Gas compressor

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Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, while some can be compressed, the main action of a pump is to pressurize and transport liquids.



[edit] Types of compressors

The main types of gas compressors are illustrated and discussed below:

Compressor Types.png

[edit] Hermetically sealed, open, or semi-hermetic

A small hermetically sealed compressor in a common consumer refrigerator or freezer; it typically has a rounded steel outer shell that is permanently welded shut, and which seals operating gases inside the system. There is no route for gases to leak, such as around motor shaft seals. On this model, the plastic top section is part of an auto-defrost system which use motor heat to evaporate the water.

Compressors are often described as being either open, hermetic, or semi-hermetic, to describe how the compressor and motor drive is situated in relation to the gas or vapour being compressed. The industry name for a hermetic is hermetically sealed compressor, while a semi- is commonly called a semi-hermetic compressor.


The difference between the hermetic and semi-hermetic, is that the hermetic uses a one-piece welded steel casing that cannot be opened for repair; if the hermetic fails it is simply replaced with an entire new unit. A semi-hermetic uses a large cast metal shell with gasketed covers that can be opened to replace motor and pump components.

The primary advantage of a hermetic and semi-hermetic is that there is no route for the gas to leak out of the system. Open compressors rely on either natural leather or synthetic rubber seals to retain the internal pressure, and these seals require a lubricant such as oil to retain their sealing properties.

An open pressurized system such as an automobile air conditioner can leak its operating gases, if it is not operated frequently enough. Open systems rely on lubricant in the system to splash on pump components and seals. If it is not operated frequently enough, the lubricant on the seals slowly evaporates, and then the seals begin to leak until the system is no longer functional and must be recharged. By comparison, a hermetic system can sit unused for years, and can usually be started up again at any time without requiring maintenance or experiencing any loss of system pressure.

The disadvantage of hermetic compressors is that the motor drive cannot be repaired or maintained, and the entire compressor must be removed if a motor fails. A further disadvantage is that burnt out windings can contaminate whole systems requiring the system to be entirely pumped down and the gas replaced. Typically hermetic compressors are used in low-cost factory-assembled consumer goods where the cost of repair is high compared to the value of the device, and it would be more economical to just purchase a new device.

An advantage of open compressors is that they can be driven by non-electric power sources, such as an internal combustion engine or turbine. However, open compressors that drive refrigeration systems are generally not totally maintenance free throughout the life of the system, since some gas leakage will occur over time.

[edit] Centrifugal compressors

Figure 1: A single stage centrifugal compressor


Many large snowmaking operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines. Sometimes the capacity of the compressors is written in NM3/hr. Here ‘N’ stands for normal temperature pressure (20°C and 1 atm ) for example 5500 NM3/hr.

[edit] Diagonal or mixed-flow compressors

Diagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to an axial rather than radial direction.

[edit] Axial-flow compressors

Main article: Axial-flow compressor

An animation of an axial compressor.

Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like airfoils to progressively compress the working fluid. They are used where there is a requirement for a high flow rate or a compact design.

The arrays of airfoils are set in rows, usually as pairs: one rotating and one stationary. The rotating airfoils, also known as blades or rotors, accelerate the fluid. The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage.[1] Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation.


[edit] Reciprocating compressors


Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines.[1][4][5] Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over 1,000 hp (750 kW) are commonly found in large industrial and petroleum applications. Discharge pressures can range from low pressure to very high pressure (>18000 psi or 180 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, and more costly than comparable rotary units.[6] Another type of reciprocating compressor is the swash plate compressor, which uses pistons which are moved by a swash plate mounted on a shaft – see Axial Piston Pump.

Household, home workshop, and smaller job site compressors are typically reciprocating compressors 1½ hp or less with an attached receiver tank.

[edit] Rotary screw compressors

Diagram of a rotary screw compressor


[edit] Rotary vane compressors


With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e.g., 2 bar or 200 kPa; 29 psi) for bulk material movement while oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bar (1,300 kPa; 190 psi) in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.

Rotary vane compressors can have mechanical efficiencies of about 90%.[9]

[edit] Scroll compressors

Main article: Scroll compressor

Mechanism of a scroll pump

A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involute, archimedean spiral, or hybrid curves.[10][11][12] They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range


This type of compressor was used as the supercharger on Volkswagen G60 and G40 engines in the early 1990s.[citation needed]

[edit] Diaphragm compressors

Main article: Diaphragm compressor



A three-stage diaphragm compressor

The photograph included in this section depicts a three-stage diaphragm compressor used to compress hydrogen gas to 6,000 psi (41 MPa) for use in a prototype compressed hydrogen and compressed natural gas (CNG) fueling station built in downtown Phoenix, Arizona by the Arizona Public Service company (an electric utilities company). Reciprocating compressors were used to compress the natural gas.


[edit] Air bubble compressor

A mixture of air and water generated through turbulence is allowed to fall into a subterranean chamber where the air separates from the water. The weight of falling water compresses the air in the top of the chamber. A submerged outlet from the chamber allows water to flow to the surface at a lower height than the intake. An outlet in the roof of the chamber supplies the compressed air to the surface. A facility on this principal was built on the Montreal River at Ragged Shutes neary Cobalt, Ontario in 1910 and supplied 5,000 horsepower to nearby mines. [13]

[edit] Temperature

Main article: Gas laws

Compression of a gas naturally increases its temperature, often referred to as the heat of compression. W = \int_{V_1}^{V_2} P dV = P_1 V_1^n \int_{V_1}^{V_2} V^{-n} dV where

\frac { P_2 }{ P_1 }\ = \left( \frac{ v_1 } { v_2 }\ \right) ^ n


W = \frac {{P_1} {V_1^n}} {1-n}\ ( {V_2^{1-n}} - {V_1^{1-n}} )

with n taking different values for different compression processes (see below).


