Reciprocating Compressors #1

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A reciprocating compressor is a positive-displacement machine in which the compressing and displacing element is a piston moving linearly within a cylinder. Figure 10-1 shows the action of a reciprocating
compressor.

Reciprocating compressor action.

In Position 1 the piston is moving away from the cylinder head and the suction valve is open, allowing the cylinder pressure to equal suction pressure and gas to enter the cylinder. The discharge valve is closed. At Position 2 the piston has traveled the full stroke within the cylinder and the cylinder is full of gas at suction pressure. The piston begins to move to the left, closing the suction valve. In moving from Position 2 to Position 3, the piston moves toward the cylinder head and the volume is reduced. This increases pressure until the cylinder pressure is equal to the discharge pressure and the discharge valve opens. The piston continues to move to the end of the stroke near the cylinder head, discharging gas.
Pressure in the cylinder is equal to discharge pressure from Position 3 to Position 4. As the piston reverses its travel the gas remaining within the cylinder expands until it equals suction pressure and the piston is again in Position 1.

Reciprocating compressors are classified as either “high speed” or “slow speed.” Typically, high-speed compressors run at a speed of 900 to J 200 rpm and slow-speed units at speeds of 200 to 600 rpm.

Figure 10-2 shows a high-speed compressor frame and cylinders. The upper compressor is called a two throw machine because it has two cylinders attached to the frame and running off the crank shaft. The lower compressor is a four-throw machine because it has four cylinders attached to the frame. The number of “throws” refers to the number of pistons.

High-speed reciprocating compressor frames and cylinders. (Courtesy or Dresser-Kara? Company.)

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