Page 208 - Reciprocating Compressors Operation Maintenance

194 Reciprocating Compressors: Operation and Maintenance

change capacity. Within the limits of its maximum and minimum speed,
the capacity of such a compressor is infinitely variable.
The majority of compressors sold, however, operate at fixed speeds
and use synchronous or induction motor drives. Because of the constant
speed nature of these prime movers, the capacity of the compressor must
be altered by some means other than speed.
Compressors are controlled to limit either capacity or horsepower. The
need to limit horsepower is probably the lesser known reason for com-
pressor controls. Figure 3-52 illustrates an application of a compressor
that is controlled so as not to exceed available horsepower.

800
600
BMP
400
200
DISCHARGE PRESS
FIGURE 3-52. Capacity unloading to limit power draw.

In this case, the motor had to be loaded between 90% and 100% over a
wide range of discharge pressures. When the motor becomes fully
loaded, as the discharge pressure is increased, the capacity is reduced
slightly before the discharge pressure is increased further.
CAPACITY CONTROL

Controlling a compressor for reasons of capacity accounts for the
largest number of control applications.
Compressor regulation falls into two categories—manual and automat-
ic—and involves elements needed to accomplish three basic functions:

1. Sense pressure changes
2. Relay pressure changes
3. Unloading mechanisms