Honeywell AUTOMATIC CONTROL, Engineering Manual

Page: 253 / 518

ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
243
DEHUMIDIFICATION CONTROL PROCESSES
M10472
MA
SA
9
6
5
2
100
32
SUPPLY
FAN
COOL
HEAT
N.O.
ON
1
CONTROL
PROGRAM
3
4
8
7
76
55
55
72
N.O.
74
Item
No. Function
1 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL
).
2-4 Space temperature PI control loops have heat
and cool setpoints with deadband.
5,6 Space humidity sensor and setpoint enable
dehumidification.
7,8 Heating and cooling valves position for heat,
cool, and dehumidify cycles. The valves close
when the fan is off.
9 Control program coordinates cooling, heating,
dehumidifying, and fan and hot water
interlock control.
FEATURES
1. This application uses two-position control of a direct
expansion or chilled water coil for dehumidification.
2. This application controls both space temperature (with a
dead band) and dehumidification.
3. Dehumidification cycle is disabled if hot water flow is
not available.
4. Heating setpoint raised slightly during dehumidification
cycle to reduce cooling (dehumidifying) energy.
CONDITIONS FOR SUCCESSFUL OPERATION
1. If a chilled water coil is used, the water supply is cold
enough to produce the lowest required dew point.
2. The cooling coil valve should be two-position controlled
for the dehumidification cycle for quick and maximum
dehumidification. A space RH differential is required for
two-position control, such as dehumidification on a rise
to 60% RH and off on a drop to 55% RH. Modulating
dehumidification control of a chilled water valve, which
delays moisture removal until the chilled water flow gets
high enough to cause condensation, is also satisfactory.
3. Hot water flow is available for reheat or the space will
get very cold and humid during periods when
dehumidification is demanded and the cooling load is
light.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
1. Anytime the supply fan runs, control system shall
be enabled.
2. The chilled water valve and hot water valve shall be
modulated as required to maintain their separate heating
and cooling space temperature setpoints.
3. Anytime the space relative humidity rises above it’s
setpoint, the cooling valve shall position full open for
maximum dehumidification. In this mode, the heating
space temperature setpoint shall be midpoint between the
heating and cooling setpoints, and the hot water valve
shall be modulated to prevent the dehumidification cycle
from overcooling the space.
4. The dehumidification cycle shall be disabled anytime
there is no hot water flow or temperature.
PSYCHROMETRIC ASPECTS
In comfort air conditioning, dehumidification is typically
required when the OA is cool and moist and the solar load is
too low to demand enough cooling for dehumidification. The
space relative humidity is maintained at or below a desired
value, depending on the moisture content of the air entering
the cooling coil.
The following applications show various methods of
controlling dehumidification in air conditioning systems.
DIRECT EXPANSION OR WATER
COIL SYSTEM CONTROL
Functional Description