Air compressor terminology guide
November 25, 2023
As you conduct your air compressor research, it is crucial to understand the basic air compressor terminology involved. These terms form the core of air compressor technology and are of vital importance when you buy or wholesale air compressors from the air compressor manufacturers. Accurate terminology prevents costly ordering mistakes, speeds up communication with manufacturers and suppliers, and helps buyers correctly match equipment to their application. A glossary also shortens training time for new technicians, improves accuracy in technical documentation and contracts, and supports faster troubleshooting since teams can pinpoint issues using precise, shared vocabulary instead of guesswork. It also helps clarify cost-related terms like Life Cycle Cost, which accounts for the full ownership expense of running a compressor—covering the initial investment, energy use, maintenance, and downtime—rather than just the price tag on the unit, so you can judge true value over the life of the machine.
By reading the BISON Air compressor terminology guide, becoming proficient in these terms will enable you to thrive in the air compressor business and provide the highest quality service to your customers.
Common air compressor terminology Glossary
What is Absolute pressure?
The measurement of pressure relative to the absolute zero pressure of a space — e.g., a vacuum. Absolute pressure combines atmospheric pressure and gauge pressure to represent the true pressure of compressed air relative to a perfect vacuum.
What is absolute temperature?
The temperature of air or gas is referred from absolute zero and is also known as Rankine temperature (Fahrenheit plus 459.6) and Kelvin temperature (Celsius plus 273).
What is absorption process?
An action that causes a desiccant to liquefy by attracting condensed moisture.
What is actual capacity (FAD)?
This is the amount of compressed gas delivered (at rated speed and conditions) to the discharge system.
What is adiabatic head?
Expressed in foot-pounds (ft. lbs.), this is the amount of energy required to move one pound of gas to different pressure levels adiabatically (without heat transfer).
What is adsorption?
A process in which a porous desiccant (regenerable) attracts moisture from the compressed air and removes it.
What are air receivers?
Tanks or vessels that store pressurized gas and dampen discharge pulsations. Pairing a smaller rotary screw compressor with an air receiver tank cost-effectively meets peak demand spikes without oversizing. Larger compressed air systems may have both primary and secondary receivers.
What are Air (gas) bearings?
Hydrostatic or hydrodynamic machine elements can carry loads due to high pressure. Bearings allow a degree of movement with air (or other gas) acting as a lubricant.
What is aftercooler?
A heat exchange device that cools the exhaust air after compression. After cooling, any resulting condensate can be removed using a moisture separator.
What is Automatic sequencer?
A control device or tool programmed to operate air compressors in a predetermined sequence.
What is brake horsepower (bhp)?
The horsepower required at the input shaft of the compressor for proper performance.
What is capacity gauge?
This tool is used to measure the amount of airflow inside rotary screw compressors. Results are usually displayed as a percentage of capacity.
What is check valve?
A check valve is a device used to allow a one-way flow of air inside the compressor. It permits airflow in one direction while preventing backflow or reverse flow in the opposite direction.
What is compression ratio?
Expresses the ratio between inlet absolute pressure and discharge absolute pressure. The critical pressure ratio defines the point at which gas flow reaches sonic velocity, capping maximum flow rate regardless of further pressure increase.
What does “bar” mean on an air compressor?
Bar is a metric unit of pressure equal to 100,000 pascals, roughly matching atmospheric pressure at sea level (about 14.5 psi). It appears on compressor gauges and spec sheets, especially outside the United States, to indicate tank or discharge pressure. When comparing equipment internationally, converting bar to psi ensures accurate comparisons between compressor models and specifications.
What is CFM ?
CFM stands for Cubic Feet per Minute. It is a unit of measurement used to quantify the volumetric flow rate of air or gas, indicating how much air passes through a specific point within one minute. CFM is commonly used to assess air compressor capacity and performance. Air delivery is measured via the low pressure orifice test method, recognized by ASME and ANSI standards.
What is Free Air CFM?
The airflow rate at a specified point and condition is then converted to ambient conditions.
What is Actual CFM?
Air flow rate at a specified point and onditions, using standardized testing and certification methods to ensure consistent, comparable results.
What is Inlet CFM ?
The air that flows through the inlet filter or valve of the compressor (under rated conditions).
What is Standard CFM?
Free airflow measured and converted to uniform reference conditions (68°F, 0% humidity, 14.5 psia). ACFM measures airflow at the specific measurement point; SCFM normalizes to standardized conditions for consistent comparison across compressors.
What is Cut-in pressure?
The minimum discharge pressure that, when reached, switches the compressor from unload to load operation.
What is Cutout pressure?
The maximum discharge pressure that, when reached, switches the compressor to unload.
What is Duty Cycle?
The air compressor duty cycle refers to the length of time the machine can continue to operate within a specified period of time, usually expressed as a percentage.
