Reciprocating compressor is the earliest compressor type with wide range of application from consumer to industrial usages. These compressors are manufactured in the broad range of configurations and the pressure range is wider than other types of compressors beginning from vacuum to 40000 psig. Higher maintenance cost and lower capacity of reciprocating compressors in comparison with centrifugal compressors have reduced the population of this type since late 1950s to the mid-1970s. But recent rises in energy cost and the advent of new specialty process plants have given the reciprocating compressors a more prominent role in the design of new plants.
The basic components of a reciprocating compressor are a crankshaft, crossheads, piston rod packing, cylinders, pistons, suction valves, and discharge valves.
Reciprocating compressor mechanism:
A prime mover rotates the crankshaft. In big compressors a flywheel can reduce large oscillations in torque and gives even more rotation to crankshaft. Then the crankshaft converts the rotary motion of the prime mover into reciprocating motion of the pistons by the use of connecting rod. Pistons, by their reciprocating motions in the cylinders, compress air or any other gas.
The compression cycle of a reciprocating compressor consists of two strokes of the piston: the suction stroke and the compression stroke. The suction stroke begins when the piston moves away from the inlet port of the cylinder. The gas in the space between the piston and the inlet port expands rapidly until the pressure decreases below the pressure on the opposite side of the suction valve. The pressure difference across the suction valve causes the suction valve to open and let the gas into enter the cylinder. The gas flows into the cylinder until the piston reaches the end of its stroke. The compressor stroke starts when the piston starts its return movement. When the pressure in the cylinder increases above the pressure on the opposite side of the suction valve, the suction valve closes to trap the gas inside the cylinder. As the piston continues to move toward the end of the cylinder, the volume of the cylinder decreases and the pressure of the gas increases. When the pressure inside the cylinder reaches the design pressure of the stage, the discharge valve opens and discharges the contents of the cylinder to the discharge flow.