Gas compressor

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A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Compression of a gas naturally increases its temperature.

Compressors are closely related 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 generally incompresible, so the main result of a pump is to move the liquid elsewhere.

Contents

[edit] Compressor designs

Figure 1: Small reciprocating air compressor. Air is compressed by two pistons into the storage tank. It is powered by an electric motor (blue).
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Figure 1: Small reciprocating air compressor. Air is compressed by two pistons into the storage tank. It is powered by an electric motor (blue).

Some important designs of compressors include:

  • 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. Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors up to 1000 hp are still commonly found in large industrial applications, but their numbers are declining as they are replaced by less costly rotary screw compressors. Discharge pressures can range from low pressure to very high pressure (>5000 psi or 35 MPa).
  • Rotary screw compressors — use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 5 hp (3.7 kW) to over 500 hp (375 kW) and from low pressure to very high pressure (>1200 psi or 8.3 MPa). They are commonly seen with roadside repair crews powering air-tools. This type is also used for many automobile engine superchargers because it is easily matched to the induction capacity of a piston engine.
  • Axial-flow compressor — use a series of fan-like rotating rotor blades to progressively compress the gasflow. Stationary stator vanes, located downstream of each rotor, redirect the flow onto the next set of rotor blades. The area of the gas passage diminishes through the compressor to maintain a roughly constant axial Mach number. Axial-flow compressors are normally used in high flow applications, such as medium to large gas turbine engines. They are almost always multi-staged. Beyond about 4:1 design pressure ratio, variable geometry is often used to improve operation.
  • Centrifugal compressors — use a vaned rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. These are for continuous, heavy industrial uses and are usually stationary. Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 lbf/in² (69 MPa). Many large snow-making 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.
  • Diagonal or mixed-flow compressor — are similar to a centrifugal compressor, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent CF compressor. A diagonal flow compressor is used in the Pratt & Whitney Canada PW600 turbofan.
  • Scroll compressor—similar to a rotary screw device, it includes two interleaved spiral-shaped scrolls to compress the gas. Its output is more pulsed than that of a rotary screw compressor, and this has caused its declining industrial use. It can be used as an automotive supercharger, and in an air conditioner condensing unit.
  • Diaphragm Compressor is used for hydrogen and compressed natural gas (CNG).

Air compressors sold to and used by the general public are often attached on top of a tank for holding the pressurized air. Oil-lubricated and oil-free compressors are available. Oil-free compressors are desirable because without proper consideration (additional parts) oil can make its way into the air stream. In a given use, for example as a diving air compressor, even minimal oil may be unacceptable.

[edit] Applications

Gas compressors are used in various applications where either higher pressures or lower volumes of gas are needed:

The plots below (Figure 2, Figure 3, Figure 4 and Figure 5) contain: a compressor/compression schematic layout, a compressor/compression entropy vs temperature diagram, a gas compressor/compression specific power input vs pressure ratio and a compressor/compression power input vs mass flow rate.

Figure 2: Schematic layout
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Figure 2: Schematic layout
Figure 3: Entropy vs temperature diagram
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Figure 3: Entropy vs temperature diagram
Figure 4: Specific power input vs pressure ratio
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Figure 4: Specific power input vs pressure ratio
Figure 5: Power input vs mass flow rate
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Figure 5: Power input vs mass flow rate

[edit] Temperature

Charles's law says "when a gas is compressed temperature is raised".

There are three possible relationships between temperature and pressure in a system (a volume of gas) undergoing compression:

  • isothermal - gas remains at constant temperature throughout the process. In this cycle, internal energy is removed from the system as heat at the same rate it is added by the mechanical shaft work of compression. This is impractical for a working machine.
  • adiabatic - In this process there is no heat transfer to or from the system, and all supplied shaft work is added to the internal energy of the gas, resulting in increases of temperature and pressure. Theoretical temperature rise is T2 = T1·Rc((K-1)/K)), with T1 and T2 in degrees Rankine or kelvins, and K = ratio of specific heats (approximately 1.4 for air). The rise in air and temperature ratio means compression does not follow a simple pressure to volume ratio. This is less efficient, but quick. In practice there will always be a certain amount of heat flow, as to make a perfect adiabatic system would require perfect heat insulation of all parts of a machine.
  • Polytropic - 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). Cycle efficiency is then the ratio of temperature rise at theoretic 100 percent (adiabatic) vs. actual (polytropic).

[edit] Staged compression

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 cause condensation meaning water separators with drain valves are present. The compressor flywheel may drive a cooling fan.

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 x 7 x 7 = 343 Atmospheres).

[edit] Prime movers

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

[edit] See also

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