This directly affects running time and cooling time. Different compressor types have greatly different duty cycles—a key factor for long-term efficient operation and service life. Operating beyond the specified cycle causes overheating and early wear. A 50% duty cycle unit must rest half the time to avoid overheating, unlike 100% continuous-duty units. Secondary specs like voltage, phase, tank size, and noise level also vary and should be checked against duty cycle rating.
What is Desiccant?
A material with a very porous surface that can draw in water vapour from the surrounding air and then remove it.
What is Dew point?
The temperature at which moisture begins to condense if air cools at constant pressure. Water content in compressed air is quantified by dew point or ppm; inadequate control causes corrosion and downstream equipment damage.
What is Diaphragm?
A static element of a multistage compressor is found between the compressor stages. May add guide vanes.
What is Diaphragm cooling?
A heat removal method that circulates coolant through the diaphragm.
Displacement
Expressed in cfm or cubic centimeters, this is the volume displaced by the piston or rotor.
Compression efficiency
Compares the theoretical power to the power delivered to the air or gas by the compressor.
Isothermal efficiency
Compares the theoretical work to the actual work transferred to the gas during compression.
Mechanical efficiency
Compares power delivered (from air or gas) to brake horsepower.
Polytropic efficiency
Compares polytropic energy transferred to gas versus actual transferable energy. The polytropic process follows PV^n=constant, where exponent n varies with heat transfer during compression, distinguishing it from ideal isothermal or adiabatic models.
Volumetric efficiency
Compares the compressor’s actual capacity to its piston displacement.
What is Exhauster?
A compressor whose inlet pressure is lower than its atmospheric pressure.
What are Expanders?
Engines or turbines in which the expanding gas works and then undergoes a temperature drop.
What are Air compressor Filters?
Air compressor filter is a device used to remove moisture, particles and lubricants from the surrounding air. ISO 8573 is the international standard for compressed air quality classification. Medical air requires strict purity standards—oil-free, low-contaminant compressed air for hospital and dental applications.
Free air
Refers to atmospheric air that is not affected by the compressor.
Full load
Refers to the full-speed operation of an air compressor, providing the maximum airflow through a wide-open inlet and discharge.
Gauge pressure
It’s usually expressed in pounds per square inch (psig). This is the pressure most instruments or tools are used to determine.
Inlet pressure
It’s the actual pressure at the inlet flange of the compressor.
Intake pressure
The total pressure at the inlet connection point of the compressor.
Intercooling
The process in which heat is removed from the gas or air between compression stages.
Load factor
The relationship between the average compressor load and the maximum compressor load during a specified period.
Load/unload control
A control method that allows compressors to operate at no load or full load while the driver speed remains constant. Bypass regulation diverts excess air around the compressor to control pressure, avoiding cycling losses. A variable frequency drive (VFD) adjusts motor speed to match real-time air demand rather than running at fixed speed, improving energy efficiency.
What is Piston displacement?
The volume of air displaced by the piston. The first-stage piston displacement is the overall piston displacement for multistage compressors.
What is Pneumatic tools?
Special tools powered by compressed air. Most air tools require around 90 PSI, but heavy-duty applications may demand 100-150 PSI or higher.
What is Polytropic head?
Expressed in foot-pounds (ft. lbs.), this is the energy required to compress air polytropically (reversibly) to move one pound of gas from one pressure to another. The universal gas constant used in compressor thermodynamic equations is defined as R = 8.314 J/mol·K.
Pressure drop
It means pressure loss in a compressed air system due to restriction or friction. Pressure loss in a compressed air system due to restriction or friction. Equivalent pipe length accounts for fittings and valves by expressing them as additional straight-pipe length for more accurate pressure drop calculations. Reynolds’ number determines whether airflow in piping is laminar or turbulent.
Relative humidity
The comparison between the partial pressure of a vapor and its saturation pressure at the dry-bulb temperature.
Rotor
A compressor rotating element consisting of one or more impellers and shafts in moving compressors.
What is Specific humidity?
In a mixture of air vapor, it is the weight of water vapor per pound of dry air.
What is Standard air?
Defined in ISO standards as 68°F (20°C), dry (0% relative humidity), and 14.5 psia. The Normal Litre standardizes gas volume measurement at 0°C and 1 atm for consistent comparison of air consumption across compressor systems.
What is Static temperature?
A moving thermometer at a constant velocity indicates the actual temperature of a moving gas stream.
Temperature rise ratio
Comparison between the temperature rise and the total temperature rise during the compression process. For a perfect gas, it is equal to the ratio of the actual increase in enthalpy to the rise in isentropic enthalpy.
Theoretical (ideal) horsepower
The amount of horsepower required to isolate gas or air under specified conditions.
What are Vacuum pumps?
