Screw compressor glossary

While many of us have been around screw compressors for a long time, there may be some terminology whose meaning we are not sure of. This glossary attempts to describe the most common terms associated with screw compressors and compressed air systems. — A — Absolute micron rating. All (not just 98%) particles larger than the stated micron size that will be removed from the fluid be...


Key Concepts
  • Common terms

  • Acronyms

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— B —
— C —
— D —
— E —
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— O —
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— T —
— U —
— W —

While many of us have been around screw compressors for a long time, there may be some terminology whose meaning we are not sure of. This glossary attempts to describe the most common terms associated with screw compressors and compressed air systems.

— A —

Absolute micron rating. All (not just 98%) particles larger than the stated micron size that will be removed from the fluid being filtered.

Absorb. A method to trap liquids or gases by causing them to penetrate into the absorbent material.

ACFM. Actual cubic feet per minute

Activated alumina. An adsorption-type desiccant.

Actual capacity. Quantity of gas actually compressed and delivered to the discharge system at rated compressor speed and under rated conditions.

Adsorbent filter. A filter medium primarily intended to hold soluble and insoluble contaminants on its surface by molecular adhesion.

Aerosol. A suspension of fine solid or liquid particles in a gas.

Aftercooler. Heat exchanger for cooling air or gas discharge from a compressor. Designed to reduce the temperature and liquefy condensate vapors. Both air and water-cooled versions are available.

Air-cooled compressor. A compressor cooled by ambient air circulated around the casing.

Air dryer. A device for drying compressed air by means of condensation obtained by over-compression, cooling, absorption, adsorption, selective permeation or a combination of these methods.

Air receiver. A tank that serves to store compressed air for large demands in excess of compressor capacity.

Ambient. Undisturbed environmental surroundings, particularly air and temperature.

Anhydrous. Devoid of water.

Artificial demand. Running a compressed air system at a pressure higher than needed.

Automatic sequencer. A device that operates compressors in sequence according to a programmed schedule.

— B —

Bar. A unit of pressure equal to 14.5 psi or 0.99 atmospheres.

Beta ratio. Efficiency measurement of a filter expressed by the number of particles, of a given size, upstream of the filter, divided by the number of particles of that size downstream of the filter.

Breakdown maintenance. Maintenance performed after a machine has failed to return to an operating state.

Breathing air. Special compressed air treatment meeting OSHA Grade D air requirements.

— C —

Capacity. The full rated volume of air compressed and delivered at a certain set of conditions.

Capacity filtration. The amount of air a filter can handle, expressed in cfm.

Capacity gauge. A gauge that measures air flow as a percentage of capacity, used in rotary screw compressors as an estimator during modulation.

CFM. Cubic feet per minute.

Chatter. Abnormal, rapid, reciprocating movement of the disk on the seat of a pressure relief valve.

Cleanable. A filter element which, when loaded, can be restored to an acceptable percentage of its original dirt capacity.

Coalescing filter. A filter unit that combines three methods to filter out oil aerosols: direct interception , a sieving action; inertial impaction , collision with filter fibers; diffusion , particles travel in a spiral motion, presenting an effective frontal area capturing particles within the filter medium.

Condensate system. A device that removes enough oil and contaminants from condensate drains to allow the condensate to enter the sewage system.

Contaminant capacity. The weight of a contaminant that must be added to the influent to produce a given differential pressure across a filter at specified conditions. Used as an indication of relative service life.

Cut in/cut out pressure. The settings on a pressure switch used to load or unload an air compressor in a constant-speed application or start and stop a compressor in start/stop application. The cut out pressure or maximum pressure is the point at which no air is being delivered. The cut in pressure or minimum pressure is the pressure the system is allowed to fall to before air volume is required.

Cyclone. A type of separator for removing large particles from an air stream using gravitational and centrifugal forces.

— D —

Deliquescent. Melting and becoming a liquid by absorbing moisture.

Desiccant. An adsorption type material used in air dryers, usually activated alumina, silica gel, or molecular sieve.

Dew point. The temperature at which vapor in a space will start to condense if the air is cooled at constant pressure.

Drain valve. A device that removes surplus liquid from a compressed air system.

Dripleg. A pipe extending downward from the bottom of an airline to collect condensate.

Dual control. Load/unload control system that tries to maximize compressor efficiency by matching air delivery and air demand.

