Chapter 6REFRIGERATION AND AIR CONDITIONING
manufacturers technical manual or the directions
enclosed with every new seal.
Controlling the capacity of the compressor is
accomplished by unloading and loading the
cylinders. This is a very desirable design feature
of the unit, because if the compressor is to be
started under a load (all cylinders are working),
there is a much greater amount of torque required
and it is necessary to have a much larger drive
motor. Also, if the compressor is running at a con-
stant capacity or output, it will reach the low
temperature or pressure limits and will be con-
stantly starting and stopping, thereby putting ex-
cessive work on the unit.
Unloading of the cylinders in the compressor
is accomplished by lifting the suction valves off
their seats and holding them open. This method
of capacity control unloads the cylinders com-
pletely and allows the compressor to work at as
little as 25% of its rated capacity.
When the compressor is not in operation, the
unloader power element mechanism, which is
operated by oil pressure from the capacity con-
trol valve, is in the unloaded position (figure 6-6).
The unloader spring pushes against the unloader
piston. This action moves the unloader rod to the
left, thereby rotating the cam rings. As the cam
rings are rotated, the lifting pins are forced up-
ward, raising the suction valve off its seat. The
suction valve is held in this position until the com-
pressor is started and oil pressure of approxi-
mately 30 psi is reached. At this time, the oil
pressure from the capacity control valve pushes
the unloader piston back to the right against the
unloader spring. The motion transmitted through
the push rod rotates the cam ring, thus lowering
the lifting pins and allowing the suction valve to
close or operate normally and the cylinder to
become loaded (figure 6-7). On most compressors
unloaders are connected to the cylinders in pairs.
Figure 6-7.Unloader mechanism in loaded position.