ABS: A non-toxic plastic sometimes used as a piping material in compressed air installations

Absolute Pressure: A pressure at absolute zero (i.e. a perfect vacuum.) Absolute Pressure equals the sum of the Atmospheric Pressure and Gauge Pressure

Absolute Temperature: The temperature measured from Absolute Zero, using the Kelvin Scale

Activated Carbon: A form of carbon characterised by high adsorptive capacity for gases, vapours or solids

Adsorption: The natural action of a gas, vapour or liquid being attracted to and held on the surface of a solid

After Cooler: A heat exchanger for the reduction of temperature of air after compression

After Cooling: The removal of heat from the air after compression is complete

Air Compressor: A machine which compresses air to a pressure higher than its inlet pressure

Air Receiver: A tank in which compressed air is stored, also called a pressure vessel

Ambient Temperature: The temperature of the surrounding environment in which the equipment is working in

Atmospheric Pressure: The pressure exerted by the atmosphere

Ball Valve: A device that shuts off flow by means of a hollow spherical component (ball)

Bourdon Tube Gauge: An instrument that senses and records pressure. The pressure energy detected is converted into mechanical movement, i.e. moving a pointer on a dial

Carbon Dioxide: A colourless, odourless, incombustible gas. It is a major greenhouse gas, contributing to climate change (global warming)

Carbon Monoxide: A colourless, odourless, incombustible, toxic gas

Celsius: A temperature scale on which the freezing point of water is 0 degrees Celsius and the boiling point of water is 100 degrees Celsius (at one atmosphere of pressure. Celsius is the term preferred for Centigrade)

CE Mark: A mark that denotes compliance with European directives

CFM: A commonly used abbreviation for ‘cubic feet per minute’, a figure applied to the output of an air compressor

Chemical Energy: The energy in chemical bonds holding the atoms of a compound together. Food, wood, batteries, and fossil fuels possess chemical energy

Coalesce: This is the process of bringing together fine liquid particles or mists together to form larger particles as liquid droplets

Competent Person: A legal phrase to identify a person who performs the activities required by the Pressure Systems Safety Regulations for compressed air systems. The competent person may be required to produce schemes of examination or to do examinations

Compressor Capacity: The actual volume rate of flow compressed and delivered at the standard discharge point, at stated inlet conditions, usually expressed in terms of Free Air Delivered

Condensate: The water formed in a compressed air system from water vapour due to a decrease in air temperature and or an increase in pressure. Often contaminated with oil and solid particles

Constant: A quantity that does not change its value, no matter what happens to other related variables, usually represented by the letter K

Coolant: A fluid (liquid or gas) used to remove heat from compressed air

Demand: The amount of compressed air required by tools and equipment

Desiccant: A chemical possessing a strong affinity for water

Dew Point: The temperature at which air is fully saturated with water vapour at the prevailing pressure

Directly Proportional: The process by which two quantities decrease or increase with each other

Discharge Outlet (port or valve): A hole or valve through which gas or air leaves the compressor

Displacement: The volume displaced by the compression element of the first stage per unit of time (e.g. litres per second, cubic feet per minute)

Distribution System: A network of pipes installed from one point to another in a factory that delivers compressed air to a point of use

Drains: Devices that collect condensate from a compressed air system. These are often fitted to air receivers, filters, dryers or piping

Dryer: A device that reduces the water vapour content of the compressed air

Efficiency: Something that is produced with minimum effort or energy

Filter: A device that removes contaminants from compressed air

Filtration: A physical process that selectively removes gases, vapours, liquids or solids from compressed air by providing a barrier to such particles

Fahrenheit: A temperature scale on which the freezing point of water is 32 degrees F and the boiling point of water is 212 degree F (at one atmosphere of pressure)

Flow Controller: A device which varies the amount of air flowing through it

Fossil Fuels: Carbon-rich fuel formed from the remains of ancient animals and plants. Coal, oil, and natural gas are all fossil fuels

Foul Sewer: The main drains used for the disposal of waste water of domestic or industrial origin, that comes under the responsibility of the local water authority

Free Air: Air that is under atmospheric conditions at the inlet point which is unaffected by the compressor

Free Air Delivered (FAD): The free air delivered by a compressor is the actual volume which it will compress and deliver at the stated pressure, at the intake temperature and pressure. FAD is expressed in cubic feet per minute or litres per second.

