41
Checking Evaporator Superheat
Measure the temperature of the suction line
close to the evaporator.
Read gauge pressure at the suction line close
to the evaporator.
Convert the pressure obtained to a saturated
temperature using the appropriate refrigerant
temperature-pressure chart.
Subtract the saturated temperature from the
measured suction line temperature to
determine the evaporator superheat.
Compare calculated superheat to the
acceptable cooling mode superheat values
between 4.4 and 8.3°C (8 - 15°F) for all
system types. Superheat will increase with
long suction line runs.
For refrigeration systems with tandem
compressors, it is critical that the suction
superheat setpoint on the TXV is set with one
compressor running. The suction superheat
must be 5.6 and 7.2°C (10-13°F) with one
compressor running. The suction superheat
will increase with both compressors in a
tandem running. Inadequate suction
superheat can allow liquid refrigerant to
return to the compressors which will wash the
oil out of the compressor. Lack of oil
lubrication will destroy a compressor.
Measure liquid sub-cooling with both
compressors in a refrigeration system
running.
Adjusting Sub-Cooling and Superheat
Temperatures
The system is overcharged if the sub-cooling
temperature is too high and the evaporator is
fully loaded (low loads on the evaporator
result in increased sub-cooling) and the
evaporator superheat is within the
temperature range as shown in the table
above (high superheat results in increased
sub-cooling).
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