Air-Pak · Resources

Compressed air definitions.

Plain-language definitions for the terms you’ll see across compressed air, drying, and filtration, from absorb to volumetric efficiency.

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88 terms
Dryers, desiccants, pressure, flow, and compressor basics.

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88 terms


A

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Absorb
A method of trapping liquids or gases by causing them to penetrate into the absorbent material.
Activated alumina
An adsorption-type desiccant.
Adsorb
A method that causes a liquid or gas to condense only on the surface of an adsorbing material.
Air dryer
A device for drying compressed air by condensation (through over-compression or cooling), absorption, adsorption, or a combination of these methods.
Air flow
The motion of air relative to a body within it.
Ancillary equipment
Components subordinate to the compressor.
Atmospheric dew point
The temperature at which water vapor begins to condense at atmospheric pressure. The same as dew point, but related to atmospheric air only.

B

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Bag house
A dust-collection chamber containing numerous permeable fabric filters through which exhaust gases pass. Fine particulates carried in the exhaust stream are collected in the filters for later treatment or disposal.
Bar
A unit of pressure equal to approximately 0.987 atmospheres or 14.504 psi.
Breathing air
Specialty compressed air treatment meeting OSHA Grade D breathing air requirements.
Bubble
In spray painting, air trapped in a paint film, caused by poor atomization during spraying.

C

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Capacity
The full rated volume of gas flow a compressor compresses and delivers at set conditions.
CFM
Acronym for cubic feet per minute. An airflow measurement of volume.
Compressed air
Air under pressure greater than that of the atmosphere.
Compressor
A machine that compresses air or gases.
Condensate
The liquid that separates from a vapor during condensation.
Contaminant
Foreign matter carried in the air, gas, or fluid that must be filtered out. Includes airborne dirt, metal particles from wear of the compressor’s moving parts, and rust from metal pipelines.
Control valve
A valve that controls flow in air lines.
Cubic feet per minute (CFM)
An airflow measurement of volume.
Cut-in / cut-out pressure
The settings on a pressure switch used to load or unload the compressor on a constant-speed application, or to start or stop it on a start/stop application. The cut-out pressure, also called the maximum pressure, is the point at which no air is delivered. The cut-in pressure, also called the minimum pressure, is the pressure the system is allowed to fall to before more air volume is required.
Cycle
A single complete operation made up of progressive phases, starting and ending at the neutral position.
Cycle time
The time it takes a compressor to complete one cycle.
Cylinder
The piston chamber in a compressor or actuator.

D

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Delta P
The pressure drop through a component; the difference in pressure between two points.
Demand
The flow of air, under specific conditions, required at a particular point.
Desiccant
An adsorption-type material used in compressed air dryers. Industry standards are activated alumina, silica gel, and molecular sieves.
Desorption
The opposite of absorption or adsorption. In filtration, the downstream release of particles previously retained by the filter.
Dew point
The temperature at which the vapor in a space (at a given pressure) starts to condense, forming dew. For a gas mixture, it is the temperature at which the constituent with the highest boiling point starts to condense.

F

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Filter
A device that removes solid contaminants, such as dirt or metal particles, from a liquid or gas (including air), or that separates one liquid from another or a liquid from a gas. The term covers the complete unit: housing, filter element, and internal bypass.
Filter separator
A filtering unit that separates solids and liquid droplets from a gas such as air. Widely used to remove oil from gas or air.
Flow
The volume of a substance passing a point per unit of time, for example cubic feet per minute or gallons per hour.
Flow control valve
A valve that controls the flow of air passing through it. Often used in retardation or timing circuits, and especially to regulate piston speed in cylinders.
Flow meter
An instrument that measures the air flow of a compressor, in CFM.
Flow rate
The quantity of a substance moved per unit of time, such as liters or gallons per minute, or cubic meters or cubic feet per second. Air flows are usually expressed in CFM, SCFM, ACFM, or ICFM.
Full load
Reached when the compressor runs at full RPM with a fully open inlet and discharge, delivering maximum volume at rated pressure.

G

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Gauge
An instrument for measuring, testing, or registering.
Gauge pressure
Pressure as read by most instruments and gauges.

H

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Heatless dryer
A heatless reactivated dryer. Dry compressed air is expanded to near atmospheric pressure inside the regenerating tower, where it picks up moisture from the saturated desiccant bed and is then purged to atmosphere.
Horsepower (HP)
A unit of power equal to 33,000 foot-pounds per minute, 550 foot-pounds per second, or 746 watts.
Humidity
The moisture content of air.
Humidity, relative
The ratio of the partial pressure of the vapor in a gas (or air) vapor mixture to the vapor saturation pressure at the mixture’s dry-bulb temperature.
Humidity, specific
The weight of water vapor in an air-vapor mixture per pound of dry air.

I

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Inlet pressure
The total pressure (static plus velocity) at the compressor’s inlet flange.
Inlet temperature
The temperature at the compressor’s inlet flange.

L

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Leak (air)
A crack or hole that accidentally admits or releases a gas.

