THE PISTON COMPRESSION PROCESS

THE PISTON COMPRESSION PROCESSThe piston type is very widely used, being adaptable in size, number of cylinders, speed and method of drive. It works on the two-stroke cycle (see Figure 4.4). Automatic pressure-actuated suction and discharge valves are used as shown in Figure 4.4 . As the piston descends on the suction stroke, the suction valve opens to admit gas from the evaporator. At the bottom of the stroke, this valve will close again as the compression stroke begins. When the cylinder pressure becomes higher than that in the discharge pipe, the discharge valve opens and the compressed gas passes to the condenser.

THE PISTON COMPRESSION PROCESS

THE PISTON COMPRESSION PROCESS

(c) (d)

Figure 4.4 Reciprocating compressor: (a) suction stroke, (b) discharge stroke, (c) piston at top of discharge stroke and (d) re-expansion during first part of suction stroke (Emerson Climate Technologies)

Gas left in the clearance at the top of the stroke must re-expand before a fresh charge can enter the cylinder, see Figure 4.4d. The suction valve will not open until the cylinder pressure is lower than the suction pressure. A larger re-expansion or clearance volume means the piston must travel further down the cylinder before the pressure falls below suction pressure. The further the piston travel, without the valve opening, the higher the losses.

The process is represented on a pressure-volume or indicator diagram (Figure 4.5). The area of the diagram represents the work required to compress and discharge the gas.

THE PISTON COMPRESSION PROCESS

Figure 4.5 Reciprocating compressor indicator diagram

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