Full Load: The state in which a compressor is producing air and operating at full capacity

Galvanized Steel: This is steel coated with zinc to reduce the effects of corrosion

Gauge Pressure: The pressure that is shown or indicated on a gauge or instrument

Generator: A device or machine that converts mechanical energy into electrical energy

General Gas Law: This is a law which states how pressure, volume and temperature interact

Hazard: This is anything that can cause harm e.g. in the workplace: lifting, machining, working from ladders

Heat Recovery: A process where heat which is generated by a compressor, that would otherwise be wasted, is saved and re-used

High Pressure: This is system pressure which is over 40 bar

Higher Efficiency Motor (HEM): A Motor that uses up to 3% less electricity than a standard motor, by virtue of an improved design

Humidity: The degree of water vapour in the atmosphere

Inlet Port (or valve): A hole (or valve) through which air enters the compressor

Inter Cooling: The removal of heat from the air between stages in multi-stage compression

Inversely Proportional: The process by which one quantity decreases when another increases

Kinetic Energy: The energy possessed by a moving object

Low Pressure: This is pressure generally considered to be between atmospheric pressure and 20 bar

Lubricant: A fluid (generally an oil) used to overcome friction between surfaces that come into continuous moving contact

Normal: This is a term represented by the prefix ‘n’ denoting a standard condition of temperature and humidity

Non-Renewable Energy Resource: This is an energy resource that is either replenished very slowly or is not replenished at all by natural processes

Off Load: This refers to a compressor that is switched on i.e. running but the compressor is not producing air

Oil Injected (lubricated): An air compressor in which oil is injected into the chamber during the compression process to limit the temperature rise in the chamber

On Load: This refers to a compressor that is producing air, either at part load or full load

Over Pressurisation: This is when the pressure of a compressed air system is exceeded

Packaged Air Compressor: A self-contained unit consisting of a compressor and prime mover together with accessories for its safe operation

Parameter: A physical quantity that can be measured, e.g. volume, pressure, etc.

Part Load: This refers to a compressor when it is producing air, but not at full capacity

Pattern of Use: This describes the way in which tools and equipment are used

Piping: A pipe or system of pipes used for the conveyance of compressed air

Piston: A mechanical device which has a plunging or thrusting motion

Pneumatics: This refers to the science systems driven by compressed air

Point of Use: The point at which the compressed air performs its useful work, i.e. powering air tools or equipment

Positive Displacement: The action of a compressor type that traps and compresses a measured volume of air between its fixed and moving parts that causes the air’s pressure to rise and reduce its volume and delivery to a system

Potential Energy: This is stored energy that can be released or harnessed to do work (chemical energy is a form of potential energy)

Power: The rate at which energy is transferred or converted per unit of time. It is quoted in Watts

Pressure Band: The difference between the specified upper and lower limit of pressure in which compressors are required to operate

Pressure Drop: The drop in pressure between any two specified points in a system

Pressure Ratio: (total). The ratio between the absolute discharge pressure and the absolute inlet pressure

Pressure Relief (Safety) Valve: A device to limit the maximum system pressure by allowing the escape of the compressed air into the atmosphere when a dangerous limit has been reached. This device is also referred to as a safety valve or a pressure relief valve (PRV)

Pressure Reducing Valve (Pressure Regulator): A device which reduces the incoming pressure to a lower level and maintains it irrespective of inlet pressure and outlet flow rate changes

Pressure Vessel: This is an air storage tank or air receiver

Primary Energy Source: An energy source available directly from the environment e.g. coal, oil, natural gas, sun, wind, geothermal energy and hydropower

Prime Mover: A self-propelled machine, e.g. an electric motor or internal combustion engine, used to drive machinery

Purity: This is something that is free from contaminants. Air purity is the degree of cleanliness required for an application (often referred to as “air quality”)

Reciprocating: A machine in which a piston moves alternately backward and forward in a cylinder

Refrigerant: A fluid used for cooling that, when changing from liquid to gas, takes heat from its surroundings (in this case compressed air)

Refrigerant Dryer: A device which recycles a fluid (a refrigerant) between liquid and gas stages producing a cooling effect on the compressed air

Renewable Energy: An energy resource that can be replenished within a finite time

Ring Main: An air main that begins and ends at the compressor so that every usage point has two possible routes of supply

Risk: The chance, great or small, that someone will be harmed by a hazard

Roots Blower: A positive displacement rotary machine used for compressed air applications at low pressures up to 1.5 bar

Rotary: Compressors where the air is compressed by circular motion

Run-On Timer: A time related control that switches off the prime mover when the compressor has been on off-load for a specified period of time

Secondary Energy Source: These are energy sources derived from a primary energy source. The most common secondary energy source is electricity

Sensor: An electronic device used to measure a physical characteristic such as pressure or temperature and converts the information into an electronic signal

Service Pipe: A pipe that delivers gas, oil, water, steam or air to and around a site

Shut Off Valve: A device incorporating a means to close off supply of air flow

Signal Switch: A switch (mechanical or electronic) which responds to the signal from a sensor

Single Line Main: Piping arrangement whereby various branches are taken from a single pipe such that each usage point has one route of supply

Standard: This indicates that the figures quoted are derived at a standard temperature and humidity, the ‘s’ prefix is used, e.g. scfm – standard cubic feet per minute

System Pressure: This is the normal operating pressure, often referred to as “working pressure”

Tertiary Energy Source: An energy source derived from secondary energy sources

Thermodynamics: This is the science of the conversion of heat to other forms of energy

Utility: A domestic or industrial service company, e.g. gas, oil, water, electricity company

Variable Speed Drive (VSD): This is an electronic device which when fitted to a motor can vary the speed of a compressor and vary its air output

Water Vapour: This is the gaseous state of water