Absolute Pressure – The measure of pressure compared to the absolute zero pressure of an empty space—e.g., a vacuum.

Absolute Temperature – Air or gas temperature referenced from absolute zero. Known as the Rankine temperature (Fahrenheit plus 459.6) and the Kelvin temperature (Centigrade plus 273).

Absorption – A process in which a desiccant liquefies by drawing in condensed moisture.

Actual Capacity – Also known as Free Air Delivered (FAD), this is the amount of gas actually compressed and delivered (at rated speeds and conditions) to a discharge system.

Adiabatic Head – Conveyed in foot pounds (ft. lbs.), this is the amount of energy needed to deliver a pound of a gas to different pressure levels adiabatically (without heat transfer).

Adsorption – A process in which a highly porous desiccant (capable of regeneration) attracts moisture from compressed air, and removes it.

Air Receivers – Tanks or vessels utilized for the storage of pressurized gases and damping of discharge line pulsations. Larger compressed air systems can have both primary and secondary receivers.

Air Bearings (or Gas Bearings) – Hydrostatic or hydrodynamic machine elements that are capable of carrying loads due to high pressurization. Bearings permit a degree of motion with air (or another gas) as lubricant.

Aftercooler – A heat exchanging device that cools discharged air after compression. Following the cooling process, any resulting condensate can be removed using a moisture separator.

Atmospheric Pressure – The measured surrounding pressure of a particular location and its altitude.

Automatic Sequencer – A control device programmed to operate air compressors in a pre-specified order.

Brake Horsepower (bhp) – The horsepower needed at a compressor’s input shaft for proper performance.

Capacity – Often expressed in cubic feet per minute (cfm), this is the quantity of air flow that is delivered through particular conditions.

Capacity Gauge – This tool is used for measuring the amount of air flow within rotary screw compressors. Results are displayed as a percentage of capacity.

Check Valve – A device used to permit the unidirectional flow of air within a compressor.

Clearance – Typically expressed as a percentage, this is the displacement volume per stroke subtracted by the maximum cylinder volume in a compressor.

Clearance Pocket – A supplementary volume which can be opened to temporarily increase clearance, resulting in the reduction of a compressor’s volumetric efficiency.

Compressibility – A factor that is used to convey how a gas deviates from thermodynamic laws.

Adiabatic Compression – A type of compression with no heat transfer during the entire process.

Isentropic Compression – A type of adiabatic compression that is reversible—it has no increase in entropy.

Isothermal Compression – A type of compression with a constant gas temperature.

Polytropic Compression – A type of compression this is expressed by the equation PVn, where the connection between pressure and volume is a constant.

Compression Ratio – Expresses the comparison between two absolute pressures: discharge pressure and inlet pressure.

Constant Speed Control – A system used in the constant operation of a compressor. By varying the load, compressors are able to parallel air supply and air demand.

Critical Pressure – The saturation pressure at the time when saturation temperature reaches its critical point.

Critical Temperature – The highest possible temperature that liquid and vapor states can exist in; this is often defined as the highest temperature needed to liquefy gas solely using pressure.

Cubic Feet Per Minute (cfm) – The volumetric measure of the rate of air flow within a compressor.

Free Air cfm – Air’s flow rate at a specified point and condition, which is then converted to surrounding conditions.

Actual cfm – Air’s flow rate at a specified point and condition.

Inlet cfm – Air that is flowing through the inlet filter or valve of a compressor (under rated conditions).

Standard cfm – The flow rate of free air that is measured and then converted to a uniform set of reference conditions.

Cut-In Pressure – Minimum discharge pressure that, when reached, switches the compressor from unload to load operation.

Cut-Out Pressure – Maximum discharge pressure that, when reached, switches the compressor from load to unload operation.

Cycle – The steps performed by an unloading compressor. The steps are fully loaded, modulating, unloaded, and idle.

Cycle Time – Total amount of time needed for an air compressor to finish an entire cycle.

Deliquescent – The process of melting and changing into a liquid through moisture absorption.

Desiccant – A substance with a highly porous surface area that can attract water vapor, and then remove it from ambient air.

Dew Point – The temperature point at which moisture starts to condense in the air, if the air continues to be cooled at a single pressure.

