AAON CB-060 - Manual (Page 22)

22
Units equipped with hot gas reheat must be
charged with the hot gas valves closed while
the unit is in cooling mode. 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 Table 3 below when
determining the proper sub-cooling.
The vertical rise of the liquid line must be
known in order to adjust the sub-cooling
range for proper charge.
Checking Liquid Sub-cooling
Measure the temperature of the liquid line as
it leaves the condenser coil.
Read the gauge pressure reading of 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 or Table 5 below.
Subtract
the measured liquid line
temperature from the saturated temperature
to determine the liquid sub-cooling.
Compare calculated sub-cooling to Table 3
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 or Table 5 below.
Subtract the saturated temperature from the
measured suction line temperature to
determine the evaporator superheat.
Compare calculated superheat to Table 3
below for the appropriate unit type and
options.
Table 3 - Sub-cooling and Superheat
Air-Cooled Condenser
Sub-Cooling* 12-18°F
Sub-Cooling with
Hot Gas Reheat*
15-22°F
Superheat** 8-15°F
Air-Source Heat Pump
Sub-Cooling* 2-4°F
Sub-Cooling with
Hot Gas Reheat*
15-22°F
Superheat** 16-22°F
* Sub-cooling must be increased by 2°F per
20 feet of vertical liquid line rise for R-410A
** 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 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 3
above (high superheat results in increased
sub-cooling)