9
STEAM INJECTION INSTALLATION, OPERATION, AND MAINTENANCE MANUAL
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Sensor placement
OVERVIEW
DC-1084
Outside air
Relief air
Return air
Air handling
unit
8' to 12'
(2.4 m to 3.7 m)
min.
Humidifier dispersion assembly
Turning vanes
Window
Doorway
Window
Point of vapor absorption
Vapor absorption has taken place
Airflow switch or differential pressure switch (sail
type recommended for VAV applications)
High limit humidistat or high limit transmitter (set at
90% RH maximum) for VAV applications
Damper control
C
A
E
F
D
B
E
F
G
F
Wall or
partition
Other factors affecting humidity control
Humidity control involves more than the
controller’s ability to control the system. Other
factors that play an important role in overall
system control are:
• Size of humidification system relative to
load
• Overall system dynamics associated with
moisture migration time lags
• Accuracy of humidistats and humidity
transmitters and their location
• Dry bulb temperature accuracy in space
or duct
• Velocities and airflow patterns in ducts
and space environments
• Electrical noise or interference
mc_072011_1656
SENSOR LOCATION IS CRITICAL
Sensor location has a significant impact on humidifier performance. See the
recommendations below and Figure 9-1.
Note: DriSteem recommends that you do not interchange room and duct
humidity devices. Room humidity devices are calibrated with zero or
little airflow, whereas duct humidity devices require air passing across
them.
Recommended humidity control (transmitter/humidistat) locations:
A Ideal. Ensures the best uniform mix of dry and moist air with stable
temperature control.
B Acceptable, but room environment can affect controllability, such as
when sensor is too close to air grilles, registers, or heat radiation from
room lighting.
C Acceptable. Provides uniform mixture of dry and moist air. If extended
time lag exists between humidity generation and sensing, extend
sampling time.
D Acceptable (behind wall or partition) for sampling entire room if sensor
is near an air exhaust return outlet. Typical placement for sampling a
critical area.
E Not acceptable. These locations might not represent actual overall
conditions in the space.
F Not acceptable. Do not place sensors near windows, door
passageways, or areas of stagnant airflow.
Recommended safety (airflow and high limit) sensor location:
G Best sensing location for high limit humidistat or humidity sensor and
airflow proving switch.
FIGURE 9-1: RECOMMENDED SENSOR LOCATIONS
