13
PERFECT HUMIDITY]
DEHUMIDIFICATION SYSTEM
Units with the factory−equipped Perfect Humidity
option are capable of providing multiple modes of
improved dehumidification as a variation of the normal
cooling cycle. See Fig 17. The design of the Perfect
Humidity system allows for two humidity control
modes of operation of the rooftop unit, utilizing a
common subcooling/reheat dehumidification coil located
downstream of the standard evaporator coil. This allows
the rooftop unit to operate in both a dehumidification
(Subcooling) mode and a hot gas (Reheat) mode for
maximum system flexibility. The Perfect Humidity
package is factory−installed and will operate whenever
there is a dehumidification requirement present.
The Perfect Humidity system is initiated based on an
input from a discrete input from a mechanical space or
return air humidistat.
Perfect Humidity] Modes
Normal Cooling for Units A17 − A30
During the Normal Cooling mode, the liquid refrigerant
flows from the outdoor condenser through the normally
open (NO) Cooling System Valve (CSV) to the expansion
device. Both the Reheat1 (RH1.x) and Reheat2 (RH2)
valves are closed during the normal cooling mode.
During the Normal Cooling mode, the refrigerant flows
from the outdoor compressor through the condenser coil.
The Reheat2 (RH2.x) is closed, preventing the refrigerant
from bypassing the condenser coil. The refrigerant then
flows through the open Reheat2 (RH1.x) 3−way valve to
the TXV Metering Device, bypassing the Perfect
Humidity coil, and finally passing through the
evaporator coil before returning to the outdoor
compressor. See Fig 17.
Reheat1 (Subcooling Mode) for Units A17 − A30
Th Reheat1 or Subcooling mode will be engaged to
satisfy part−load−type conditions when there is a space
call for cooling and dehumidification. Although the
temperature could have dropped and decreased the
sensible load in the space, the outdoor and/or space
humidity levels could have risen. A typical scenario could
be when the outside air is 85F (29C) with 70% to 80%
relative humidity (RH). Desired Sensible Heat Ratio
(SHR) for equipment in this scenario is typically from 0.4
to 0.7. The Perfect Humidity unit will initiate the
Dehumidification mode when both the space temperature
and humidity are above the temperature and humidity
setpoints while attempting to meet both setpoint
requirements.
Once the humidity requirement is met, the unit can
continue to operate in normal cooling mode to meet any
remaining sensible capacity load. Alternatively, if the
sensible load is met and humidity levels remain high the
unit can switch to Hot Gas Reheat mode or Reheat2 mode
to provide neutral, dehumidified air.
During the Reheat1 or Subcooling mode, the liquid
refrigerant flows from the outdoor compressor through the
condenser coil to the Reheat1 (RH1.x) 3−way valve and
on to the Perfect Humidity coil. The Reheat2 (RH2.x)
valve is closed. The liquid refrigerant then passes through
the Perfect Humidity coil and then a metering device or
Thermostatic Expansion Valve (TXV). From the TXV, the
liquid refrigerant passes through the evaporator coil and
back to the outdoor compressor. See Fig 18.
Reheat2 (Hot Gas Reheat Mode) for A17 − A30
This Reheat2 or Hot Gas Reheat mode is used when dehu-
midification is required without a need for cooling, such
as when the outside air is at a neutral temperature, but
high humidity exists. This situation requires the equip-
ment to operate at a low SHR of 0.0 to 0.2. With no cool-
ing requirement calling for dehumidification, the Perfect
Humidity dehumidification system will energize both
compressors, opening the two hot gas bypass valves, al-
lowing refrigerant flow to the Perfect Humidity coil to
reheat the unit’s supply air to a neutral temperature.
The hot bypassed refrigerant liquid (gas or two−phase
mixture) exits the outdoor compressor and passes through
the open Reheat1 (RH1.x) at the same time it passes
through the condenser coil to the open Reheat2 (Rh2.x) to
the Perfect Humidity coil. After the refrigerant passes
through the Perfect Humidity coil, it enters a TXV
metering device, decreasing the air pressure, and on to the
evaporator coil. The refrigerant is subcooled in this coil to
a temperature approaching the evaporator leaving air
temperature. The liquid refrigerant then returns to the
outdoor compressor. See Fig. 19.
The refrigerant enters the TXV and evaporator coil at a
temperature lower than the temperature in the standard
cooling operation. This lower temperature increases the
latent capacity of the evaporator. The refrigerant passes
through the evaporator turning it into a superheated vapor.
The air passing over the evaporator coil becomes colder
than it would during normal operation. As this same air
passes over the Perfect Humidity Reheat Coil, it will be
warmed to the neutral supply air temperature.
Perfect Humidity] System Components
The Perfect Humidity System uses the standard unit
compressor(s), evaporator coil and Round Tube−Plate Fin
(RTPF) condenser coil. Additional refrigeration system
hardware includes a subcooler/reheat coil and control
solenoid valves. On some models, the evaporator coil
includes a TXV as a standard feature. Units with Perfect
Humidity FIOP also include a factory−installed head
pressure control system (Motormaster I) to provide proper
liquid pressure during reheat modes. Unique system
controls include a reheat relay mode, and evaporator coil
freezestat, and secondary low pressure switch.