ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
235
HUMIDIFICATION CONTROL PROCESS
Item
No. Function
1 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
2,3 Space humidity PI control modulates the
humidifier to maintain the minimum space
relative humidity setpoint.
4,5 Humidifier leaving air humidity high limit
proportional control modulates humidifier off,
if necessary, to prevent relative humidity from
exceeding the setpoint.
6 Humidifier is off whenever the fan is off.
7 Control program coordinates digital control
loops and fan interlock.
FEATURES
1. Moisture is added with only a slight increase in dry bulb
temperature (steam humidifier).
2. Humidification is turned off when the fan is stopped to
prevent accumulation of moisture in the ducts.
3. Duct humidity high-limit keeps air below saturation
moisture-condensing point.
CONDITIONS FOR SUCCESSFUL OPERATION
1. The steam pressure to the valve is kept at a constant value
between 35 and 85 kPa.
2. The air passing through the humidifier is warm enough
to absorb the required amount of moisture. The best
humidifier location is after a heating coil.
3. The high-limit humidistat is set relatively high, about 90
percent relative humidity.
4. If the digital controller is in a network and the OA
temperature value is available, the comfort humidification
system is disabled when the OA temperature is above a
summer value (18°C).
5. Check recommended applications for specific humidifier
furnished.
6. Where humidifiers have a separate steam jacket, a
separate valve may be added to shut down the jacket steam
during prolonged off periods to minimize heat loss. The
jacket keeps the humidifier hot so when humidification
is required, the humidifying steam does not condense
inside the humidifier and enter the duct as water.
Humidification is a process of adding moisture to air. The most
commonly used humidifier type is the steam jet. Humidifier
requirements vary; check manufacturers recommendations.
Although steam jet humidifiers are depicted, other modulating
types control similarly. On-off humidifiers require a differential
in addition to a setpoint.
Generally, humidifiers should be locked off during periods
of mechanical cooling.
CONTROL OF MODULATING HUMIDIFIER
Functional Description
M15168
OA
RA
SA
7
6
4
5
52
N.C.
ON
1
CONTROL
PROGRAM
3
2
45
88
52
52
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