Caution – AAON LL-540 User Manual
Page 44
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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 9 for
the appropriate unit type and options.
Table 9 - Acceptable Refrigeration Circuit
Values
Air-Cooled Condenser with Scroll
Compressors
Sub-Cooling
2
12-18°F
Superheat
1
10-15°F
Evaporative-Cooled Condenser with
Scroll Compressors
Sub-Cooling
2
6-10°F
Superheat
1
10-15°F
1
One compressor running in tandem
2
Two compressors running in tandem
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 9 (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.
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 expansion valve
may need adjustment to correct the
superheat.
Special Low Ambient Option Charging
Instructions
For units equipped with low ambient
refrigerant flood back option being charged
in
the
summer
when
the
ambient
temperature is warm:
Once enough charge has been added to get
the evaporator superheat and sub-cooling
values to the correct setting more charge
must be added. Add approximately 80% of
the receiver tank volume to the charge to
help fill the receiver tank. The additional
charge is required for the system when
running in cold ambient conditions.
For units equipped with low ambient
refrigerant flood back option being charged
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