14
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.
Refrigerant Charging.
When charging a system with refrigerant that is in
a vacuum with vacuum pump, the above process
is very important to remove moisture inside
system. The moisture can cause system damage or
failure.
Charge refrigerant into a system through a
