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44
Heating -- Units With Economi$er
IV
When the room temperature calls for heat, the heating controls are
energized as described in the Heating, Units Without Economizer
section. During the occupied mode, whenever the indoor fan
contactor is energized, the economizer damper moves to the
minimum position.
Units With Perfect Humidity
Adaptive
Dehumidification
System
Normal Design Operation
When the rooftop operates under the normal sequence of
operation, the compressors will cycle to maintain indoor
conditions. (See Fig. 44.)
The Perfect Humidity adaptive dehumidification system includes
a factory-installed Motormaster low ambient control to keep the
head and suction pressure high, allowing normal design cooling
mode operation down to 0F.
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. 45.)
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 further
subcooled to a temperature approaching the evaporator
leaving-air temperature. The liquid then enters a thermostatic
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 refrigerant 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
temperature is warmer than 40_F. A factory-installed temperature
switch located in the condenser section will lock out subcooling
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 additional
refrigerant charge required by the subcooling/reheat coil.
When in subcooling mode, there is a slight decrease in system
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 control, 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. 46.)
This hot gas will mix with liquid refrigerant leaving the
condenser coil and flow to the subcooling/reheat
dehumidification coil. Now the conditioned air coming off the
evaporator will be cooled and dehumidified, but will be warmed
to neutral conditions (72_Fto75_F) by the subcooling/reheat
dehumidification coil.
C06135
Fig. 44 --- Perfect Humidityt Normal
Design Cooling Operation
C06136
Fig. 45 --- Perfect Humidity Subcooling
Mode Operation
C06137
Fig. 46 --- Perfect Humidity Hot Gas
Reheat Mode Operation
See T able 37 for the Perfect Humidity adaptive dehumidification
system sequence of operation.
581B,C