Airflow Control Applications; Central Fan System Control; Supply Fan Control For Vav Systems; General - Honeywell AUTOMATIC CONTROL Engineering Manual

Figure 22 illustrates an airflow pickup station typically used
in the primary air inlet to a VAV air terminal unit. The pickup
station consists of two tubes that measure differential pressure.
This measurement can be used in an airflow calculation.

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
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
EXFILTRATION (EX)
BUILDING
EXHAUST
SPACE
FAN
EXHAUST
AIR (EA)
SA = RA +EX +EA
C2618
VAV BOX
INLET
SLEEVE
AIRFLOW
PICKUP
Fig. 22. Airflow Pickup Station for VAV Box Applications.
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 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.
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ENGINEERING MANUAL OF AUTOMATION CONTROL
DAMPER
AXLE
M12214