Reciprocating Compressors

Reciprocating Compressors are one of the most widely used types of compressors for refrigeration and air conditioning applications. The reciprocating compressors comprise of the piston and the cylinder arrangement similar to the automotive engine. While the engine generates power after consuming fuel, the reciprocating compressor consumes electricity to compress the refrigerant. Inside the cylinder the piston performs reciprocating motion which enables the compression of refrigerant inside it.

Principle of Working of the Reciprocating Compressors

Apart from the piston and the cylinder arrangement, the reciprocating compressor also comprises of the crankshaft, connecting rod and other small connecting elements. The crankshaft is connected to the electric motor directly by coupling or by belt and driven by the pulley arrangement. The rotary motion of the crankshaft is converted into the reciprocating motion of the piston inside the cylinder via the connecting rod. Let us see the various strokes of the piston inside the cylinder (refer the figures below):


Working of Reciprocating Compressor

1) Piston at Top Dead Center (TDC) Position:

Let us suppose that initially the piston is at the top position inside the cylinder; this is called as the top dead center (TDC) position of the piston. At this position the refrigerant that has already been compressed is delivered from the discharge valve. From the top dead center position the piston starts moving towards the downward direction. At this instance the discharge valve is already closed, while the suction valve opens due to suction pressure of the refrigerant from the suction pipeline.

The refrigerant from the suction pipeline is taken inside the cylinder of the compressor via the suction valve. As the piston moves downwards, the amount of the refrigerant taken inside the cylinder increases. When the piston reaches bottom most position it is said to be in bottom dead center position (BDC). At this position the maximum amount of the refrigerant is sucked by the cylinder or compressor.

2) Piston at Bottom Dead Center (BDC) Position:
At the BDC position the maximum amount of the refrigerant has been taken inside the cylinder from the suction line of the refrigeration or air conditioning system. The piston now starts moving in the upward direction due to which the volume of the refrigerant inside the cylinder starts reducing, that means the refrigerant starts getting compressed and its pressure starts increasing. Due to high pressure of the refrigerant inside the cylinder, its suction valve closes. Due to crankshaft motion the piston continues moving upwards and compressing the refrigerant.

The pressure of refrigerant goes on increasing as it gets more and more compressed. At the end of the compression stroke the discharge valve opens and the refrigerant is delivered to the discharge pipeline or tubing of the refrigeration or the air conditioning system. Due to the rotary motion of the crankshaft the reciprocating motion of the piston continues inside the cylinder and it finally reaches the TDC position, where all the compressed refrigerant inside the cylinder is delivered to the discharge line and the discharge valve closes. From here on the piston starts moving again to the BDC position and the operation of the compressor continues.

When moving from BDC to the TDC position, the piston does not touch the cylinder at the top position, rather some volume is remains vacant between the top position of the piston and the cylinder, this volume is called as the clearance volume. Such clearance volume is also present at the bottom BDC position.

Thus there are two strokes of the piston inside the cylinder, the suction stroke and the compression stroke. For each revolution of the crankshaft one suction and one discharge stroke of the piston inside the cylinder is produced.