M

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Maximum operating pressure
The highest operating pressure a system or component is designed to withstand.
Mayonnaise
The oily condensate discharged by lubricated air compressors, named for its appearance. Normally, oily condensate should be just cloudy, like a little milk in a bucket of water. When a lubricated compressor has a problem, the condensate turns thick and sticky, almost identical to its namesake.
Membrane dryer
A dryer that reduces dew point by passing compressed air through a bundle of hollow membrane fibers. Water vapor and a portion of the compressed air permeate the membrane walls and vent to atmosphere.
Micron
A micrometer, or one millionth of a meter, sometimes shown in filtration as the Greek letter µ (mu). A micron is 0.000039 in. Contaminant particles are measured by micron size and count.
Micron rating
A measurement applied to filters or filter media indicating the particle size above which suspended solids will be removed.
MLT
Micro Logic Timer, designed specifically for SFD dryer systems with two-minute (120-second) cycles, controlling the charge and regeneration drying cycles.
Molecular sieves
A solid adsorbent used for drying compressed air.
Molecular theory
All matter consists of molecules in constant motion, held together by molecular forces. In a solid, the molecules are closely packed in a pattern where those forces are very strong, which gives the solid its consistency and form; molecular motion is mostly oscillation around points of equilibrium.
In a liquid, the molecules are about as close as in a solid, but they aren’t arranged in a lattice and the cohesive forces are weaker. The molecules move more freely relative to one another, so the liquid conforms to the walls of its container and its free surface settles horizontally under gravity.
In a gas, the molecules are farther apart and move freely because the molecular forces are weaker. A gas therefore expands through space and mixes with other gases present. Because the total volume of the molecules is very small relative to the volume of the gas, a gas can be compressed into a small part of its original volume.

N

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Normal air
The term for average atmospheric air at sea level in a temperate zone, where it contains some moisture. The ASME Test Code for Displacement Compressors defines it as 14.696 psia, 68 °F, and 36% RH, weighing 0.075 lb/cu ft, with a k value of 1.395.

O

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Oil-free compressor
A positive displacement air compressor with no oil injected into the compression chamber for lubrication, cooling, or sealing.
Operating pressure
The gauge pressure at which a pressure vessel is maintained in normal operation.
Orifice
An opening such as a hole or vent through which air can pass, or a restricted opening placed in a pipeline to control or measure flow.

P

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Piston displacement
The net volume actually displaced by the compressor piston at rated machine speed, generally expressed in CFM. For multistage compressors, the first-stage displacement is commonly stated as that of the entire machine.
Pneumatic
Of, relating to, or using air; moved by air pressure; filled with compressed air.
Pneumatic tools
Tools that operate by air pressure.
Pneumatics
The engineering science of gas pressure and flow.
Point of use
A single outlet, or a limited number of outlets, in a building used to connect tools or equipment to the air system.
Positive displacement compressors
Compressors that confine successive volumes of air or gas within a closed space and compress them. They may be reciprocating or rotary. In short, they trap air and squeeze it to the desired pressure.
Pounds per square inch
A unit of pressure, abbreviated PSI.
Pressure
Force per unit area, usually expressed in pounds per square inch (PSI) or bar.
Pressure dew point
The temperature at which moisture begins to condense in a compressed air system.
PSI
Pounds per square inch.
PSIG
Pounds per square inch, gauge: pressure as indicated by a pressure gauge.
Purge air
The portion of dry, full-line-pressure compressed air taken from the drying tower of a dual-tower desiccant dryer, expanded to very low pressure, and passed across the wet desiccant in the regenerating tower to strip out its moisture. In an external blower-type dryer, purge air is atmospheric air compressed by a blower and heated by an external heater to strip moisture from the wet desiccant bed.

Q

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Quick coupler
A coupling device with a spring-loaded shutoff valve and a positive locking mechanism, used to connect tools, hoses, and other accessories. Also known as a quick disconnect.

S

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Safety relief valve
An automatic pressure-relieving device, actuated by static pressure upstream of the device, that opens in proportion to the increase in pressure over its opening pressure.
Saturated air-vapor mixture
A mixture whose space is saturated with water vapor at the mixture temperature.
Saturated vapor pressure
The pressure in a closed vessel containing a liquid and its vapor, at a given temperature, once equilibrium is reached. It depends only on temperature and must be determined experimentally.
Saturation
Occurs when a vapor is at the dew point, or saturation temperature, corresponding to its partial pressure. A gas is never saturated with a vapor; however, the space occupied jointly by the gas and vapor may be.
SCFM
Standard cubic feet per minute: flow of free air measured at a reference point and converted to a standard set of reference conditions. Standards vary, so care is needed. In the United States, the usual standard is 14.696 psia and 60 °F. Some chemical engineers use one atmosphere and 0 °C, usually stating the reference point, and Europeans normally use one atmosphere and 0 °C. Because the standard isn’t the same everywhere, the reference pressure and temperature should always be specified.
Screw compressor
A positive displacement rotary compressor.
SCUBA
Self-Contained Underwater Breathing Apparatus.
Silica gel
A desiccant most commonly used in heat-regenerative dryers.
Single-stage compressors
Machines in which air or gas is compressed in each cylinder or casing from initial intake pressure to final discharge pressure.
Standard air
Air at 68 °F, 14.70 psia, and 36% relative humidity (0.0750 lb/cu ft density), per ASME. In the gas industry, the temperature of standard air is usually given as 60 °F unless specifically stated otherwise.

T

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Temperature, discharge
The temperature at the compressor’s discharge port.
Temperature, inlet
The temperature at the compressor’s inlet flange.
Temperature, intake
The total temperature at the compressor’s intake flange.
Two-stage compressor
A machine in which air or gas is compressed from initial pressure to an intermediate pressure in one or more cylinders or casings, then to final discharge pressure in a second stage.

U

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Unload
The compressor keeps running, usually at full RPM, but delivers no air because the inlet is closed off or modified so no inlet air is trapped.
Unloaded horsepower
The power consumed to overcome frictional losses when operating unloaded.

V

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Venturi
A tube with a narrow throat that increases the velocity and lowers the pressure of the fluid passing through it, creating a partial vacuum just past the constriction. That suction is commonly used to introduce a liquid (such as a regenerant) or a gas (such as air) into a flowing stream.
Volumetric efficiency
The ratio, as a percentage, of actual delivered capacity (measured at inlet temperature, pressure, and gas composition) to piston displacement.

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