Reference

Compressed Air Glossary

Clear, practical definitions for the terminology you will encounter when specifying, operating, and maintaining compressed air systems. From aftercoolers to Written Schemes of Examination.

A

Aftercooler
A heat exchanger fitted after the compression stage that cools compressed air and causes moisture to condense out. Reduces the temperature of discharged air before it enters the receiver or distribution system.
Air audit
A systematic assessment of a compressed air system covering supply, demand, distribution, and energy usage. Used to identify leaks, inefficiencies, and opportunities to reduce operating costs.
Air receiver
A pressure vessel that stores compressed air downstream of the compressor. Provides a buffer to meet short-term demand peaks, stabilises system pressure, and allows moisture to settle out before reaching the point of use.
Air treatment
The process of conditioning compressed air after compression. Includes filtration, drying, and oil removal to meet the required air quality standard for the application.

B

Bar
A unit of pressure commonly used in compressed air systems. 1 bar equals approximately 14.5 psi. Most industrial compressors operate between 7 and 13 bar.
Bearing
A mechanical component that supports the rotating elements of a compressor (rotors, shafts, motors). Bearing wear is one of the most common causes of compressor failure and is monitored during planned maintenance.
Blow-off valve
A safety valve that releases compressed air to atmosphere when system pressure exceeds a set limit. Protects the compressor and downstream equipment from overpressure.
Breathing air (BS EN 12021)
Compressed air that meets the requirements of BS EN 12021 for human respiration. Used in applications such as spray painting, diving, and medical environments. Requires specific filtration, drying, and regular air quality testing to verify compliance.

C

CFM (cubic feet per minute)
A volumetric flow rate measurement for compressed air, commonly used in the UK alongside litres per second. Indicates how much air a compressor can deliver at a given pressure. Always check whether the quoted figure is FAD or displaced.
Competent Person
Under PSSR 2000, the individual or organisation responsible for drawing up and certifying the Written Scheme of Examination for a pressure system. Must have sufficient knowledge and experience of the specific type of equipment being examined.
Condensate
Water that forms when compressed air is cooled below its dew point. Contains oil, particulates, and other contaminants. Must be collected and disposed of correctly, usually through an oil/water separator, to meet environmental regulations.
Condensate drain
An automatic or manual valve fitted to receivers, dryers, and filters to remove accumulated condensate from the compressed air system. Timed drains, float drains, and zero-loss drains are the most common types.
Compressor oil
Specially formulated lubricant used in oil-injected rotary screw and rotary vane compressors. Provides sealing, cooling, and lubrication. Must be changed at manufacturer-specified intervals to prevent premature wear and overheating.

D

Dew point
The temperature at which moisture in compressed air begins to condense into liquid water. A lower dew point means drier air. Pressure dew point (PDP) is measured at system pressure, while atmospheric dew point is measured at ambient pressure.
Drive belt
A belt connecting the motor to the compressor element in belt-driven machines. Requires regular tension checks and replacement as part of planned maintenance. Worn belts reduce efficiency and can cause unexpected downtime.
Dryer (desiccant)
An air dryer that uses an adsorbent material (typically activated alumina) to remove moisture from compressed air. Achieves very low dew points (typically minus 40 degrees C PDP) for applications requiring extremely dry air.
Dryer (refrigerant)
An air dryer that cools compressed air using a refrigeration circuit, causing moisture to condense and drain away. Typically achieves a pressure dew point of 3 degrees C. The most common dryer type in general industrial applications.

F

F Gas
Fluorinated greenhouse gases used as refrigerants in refrigerant dryers and some cooling systems. Subject to F Gas Regulations requiring leak checks, record keeping, and handling by certified technicians.
FAD (free air delivery)
The actual volume of compressed air delivered by a compressor, measured at inlet conditions (ambient temperature and atmospheric pressure). FAD is the honest measure of compressor output and should be used when comparing machines.
Filtration
The removal of contaminants (particulates, oil aerosol, oil vapour) from compressed air using inline filters. Filter grades range from general purpose to sterile, selected based on the air quality class required by the application.
Fixed speed
A compressor that runs at a constant motor speed regardless of air demand. Efficient when demand is constant and close to full output. Less efficient than VSD at partial loads because it must load and unload or run on idle.

I

Inlet valve
The valve that controls airflow into the compressor element. On load/unload machines, the inlet valve opens under load and closes when the compressor unloads. Inlet valve failure is a common service issue.
Intercooler
A heat exchanger between compression stages in a multi-stage compressor. Cools the air between stages to improve compression efficiency and reduce the final discharge temperature.
ISO 8573-1
The international standard for compressed air quality. Defines purity classes for particles, water, and oil content. Specifying the correct ISO 8573-1 class ensures the air quality matches the application requirements.

