Compressor, abbreviated as COMPR, is a device that forces air or gas into a smaller area increasing the volume and creating a usable force of energy.  Compressors come in many configurations and flow rates and can be found in every industry from refrigerating to welding, even your home. Compressors are split into two categories: positive displacement and dynamic.


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compressor Types

  • Dynamic Compressors  -  The gas is first accelerated to high velocity, and then it passed through a diffuser.  The kinetic energy of the air is converted to pressure energy at the diffuser before it leaves through the outlet.
    • Axial Compressor  -  Used where continuous flow and large amount of air is required.  There are a series of blades on a shaft, the shaft having a number rows.  The number of rows is referenced as a 10 stage, 15 stage and so on.  The shaft is inside a tapered housing which compresses the air as it flows through.  As the blades turn the air is pulled in the front, being compressed tighter as it exits.
    • Centrifugal Compressor  -  Often used offshore for field gas compression and are often used in gas plant applications.  These are used when there is limited space available, when vibrations could be a problem (offshore) or a low weight to horsepower ratio is required.
  • Positive Displacement  -  Air is drawn into a chamber.  The trapped air is then compressed, physically reducing the volume of the chamber.

    • Reciprocating Compressor  -  Uses a crankshaft driven piston and cylinder to compress the air.  Primarily used in field gas compression and have a very large range of capacity and horsepower characteristics.
      • Single Acting  -  This compressor only have valves on top of the cylinder, so there is one compression cycle for every turn of the crankshaft.
      • Double Acting  -  This compressor is similar to single-action, but they have inlet and discharge valves on both sides of the cylinder.  This gives you two compression cycles for every turn of the crankshaft.
      • Diaphragm  -  Compression of air is by the forward and backward movement of a crankshaft which brings about the movement of a special membrane inside the compression box.  They are useful in processes where leaks through packing cannot be tolerated. 
    • Rotary Compressor  -  Compression system that includes a pair of matching helical screws.
      • Liquid Ring  -  An offset rotar is placed in a cylindrical housing and the liquid is fed into the housing.  As the rotar spins, the water is thrown by centrifugal force to the outside.  The void created in the center holds gas drawn in through the inlet port of the pump.  As gas travels around the pocket of air in the center, it is compressed, then fed back out the discharge port.
      • Lobe  -  They have two intermeshing rotors mounted on parallel shafts.  In a twin-lobe compressor, each rotor has two lobes, four lobes per compressor. In a tri-lobe machine each rotor has three lobes, six lobes per compressor.
      • Screw  -  The twin elements include male and female parts rotating in opposite directions.  Air fills the space between the rotors and, as they rotate, the volume between them and the surrounding housing decreases, squeezing or compressing the air into a smaller space.  The length, pitch of the screw, and the form of the discharge port collectively determine the pressure ratio.
      • Scroll  -  Uses two interleaved spiral-like vanes to pump or compress the gas. One of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of gas between the scrolls.  They operate more smoothly, quietly, and reliably than other types of compressors. These compressors are usually operated in the lower volume range
      • Vane  -  Often used in tank vapor recovery (TVR) operations.  These compressors take gas at very low suction pressures and raise them to pressures required for gas sales.  If they cannot get them to the high pressures, they will often discharge into another compressor that can.


Compressor Installation

  • Foundations  -  Foundations are very important to the installation of a compressor. This is because compressors produce a huge amount of vibration and the foundation acts as a mass dampener.  The cement foundation is usually very large and very well reinforced.
  • Grouting  -  Grouting ensures that the entire compressor skid is equally supported at all points.  Grout is usually epoxy grout or cement based grout.
  • Alignment  -  Setting the unit on the foundation is very critical.  If this is done hap-hazardously, the skid or piping can become deformed and not affect the operation of the equipment.  Alignment of the compressor with respect to the driver absolutely must be done before grouting the unit.  This should be completed by an expert and should be verified after the unit has been running for a given length of tme.


Compressor standards

ISO Standards

  • ISO 5388 - Stationary air compressors  -  Safety rules and code of practice


Compressor Supporting Equipment

A gas compressor does not run by itself.  It needs quite a bit of equipment to keep it functioning properly.  Examples of this equipment are:

  • Drivers
    • Internal Combustion Engine
    • Electric Motors may be attached to a gear set and / or a Variable Frequency Drive
    • Gas Turbines
  • Accessories
    • Lube Oil
    • Gas Cooler, typically an Air Cooled Heat Exchanger (as gas is compressed, heat is added and is usually removed)
    • Gas Scrubber for removing liquids before the gas enters the compression chamber.
    • Water Jacket Cooling for keeping the compressor, proper, cool.
    • Fuel system for driver
    • Control Panel
    • Starters


Compressor Glossary


  • Adiabatic Compression  -  A process during which no heat is added or removed from the gas.
  • Aftercooling  -  The cooling of gas in a heat exchanger following the completion of compression to reduce the temperature and liquefy condensable vapors.
  • Air Receivers  -  A tank into which the compressed air or gas is discharged from the compressor.  Receivers help to eliminate pulsations in the discharge line and also act as storage capacity during intervals when the demand exceeds the capacity of the compressor.
  • Ambient Temperature  -  When outdoors the ambient temperature is the current surrounding environment air temperature.  This temperature has nothing to do with high or low forcasts.

  • Avogadro's Law  -  When temperature and pressure are held constant, the volume of a gas is proportional to the number of moles of gas present.


  • 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.
  • Butterfly Valve  -  A quarter turn valve (90° or less) with a circular disk as its closing element.s of gas present.


  • Check Valve  -  A valve that are designed to allow the process fluid to flow in only one direction to prevent backflow.

