Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual page 255

FEATURES
1. Better dehumidification by having the chilled water flow
sequence slightly ahead of the face damper opening to
keep a low dew point temperature.
2. This application controls heating, cooling (with a dead
band), and dehumidification.
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. To prevent frosting the coil under light load, if direct
expansion coil is used, proper valve cut-in, staging, and
cut-out settings related to the face and bypass damper
position are determined.
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.
4. During some chilled water coil loadings, the coil leaving
water temperature may drop quite low. If this is
undesirable to the chiller plant, a coil leaving air
temperature low limit PID loop will reduce the effects.
5. If a DX coil is used, the liquid line solenoid valve is staged
with the damper position and software timers are provided
to prevent frosting the coil and to prevent short cycling
the refrigeration system.
NOTE: The balance of this discussion is for a chilled
water coil system.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, the control system shall
be enabled.
The hot water valve shall be modulated as required to
maintain the heating space temperature setpoint. Anytime the
chilled water valve is full open, because of a demand for
dehumidification, the actual heating space temperature setpoint
shall be midpoint between the heating and cooling setpoints.
As the space temperature cooling PI loop or the space
humidity high limit PI loop demand (whichever demand is
greater) for cooling varies from 0 to 60 percent, the chilled
water valve shall position from closed to open. As the space
temperature or the humidity PI loop demand (whichever
demand is greater) for cooling varies from 10 to 100 percent
demand the face and bypass dampers shall position from closed
to open to the coil face.
The dehumidification cycle shall be disabled anytime the hot
water pump is off or the hot water temperature is inadequate.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
PSYCHROMETRIC ASPECTS
The space relative humidity is maintained at or below a
desired value depending on the moisture content of the air
entering the coil.
In the following chart it is assumed that:
1. Chilled water (not DX) is used
2. Desired space condition is 27 C DB and 50% RH.
3. Outdoor air condition is 27 C DB and 24 C WB.
4. Air entering the system is from the MIXED AIR
CONTROL WITH ECONOMIZER CYCLE application.
The system operates on 25 percent outdoor air during
the cooling cycle.
5. Cooling coil leaving air temperature is 13 C.
3
COOLING COIL
DISCHARGE
13 C DB
The following results are obtained:
Item
No. Explanation
1 Mixed air temperature.
2 Air entering the cooling 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 This point represents air entering the heating
coil which is a mixture of cooling coil leaving
air through the face damper and the mixed air
through the bypass damper. If the cooling or
dehumidification load decreases, air entering the
heating coil will move toward the mixed air
condition. If the cooling or dehumidification
load increases, air entering the heating coil will
move toward the cooling coil leaving air
condition.
4 As the cooling sensible load decreases, the
heating coil valve will open and Point 4 moves
on a constant moisture line from Point 3.
245
OA 27 C DB,
24 C WB
2
1
RA 27 C DB,
4
50% RH
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