Cooling Control Processes; Control Of Modulating Chilled Water Coil Three-Way Valve - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

AIR HANDLING SYSTEM CONTROL APPLICATIONS
SPECIFICATIONS
1. The humidifier shall be modulated by a space humidity
PI control loop to maintain the humidity setpoint.
2. A humidifier leaving air humidity high limit control loop
shall disable the humidifier if the humidity rises above
the high-limit setpoint.
3. The humidifier shall be off upon loss of actuator motive
force and when the fan is off.
PSYCHROMETRIC ASPECTS
The steam humidification process is almost isothermal.
In the following chart it is assumed that:
1. Space conditions are 22 C and 45% RH.
2. Mixed air (entering the heating coil) is 16 C and 6 C
DP.
3. Additional moisture required to maintain space conditions
is 3.0 grams per kg of dry air.

COOLING CONTROL PROCESSES

The following are common control arrangements for cooling.

CONTROL OF MODULATING CHILLED WATER COIL THREE-WAY VALVE

Functional Description
3
27
CONTROL
PROGRAM
27
2
50
MA
5
SA
4
1
ON
SUPPLY
FAN
M15169
STEAM HUMIDIFICATION PROCESS
DETERMINES SLOPE OF THIS LINE
ENTHALPY
=
HUMIDITY RATIO
MA
1
OA
4 C DB, 1.5 C WB
Item
No. Explanation
1 RA and dry OA mix and enter a heating coil.
2 Heating coil leaving air; air gains
sensible heat.
3 Humidifier leaving air; air gains mostly
latent heat.
4 RA; cooled via heat loss and humidity
reduced.
Item
No. Function
1 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
2, 3 Space temperature PID control loop
modulates the three-way valve to maintain
space temperature setpoint.
4 Chilled water valve directs flow through or
around coil as needed to furnish proper
amount of cooling.
5 Control program coordinates temperature
control and fan status.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
236
RA 22 C DB,
45% RH
SA 25 C DB,
3
4
16 C WB
2
COIL
DISCHARGE
C3248