Various compressors operate with a volume below atmospheric pressure and discharge at or above that pressure.
What is Valves?
Devices are used to direct or stop airflow within a system.
Water-cooled compressor
A water-cooled compressor that is cooled by water circulation through jackets near the cylinder, or casing, between and after the compression stages. Water cooling is more efficient than air cooling for high-capacity, continuous-duty industrial compressors, as it handles sustained thermal loads better. Requires access to water supply and drainage, making it suited to stationary industrial installations rather than portable shop setups.
Reciprocating vs Rotary Screw Compressors
The two dominant mechanisms found in most air compressor terminology are reciprocating (piston) and rotary screw designs. Understanding the distinction helps make sense of many other terms in this glossary, since specifications like duty cycle, displacement, and compression ratio behave differently depending on which mechanism is being described.
Reciprocating (piston) compressors use one or more pistons driven by a crankshaft to compress air inside a cylinder, similar in principle to a pump. Common variants include:
- Single-stage — compresses air to final pressure in one piston stroke; typically used for lower-pressure applications.
- Two-stage — compresses air across two strokes with intercooling between stages, allowing higher pressures with less heat buildup, making them suited to more demanding tasks than single-stage units.
- Oil-lubricated — uses oil to lubricate the piston and cylinder, generally supporting longer duty cycles.
- Oil-free (oil-less) — uses self-lubricating materials instead of oil, reducing the risk of oil contamination in the air supply.
- Double acting — compresses air on both sides of the piston during each stroke cycle, effectively doubling output capacity compared to single-acting designs.
Rotary screw compressors use two interlocking helical rotors that compress air continuously as they turn, rather than in discrete strokes. Variants include:
- Oil-injected — oil is injected into the compression chamber for lubrication, sealing, and cooling; oil-flooded designs use continuous oil circulation for enhanced cooling and sealing performance.
- Oil-free — rotors are precision-machined to compress air without oil contact in the compression chamber, eliminating oil from the compression chamber entirely, which makes them essential for food, pharmaceutical, and electronics manufacturing where contamination is unacceptable.
- Fixed-speed — runs at a constant motor speed regardless of demand.
- Variable-speed drive (VSD) — adjusts motor speed to match air demand, which can reduce energy use during partial-load operation.
| Feature | Reciprocating | Rotary Screw |
|---|---|---|
| Compression action | Intermittent (piston strokes) | Continuous (rotating screws) |
| Typical duty cycle | Lower, suited to intermittent use | Higher, suited to continuous use |
| Noise/vibration | Generally higher | Generally lower |
| Common lubrication | Oil-lubricated or oil-free | Oil-injected or oil-free |
In conclusion
As a professional air compressor manufacturer, BISON knows that this terminology guide may not cover all professional terms. We understand that a thorough understanding of the terminology of the air compressor industry is critical to your career development. If you encounter confusion during the exploration process or need to know more about other air compressor knowledge, please feel free to contact BISON for help at any time. We are happy to discuss air compressor expertise with you and provide strong support for your business.
frequently asked questions about Air compressor terminology guide
How many PSI is a CFM?
CFM measures airflow at a given PSI and increases directly with horsepower. A small at-home compressor delivers about 2 CFM; industrial units with 200 HP achieve roughly 1,000 CFM at 100 PSI. As a rule of thumb, a well-designed compressor produces about 4 CFM at 100 PSIG per horsepower. If a compressor’s rated CFM falls short of that ratio relative to horsepower, the motor may be oversized, wasting energy rather than boosting performance.
What is 1 CFM?
A cubic foot is 28.3 liters. 1 CFM is a flow rate of 28.3 liters per minute.
How to read air compressor specs?
Focus on matching CFM to PSI together, not separately, since airflow output changes with pressure. Check tank size for how long high-demand tasks can run, horsepower for raw power, duty cycle for continuous-use capability, and noise rating (dB) if working indoors. Also confirm voltage/phase requirements including three-phase power availability and outlet fittings match your existing hoses and pneumatic tools before purchasing. Larger motors on bigger units may also involve starting methods like star/delta starting, which initially wires the motor windings in a star configuration to reduce starting current before switching to delta for full-power running—worth knowing if you ever hear a technician mention it during setup or servicing.
Is a higher PSI better?
No, higher PSI isn’t always better—for tires, the recommended pressure is the best compromise between comfort, handling, safety, and fuel economy. But it’s okay to go over recommended inflation by a psi or two. And going up is always better than going down.
What are the parts of an air compressor called?
Core components include the motor or engine, the compression pump (cylinder-piston or rotor set), the tank or receiver, a pressure switch, a safety relief valve, a pressure regulator, gauges, an intake filter, a check valve, an unloader valve, and a drain valve. Each part manages airflow, pressure control, or safety, and understanding them helps with maintenance and troubleshooting.