 T_2 = T_1 . {R_c}^{(k-1)/k} with T1 and T2 in degrees Rankine or kelvins, and k = ratio of specific heats (approximately 1.4 for air). Rc is the compression ratio; being the absolute outlet pressure divided by the absolute inlet pressure. The rise in air and temperature ratio means compression does not follow a simple pressure to volume ratio. This is less efficient, but quick. Adiabatic compression or expansion more closely model real life when a compressor has good insulation, a large gas volume, or a short time scale (i.e., a high power level). In practice there will always be a certain amount of heat flow out of the compressed gas. Thus, making a perfect adiabatic compressor would require perfect heat insulation of all parts of the machine. For example, even a bicycle tire pump’s metal tube becomes hot as you compress the air to fill a tire. The relation between temperature and compression ratio described above means that the value of n for an adiabatic process is k (the ratio of specific heats).


For an isothermal process, n is 1, so the value of the work integral for an isothermal process is:

W = - {P_1} {v_1} \ln \left( \frac {P_2} {P_1}\ \right)

When evaluated, the isothermal work is found to be lower than the adiabatic work.

  • Polytropic – This model takes into account both a rise in temperature in the gas as well as some loss of energy (heat) to the compressor’s components. This assumes that heat may enter or leave the system, and that input shaft work can appear as both increased pressure (usually useful work) and increased temperature above adiabatic (usually losses due to cycle efficiency). Compression efficiency is then the ratio of temperature rise at theoretical 100 percent (adiabatic) vs. actual (polytropic). Polytropic compression will use a value of n between 0 (a constant-pressure process) and infinity (a constant volume process). For the typical case where an effort is made to cool the gas compressed by an approximately adiabatic process, the value of n will be between 1 and k.

[edit] Staged compression

In the case of centrifugal compressors, commercial designs currently do not exceed a compression ratio of more than a 3.5 to 1 in any one stage (for a typical gas). Since compression generates heat, the compressed gas is to be cooled between stages making the compression less adiabatic and more isothermal. The inter-stage coolers typically result in some partial condensation that is removed in vapor-liquid separators.


Because rotary screw compressors can make use of cooling lubricant to remove the heat of compression, they very often exceed a 9 to 1 compression ratio. For instance, in a typical diving compressor the air is compressed in three stages. If each stage has a compression ratio of 7 to 1, the compressor can output 343 times atmospheric pressure (7 × 7 × 7 = 343 atmospheres). (343 atm/34.8 MPa; 5.04 ksi)

[edit] Prime movers

There are many options for the “prime mover” or motor which powers the compressor:

  • gas turbines power the axial and centrifugal flow compressors that are part of jet engines
  • steam turbines or water turbines are possible for large compressors
  • electric motors are cheap and quiet for static compressors. Small motors suitable for domestic electrical supplies use single phase alternating current. Larger motors can only be used where an industrial electrical three phase alternating current supply is available.
  • diesel engines or petrol engines are suitable for portable compressors and support compressors. Common in automobiles and other types of vehicles (including piston-powered airplanes, boats, trucks, etc.), diesel or gasoline engines can power compressors using their own crankshaft power (this setup known as a supercharger), or, using their waste exhaust gas to spin a turbine connected to the the compressor (this setup known as a turbocharger).

[edit] Multiple compressor configurations

It is possible to connect two or more compressors in tandem, effectivly doubling their CFM output and air storage capacity. The advantages of such a configuration are portability and cost savings. It is significantly less expensive to purchase smaller compressors and rig them together than it would be to purchase one large compressor with the same output. See:

[edit] Applications


[edit] See also

[edit] References

  1. ^ a b c d e f Perry, R.H. and Green, D.W. (Editors) (2007). Perry’s Chemical Engineers’ Handbook (8th ed.). McGraw Hill. ISBN 0-07-142294-3.
  2. ^ Dixon S.L. (1978). Fluid Mechanics, Thermodynamics of Turbomachinery (Third ed.). Pergamon Press. ISBN 0-08-022722-8.
  5. ^ Reciprocating Compressor Basics Adam Davis, Noria Corporation, Machinery Lubrication, July 2005
  6. ^ Introduction to Industrial Compressed Air Systems
  7. ^ Screw Compressor Describes how screw compressors work and include photographs.
  8. ^ Technical Centre Discusses oil-flooded screw compressors including a complete system flow diagram
  9. ^ Mattei Compressors
  13. ^ Maynard, Frank (November 1910). “Five thousand horsepower from air bubbles”. Popular Mechanics: Page 633.
  14. ^ Perry’s Chemical Engineer’s Handbook 8th edition Perry, Green, page 10-45 section 10-76
  15. ^ Millar IL; Mouldey PG (2008). “Compressed breathing air – the potential for evil from within.”. Diving and Hyperbaric Medicine. (South Pacific Underwater Medicine Society) 38: 145–51. Retrieved 2009-02-28.
  16. ^ Harlow, V (2002). Oxygen Hacker’s Companion. Airspeed Press. ISBN 0967887321.

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