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.