Dehumidification - Daikin MicroTech III Operation Manuals

Dehumidification

In the dehumidification mode, mechanical cooling is used to
cool air low enough to wring moisture out of it . Hot Gas Reheat
or the standard unit heating equipment (Gas, SCR Electric
or Steam/Hot water) is used to raise the temperature of this
cooled air to a desirable value .
Dehumidification Initiation
An analog sensor is mounted in the return duct, the space, or
outdoor to sense relative humidity . The location is selected by
setting the Sensor Location value on the keypad to Return,
Space, or OAT .
NOTE: This setting is used to dictate which temperature
sensor is used to calculate the dewpoint . OAT
can only be selected for units with DAT control .
The relative humidity and a calculated dewpoint
temperature are both displayed on the keypad . The
calculated dewpoint temperature is based on the
relative humidity and Return, Space, or OAT value
depending on the location of the humidity sensor .
Humidity control is disabled if cooling is disabled for
any reason. Dehumidification operation is initiated
when Humidity Control is set to either relative
humidity or dewpoint and that value rises above the
appropriate setpoint by more than half its deadband .
Dehumidification Termination
Dehumidification is terminated if the selected variable, Relative
Humidity or Dew Point, drops below the appropriate humidity
setpoint by more than half its deadband. Dehumidification is
also terminated if cooling is disabled for any reason . Other
ways that dehumidification may be terminated are if Dehum
Method is set to "None" on the keypad, or if the unit goes to Off
Net, Off Sw, Off Alm, or Off Man Ctrl .
When Dehumidification is terminated, all mechanical
cooling is turned off except when the unit is in the Cooling
state . Modulated cooling reverts to normal control when
dehumidification is terminated in the Cooling state. For
units with compressors, the number of cooling stages is
reduced by one and control reverts to normal control when
dehumidification is terminated in the Cooling state. Another
compressor stage change could then occur after one Cooling
Stage Time has elapsed
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Dehumidification Mechanical Cooling Control
During dehumidification, control of mechanical cooling is
based on the following two editable values of the Leaving Coil
Temperature setpoint .
• Mx Lvg Coil T (Default = 52°F)
• Mn Lvg Coil T (Default = 45°F)
For compressorized units, the number of compressor stages
increases when all of the following are true:
• The time since the last stage change exceeds the
Clg Stage Time
• Leaving Coil Temperature is greater than the
Mx Lvg Coil T
• The current cooling stage is less than the available
number of stages
During the dehumidification mode, the number of compressor
stages decreases when all of the following are true:
• The time since the last stage change exceeds the
Clg Stage Time
• Leaving Coil Temperature is less than Mn Lvg Coil T Spt
• The current cooling stage is greater than zero
Units with modulated cooling are controlled by a PI Loop to
maintain the temperature leaving the coil at the Mn Lvg Coil T .
The normal DAT Cooling Deadband and PI Loop parameters
are used in this PI Loop .
MPS HGBP Valve
When the Unit Type is MPS and the unit is equipped with
modulating hot gas reheat control an HGBP valve output
(MCB-DO6) is controlled to make sure hot gas is not flowing
through both the hot gas bypass and hot gas reheat circuits
simultaneously .
The HGBP Valve Output is ON (energized) whenever Unit
State is Cooling and dehumidification operation is inactive. The
HGBP Valve Output is OFF (de-energized) any other time .
97 OM 920-6 • MICROTECH UNIT CONTROLLER
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