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Compressor Professor
Gas compressor
From Wikipedia, the free encyclopedia
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.
Compressor designs
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 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 airtools. This type
is also used for many automobile engine superchargers because it is easily
matched to the induction capacity of a piston engine.
* 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.
* Axial-flow compressor—a series of fans spinning on a shaft in a tapered
tube draw in gas at one end and, with the aid of interstage stators (a series
of convergent and divergent ducts), compress it and output it at the other
end. Usually for very high flow applications. Almost always multi-staged. Variable
geometry often used to improve handling beyond about 4:1 design pressure ratio.Most
common compressor in large gas turbine engines.
* 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.
* 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. Can be found in automotive use as a supercharger.
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.
Applications
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 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
* at vehicle service stations for providing compressed air for filling pneumatic
tires
Temperature
Charles' law says "when a gas is compressed temperature is raised".
There are three possible relationships between temperature and pressure in a gas undergoing compression:
* isothermal - gas at final stage of compression is same temperature
as at beginning of compression. In this cycle, heat is removed at the
same rate as it is added by the work of compression. This is impractical
for a working machine.
* adiabatic - This assumes that there is no heat transfer, into or out
of the process, and that all work added is expended in creating a pressure
rise. 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.
* Polytropic - This assumes that heat may enter or leave the process, and that
work added can appear as both increased pressure (useful work) and increased
temperature above adiabatic (losses due to cycle efficiency). Cycle efficiency
is then the ratio of temperature rise at theoretic 100 percent (adiabatic)
vs. actual (polytropic).
Staged compression
Since compression generates heat, the compressed air 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).
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 used as superchargers from their own crankshaft power.
They use exhaust gas energy to power turbochargers
