Cold Link Africa VOL. 01 - No.01 | September / October 2015 | Page 36

TALKING TECHNICALLY
INCORPORATING COLD CHAIN

Capacity requirements can vary due to
varying loads, lower ambient temperatures
or even the amount of usage of, for
instance, a cold-room. Because of this,
the capacity of the plant must often be
reduced during periods when design loads
are not experienced. This can be done in
many different ways i.e.:
a. Stopping and starting the compressor;
b. Cylinder overloading;
c. Hot gas bypass;
d. Speed control;
e. Crankcase pressure regulation; and
f. Multiple compressors.

Speed control
This is possibly the best and most
efficient method of capacity control.
When the compressor is driven by an
electric motor, a two-speed motor can
be installed at reasonable cost. Full
electronic speed control is available, but
expensive. The speed of compressors
driven by turbines or internal combustion
engines can, of course, be easily
regulated. The capacity of a compressor
as well as its power requirements are

almost directly proportional to the speed
at which it runs.
Back-pressure regulation
Capacity control can be provided by
the installation of a pressure regulator in
the suction line between the evaporator
and the compressor. As the refrigeration
load in the evaporator decreases, less
refrigerant is boiled off and the pressure in
the evaporator drops. This will cause the
pressure regulation to close, which in turn

Hot gas bypass sytem.

Stopping and restarting
Stopping and restarting the compressor
is by far the simplest method of capacity
control. This is called cycling. This works
well for small systems. Its advantage is that
when the compressor runs, it operates on
the design load which is the point at which
the compressor is usually most efficient.
The disadvantage of the stop-start
method is that only 100% or no capacity is
available. In plants where long periods of
part load operation are required in addition
to reasonably accurate temperature
control, the compressor would be cycled
frequently. This is detrimental to the
compressor and its motor.
Cylinder unloading
On multiple cylinder compressors, an
individual cylinder can be unloaded
by ‘lifting’ the suction valve so that
compression cannot take place. If this is
done, the piston, on its downstroke, will
suck vapour in which on the upstroke will
be expelled again into the suction system.
In doing this, the swept volume of the
compressor and hence the capacity will
be reduced. The unloading of cylinders will
also reduce the power required to drive
the compressor. This is one of the most
efficient methods of capacity control.
Compressors are generally controlled
in steps down to ¼ (on four-cylinder
machines) or ⅓ (on six-cylinder machines).
The higher the number of cylinders in a
compressor, the higher the number of
steps of unloading can be provided.
It must be noted that a compressor
running unloaded for long periods can
overheat due to the compression and
re-compression effect of the vapour in the
unloaded cylinders.
The following shows the number of steps
available on multi-cylinder compressors
as well as the power requirements at the
reduced capacities.
Hot gas bypass
Hot gas bypass is another method of
reducing the capacity of a compressor.
In this system, the hot discharge gas is
bypassed back into the section side of
the compressor when the suction pressure
falls below a predetermined minimum
value. Because the refrigerant is still being
compressed, which requires energy,
the power reduction is minimal. An
additional disadvantage is that the hot
gas can cause excessive superheating
of the vapour which, in turn, can cause
overheating of the compressor. Due to
its simplicity, it is often used to provide
unloaded starting of the compressor.

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COLD LINK AFRICA • September | October 2015