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Compressed Air Encyclopedia

Piston Compressors (Reciprocating Compressors)
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Piston compressors consist of a piston reciprocating inside a cylinder to compress air.

Piston compressors are constructed similar to an internal combustion engine. The piston is connected to a connecting rod and a crankshaft while it travels up and down within a cylinder. Valves control the air entering and discharging from the cylinder.

As the piston travels down, the inlet valve opens and air is draw into the cylinder. At the bottom of the piston’s travel the inlet valve closes trapping the air within the cylinder. As the piston changes it’s direction and returns to the top of the cylinder it starts to compress the air. At the top of it’s stroke, the exhaust valve opens and allows the compressed air to exit the cylinder.

A two-stage compressor contains a ‘low-pressure’ cylinder and a ‘high-pressure’ cylinder. The air is first compressed to an intermediate pressure in the ‘low-pressure’ cylinder. The partially compressed air then passes through an intercooler and enters the ‘high-pressure’ cylinder. The intercooler removes most of the heat that is generated by compressing the air.

Piston compressors are classified as being either single-acting or double-acting. A single-acting piston compressor compresses air only on the upper side of the piston. A double-acting piston compressor compresses air on both the upper and lower sides of the piston. Double-acting compressors are more efficient than single-acting compressors since they accomplish more work with each rotation of the crank shaft.

Piston compressors are available as either air-cooled or water-cooled. An air-cooled compressor relies on ambient air blowing over the cylinder head to cool the compressor. To aid in cooling, the cylinder and intercooler have cooling fins mounted on them.

A water-cooled compressor has a jacket of cooling water around the cylinder head. Water-cooled compressors can dissipate the heat generated through compressing air much more efficiently than air-cooled units can.

Piston compressors generate unbalanced loads and vibrations. As such larger units may require a special support structure.

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