Balancing of a Water and Air System (PDF

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static pressure supplied by the unit manufacturer.
3. Normally, about three complete passes around the entire system are required for
proper adjustment. Make a final pass without adjustments to record the end result.
4. To provide the quietest possible operation, adjust the fan to run at the slowest speed
that provides sufficient nozzle pressure to all units with minimum throttling of all unit and
riser dampers.
5. After balancing each induction system with minimum outside air, reposition to allow
maximum outside air and check power and static pressure readings.
Duct System Pressure
The duct system designer calculates the static pressure losses of the straight section of
ductwork using engineering tables and charts. To these losses, the losses of the entire
duct fitting are calculated and added along with the pressure loss data of all
manufactured items such as filters, coils, dampers and diffusers or grills. The duct
system fan(s) are selected from the total static pressure losses of the longest run(s) of
the supply air and return air ducts connected to the fan(s).
The three duct pressure that technicians measure in the field are total pressure (TP),
static pressure (SP), and velocity pressure (Vp). The three pressures are related by the
following equation:
Total pressure in w.g. (TP) = Static pressure in w.g. (SP) + Velocity pressure in w.g. (Vp)
Static pressure
Static pressure (SP) is exerted equally in all directions at any point or cross section of
the duct. It also is a measure of the potential energy to produce and maintain airflow
against duct resistance. Static pressure may be positive or negative to the atmosphere,
but it can be measured indirectly by subtracting the velocity pressure from the total
pressure (SP = TP – Vp).
Velocity pressure
Velocity pressure (Vp) is exerted only in the direction of airflow and is a measure of
kinetic energy resulting from the airflow. Velocity pressure cannot be measured directly
by a pitot tube and pressure gauge or manometer, but it can be measured indirectly by
subtracting the static pressure from the total pressure (Vp = TP – SP).
Vp = (Velocity in fpm / 4005)²
Total pressure
Total pressure (TP) of a duct is measured by the impact of the moving air stream on the
end of a Pitot tube directly facing and perpendicular to the airflow. Total pressure
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