Air Compressors
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 |
