AAON CN-140 - Manual (Page 31)

31
For units equipped with low ambient (0°F)
option see the special charging instructions
at the end of this section.
Checking Liquid Sub-cooling
Measure the temperature of the liquid line as
it leaves the condenser coil.
Read the gauge pressure at the liquid line
close to the point where the temperature was
taken. You must use liquid line pressure as it
will vary from discharge pressure due to
condenser coil pressure drop.
Convert the pressure obtained to a saturated
temperature using the appropriate refrigerant
temperature-pressure chart.
Subtract
the measured liquid line
temperature from the saturated temperature
to determine the liquid sub-cooling.
Compare calculated sub-cooling to the table
below for the appropriate unit type and
options.
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 line temperature to
determine the evaporator superheat.
For refrigeration systems with tandem scroll
compressors, it is critical that the suction
superheat setpoint on the expansion valve 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 7 for
the appropriate unit type and options.
Table 7 - Acceptable Refrigeration Circuit
Values
Air-Cooled Condenser
Sub-Cooling 12-18°F
Sub-Cooling with
Hot Gas Reheat
15-22°F
Superheat 8-15°F
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 Table 7 (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.
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