Exhaust System Control - Honeywell AUTOMATIC CONTROL Engineering Manual

OUTDOOR
SUPPLY
STATIC
FAN
PRESSURE
SENSOR
Fig. 40. Control of Return Dampers in
Zone Airflow Control.
Airflow tracking is preferred for zone control on the first
floor. Direct space static pressure control is difficult to stabilize
because of sudden static pressure changes that occur whenever
doors open. Also, the leakage around doors would require
pressure control at very low setpoints, which are difficult to
measure. On upper floors where building permeability is tight,
airflow tracking control is not as viable. Direct space static
pressure control is preferred for zone control on these floors.
Relatively small differences in airflow tracking differentials of
tightly sealed zones result in large pressure differentials. Also,
all exhaust airflows must be included when dealing with airflow
tracking, which makes the control more complex.
Both airflow tracking and direct space static pressure control
require accurate sensing. In airflow tracking, the airflow sensors
are located in supply and return ducts to sense total airflow.
Minimum velocities and location of the airflow sensor relative
to any variations from a straight duct are critical considerations.
In direct space static pressure control, the indoor static pressure
sensor should be in the largest open area and away from doors
that open to stairways and elevators. The outdoor static pressure
sensor should be at least 15 feet above the building (depending
on surrounding conditions) and be specifically designed to
accommodate multidirectional winds.
For zone control using airflow tracking or direct space static
pressure, return fan control should hold duct pressure constant
at a point about two-thirds of the duct length upstream of the
return fan (Fig. 41). This control is the same as that used to
control the supply fan, except that the duct pressure is negative
relative to the ambient surrounding the duct.
INDOOR RETURN
FAN
STATIC
PRESSURE
SENSORS
(2)
CONTROLLERS
(3)
AIRFLOW
SENSOR (2)
C2664
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
Fig. 41. Control of Return Fan in Zone Airflow Control.
To ensure minimum outdoor airflow, an airflow sensor
enables control and provides information on the quantity of
outdoor air. The airflow sensor is located in the duct having
minimum outdoor airflow (Fig. 42). The control modulates the
outdoor air, return air, and exhaust air dampers to provide
outdoor air as needed. Normally, the difference between total
supply and return airflows, as determined by zone controls,
provides the minimum outdoor air. Since each zone is set to
provide proper pressurization and buildings which are sealed
tightly require less outdoor air for pressurization, this control
scheme ensures minimum outdoor air. If minimum outdoor air
is increased, it does not affect building pressurization.
AIRFLOW SENSOR
CONTROLLER
SUPPLY
FAN
Fig. 42. Minimum Outdoor Air in Zone Airflow Control.
287
ENGINEERING MANUAL OF AUTOMATION CONTROL
RETURN
FAN
CONTROLLER
DUCT
STATIC
REFERENCE
STATIC
C2665
RETURN
FAN
C2666