AAON V3-A Installation, Operation & Maintenance Instructions Manual page 53

Checking Evaporator Superheat
Measure the temperature of the suction line
close to the compressor.
Read gauge pressure at the suction line close
to the compressor.
Convert the pressure obtained to a saturated
temperature using the appropriate refrigerant
temperature-pressure chart.
Subtract the saturated temperature from the
measured suction
determine the evaporator superheat.
Compare calculated
acceptable cooling mode superheat values of
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
should be 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.
Liquid sub-cooling should be measured with
both compressors in a refrigeration system
running.
CAUTION
Thermal expansion valve must be
adjust to approximately 8-15°F of
suction superheat. Failure to have
sufficient superheat will damage the
compressor and void the warranty.
line temperature to
superheat to the
Adjusting Sub-Cooling
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
temperature range as shown in the table
above (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.
CAUTION
DO NOT OVERCHARGE!
Refrigerant overcharging leads to
excess refrigerant in the condenser
coils resulting in elevated compressor
discharge pressure.
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. Before adjusting the
TXV, verify the sensing bulb is in the correct
position according to Figure 16 and follows
the guidelines below.
1. The suction line is clean where the sensing
bulb is attached.
2. The entire length of the sensing bulb is in
contact with the suction line.
and Superheat
is within the
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