Pressurization; Building Pressure Balance; Containment Pressurization - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
Theoretically, the mixing dampers may be modulated to
maintain a constant OA volume during occupied periods using
an OA duct airflow measuring station. Since, in these examples,
the OA duct is sized for 100 percent OA, the minimum is usually
20 to 25 percent of the maximum. The airflow velocity at
minimum airflow is extremely low, and velocity pressure
measurement is usually not practical. Hot wire anemometer
velocity sensing at these velocities is satisfactory, but costly,
and requires that the sensing element be kept clean to maintain
accuracy. (Filtering the entering OA is helpful.) A smaller,
minimum OA damper and duct may also be used to assure
adequate airflow velocity for velocity pressure measurement.
If a return fan and volumetric tracking return fan control are
used, and no relief/exhaust dampers exist, or if the RA damper
is closed and the MA damper is open during occupied non-
economizer modes, the OA volume equals the SA volume minus
the RA volume. This method is simple and low cost but is only
applicable when building exhaust and exfiltration meets
minimum ventilation requirements.
Where minimum OA only is provided (no economizer
dampers), variations of any of these methods may be used. See
the Air Handling System Control Applications section for
further information.

PRESSURIZATION

Building pressurization and ventilation are important aspects
of airflow control. A building or areas within a building can be
pressurized for positive, negative, or sometimes neutral static
pressure to control the flow of air from one area to another. A
building can use positive static pressure to eliminate infiltration
drafts, possible dirt, and contamination from outside sources.
Areas within a building such as laboratories can use negative
pressure to prevent contamination and exfiltration to adjacent
spaces and zones. Proper building pressurization must also
consider the effects of outdoor wind pressure and stack effect
or vertical air differences.

BUILDING PRESSURE BALANCE

The pressures in a building must be balanced to avoid airflow
conditions which make it difficult to open and close doors or
conditions which cause drafts. Buildings have allowable maximum
and minimum static pressures that control these conditions. A
force of 130 through 220 newtons is considered the maximum
reasonable force to open a door. The equation used to calculate
the force to overcome the pressure difference across a door as
well as to overcome the door closer is as follows:
Kd x W x A x p
F = F DA +
2 x (W – D)
Where:
F = Total door opening force applied at the knob
in newtons (N)
F = Force to overcome the door closer, applied
DA
at the knob, in newtons (N). This force is
normally between 13 and 90N.
W = Door width in meters (m)
A = Door area in square meters (m
P = Differential static pressure across the door in
pascals (Pa)
D = Distance from the door knob to the knob
side of the door in meters (m)
Rearranging the equation to calculate the differential pressure
results in the following:
(F - F DA ) x [2 x (W – D)]
p =
EXAMPLE:
Calculate the differential pressure for a 0.9m wide x 2.1m
high door that has a 130N opening force, a 45N force to
overcome the door closer, and 0.08m between the door knob
and the door edge.
(130 – 45) x [2 x (0.9 - 0.08)]
p =
p = 82.0 Pa
Similarly, the maximum pressure difference which overcomes
a 45N door closer is as follows:
(0 - 45) x [2 x (0.9 - 0.08)]
p =
p = 43.4 Pa

CONTAINMENT PRESSURIZATION

In an airflow system for a building that requires containment
pressurization, the direction of infiltration is toward the space
with the contaminants. The direction is controlled by regulating
the pressure differentials between the spaces. For example, a
laboratory is typically kept at a negative pressure relative to
surrounding spaces so that any infiltration is into the laboratory.
The static pressure difference required for containment varies
with the specific application. In buildings with areas that require
smoke control, a minimum static pressure difference of 5.0
through 10.0 Pa is suggested to control cold smoke. (Cold smoke
is generated when water spray from a sprinkler system cools
smoke from a building fire.)
270 ENGINEERING MANUAL OF AUTOMATIC CONTROL
)
2
W x A
0.9 x 1.89
0.9 x 1.89