Low Ambient Operation
During low ambient temperatures, the vapor
refrigerant will migrate to the cold part of
the system (condenser) and condense into
liquid. All CB 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 55°F
ambient temperature.
AAON head pressure control units can
operate down to 35°F ambient temperature.
Two different head pressure control options
available are adjustable fan cycling or ECM
condenser fan. See detailed information
following.
The AAON low ambient (condenser flood-
back) system is used to operate a refrigerant
system down to 0°F outside air temperature.
See detailed information below.
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 (100-470 psi) and pressure
differential (35-200 psi) can be field
adjusted using a flathead screwdriver. For
example, if the head pressure is set to
300psi, and the differential is set to 100psi,
then fans will cut in at 300psi and cut out at
200psi. Fan cycling and variable speed
condenser fan head pressure control options
allow mechanical cooling with ambient
temperatures down to 35°F.
Figure 9 - 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 to an electronically
commutated motor a variable signal in
relation to the refrigerant circuit head
pressure of the system. 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 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.
Without ambient control, the system would
shut down on low discharge pressure. The
flooded condenser method of low ambient
control fills the condenser with liquid
refrigerant, decreasing the heat transfer
capacity of the coil, which allows the coil to
operate at an acceptable discharge pressure.
The condenser coil will not be flooded
during summer ambient temperatures, so a
receiver is included to store the additional
liquid refrigerant required to flood the
condenser in low ambient.
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