Page 214 - Reciprocating Compressors Operation Maintenance

2OO Reciprocating Compressors: Operation and Maintenance

starter circuit. This allows the unit to unload for only a few seconds of
starting, or long enough for the motor to attain full speed.

DUAL CONTROL

Sometimes, the conditions of service within a plant favor both types of
control, bypass and start-stop. For instance, the air requirements may be
reduced to only a small fraction of the compressor capacity on Saturday
and Sunday. Plant-specific conditions obviously call for a combination
control employing the best elements of the two previously mentioned
control types, bypass and start-stop.

Three-Step Variable Control

This control, which uses suction valve unloading, regulates capacity in
steps of 100%, 50% and 0% of full capacity and is activated by some sort
of governor. The actual unloading of the compressor can take place elec-
trically and pneumatically, or pneumatically only.
Figure 3-55 shows a pneumatic control panel including controls for
electric auxiliaries. Included are switches and pressure gauges for indi-
cating the intercooler and system and oil pressures.
Normally, for a 100 psig air system, the pressure switches would be set
to completely unload at 100 psig. When the pressure falls to 95 psig, the
compressor would load to 50% capacity, and if the pressure continues to
fall to 93 psig, the compressor would be completely loaded.
Rising pressure would cause the compressor to unload to 50% capacity
at 98 pisg and completely unload for automatic initial unloading. Thus,
whenever the compressor is started, it will always start unloaded. It is
also unloaded when stopped.
Figure 3-56 shows diagrammatically the sequence of operation of
unloading with the three-step control.

CLEARANCE CONTROL

Although bypass control constitutes the major portion of the capacity
control devices, other methods are available and are often used. There are
many instances when bypass control does not offer the necessary flexi-
bility for a given control application.