From Table 12 - 18 lbs refrigerant charge per
circuit (tandem compressors would have 2
circuits in this size of CF).
From Table 14 - 24% of Table 12 charge
Additional charge needed for a unit with low
ambient flooded condenser controls = 18 lbs
* 0.24 = 4.32 lbs additional refrigerant charge
per circuit
Low Ambient Operation
During low ambient temperatures, the vapor
refrigerant will migrate to the cold part of the
system and condense into liquid. All CF
Series compressors are provided with factory
installed crankcase heaters to help prevent
liquid refrigerant from slugging the
compressors during startup in low ambient
conditions. The condenser or condensing unit
must have continuous power 24 hours prior
to startup. This ensures the compressor will
receive sufficient refrigerant vapor at startup.
Standard units can operate down to 12.8°C
(55°F) ambient temperature.
AAON condenser fan head pressure control
units can operate down to 1.7°C (35°F)
ambient temperature. Three different
condenser fan head pressure control options
available are adjustable fan cycling, ECM
condenser fan, or VFD controlled condenser
fans. See detailed information following.
The AAON low ambient (condenser flood-
back) system is used to operate a refrigerant
system down to -17.8°C (0°F) outside air
temperature. See detailed information
following.
Fan Cycling Low Ambient
Adjustable fan cycling is a low ambient head
pressure control option that cycles the
condenser fans to maintain refrigerant circuit
head pressures at acceptable levels during
cooling operation. The head pressure set
point (689.5 – 3240.5 kPa [100-470 psi]) and
pressure differential (241.3 – 1379 kPa
[35-200 psi]) can be field adjusted using a
flathead screwdriver. For example, if the
head pressure is set to 2068.4 kPa (300psi),
and the differential is set to 689.5 kPa
(100psi), then fans will cut in at 2068.4 kPa
(300psi) and cut out at 1379 kPa (200psi).
Fan cycling and variable speed condenser fan
head pressure control options allow
mechanical cooling with ambient
temperatures down to 1.7°C (35°F).
Figure 7 - Adjustable Fan Cycling Switch
Variable Speed Low Ambient
Variable speed condenser fan head pressure
control is a low ambient head pressure
control option that sends a variable signal in
relation to the refrigerant circuit head
pressure of the system to an electronically
commutated motor (ECM) or VFD. The
motor either speeds up or slows down air
flow accordingly in order to maintain
constant head pressure. Fan cycling and
variable speed condenser fan head pressure
control options allow mechanical cooling
with ambient temperatures down to 10.6°C
(35°F).
Flooded Condenser Low Ambient
Flooded condenser low ambient control
maintains normal head pressure during
periods of low ambient. When the ambient
temperature drops, the condensing
temperature and therefore pressure drops.
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