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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 5.6-7.2°C (10-13-) 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. Liquid
sub-cooling must be measured with both
compressors in a refrigeration system
running.
Compare calculated superheat to the
acceptable cooling mode superheat values of
4.4-8.3°C (8-15°F) for all system types.
Superheat will increase with long suction line
runs.
Adjusting Sub-cooling and Superheat
Temperatures
The system is overcharged if the sub-cooling
temperature is too high compared to Table 3
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 of 4.4-8.3°C
(8-15°F) (high superheat results in increased
sub-cooling)
Correct an overcharged system by reducing
the amount of refrigerant in the system to
lower the sub-cooling.
The system is undercharged if the superheat
is too high and the sub-cooling is too low.
Correct an undercharged system by adding
refrigerant to the system to reduce superheat
and raise sub-cooling.
If the sub-cooling is correct and the superheat
is too high, the TXV may need adjustment to
correct the superheat.
DO NOT OVERCHARGE!
Refrigerant overcharging leads to
excess refrigerant in the condenser
coils resulting in elevated compressor
discharge pressure.
EXPANSION VALVE ADJUSTMENT
Thermal expansion valves must be
adjusted to approximately 4.4-8.3°C
(8-15°F) of suction superheat. Failure
to have sufficient superheat will
damage the compressor and void the
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