Air compressors are a common piece of equipment found in most maintenance shops. There are a number of
different types of compressors available, but the two most common types are the reciprocating and the rotary
5.2 Reciprocating Air Compressors
Reciprocating compressors have been available for many years in a variety of sizes and configurations and
make up the majority of air compressors found in plants and maintenance shops. Reciprocating compressors
are efficient and relatively simple to operate and maintain. Most reciprocating compressors can be completely
overhauled with a minimum of tools and parts.
A reciprocating compressor compresses air in a cylinder, against a cylinder head, by a reciprocating piston.
While all reciprocating compressors operate in basically the same manner, there are many variations in their
construction. For example, a reciprocating compressor can be single or multicylinder, single or double acting,
single or multistage, air or water cooled, and can have a horizontal, vertical, or angled cylinder arrangement.
Other variations are possible depending on the application.
Single acting compressors utilize automotive type pistons, connected directly to the crankshaft by connecting
rods, and compress air on one side of the piston only. Double-acting compressors have a double-acting piston,
compressing air on both sides, driven by a piston rod which extends through a packing box. The piston rod
is connected to a crosshead which is connected to the crankshaft by a connecting rod. Both single and double-
acting compressors are available as single or multistage. Multistage compressors develop their final pressure
in steps, by connecting the discharge of the first stage, through an intercooler, to the intake of the second stage.
The intercooler removes the heat of compression of the first stage.
5.3 Rotary Screw Air Compressors
A rotary screw air compressor utilizes two meshing helical shaped rotors to compress the air. As the rotors
turn, air is compressed by the advancing helix. The rotor may either be oil flooded or dry. Dry rotor
compressors require the use of timing gears to maintain the proper clearance between the rotors. The oil in the
oil flooded type compressor lubricates and seals the rotors and acts as a coolant to remove the heat of
compression. The oil flooded type does not require timing gears as the oil film prevents contact of the rotors,
but an air-oil separator is necessary to remove the oil suspended in the compressed air as it leaves the
Rotary screw compressors have fewer moving parts than reciprocating compressors and provide a smooth,
nearly pulse free air supply. Rotary screw compressors are usually supplied in a "package" requiring only
connection to electrical power and to the air system. Since there is little vibration, they do not require the
massive foundation a comparable reciprocating compressor would need. They are also very popular in trailer
mounted, internal combustion engine driven portable compressors.
The construction of a rotary screw compressor is such that little maintenance can be accomplished in the field
by plant personnel. The lubricating oil filtration system must be maintained regularly as the tight tolerances