Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual page 266

AIR HANDLING SYSTEM CONTROL APPLICATIONS
23 Control program optimizes duct pressure
setpoint.
24 OA shaft static pressure point shared from OA
fan control system for this graphic (not a
physical point as shown) and provided here
for operator information.
NOTE: This system is often found on each floor of a building,
and often includes an outdoor air fan (preferably with
filtration to protect the AHU OA air flow elements)
and shaft, and a water-side economizer. The water-
side economizer provides chilled water year round (in
cold weather the cooling tower provides chilled water
via heat exchangers without the need for the chillers).
Dual equal sized chillers and boilers (non-redundant)
are assumed.
FEATURES
1. Supplies constant temperature variable volume (energy
conserving) air to VAV boxes.
2. Provides constant airflow of OA with varying supply
airflow.
3. Provides 100% RA during warm-up periods
(preoccupancy purge may be required for IAQ).
4. Perimeter boxes provide space heat when required during
occupied periods.
5. AHU provides heating during warm-up periods, and when
OA temperature effects cause low SA temperatures. If
boxes have electric heat and the AHU hot water is from
lower cost gas/oil-sourced heat, box heaters may be
disabled during warm-up periods.
6. Duct static pressure setpoint is lowered anytime all VAV
box dampers are less than 90% open (If this strategy is
used, the duct static pressure pickup need not be at the
end of the longest run, but may be at any location where
the air is not too turbulent. The maximum setpoint is
higher than if the pickup had been at the end of the run
because of the pickup location).
7. Reduced fan airflow is provided during warmup, night
purge, and cool-down periods to reduce fan energy, which
varies with the cube of the fan airflow.
8. Reduced fan airflow and staged AHU startup during cool-
down periods requires only one chiller and keeps the
chiller pull-down ahead of the AHU cooling demands
such that AHUs get cold (not cool) water for effective
cooling and dehumidification, and allows the chiller to
operate at an efficient loading (less than 100%).
CONDITIONS FOR SUCCESSFUL OPERATION
1. Airflow element and transducer must be kept clean and
calibrated.
2. OA fan must provide adequate OA shaft pressure.
Alternatively, if there is no OA fan, the OA airflow
setpoint may be maintained by modulating the RA
damper which would have to be added.
3. A controller network and adequate software and
programming provided to support communication between
the box controllers and the fan controller to allow static
pressure reset and to position the box dampers properly
during night purge, warmup, and cool-down periods.
4. If any VAV box whose damper position is a program input
can never satisfy its space cooling demand for any reason
(and its damper is always open), the duct static pressure
reset program will not lower the duct static pressure setpoint.
The duct static pressure reset program works best when
there are no more than thirty monitored VAV boxes per fan
system (with great quantities of boxes, it is likely that at
least one box damper will always be full open).
For example, if an interior zone is always under a full
cooling load, static pressure reset will not occur unless
that zone (and similar zones) is oversized. The oversized
zone would then throttle back when the building is at
full load when the duct static pressure is at design.
5. All AHUs must be near the same normal occupancy
schedule or the cool-down start-up specification edited.
6. Boiler, chiller, pumping system, and OA fan controls
carefully networked into the AHU control schemes to
assure smooth and efficient building operations.
7. All specified values and setpoints are tuned to specific
project requirements.
SPECIFICATIONS
NOTE: A set of 16 similar sized AHUs are assumed.
See FAN SYSTEM START-STOP CONTROL.
Anytime any AHU starts in the optimum start cool-down
mode, three to four AHUs shall start and the remaining AHUs
shall stage on at five minute intervals (unless they similarly
start under their optimum start programs). Any time any AHU
operates in the night purge, warmup, or cool-down modes of
operation, all associated perimeter VAV boxes shall operate at
60% of their maximum airflow setpoint, and all associated
interior VAV boxes shall operate at 25% of their maximum
airflow setpoint, unless the OA temperature is less than –9.5 C
in which case the perimeter VAV boxes shall operate at their
maximum airflow setpoint.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
256