Demand – Air flow needed at a specified point or generally needed by a facility.

Diaphragm – A static element of a multi-stage compressor that is found between the compressor’s stages. Can include guide vanes.

Diaphragm Cooling – This is a heat removal method that circulates coolant throughout the passages of a diaphragm.

Diffuser – A static passage in which velocity pressure is converted into static pressure by an impeller.

Discharge Pressure – The total air pressure produced at a specified point within a compressed air system.

Displacement – Expressed in cfm, this is the volume removed by the piston or rotor.

Droop – During demand for air, this is the pressure drop at a pressure regulator’s outlet.

Dynamic Type Compressors – Machines that compress air or gas by using energy transferred from rotating vanes or impellers to the flowing medium (air).

Efficiency – The relation between the useful amount of work performed by a machine and the energy expended by it during a given process.

Compression Efficiency – Compares theoretical power to actual power that is imparted and delivered to air or gas via a compressor.

Isothermal Efficiency – Compares theoretical work to actual work that is transferred to a gas during the compression process.

Mechanical Efficiency – Compares imparted power (from air or gas) to brake horsepower.

Polytropic Efficiency – Compares the amount of compressed polytropic energy (transferred to gas) with actual transferred energy.

Volumetric Efficiency – Compares the actual capacity of a compressor to its piston displacement.

Exhauster – A compressor that has lower inlet pressure than its atmospheric pressure.

Expanders – Engines or turbines in which an expanding gas does work, and then undergoes a temperature drop.

Filters – Devices used to remove particles, moisture, and lubricants from surrounding air.

Flange Connection – Used to join a compressor’s inlet or discharge to specific piping with bolted rims.

Fluidics – Blanket term for instruments and controls (without moving parts) that depend on either low rate air flow or low-pressure gas as their operating medium.

Free Air – Refers to atmospheric air that isn’t affected by a compressor.

Full-Load – Refers to the full speed operation of an air compressor, which provides the maximum amount of air flow through a wide-open inlet and discharge.

Gas – A basic phase of matter that usually refers to other gases, and not air.

Gauge Pressure – Normally expressed in pounds per square inch (psig), this is the pressure most instruments are used to determine.

Guide Vane – A static, sometimes adjustable element used to direct a flowing medium as it gets closer to an impeller’s inlet.

Hysteresis – The lag time of a pressure regulator in response to a demand for air within the system.

Impeller – The multi-bladed part of a dynamic compressor’s rotating element that provides energy to the flowing medium via centrifugal force.

Indicated Power – The amount of power in a compressor specified by compressor indicator diagrams.

Indicated Horsepower – The amount of horsepower in a displacement compressor specified by compressor indicator diagrams.

Indicator Card – A pressure volume (PV) diagram produced by an indicator; it corresponds to a compressor or an engine cylinder.

Inducer – An impeller’s curved inlet section.

Inlet Pressure – The actual pressure at a compressor’s inlet flange.

Intake Pressure – The total pressure at the connection point of a compressor’s inlet.

Intercooling – Process in which heat is removed from gas or air between the stages of compression.

Degree of Intercooling – The Difference in temperatures (of air or gas) between a compressor’s inlet and an intercooler’s outlet.

Perfect Intercooling – The condition when air or gas temperatures exiting an intercooler matches air or gas temperatures entering a compressor.

Leak – An unintentional loss of compressed air to surrounding conditions of a compressor.

Liquid Piston Compressor – A type of rotary compressor with a vaned rotor revolving in an elliptical stator. Spaces between these two components (the stator and rotor) are sealed with a liquid ring that rotates with the compressor’s impeller.

Load Factor – Relation between average compressor load and maximum compressor load over a specified time period.

Load/Unload Control – A control method allowing compressors to operate at either no load or full-load while the driver’s speed remains constant.

Modulating Control – System that throttles the compressor inlet proportionally to match varying demand.

Multi-Casing Compressor – Two or more compressors (with separate casings) that form a single unit. The compressors are driven by a single driver.

Multi-Stage Axial Compressor – A dynamic compressor consisting of multiple rows of rotating elements. The elements in this compressor operate in series with a single rotor and casing.

Multi-Stage Centrifugal Compressor – A dynamic compressor consisting of multiple impellers that operates in series in a single casing.

