pressure exerted by that portion of the mixture.

Peak Horsepower (see Horsepower)

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 as cfm. For single –acting cylinders it is the displacement of the compressing end only. For multistage compressors, the displacement of the first stage only is common stated as that of the entire machine.

Polytropic Process (see Process)

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

Potential Energy (see Energy)

Power (see Horsepower)

Pre-cooler is a heat exchanger located immediately preceding an ejector to condense and remove a portion of the vapor in the mixture and thus reduce the total lb/hr to be handled.

Pressure is force per unit area.

A Process occurs whenever the system undergoes either a change in state or an energy transfer at a steady state.

Reversible process is an ideal process that may be stopped and made to retrace its steps and restore to the system or surroundings cannot be returned to their original state.

Adiabatic Process is one during which there is no heat added to or removed from the system.

Isentropic Process is one wherein the entropy remains constant.

Polytropic Process is one in which changes in gas characteristics during compression are considered.

Pseudo Critical Pressure (see Critical Pressure)

Psychrometry has to do with the properties of air-water vapor mixtures in the atmosphere.

Pumping is the reversal of flow within a dynamic compressor that takes place when the capacity being handled is reduced to a point where insufficient pressure is being generated to maintain flow.

Ratio of Specific Heats is the ration of the specific heat at constant pressure to the specific heat constant volume. It may vary considerably with pressure and temperature.

Recovery Pressure is that pressure of either motive fluid or discharge at which an ejector returns to stable operation following a period of unstable operation due to having previously reached the breaking pressure. There are two recovery pressures, one for motive fluid and one for the discharge pressure.

Reduced Pressure is the ratio in absolute units of the actual gas temperature to the absolute critical pressure.

Relative Humidity (see Humidity)

Saturation occurs when the vapor is at the dew point 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 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 experimentally determined 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). It is an experience factor.

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 0F> It should also take into account any compressibility deviation 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 a constant volume. The values are different and are known as C p and C v respectively.

Specific Humidity (see Humidity)

Specific Volume is the volume of a given weight of gas usually expressed as cu ft/lb at SPT conditions.

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 the absolute static pressure prevailing at the suction of the ejector.

Super Compressibility (see Compressibility)

Superheated Air-Vapor Mixture is one in which the space occupied by the mixture is above the saturation temperature at the mixture temperature.

Surge (see Pumping)

Temperature is the property of a substance that gauges the potential or driving force for the flow of heat.

Theoretical Horsepower (see Horsepower)

A Thermal Compressor is an ejector used to compress waster or exhaust system or any other gas through a moderate range of compression above atmospheric pressure.

Vapor Pressure is the pressure exerted y 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 to the piston displacement.

Volumetric Efficiency is the ratio in percent of the actual delivered capacity to the piston displacement.

Wet Bulb Temperature is used in psychrometry and temperature recorded by a thermometer whose bulb has been covered with a wetted wick and whirled on a sling psychrometer. Taken with the dry bulb, 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 are at its saturated vapor pressure.

A Wet Helical-Lobe Unit is one, which handles a small constant flow of liquid with the gas, utilizes evaporative cooling or circulates a liquid for sealing and/or cooling. The last may or may not be evaporative cooling.