19-02-2013, 02:50 PM
Air Compressor Types and Components
Air Compressor.pdf (Size: 203.51 KB / Downloads: 134)
There are three basic types of air compressors:
• Reciprocating (Recip)
• Rotary Screw (Screw)
• Rotary Centrifugal (Centrifugal)
Reciprocating Units
Reciprocating air compressors are positive
displacement compressors. This means they are
taking in successive volumes of air which is confined
within a closed space and elevating this air to a higher
pressure. The reciprocating air compressor
accomplishes this by using a piston within a cylinder as
the compressing and displacing element.
The reciprocating air compressor is considered single
acting when the compressing is accomplished using
only one side of the piston. A compressor using both
sides of the piston is considered double acting.
The reciprocating air compressor uses a number of
automatic spring loaded valves in each cylinder that
open only when the proper differential pressure exists
across the valve.
Inlet valves open when the pressure in the cylinder is
slightly below the intake pressure. Discharge valves
open when the pressure in the cylinder is slightly
above the discharge pressure.
A compressor is considered to be single stage when
the entire compression is accomplished with a single
cylinder or a group of cylinders in parallel. Many
applications involve conditions beyond the practical capability of a single compression stage. Too great a compression
ration (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature or other
design problems.
For practical purposes most plant air reciprocating air compressors over 100 horsepower are built as multi-stage units
in which two or more steps of compression are grouped in series. The air is normally cooled between the stages to
reduce the temperature and volume entering the following stage.
Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated
configurations, may be packaged, and provide a wide range of pressure and capacity selections
Rotary Screw Compressors
Rotary air compressors are positive displacement compressors.
The most common rotary air compressor is the single stage
helical or spiral lobe oil flooded screw air compressor. These
compressors consist of two rotors within a casing where the
rotors compress the air internally. There are no valves. These
units are basically oil cooled (with air cooled or water cooled oil
coolers) where the oil seals the internal clearances.
Since the cooling takes place right inside the compressor, the
working parts never experience extreme operating
temperatures. The rotary compressor, therefore, is a continuous
duty, air cooled or water cooled compressor package.
Because of the simple design and few wearing parts, rotary
screw air compressors are easy to maintain, operate and
provide great installation flexibility. Rotary air compressors can
be installed on any surface that
will support the static weight.
Centrifugal Compressors
The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air. The rotor accomplishes this by changing the momentum and pressure of the air. This momentum is converted to useful pressure by slowing the air down in a stationary diffuser. The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents. The centrifugal is a continuous duty compressor, with few moving parts, that is particularly suited to high volume applications, especially where oil free air is required. Centrifugal air compressors are water cooled and may be packaged; typically the package includes the after-cooler and all controls.
Filters
Coalescing filters are the most common form of compressed air purification. These filters remove liquid water and lubricants from compressed air and are installed downstream in a refrigerated air dryer system or upstream in a esiccant dryer system.
Filters are rated according to liquid particle retention size (micron) and efficiency, such as 0.50 micron and 99.99% .O.P. efficient, or 0.01 micron and 99.9999% D.O.P efficient.
Coalescing filters can only remove previously condensed liquids; they do not remove water or lubricant vapors from the compressed air. Any condensation produced from subsequent compressed air cooling will have to be eliminated. When seeking to remove water and lubricant vapors from compressed air, specify an air dryer.