Reciprocating compressor is a direct volume-reduction machine. It is a positive displacement compressor. Usually compressor takes refrigerant vapor at a low temperature & pressure and raises it to a higher level. Then it pumps refrigerant throughout the system. In the reciprocating compressor, a piston travels back and forth in a cylinder. A rod connects the piston to a crankshaft. As the motor turns, the crankshaft forces the piston up and down. A suction valve between the suction line and cylinder lets refrigerant vapor enter the cylinder during the suction stroke. The discharge valve between the discharge line and the cylinder lets refrigerant vapor exit the condenser during the discharge stroke. There are only two real problems with reciprocating compressors: pulsation and mechanical reliability.