Reciprocating compressors use crankshaft-driven pistons to compress gases used in various processes. They fall into the category of positive-displacement compressors. Positive displacement compressors deal with a specific quantity of air or gas is contained in a compression chamber and its volume is mechanically reduced, thus increasing its pressure.
Figure 1 - Typical compressors classification table
Similar to internal combustion engines, the rotary motion of the crankshaft is transformed into linear motion of the piston. As the piston moves, it sucks in low pressure gas and increases its pressure.
Structure of reciprocating compressors
Compression cylinders, also known as stages, confine the process gas during compression. Arrangements may be of single-or dual-acting design: in the dual-acting design, compression occurs on both sides of the piston.
Unlike internal combustion engines, there is no ignition involved: the gas just leaves the compressor cylinder at a highet pressure than the suction pressure.
Reciprocating compressors are typically used where high compression ratio (ratio of discharge to suction pressures) is required at relevantly small flow rates, and the process fluid is relatively dry. Axial flow compressors are mostly suitable for high flow, low compression ratio processes. Reciprocating compressors intended for higher compression ratio are designed as multistage compressors, that is with several working cylinders working in sequence.
These cylinders are driven by the same shaft, connected to a driver motor. Reciprocating compressors are typically low-speed devices and are most often direct- or belt-driven by an electric motor. Some times the motor is integral to the compressor, thus the motor shaft and compressor crankshaft are one piece, therefore eliminating the need for interconnecting coupling. At constant speed, the air flow remains almost constant with variations in discharge pressure.
Applications of reciprocating compressors
Reciprocating compressors are used to compress natural gas (gas transmission pipeline applications), supply high-pressure gas for oil well drilling for gas lift, as well as in various industrial or chemical applications involving air and refrigerant compression, e.g. in refrigeration plants.
Figure 2 - Motor-driven six-cylinder reciprocating compressor