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Subcooling Mode
When subcooling mode is initiated, this will energize (close)
the liquid line solenoid valve (LLSV) forcing the hot liquid
refrigerant to enter into the subcooling coil (see Fig. 41).
As the hot liquid refrigerant passes through the subcooling/
reheat dehumidification coil, it is exposed to the cold supply
airflow coming through the evaporator coil. The liquid is fur-
ther subcooled to a temperature approaching the evaporator
leaving-air temperature. The liquid then enters a thermo-
static expansion valve (TXV) where the liquid drops to a
lower pressure. The TXV does not have a pressure drop great
enough to change the liquid to a 2-phase fluid, so the liquid
then enters the Acutrol™ device at the evaporator coil.
The liquid enters the evaporator coil at a temperature lower
than in standard cooling operation. This lower temperature
increases the latent capacity of the rooftop unit. The refriger-
ant passes through the evaporator and is turned into a
vapor. The air passing over the evaporator coil will become
colder than during normal operation. However, as this same
air passes over the subcooling coil, it will be slightly warmed,
partially reheating the air.
Subcooling mode operates only when the outside-air temper-
ature is warmer than 40 F. A factory-installed temperature
switch located in the condenser section will lock out subcool-
ing mode when the outside temperature is cooler than 40 F.
The scroll compressors are equipped with crankcase heaters
to provide protection for the compressors due to the addi-
tional refrigerant charge required by the subcooling/reheat
coil.
When in subcooling mode, there is a slight decrease in sys-
tem total gross capacity (5% less), a lower gross sensible
capacity (20% less), and a greatly increased latent capacity
(up to 40% more).
Hot Gas Reheat Mode
When the humidity levels in the space require humidity con-
trol, a hot gas solenoid valve (specific to hot gas reheat mode
only) will open to bypass a portion of hot gas refrigerant
around the condenser coil (see Fig. 42).
This hot gas will mix with liquid refrigerant leaving the con-
denser coil and flow to the subcooling/reheat dehumidifica-
tion coil. Now the conditioned air coming off the evaporator
will be cooled and dehumidified, but will be warmed to neu-
tral conditions (72 F to 75 F) by the subcooling/reheat dehu-
midification coil.
NOTE: The 551B090-150 rooftop units can operate one cir-
cuit in subcooling mode and one circuit in hot gas reheat
mode or both circuits in hot gas reheat mode, or both in nor-
mal design cooling mode.
The net effect of the rooftop when in hot gas reheat mode is
to provide nearly all latent capacity removal from the space
when sensible loads diminish (when outdoor temperature
conditions are moderate). When in hot gas reheat mode, the
unit will operate to provide mostly latent capacity and
extremely low sensible heat ratio capability.
Similar to the subcooling mode of operation, hot gas reheat
mode operates only when the outside air temperature is
warmer than 40 F. Below this temperature, a factory-installed
outside air temperature switch will lock out this mode of
operation.
See Table 29 for the Perfect Humidity dehumidification sys-
tem sequence of operation.
Fig. 40 — Perfect Humidity™ System
Normal Design Cooling Operation
Fig. 41 — Perfect Humidity System
Subcooling Mode Operation
Fig. 42 — Perfect Humidity System
Hot Gas Reheat Mode Operation