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FEATURES
1. The system admits outdoor air for cooling based upon
the economizer decision.
2. Space relative humidity is maintained by controlling both
humidification and dehumidification.
3. Reheat prevents subcooling on dehumidification cycle.
4. Constant temperature and humidity control. (Do not use
this where deadband temperature or humidity control is
acceptable.)
CONDITIONS FOR SUCCESSFUL OPERATION
1. Heating is available during dehumidification cycle.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
1. Anytime the supply fan runs, control system shall be
enabled, and mixing dampers shall position to minimum
ventilation position during scheduled occupancy periods.
2. Anytime the economizer decision program determines
that OA is suitable to assist in cooling, the OA/RA mixing
dampers shall be under control of the SA PI loop.
3. The space humidity PI loop shall modulate the humidifier,
subject to a humidifier leaving air high limit humidity PI
loop setpoint, to maintain the space humidity PI loop
setpoint. Humidifying control shall be disabled anytime
the chilled water valve is modulating or the fan is off.
4. The space humidity PI loop shall override the temperature
controls to modulate the chilled water valve open for
dehumidification if required to maintain the space
humidity PI loop setpoint. The dehumidifying control
loop shall be disabled anytime there is no hot water flow
or temperature.
5. As the SA PI cooling demand varies from 100 to 45%, the
cooling SA PI loop setpoint shall vary from 13 C to 24 C.
6. As the SA PI cooling demand varies from 40 to 0%, the
reheat coil hot water valve SA PI loop setpoint (chilled
water and economizer) shall vary from 11 C to 41 C.
7. The heating coil hot water valve shall modulate to prevent
the cooling coil leaving air temperature from dropping
below 11 C.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
PSYCHROMETRIC ASPECTS
For cooling conditions it is assumed that:
1. Design outdoor air condition is 35 C DB and 26 C WB.
2. RA condition is 24.5 C DB and 18 C WB.
3. System operates on 25 percent minimum outdoor air.
4. Space temperature setpoint is set at 23.5.
5. Space humidity control is set at 50 percent.
6. Coil leaving air temperature is at 13 C.
3
SPACE
23.5 C DB
13 C DB
The following results are obtained:
Item
No.
Explanation
1
Mixed air temperature at cooling design
condition.
2
Air entering the coil is cooled along a line of
constant moisture content until saturation is
approached. Near saturation the moisture con-
tent is reduced as the air is cooled. This process
involves both latent and sensible cooling.
3
Cooling coil leaving air temperature will be as
low as required to satisfy either the space
temperature controller or the space humidity
controller whichever is calling for the greatest
cooling. If dehumidification cools discharge
temperature below setpoint, the heating coil
provides reheat.
4
The space temperature is 23.5 C DB and the
RA temperature is 24.5 C DB. The 1 kelvin
DB rise is an example of sensible cooling load
which may occur in ceiling, space, and RA
ducts. The rise will be a function of system,
building, and lighting arrangement.
253
OA 35 C DB,
26 C WB
2
MA
1
RA 24.5 C DB,
4
17 C WB
1 KELVIN
COOLING
C3263
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