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SEQUENCING FAN CONTROL
VAV systems with multiple fans can use fan sequencing. This
allows the fans to operate with greater turndown. For example,
if a single fan modulates from 100 percent to 50 percent of
wide open capacity (50 percent turndown), then two fans with
exactly half the capacity of the larger fan can run with 75 percent
turndown. Also, sequencing is more efficient. Most of the year
the system is not run at full capacity. Under light load conditions,
some fans run while others are at standby. It should be taken
into account that fan power varies with the cube of the fan
airflow when determining fan staging strategies.
For single supply fan systems, fan output volume is controlled
by duct static pressure. However, the decision to turn fans on
or off is based on total supply airflow. For centrifugal fans, if
Supply Fan 1 in Figure 39 is operating near its maximum
velocity capacity, Supply Fan 2 is energized. This opens Damper
2 and Fan 2 is slowly modulated upward. As the duct static is
satisfied, Fan 1 will modulate downward until Fan 1 and Fan 2
are operating together, controlled by duct static pressure. If the
outputs of Fans 1 and 2 approach maximum capability, Supply
Fan 3 is energized. When zone load decreases and terminal
units decrease airflow, duct static increases, modulating fans
downward. When total supply flow decreases enough, Fan 3 is
turned off and Fans 1 and 2 increase in output as required to
maintain duct static. Similarly, Fan 2 may be turned off. Time
delays protect fan motors from short cycling and fan operation
may be alternated to spread wear.
RETURN
FAN
SUPPLY
FAN 1
SUPPLY
FAN 2
SUPPLY
FAN 3
CONTROL
PANEL
Fig. 39. VAV System with Fan Sequencing.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
DAMPER 1
FLOW
MEASURING
STATION
DAMPER 2
DAMPER 3
C2634
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
Vaneaxial fan sequencing is also decided by total supply flow,
but the operating fan(s) is modulated to minimum output when
the next fan is turned on. This sequence is used to avoid a stall
of the starting fan. When all requested fans are running, they
are modulated upward to satisfy duct static setpoint.
OTHER CONTROL MODES
Warm-Up Control
If warm-up control is used, it is not necessary to provide
outdoor air. The following control actions should be accom-
plished when using warm-up control:
— Exhaust and relief fans should be off.
— Building pressurization control (if used) should be
reset to zero static differential.
— Airflow tracking control (if used) should be reset to
zero differential.
— If a return fan is used, the supply fan maximum airflow
is limited to no greater than the return fan capacity.
With digital control VAV systems, this is accomplished
by commanding the VAV boxes to some percent of
their maximum airflow setpoints during this mode.
— Space thermostats should change the warm-up mode to
normal operation to prevent over or under heating.
Smoke Control
If smoke control is used, the return damper closes and the
return fan operates as a relief or exhaust fan. Controls must
prevent over pressurization of ducts and spaces.
Night Purge Control
Night purge can be used to cool a building in preparation for
occupancy and to cleanse the building of odors, smoke, or other
contaminants. Outdoor and relief air dampers must be open
and the return damper closed. If airflow tracking is used, supply
fan must be limited to the return fan volume. Some control
systems allow space thermostats to be set at lower setpoints
during this cycle to maximize free cooling. If digital control is
used, significant energy savings can be accomplished by
commanding all VAV box airflow setpoints to approximately
50 percent of their maximum values.
ZONE AIRFLOW CONTROL
AIRFLOW TRACKING/SPACE STATIC PRESSURE
Zone airflow control provides pressurization control for a
portion of a facility. Figure 40 shows a zone airflow control
example for a building. Airflow tracking or direct space static
pressure control of the return damper on each floor determines
the pressurization of each floor.
285
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