Duty cycle. Percentage of time a compressor can operate at full load over a thirty-minute period.

— E —

Element. The medium or material that does filtering or separating. May be paper, wire, mesh, cellulose, plastic, or a combination of these materials.

Environmental contaminant. All material present in and around a compressor system, such as dust, air moisture, chemicals, and heat.

— F —

FAD. Free air delivery. Air is at atmospheric conditions at the compressor site. Flow is measured at the discharge of the compressor, after the aftercooler and separator. Capacity and power consumption are corrected to ISO 1217 standard reference conditions.

Filter efficiency. The ability of a filter to remove specified contaminants under specified test conditions. Expressed as a percentage of the quantity of test contaminant.

FRL. Filter, regulator, and lubricator, usually combined into one unit.

Full load. Achieved when the air compressor is running at full speed with a fully open inlet and discharge while delivering maximum volume at rated pressure.

— H —

Heatless reactivated dryer. By expanding cold air to near atmospheric pressure inside a regeneration tower, the dryer picks up moisture from the saturated desiccant bed and purges it to atmosphere.

Heat reactivated dryer. Internal types have steam or electric heaters embedded in the desiccant bed. External types use a blower or cooler to reactivate desiccant.

— I —

ICFM. Inlet cubic feet per minute. Air volume flowing through the compressor inlet under rated conditions.

Inlet throttle. A compressor control mechanism designed to control output to meet plant demands.

Intercooling. The removal of heat from compressed air between compressor stages.

— K —

Kilowatt hour (kWh). A unit of work. The work done in one hour at the rate of 1000 watts.

Knock out. A term used to describe condensate flow rate, expressed in gpm.

— L —

Load factor. Ratio of average compressor load to the maximum rated compressor load over a given period of time.

Loaded. A filter element so full of contaminants it can no longer pass rated flow without excessive pressure differential.

Lubricator. A device designed to add lubricant into an air line.

— M —

MAWP. Maximum allowable working pressure.

Mayonnaise. The oily condensate discharged by lubricated compressors.

Membrane dryer. Water vapor and some compressed air permeate the membrane walls and vent to atmosphere.

Mesh size. The number of openings in a square inch of screen or sieve.

Micron. Micrometer or one millionth of a meter. Equals 0.000039 in.

Micron rating. A measurement applied to filters to indicate the suspended particle size that will be removed.

Modulating control. System that adapts to varying demand by throttling the compressor inlet proportionally to demand.

— N —

Nominal filter rating. Indicates the approximate particle size for which a majority will not pass. It generally means 85% of particles of this size will be retained.

Nominal micron rating. A term that means 98% of all particles larger than a stated micron size will be removed.

— O —

Oil-free compressor. A compressor that has no oil injected into the compression chamber for lubrication, cooling, or sealing.

— P —

Performance curve. A plot of expected operating characteristics such as, discharge pressure vs. inlet capacity or shaft horsepower vs. inlet capacity.

PPM. Parts per million.

Pressure dew point (PDP). Pressure dew point temperature. The temperature at which moisture begins to condense in a compressed air system.

PSI. Pounds per square inch.

PSIA. Pounds per square inch, absolute.

PSID. Pounds per square inch, differential.

PSIG. Pounds per square inch indicated by a gauge.

— R —

Refrigeration dryer. A device consisting of a refrigeration system and heat transfer system designed to remove moisture from compressed air.

Regeneration. The process of desiccants being regenerated by driving off the water in them.

— S —

SCFM. Standard cubic feet per minute. Flow of free air measured at some reference point and converted to a standard set of conditions.

Specific power. A measure of compressor efficiency, usually in the form of bhp/100 acfm.

Standard air. Air at 14.5 psia, 68 F, and 0% relative humidity.

Strainer. A device used to remove solids from an air stream.

— T —

Trim compressor. A compressor running efficiently at part load, allowing other operating compressors to run efficiently at full capacity.

— U —

Unload. The air compressor runs at full speed but no air is delivered because the inlet is closed.

Unloaded horsepower. The power consumed to overcome frictional losses when operating in an unloaded condition.

— W —

WOG. A pressure rating for valves. W = water, O = oil, G = gas.

Plant Engineering magazine extends its appreciation to Atlas Copco Compressors, Inc. and for their assistance in the preparation of this article. Atlas Copco provided the photographs and illustrations.

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