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TRG-TRC014-EN 65
period four
Fan Modulation
notes
included in the supply duct, this method ensures that the sensor is on the fan
side of the damper so that the duct is protected from high pressures. Also,
depending on the layout of the duct system, this method may eliminate the
need for multiple duct-mounted sensors. It is not, however, as energy efficient
as the other methods.
For the supply duct static-pressure control method, the static pressure
sensor is located somewhere in the supply duct system. Typically the location
of the sensor, and consequently the set point of the controller, is two-thirds of
the distance between the supply fan outlet and the critical terminal-unit inlet.
The critical terminal unit is at the end of the supply duct path that represents the
largest total pressure drop.
The sensor is field-installed, and the controller is set to maintain the pressure
corresponding to that location in the duct system at design airflow conditions.
In larger systems with many terminal units, determining the best sensor
location for all load conditions can be difficult—often determined by trial and
error and requiring multiple sensors. Field installation and adjustment of one
or possibly several duct pressure sensors increases installation cost. Without
factory testing, reliability may also suffer compared to the fan outlet method
previously discussed. However, using this method typically allows for fan
energy savings compared to the fan outlet method.
Supply Duct Static-Pressure Control
VAV
VAV
terminal units
terminal units
supply
supply
fan
fan
static
static
pressure
pressure
sensor
sensor
sensor located
2/3 down supply duct
Figure 92