ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
236
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
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.
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.
M15169
MA
SA
4
5
50
ON
1
CONTROL
PROGRAM
3
2
27
SUPPLY
FAN
27
C3248
MA
SA 25°C DB,
16°C WB
RA 22°C DB,
45% RH
1
2
3
4
OA
4°C DB, 1.5°C WB
COIL
DISCHARGE
STEAM HUMIDIFICATION PROCESS
DETERMINES SLOPE OF THIS LINE
ENTHALPY
HUMIDITY RATIO
=
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