Handbook of air conditioning and refrigeration / Shan K

23.24 CHAPTER TWENTY-THREE
FIGURE 23.8 Duct static pressure control.
Set Point of Duct Static Pressure and Sensor’s Location
The set point of the duct static pressure should be the minimum static pressure such that when added
to its associated velocity pressure, their sum—the total pressure—should overcome the maximum
total pressure loss in any of the branch takeoffs including VAV boxes, flexible ducts, diffusers, and
dynamic losses under all operating conditions. The duct static pressure is more stable than the duct
total pressure in a supply main duct when the airstream is flowing. More often, the set point of the
duct static pressure in VAV systems ranges between 0.8 and 1.5 in. WG (200 and 375 Pag)
The static pressure sensor should be located in the supply main duct in a position such that the
required supply volume flow rate is guaranteed for all VAV boxes with greatest energy savings during
part load. The nearer to the farthest branch takeoff, the greater the energy saving because �p fix
is nearly minimum. For most VAV systems, the duct pressure sensor is often located in the main
duct near the farthest branch takeoff and at a location where steady static pressure can be measured.
Duct static pressure control is discussed also in Sec. 29.5.
Warren and Norford (1993) recommended that the duct static pressure set point be reset based
on messages from each VAV box collected by a poll manager control block during operation. Such
a DDC control system is complicated and expensive.
ASHRAE/IESNA Standard 90.1-1999 specifies that duct static pressure sensors used to control
variable-air-volume fan operating characteristics shall be placed in a position such that the controller
set point is no greater than one-third of the total design fan (supply) static pressure except for direct
digital control systems with zone reset capability where the sensor may be located at the fan discharge.
For a supply duct system with major duct splits, multiple sensors should be installed in each
major branch to ensure static pressure can be maintained in each of the major branches.
For air systems with direct digital control of individual zone VAV boxes connecting to the central
system controller, the duct static pressure set point shall be reset based on the zone requiring the
most pressure, that is, the set point is reset lower until one zone damper is nearly wide open.
Comparison between Adjustable-Frequency Drives and Inlet Vanes
In VAV systems, both adjustable-frequency variable-speed drives and inlet vanes are used to modulate
the volume flow and fan total pressure of the centrifugal supply, return, relief, and exhaust fans,
as described in Sec. 15.4. Generally, fans using adjustable-frequency variable-speed drive to vary