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

  • Clearance  -  Is the volume contained in one end of the cylinder which is not swept by the movement of the piston.  It includes space between piston and head at the end of the compression stroke, space under the valves, etc., and is expressed as a percentage of the piston displacement per stroke.

  • Compressibility  -  Measures the change in volume under external forces for any liquid.

  • Compression Efficiency  -  The ratio of work requirement to the actual work required to be done on a gas.  Efficiency accounts for leakage and fluid friction losses.  
  • Compression Ratio  -  The ratio of the maximum volume to the minimum volume in a cylinder.

  • Condensate System  -  A device that removes enough oil and contaminants from condensate drains to allow the condensate to enter the sewage system.
  • Critical Pressure  -  The highest temperature at which well defined liquid and vapor states exist.  It may be defined as the highest temperature at which it is possible to liquefy a gas by pressure alone.
  • 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.


  • Degree of Intercooling  -  The difference in temperature between the inlet of the compressor and the outlet of the intercooler expressed as an inverse percentage.

  • Deliquescent  -  Melting and becoming a liquid by absorbing moisture.
  • Discharge Pressure  -  The total gas pressure (static plus velocity) at the discharge flange of the compressor.  Velocity pressure usually is considered only with dynamic pressure.
  • Discharge Temperature  -  The temperature at the discharge flange of the compressor.

  • Drain Valve  -   A device that removes surplus liquid from a compressed air system.
  • Dual Stage  -  A compressor where air is compressed from initial pressure to an intermediate pressure in one or more cylinders.

  • Dynamic Compressor  -  The gas is first accelerated to high velocity, and then it passed through a diffuser.  The kinetic energy of the air is converted to pressure energy at the diffuser before it leaves through the outlet.


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


  • 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.



  • Humidity  -  Has to do with the moisture or water vapor, in the atmosphere.


  • Inlet Pressure  -  Pressure at the intake to the compressor.  Usually expressed as absolute pressure.

  • Inlet Throttle  -  A compressor control mechanism designed to control output to meet plant demands.

  • Intercooling  -  Removal of heat of compression between stages of a multi-stage compressor.  Stage 1 - reduce the temperature,  Stage 2 - reduce the volume to be compressed in the succeeding stage, Stage 3 - liquefy condensable vapors, and Stage 4 - save power.

  • Isenttopic Compression  -  A reversible adiabatic compression.  During isentropic compression the pressure, volume and temperature of the gas all vary.
  • Isothermal Compression  -  A process during which the temperature of the gas is held constant.  As the requires continuous removal of heat during compression, this process is not practical.  Isothermal formulas are of little use except on theoretical applications.  



  • Kick-in Pressure  -  Factory set low pressure point of the pressure switch that starts the compressor to re-pressurize the tank to a higher pressure.

  • Kick-out Pressure  -  Factory set high pressure point of the pressure switch that stops the compressor from increasing the pressure in the tank above a certain level.


  • Load Factor  -  The ratio of the average compressor load during a given period of time to the maximum rated load of the compressor.
  • Load Time  -  The time it takes for a compressor to go from load to unload.

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


  • Membrane Dryer  -  Water vapor and some compressed air permeate the membrane walls and vent to atmosphere.
  • Modulating Control  -  System that adapts to varying demand by throttling the compressor inlet proportionally to demand.




  • Perfect Intercooling  -  When the temperature of air leaving the intercooler is equal to the temperature at the intake of the compressor.
  • Piston Displacement  -  The net volume displaced by the piston at rated machine speed.  For single‐action double‐acting cylinders, it is the total of both ends.  For multistage compressors, the displacement of the first stage only is commonly stated as that of the entire machine.
  • Polytropic Compression  -  Process during which change in gas characteristics are considered.  This is the normal compression cycle of dynamic units.
  • Positive Displacement  -  Air is drawn into a chamber.  The trapped air is then compressed, physically reducing the volume of the chamber.
  • Pressure  -  The force exerted perpendicular to the surface of an object and is expressed as force per unit area.

  • Pressure Dew Point (PDP)  -  The temperature at which moisture begins to condense in a compressed air system.
  • Pressure Discharge  -  The absolute total pressure at the discharge flange of a compressor.
  • Pressure Loss  -  The difference in pressure between two points, usually caused by friction resistance in the pipe, but moisture can also affect it.



  • Receivers  -  Tanks used for the storage of air discharged from compressors.  They serve also to damp discharge line pulsation.

  • Reciprocating Compressor  -  Uses a crankshaft driven piston and cylinder to compress the air.  Primarily used in field gas compression and have a very large range of capacity and horsepower characteristics.
  • Relative Humidity  -  A measure of the amount of moisture in the air with respect to the temperature.

  • Rotary Compressor  -  Compression system that includes a pair of matching helical screws.


  • Seal  -  Devices used between rotating and stationary parts to separate, and minimize leakage between areas of unequal pressures.
  • Single Stage  -  A compressor in which the air is compressed from initial pressure to final pressure in one step.
  • Specific Humidity  -  The weight of water vapor in an air-vapor mixture per unit weight of dry air.
  • Static Pressure  -  The difference in air pressure between the suction side and pressure side of a blower.  The higher the static pressure or resistance, the more energy it takes to move air through the object.
  • Super Compressibility Factor  -  A factor expressing deviation of a gas from perfect gas laws.  For practical purposes, at the pressures and temperatures normally encountered in air compressors, air can be considered to follow the perfect gas laws.


  • Thermal Compressor  -  An ejector used to compress waste or exhaust steam or any other gas through a moderate range of compression above atmospheric pressure.


  • 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.


  • Volumetric Efficiency  -  The ratio in percent of the actual delivered capacity (measured at inlet temperature, pressure and gas composition) to the piston displacement.


  • Water-Cooled Compressor  -  Machines cooled by water circulated through jackets surrounding the cylinders or casings.






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