Generally, control systems include unloading
devices that function to remove all but the fric-
tion loads on compressors. Thus starting is unaf-
fected by compression loads. Various types of
unloading devices are discussed below.
The inlet-valve-type unloader holds the inlet
valve open mechanically during both the suction
and compression strokes, thereby preventing com-
pression. Figure 11-20 illustrates a common inlet
valve unloader. The unloader is positioned above
the inlet valve. When air pressure rises to the
preset unloading pressure, a pressure switch
operates a solenoid unloader valve, which opens
and allows receiver pressure to the inlet valve
unloader. The pressure from the receiver, acting
on the diaphragm of the inlet valve unloader,
forces the yoke fingers against the inlet valve,
holding it open. The intake air is pushed back out
the inlet valve on the compression stroke so no
compression takes place.
Figure 11-21 illustrates the thin plate, low-lift
type of compressor valve. Most compressors use
this type of valve.
Figure 11-20.Inlet valve unloader.
The use of an unloader valve on each cylinder
and a pressure switch with a solenoid unloader
valve provides a step or sequenced capacity con-
trol. Figure 11-22 illustrates a flow diagram of a
Figure 11-21.Thin plate, low-lift, compressor valve assembly.