AAON RL-240 - Manual (Page 46)

46
get the proper charge. After charging, unit
should
be operated in reheat
(dehumidification) mode to check for
correct operation.
After adding or removing charge the system
must be allowed to stabilize, typically 10-15
minutes,
before making any other
adjustments.
The type of unit and options determine the
ranges for liquid sub-cooling and evaporator
superheat. Refer to the table below when
determining the proper sub-cooling.
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
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 subcooling should be measured with
both compressors in a refrigeration system
running.
Compare calculated superheat to the table
below for the appropriate unit type and
options.
Table 12 - Acceptable Sub-Cooling and Superheat
Temperatures
Air-Cooled Condenser
Sub-Cooling
2
12-18°F
Sub-Cooling with
Hot Gas Reheat
2
15-22°F
Superheat
1
10-15°F
Evaporative-Cooled Condenser
Sub-Cooling
2
6-10°F
Sub-Cooling with
Hot Gas Reheat
2
8-12°F
Superheat
1
10-15°F
Water-Cooled Condenser
Sub-Cooling
2
6-10°F
Sub-Cooling with
Hot Gas Reheat
2
8-12°F
Superheat
1
10-15°F
1
One compressor running in tandem
2
Two compressors running in tandem