Gas compressor
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, whereas the main result of a pump raising the pressure of a
liquid is to allow the liquid to be transported elsewhere.
 |
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 — uses
pistons driven by a crankshaft. They are both stationary and 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 — uses two meshed rotating positive-displacement
helical screws to force the gas into a smaller space. These are usually for continuous, commercial and industrial applications and are both stationary and 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 — a series of fan-like rotating
rotor blades progressively compress the gasflow. Stationary
stator vanes, located downstream of each rotor, redirect the flow onto the next set of rotor blades. The gas passage contracts through the compressor to maintain a roughly constant axial Mach number. Normally used in high flow applications, such as medium to large
gas turbine engines. Almost always multi-staged. Beyond about 4:1 design pressure ratio, variable geometry is often used to improve handling.
*
Centrifugal compressors — a vaned rotating disk or
impeller in a shaped housing forces the gas to the rim of impeller increasing the velocity of the gas. A
diffuser (divergent duct) section converts the velocity energy to pressure energy. These are used 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 — similar to a centrifugal compressor, but with a radial and axial velocity component at exit from the rotor. Diffuser is often used to turn diagonal flow to the axial direction. Lower diameter diffuser than equivalent CF compressor. A diagonal flow compressor is used in the
Pratt & Whitney Canada PW600 turbofan.
*
Scroll compressor—similar to a rotary screw device, this one includes two interleaved
spiral-shaped scrolls to compress a gas. Its output is more pulsed than the latter and this factor has caused its declining industrial use. They can be used as automotive superchargers, and used as air conditioner condensing units.
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, oil, even minimal oil, may be unacceptable.
Gas compressors are used in various applications where either higher pressures or lower volumes of gas are needed:
* in pressurised aircraft to provide a breathable atmosphere of higher than ambient pressure
* in
jet engines to provide the great mass of operating fluid and, at high altitudes, a high enough concentration of
oxygen for combustion of the
air and
fuel mixture. The power to turn the compressor comes from the jet's own
turbines.
* in many various industrial, manufacturing and building processes to power all types of
pneumatic tools.
* in
medicine and
manufacturing to store purified or manufactured gases in a small volume
* as a medium for transferring
energy, such as to power
pneumatic equipment
* in
refrigeration and
air conditioner equipment to move
heat from one place to another in
refrigerant cycles: see
heat pump.
* in
pipeline transport of domestic gas to move the gas from the production site to the consumer
* in
SCUBA diving,
hyperbaric oxygen therapy and other life support devices to store
breathing gas in a small volume such as in
diving cylinders
* in
submarines to store gas for later use as
buoyancy* in
turbochargers and
superchargers to increase the performance of
internal combustion engines by concentrating oxygen
* providing compressed
air for filling pneumatic
tires
* in a
Biogas powerplant to raise the gas pressure for the gas turbines
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 =
T1ยท
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).
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).
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 used as
superchargers from their own crankshaft power. They use exhaust gas energy to power
turbochargers
*
Gas compression heat pump