K

kW
Kilowatt. The standard unit of power used to rate compressor motors. A compressor's kW rating indicates its electrical input power. Energy costs are the largest component of total compressor ownership cost, typically 70% to 80% over the machine's lifetime.

L

L/s (litres per second)
A volumetric flow rate measurement for compressed air, commonly used alongside CFM in the UK. Indicates the volume of air delivered per second. 1 L/s equals approximately 2.12 CFM.
Leak detection
The process of identifying and locating compressed air leaks in a distribution system. Typically performed using ultrasonic detection equipment. A well-maintained system should have a leak rate below 10%. Many sites lose 20% to 30% of their compressed air to leaks.

M

Minimum pressure valve
A valve in the compressor that maintains a minimum internal pressure (typically around 4 bar) to ensure proper oil circulation and separator performance before air is released to the system.
Motor
The electric motor that drives the compressor element. Most industrial compressors use three-phase induction motors rated from 2.2 kW to 250 kW and above. Motor efficiency class (IE3, IE4, IE5) directly affects energy costs.

O

Oil carryover
The amount of compressor oil that passes through the oil separator and enters the compressed air supply. Measured in parts per million (ppm). Excessive oil carryover indicates a failing separator element or other service issue.
Oil separator
A component inside an oil-injected compressor that separates lubricating oil from the compressed air before it leaves the machine. The separator element is a consumable item replaced during major services.
Oil-free
A compressor design where no oil contacts the compressed air during compression. Used in applications where even trace oil contamination is unacceptable, such as food, pharmaceutical, and electronics manufacturing. Oil-free compressors typically cost more to purchase and maintain.

P

Planned preventative maintenance (PPM)
A scheduled maintenance programme carried out at defined intervals to keep compressors and air treatment equipment in reliable operating condition. Includes oil changes, filter replacements, belt checks, and system inspections. Reduces the risk of unplanned breakdowns.
Pressure drop
The reduction in air pressure between the compressor outlet and the point of use. Caused by friction in pipework, fittings, filters, dryers, and other components. Every 1 bar of unnecessary pressure drop increases energy costs by approximately 7%.
Pressure vessel
Any vessel designed to contain compressed air or gas above atmospheric pressure. Includes air receivers, oil separators, and dryer vessels. Subject to PSSR 2000 and must be included in the Written Scheme of Examination.
PSSR (Pressure Systems Safety Regulations)
UK legislation (PSSR 2000) that places duties on owners and users of compressed air systems to establish safe operating limits, maintain equipment, and arrange periodic examination by a Competent Person. The Written Scheme of Examination (WSE) is a key requirement of PSSR.

R

Receiver
See Air receiver.
Rotary screw
The most common compressor type in industrial applications. Uses two intermeshing helical rotors to compress air continuously. Available in oil-injected and oil-free variants. Typically ranges from 2.2 kW to 500 kW and above.
Rotary vane
A compressor type that uses a rotor with sliding vanes inside a cylindrical housing. Known for quiet operation, long service life, and tolerance to dusty environments. Hydrovane is the best-known rotary vane manufacturer.

S

Separator element
The consumable filter inside an oil-injected compressor that separates oil from compressed air. Also known as the oil separator or air/oil separator. Replaced during major services, typically every 4,000 to 8,000 hours.
Service kit
A packaged set of consumable parts (oil filter, air filter, separator element, and sometimes oil) supplied for a scheduled service interval. Ensures all critical components are replaced together.
Specific power (kW/m³/min)
A measure of compressor energy efficiency expressed as the electrical power consumed per unit of air delivered. Lower specific power means better efficiency. The key metric for comparing compressors of similar output.

V

VSD (variable speed drive)
A motor controller that adjusts the compressor motor speed to match air demand. Reduces energy consumption at partial loads by slowing the motor rather than loading and unloading. Most effective where demand varies significantly throughout the day.

W

Written Scheme of Examination (WSE)
A document required under PSSR 2000 that specifies what parts of a pressure system must be examined, the nature of the examination, and the intervals between examinations. Must be drawn up or certified by a Competent Person. Airmech coordinates WSE compliance as part of our PSSR service.

Need Advice on Your Compressed Air System?

45 years of practical experience. We speak plain English, not jargon.

Talk to Us Call 02476 345 658