AAON LC-054 - Manual (Page 21)

21
Compare calculated sub-cooling to the table
below for the appropriate unit type and
options.
Measure the temperature of the suction line
close to the compressor.
Checking Evaporator Superheat
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 line temperature to
determine the evaporator superheat.
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.
Compare calculated superheat to Table 2 for
the appropriate unit type and options.
Table 2 - Acceptable Refrigeration Circuit
Values
Air-Cooled Condenser
Sub-Cooling 2 12-18°F
Superheat 1 10-15°F
1 One compressor running in tandem
2 Two compressors running in tandem
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
Adjusting Sub-cooling and Superheat
Temperatures
Table 2 (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.
Thermal expansion valves must be
adjusted to approximately 10-15°F of
suction superheat. Failure to have
sufficient superheat will damage the
compressor and void the warranty.
CAUTION
DO NOT OVERCHARGE!
Refrigerant overcharging leads to
excess refrigerant in the condenser
coils resulting in elevated compressor
discharge pressure.
CAUTION