A reciprocating compressor use pistons, cylinders, and valves to compress the refrigerant. The piston moves back and forth in the cylinder. Reciprocating just means back and forth. Reciprocating compressors vary in size and capacity depending on the requirements of the system.
The compressor is a dividing point between the high and low-pressure sides of the system and houses components such as the suction and discharge valves. The evaporator is on the low pressure side and the compressor and condenser are on the high pressure side. The suction valve connects the compressor to the low pressure side of the system via the suction line that carries refrigerant into the compressor. The discharge valve connects the compressor to the high pressure side of the system via the discharge line which carries refrigerant after it has been compressed. The suction and discharge valves open and close depending on the pressure difference of them and allow the vapor refrigerant to enter and leave the compression chamber at the proper moment.
The Reciprocating Compressor Process
Well start describing the process of the compressor when the piston is at its highest possible position within the cylinder. The position is know as top dead center. At top dead center both the suction and the discharge valves are in the closed position and the refrigerant in the compression chamber is equal to the discharge pressure.
Re-expansion: The compressor motor will start to turn and the piston begins to move down in the cylinder. The piston going down, increases the amount of space or volume, the refrigerant is in. Also the pressure begins to decreases because the amount of refrigerant that was already in there is now in a larger space. This causes the refrigerant to expand. Refrigerant expanding is why this part of the process is called re-expansion.
Suction: The pressure of the refrigerant continues to drop until it reaches a point just below the suction pressure of the system. Suction pressure is the low-pressure side of the system. At that pressure the suction pressure will now be greater than the compression chamber and the suction valve will open. As the piston continues to move downward suction gas is drawn into the compression chamber. Suction will continue until the the piston stops moving in the downward direction. When the piston reaches its lowest point in the cylinder, bottom dead center, the suction portion of the cycle ends.
Compression: As the compressor continues to operate, the piston starts to move upward in the cylinder. This upward motion of the piston pushes the suction valve closed, trapping refrigerant in the cylinder. The piston continues to move upward, reducing the volume of the cylinder and increasing the pressure of the refrigerant. Compression will continue until the pressure of the cylinder is slightly greater than the pressure of the refrigerant in the discharge line.
Discharge: When the cylinder pressure is greater than the discharge pressure, the discharge valve will be pushed open, allowing the high pressure refrigerant to be pushed out of the cylinder into the discharge line as the piston continues to move upward. Discharge will continue until the piston reaches top dead center, where the discharge refrigerant will push the discharge valve closed as the piston again starts to move downward.
The cycle repeats it self as long as the system is energized.