Air Compressors

Types of Air Compressors

• Axial compressors have flow in the axial direction by accelerating air tangentially with blades attached to the rotors. This increases the kinetic energy of the air and diffuses it through static vanes to increase its pressure.
  
• Centrifugal compressors act on air with blades on a rotating impeller. The rotary motion of the air causes an outward velocity from the centrifugal force, and then a diffuser transforms this outward velocity into pressure.
  
• Compressors are mechanisms used for compressing air to higher than atmospheric levels.
  
• Diaphragm compressors achieve compression with the use of a flexing diaphragm that moves back and forth in a closed chamber; the design is an alteration of the reciprocating piston concept. The motion of the connecting rod under the diaphragm causes the flexing and only a short stroke is needed to generate similar pressure effects as those of a reciprocating piston compressor.
  
• Double acting compressors use both sides of the piston to compress the air, both the forward and the back stroke.
  
• Ejector compressors use a high-pressure jet stream. The drive of the stream is transferred to the low pressure of the air.
  
• Free piston compressors have an adjustable compression piston that moves along the length of a steel cylinder column. The guiding and compression pistons collide at the return stroke because the compressed air pushes back the compression piston in the last stage.
  
• Industrial air compressors are mechanical devices used for industrial purposes that provide air at higher than atmospheric pressure.
  
• Labyrinth compressors are oil-free and work without piston rings. A series of labyrinths creates the seal between the cylinder wall and the piston.
  
• Liquid ring compressors have only one moving part, the impeller shaft assembly. The service liquid rotating in its casing forms the liquid ring seal, and air enters through the suction port, moves between the impeller blades and is compressed before discharging.
  
• Lobe compressors use two mating lobes on different shafts that rotate in opposite directions to capture incoming air and compress it against the casing. Lobe compressors supply very high flows at pressure ranges between non-positive displacement compressors and other types of positive displacement units.
  
• Non-positive displacement compressors depend on motion to transfer energy from the compressor rotor to the air. Initial acceleration of the air produces a negative (suction) pressure at the inlet port, which draws air in.
  
• Portable air compressors do not require an electrical outlet.
  
• Positive displacement compressors work by successively trapping a volume of air and reducing it, thereby increasing the pressure. The quantity of heat produced rises proportionally to the pressure rise, resulting in substantial temperature increases of the air and the compressor itself.
  
• Reciprocating compressors move a piston to the top of a cylinder to create compression. These require either water or air cooled.
  
• Rotary compressors have no valves, and compression results from the positive action of rotating elements.
  
• Screw compressors use two contra-rotating rotors that turn in a synchronous mesh. As air enters the sealed chamber, the rotors revolve, reducing the volume of trapped air and sending it compressed through the discharge port at the designated pressure level.
  
• Swash plate compressors move pistons parallel to the crankshaft, either by a cam or by a plate mounted axially on the shaft and inclined to it.
  
• Vane compressors have an eccentrically mounted rotor that is the only moving part and rotates within the stator. As the rotor rotates, centrifugal force forces the vanes from their slots, forming compression cells, and this pumping action of the vanes sliding in and out moves the air from the inlet side of the compressor to the outlet side.

Air Compressors Terms

Aftercooling – The removal of heat when the compression process is complete.

Air Pressure Regulator – A component of an air compressor that allows the user to adjust the air pressure in the air line.

Backflow – A condition caused by a difference in pressure in which air will flow back into the distribution pipes rather than in the intended direction.

Casing – The element that houses the rotor and related internal components of an air compressor. This includes the integral inlet and discharge connections.

Collapse Pressure – The lowest amount of differential pressure something is able to withstand without deformation.

Compression/Pressure Ratio – The ratio of the absolute inlet pressure to the absolute outlet pressure. Compression/pressure ratio typically applies to a single stage of compression but could also apply to a full multistage compressor.

Cylinder – The piston compartment in an actuator or reciprocating compressor.

Discharge Piping – The piping between the aftercooler and the compressor and the air receiver and the cooler separator.

Drive – A flange-mounted belt drive, motor or direct coupling between the engine or motor and the compressor.

Full-Load – The operation of an air compressor at full speed, having a completely open inlet and discharge delivering upper limit airflow.

Guide Vane – An adjustable fixed part that directs the flow of air approaching the inlet of an impeller.

Impeller – The component of the rotating element of a dynamic compressor that gives energy to the flowing medium through centrifugal force. An impeller is comprised of blades that rotate with the shaft.

Intank Check Valve – A valve intended to stop air pressure and volume from slipping out of the compressor tank back into compressor heads when the compressor is not running.

Intercooler – Heat exchangers that eliminate heat produced during compression between the stages of a compressor.

Load Factor – The ratio of the maximum rated compressor load to the average compressor load within a certain period.

Load/Unload Control – A method of control that permits the compressor to run either at no load or at complete load at the same time that the driver remains at a constant speed. Load/unload control is an attempt to match air delivery to the demand.

Maximum Pressure Rating – The highest-pressure level recommended for a compressor.

No Load – When an air compressor is running at full RPM and is wide open, but no air is sent because the inlet is either closed off or modified and will not allow inlet air to be trapped.

Noncooled Compressor Cylinders – Compressor cylinders on a reciprocating compressor that run at low compression ratios and undergo little temperature change. These are used mainly in oil and gas field applications.

Pressure Inlet – The total pressure (static plus velocity) at the inlet flange of the compressor.

Pressure Rise – The difference between the intake pressure and the discharge pressure

Pumping/Surge – The reversal of flow in a dynamic compressor. Pumping/surge takes place when the handled capacity is reduced to an insufficient pressure in order to maintain flow.

Rotor – A revolving element of a compressor. It consists of the impeller and shaft and may have shaft sleeves and a thrust balancing device.

Shaft – The part that the rotating elements are attached to and through which energy is transferred from the prime mover.

Shaft Sleeves – Mechanisms used to position the impeller or to shield the shaft.

Sole Plate – The pad the compressor is mounted on. This is implanted in concrete and usually metallic.

Stack Up – The interaction between the stages of a centrifugal compressor. In the design of a multi-stage compressor, every stage can only run at one point of its characteristic curve, and the determination of this point is done through the design conditions of temperature, flow and pressure.

Surge Limit – The capacity in a dynamic compressor under which the process becomes unsteady.

Thrust Balancing Device – Part of a revolving element that offsets the thrust of compressor impellers.