16
System Operation.
Evaporator Superheat.
Evaporator superheat is generated from
the compressor suction line reducing
system efficiency.
Normally 6℉ to 12℉ is acceptable on
most refrigeration systems. Preferably,
6℉ to 8℉ on low temperature systems
and 8℉ to 10℉ on medium temperature
systems are desired. For systems
operating at higher temperatures, the
superheat can be adjusted to 12℉ to 15℉
as required.
The method of measuring evaporator
superheat is found by P-T method.
Obtain
evaporator
superheat
by
measuring the suction line temperature at
the expansion valve bulb. Obtain pressure
at a Schrader fitting in the evaporator
suction
connection
area,
near
the
expansion valve bulb, and convert to
temperature with a P-T chart.
Subtract the converted temperature from
the measured temperature and the
difference
is
superheat
at
the
evaporator.
Obtain
the
desired
superheat by adjusting the expansion
valve.
Evaporator superheat greater than 14℉
can substantially reduce system capacity,
while superheat less than 4℉ has the
potential for flood back.
Compressor Superheat.
Compressor superheat has an effect on
system
capacity
and
efficiency.
Compressor
superheat
affects
compressor life and recommends a
minimum of 20℉ superheat at the
compressor. Too low a compressor
superheat can permit liquid return to the
compressor causing damage.
Too high a compressor superheat can
cause
high
discharge
temperature,
resulting
in
lubricant
breakdown,
compressor overheating and can lead to
compressor damage or failure.
Compressor superheat can be changed by
adjusting the expansion valve, adding a
suction-liquid line heat exchanger.
Obtain
compressor
superheat
by
measuring the suction line temperature
about 6 to 12 inches from the compressor
service valve.
Obtain pressure at the suction service
valve and convert to temperature with a
P-T chart. Subtract the converted
temperature
from
the
measured
temperature
and
the
difference
is
superheat at the compressor.