Performance Curve – Plots the correlation between compressor operating characteristics, such as shaft horsepower versus inlet capacity, or discharge pressure versus inlet capacity.

Piston Displacement – The air volume removed by a piston. Piston displacement of the first stage is overall piston displacement for multistage compressors.

Pneumatic Tools – Special tools operated by compressed air.

Polytropic Head – Conveyed in foot pounds (ft. lbs.), this is the amount of energy needed to compress air polytropically (reversible) to deliver a pound of a gas from one pressure to another.

Positive Displacement Compressor – A type of compressor in which successive volumes are kept in a closed space and the mechanically reduced space, which causes compression.

Pressure – The measure of force per unit area, conveyed in pounds per square inch.

Pressure Dew Point – The temperature that water starts to condense out of air for a given system pressure.

Pressure Drop – A pressure loss in compressed air systems caused by restriction or friction.

Rated Capacity – At a specified point, this is the volume rate of flow at rated pressures.

Rated Pressure – The measure of the operating pressure of air compressors.

Reciprocating Compressor – A type of compressor with a piston as its compressing element.

Relative Humidity – A comparison between a vapor’s partial pressure to its saturation pressure at dry bulb temperature.

Reynolds Number (Re)– Named after physicist Osborne Reynolds in 1883, this is a dimensionless air flow parameter used to anticipate various air flow conditions.

Rotor – A compressors rotating element that consists of one or more impellers and a shaft in dynamic compressors.

Seals – These devices are used to isolate and alleviate leakage in areas of varying pressures.

Shaft – Energy is transferred to this part from the prime mover via mounted elements, then to compressed gas or air.

Sole Plate – The compressor and driver are mounted to this pad, which is often composed of metal and inserted into concrete.

Specific Gravity – Comparison between dry air and specific weight of gas or air (at equal pressures) to temperature.

Specific Humidity – In an air vapor mixture, this is water vapor’s weight per lb. of dry air.

Specific Power – Measure for determining the efficiency of air compressors, expressed as brake horsepower (bhp) /100 actual cubic feet per minute (acfm).

Specific Weight – Per unit volume, this is the weight of a gas or air.

Speed – The number of revolutions per minute (rpm) of the rotor or compressor drive shaft.

Standard Air – Used in ISO standards, this refers to air at 14.5 pounds per square inch absolute (psia), 68°F (20°C), and dry (0% relative humidity).

Start/Stop Control – A system for coordinating air supply with demand by continuously starting and stopping the compressor.

Static Pressure – A pressure measurement taken from a flowing stream that is unaffected by the stream’s velocity.

Static Temperature – A moving gas stream’s actual temperature indicated by a moving thermometer moving at an equal velocity.

Surge – An occurrence in which reduced flow rate leads to flow reversal and unsteady operating in compressors.

Surge Limit – In dynamic compressors, this is the capacity in which any surge below this point leads to poor compressor operation.

Temperature Rise Ratio – Comparison between the rise in isentropic temperature and the total temperature rise during the compression process. For a perfect gas, this equates to the ratio of rise in isentropic enthalpy to actual rise in enthalpy.

Theoretical (Ideal) Horsepower – The amount of horsepower needed to compress gas or air isothermally at certain conditions.

Torque – Usually referring to a machine or rotor’s driving couple, this is a torsional couple or movement.

Total Package Input Power – A compressor’s complete power input, which encompasses belt losses, cooling fan motors, drive motors, etc.

Total Pressure – Measured by an impact tube, this is the pressure produced by stopping moving streams of gases or liquids.

Unit Type Compressors – Compressors with 30 brake horsepower (bhp) or lower, combined with other operational components.

Unload – Operation in which air is not delivered because of a closed or modified inlet.

Vacuum Pumps – A variety of compressors operating with an intake below atmospheric pressure, and discharging to that pressure or higher.

Valves – Devices used to direct or prevent air flow within a system.

Velocity Pressure – The static pressure in a gas or air stream subtracted by its total pressure.

Volute – A spiral-shaped, static passage used for converting velocity head to pressure within a flowing gas or air stream.

Water-Cooled Compressor – A type of compressor that is cooled by water circulation via jackets close to cylinders, or casings between and after compression stages.