Page 217 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 2Q3

Manually operated, fixed-volume clearance pocket.
Upon manual opening, the fixed-volume
clearance pocket reduces compressor
capacity a specific amount in a single step
by adding a fixed amount of clearance to the
cylinder. In reverse, reducing the clearance
will increase the compressor capacity.

Pneumatically actuated, fixed-volume clearance pocket.
Pneumatic control is often used for remote,
semi-automatic operation. The pneumatic
signal can be manually controlled by three-
way valves or automatically controlled by
electrically actuated solenoid valves.

FIGURE 3-58. Fixed-volume clearance pockets.

FIVE-STEP CAPACITY CONTROL

When more control is required, it is possible to use suction valve
unloaders and a clearance pocket in each cylinder to regulate the capacity
in steps of 100%, 75%, 50%, 25%, and 0% of full compressor capacity.
With a five-step capacity control, the compressor does not normally
operate over the entire range of control. Only a sudden change in demand
would cause the compressor to unload completely. Because the compres-
sor usually floats between two steps of unloading, the pressure fluctuation
of the system is less with five-step control than with a three-step control.
Figure 3-59 shows the control sequence of a compressor with five-step
capacity control.

4 BUM r i.
100% Capodty

FIGURE 3-59. Sequence of operation with five-step control.