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BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
AIRFLOW CONTROL APPLICATIONS
CENTRAL FAN SYSTEM CONTROL
Figure 23 shows the net airflow balance for a building space
and where the return air, outdoor air, and supply fan inlet meet.
Assume that the return air damper is open, the relief air damper
is closed, and the outdoor air damper is open enough to allow
minimum design outdoor air to pass. When the outdoor and
relief dampers open further and the return damper closes further,
outdoor air increases above minimum. These conditions are
used for free cooling (economizer cycle control) and when
minimum air must be greater than the difference between supply
and return fan airflow rates.
OUTDOOR AIR
RETURN
RELIEF (OR)
FAN
RETURN
AIR (RA)
SUPPLY
FAN
SUPPLY
AIR (SA)
OUTDOOR AIR
SA = OA + (RA - OR)
INTAKE (OA)
FOR MINIMUM OA,
OR = 0 AND SA = OA + RA
Fig. 23. Central Fan Control with Net Airflow Balance.
SUPPLY FAN CONTROL FOR VAV SYSTEMS
General
The supply fan control system provides adequate duct static
pressure to all air terminal units. This duct static pressure at the
air terminal unit is used to overcome pressure drops between
the air terminal unit inlet and the controlled space. Inadequate
static limits maximum airflow to less than required, while
excessive duct static increases sound levels and wastes energy.
The location of the duct static pressure sensor is critical for
proper control of the supply fan.
Ideally, if the supply duct is one simple run with short takeoffs
to the air terminal units (Fig. 24A), the duct static pressure
sensor is located at the air terminal unit furthest from the supply
fan. However, capacity variations of the furthest air terminal
unit may adversely influence the duct static pressure sensor.
Under these circumstances, the sensor will transmit more stable
and uniform pressures if located upstream of the last three or
four air terminal units.
Typically, the supply duct is complex with multiple runs or
branches. This duct layout requires a compromise in duct static
pressure sensor location that is usually about 75 percent of the
distance between the fan and the furthest air terminal unit
(Fig. 24B). In complex duct runs where multiple branches split
close to the fan, sensors should be located in each end of the
EXFILTRATION (EX)
BUILDING
EXHAUST
SPACE
FAN
EXHAUST
AIR (EA)
SA = RA +EX +EA
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branch (Fig. 24C). Each sensed point should have its own
setpoint in the control loop. This avoids the assumption that
branches and multisensor locations have identical
requirements. The sensed point having the lowest duct pressure
relative to its own setpoint should control the supply fan.
When a long straight duct (10 diameters) is available, a single
point static sensor or pitot tube can be used. When long duct
sections are not available, use static or airflow measuring
stations which are multipoint and have flow straighteners to
provide the most accurate sensing. A reference pressure pickup
should be located outside the duct (in or near the controlled
space) and adjacent to the duct sensor to measure space static
in areas served by the duct. The static pressure sensor should
not be located at the control panel in the equipment room if
duct static is measured elsewhere. Equipment room static
pressure varies as outdoor winds change, outdoor air and relief
damper positions change, or exhaust fan operation changes.
SUPPLY
FAN
AIR
TERMINAL UNITS
SUPPLY
FAN
AIR TERMINAL
UNITS
SUPPLY
FAN
AIR TERMINAL
UNITS
Fig. 24. Locating Duct Static Pressure
Sensor for Supply Fan Control.
280
ENGINEERING MANUAL OF AUTOMATIC CONTROL
DUCT STATIC
PRESSURE SENSOR
A.
DUCT STATIC
PRESSURE SENSOR
B.
DUCT STATIC
PRESSURE SENSOR
DUCT STATIC
PRESSURE SENSOR
C.
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