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Air Compressors

Air Compressors

►Reciprocating►Rotary Screw►Rotary Centrifugal

Air Compressors

►The number of compression stages►Cooling method (air, water, oil)►Drive method (motor, engine, steam,

other)►Lubrication (oil, Oil-Free where Oil Free

means no lubricating oil contacts the compressed air)

►Packaged or custom built

Air Compressors

►Reciprocating Compressors – Capacities up to 1,000 CFM and pressures of 100 psi and above.

►Rotary Compressors – Capacities up to 10,000 CFM and pressures of up to 100 psi.

►Centrifugal Compressors – Capacities of 10,000 and above, pressures up to 100 psi.

Air Compressors

►Low Pressure Air Compressors are units having discharge pressures of 150 psi or less.

►Medium Pressure Air Compressors are units having discharge pressures between 151 to 1,000 psi

►High Pressure Air Compressors are units having discharge pressures over 1,000 psi.

Air Compressors

►Air compressor drive connections can be one of the following:

►Direct drive using a flexible coupling.►V-belts are used on compressors

driven by electric motors.►If a high speed unit such as a steam or

gas turbine is used the compressor would be driven thru a reduction gear.

Air Compressors

►Reciprocating Air Compressors are positive displacement machines

►They increase pressure of the air by reducing its volume.

►Successive volumes of air are confined within a closed space elevating the air to a higher pressure.

►Reciprocating air compressors are usually single or double stage.

Air Compressors

►Single-stage reciprocating air compressors are generally used for pressures in the range of 70 to 100 psig.

►Two-stage compressors are generally used for higher pressures in the range of 100 to 250 psig.

►Motor sizes for reciprocating air compressors usually range from 1 to 50 H.P.

►Above 100 H.P. are typically Rotary Screw or Rotary Centrifugal Compressors.

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Air Compressor Pistons

Trunk Piston Differential Piston

Reciprocating Air Compressors

►Air Compressor valves are opened and closed solely by the difference between the pressure of the air in the cylinder and the pressure of the external air on the intake valve or the pressure of the discharge air on the discharge valve.


►Most reciprocating air compressors are lubricated by a small positive displacement pump which is runs off the crankshaft.

►When the compressor is running oil is pumped to the bearings, journals, wrist pins and cylinder walls.

►The lube oil is used to lubricate, cool and remove dirt.


►Intercoolers and aftercoolers are used on reciprocating air compressors improve overall compressor efficiency.

►Density (ρ) of air at 250 °F = 0.05590 lbs/ft3

►Specific Volume (ν) = 1/ρ = 17.889 ft3/lbm

►Density (ρ) of air at 100 °F = 0.07489 lbs/ft3

►Specific Volume (ν) = 1/ρ = 13.353 ft3/lbm

►Intercoolers and aftercoolers are also used to remove moisture in the air

Intercooler


►The compressor unloader is used to remove all the compression load from the compressor while unit is starting and applies the load after the compressor is up to speed.

►The unloading system is also used on air compressors to regulate output if the compressor run continuously.


►Unloading the reciprocating air compressor may be accomplished by:

►Closing or throttling the compressor intake.

►Holding the intake valves off their seats.

►Relieving the intercoolers to atmosphere.

►Opening a bypass from the discharge to the intake.

Air Receiver►The compressed air system receivers

are classified as an unfired pressure vessel.

►The air receiver must be protected with a relief valve.

►The air receiver must be open for internal inspection at the C.O.I. Inspection every two years.

►The air receiver must have a drain valve to remove water.

►The air receiver stores air for the system and also dampens pulsations in the system.

Air Compressors

►The most common type of rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor.

►The compressor consists or two rotors within a casing where the rotors compress the air internally.

►There are no valves.►The units are basically oil cooled,

where the oil seals the internal clearances. (With the oil cooled by air or water oil coolers).

Air Compressors

►Since the cooling takes place inside the compressor, the working parts never experienced extreme operating temperature.

►The rotary compressor usually is a continuous duty, air or water cooled compressor package.

►Rotary screw air compressors are easy to maintain and operate.

Air Compressors

►Capacity control of the compressor is accomplished by variable speed or variable compressor output.

►Variable compressor output is accomplished by bypassing some of the compressed air back to the suction side.

►Advantages of the rotary screw compressor include smooth pulse-free air in a compact size with high output volume over a long life.

Source: www.tpub.com

Air Compressors►The Centrifugal Air Compressor is a

dynamic compressor which depends on the transfer of energy from the rotating impeller to the air.

►Centrifugal air compressors produce high-pressure discharge by converting angular momentum imparted by the rotating impeller.

►To do this efficiently, the centrifugal compressor must rotate at higher speeds than the other types of compressors.

Air Compressors►This type of compressor is designed

for higher capacities because flow through the compressor is continuous.

►Capacity control on a centrifugal compressor is accomplished by adjusting the position of the inlet guide vanes.

►By closing the guide vanes, volumetric flows and capacity is reduced.

►The oil lubricated running gear is separated from the air by shaft seals.