M52 Microprocessor User's Guide
M52UMCT Oct-03 8
The Dehum. Solenoid closes off refrigerant flow to part of the evaporator coil. This causes a
lower coil temperature. The coil begins to remove more moisture as a result. The air is
partially reheated, as the air through the inactive part of the evaporator is unconditioned.
This creates an overall drop in the sensible heat ratio (Rapid Dehumidification)
During dehumidification demand, some sensible cooling takes place. Under very low load
conditions, the sensible cooling effect maybe greater than the combined offsetting effect of
the heat load plus the available reheat capacity, which may cause a drop in room temperature.
If temp continues to drop and is less than (setpoint MINUS (temp dead band DIVIDED by 2)
MINUS heating proportional band) the compressors will stop overriding the demand for
Dehum. The temperature must return to set point before another cycle of dehumidification
starts.
ie: Low limit cutout =[
[[
[Set point – (temp deadband ÷
÷÷
÷2) – heating propband]
]]
]
Compressor stops when:
• Loss of demand
Compressor contactor opens
• Low pressure condition
Compressor stops when LP SW. opens. (Automatic reset) compressor contactor opens.
• Short cycling alarm
After three consecutive low-pressure alarms in 30 mins, compressor contactor opens.
Manual reset is required.
• High pressure condition
Compressor stops when HP SW. opens. (Manual reset)
Compressor contactor opens. Compressor will not start until HP SW. and controller are
reset.
• Thermal protection opens (automatic reset)
Line voltage cut-outs on all compressors except Spectre compressor where the
compressor module causes a Compressor Overload alarm and stops the compressor
through the microprocessor. Compressor module resets after 30 minutes.
• Low limit temperature
If return air temperature continues to drop and is less than (setpoint MINUS (temp dead
band DIVIDED by 2) MINUS heating proportional band) the compressors will stop
overriding the demand for Dehum.
Note: Anytime the compressor contactor opens the comp. elapse timer keeps compressor off
for 3 minutes (adjustable)
Dual Cooling Units: Series 9 only
Dual cooled units operate on DX as described above in DX Cooling Demand and DX
Dehumidification Demand. In DX mode the chilled water valve is closed. When a remote
signal contact input closes on both Chiller Ready Terminal 31 and 32 and Compressor
Disable Terminal 33 and 34 the compressors will stop and the unit will operate as described
in Chilled Water above.
Free Cooling Units: Series 9 only
Free cooling units operate on DX as described above in DX Cooling Demand and DX
Dehumidification Demand when the glycol loop temperature is above 7.2°C, (45°F). The
modulating glycol valve begins to open when the difference between the return air temperature
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