Page 216 - Reciprocating Compressors Operation Maintenance

2O2 Reciprocating Compressors: Operation and Maintenance

U0100 90 00 40 CO 0
PERCENT DISPLACEMENT

134.6 tOO 80 60 40 tO
PERCENT DISPLACEMENT

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FIGURE 3-57. The effect of adding clearance for capacity control purposes.

This effect of clearance pockets on the capacity of the compressor
offers many possibilities.
Clearance pockets, operated manually or automatically (see Figure 3-58)
are mounted in pockets cast in the cylinder or in the cylinder head. The
valve plug is located over the port opening into the cylinder bore.
Manually operated clearance pocket valves may offer a fixed clearance
increase or a variable amount of clearance increase.
In automatically operated clearance pockets, the valve is held closed
by constant pressure from the intercooler and compression of the spring.
Control pressure applied to the underside of the piston forces the piston
upward, opening the valve. When the valve is opened, the compressed air
or gas passes into the clearance pocket space on the compression stroke.
The capacity of the cylinder is thus reduced.