Outdoor Air Fan Control For Multiple Ahu's - Honeywell AUTOMATIC CONTROL Engineering Manual

AIR HANDLING SYSTEM CONTROL APPLICATIONS
OUTDOOR AIR FAN CONTROL
FOR MULTIPLE AHU'S
Functional Description
OA SA
ON 63
3
1
PERCENT
LOAD
AUTO
2
Item
No. Function
1,2 Fan runs under building system control,
subject to software on-off-auto function.
3-5 Fan loads to maintain duct static pressure
setpoint. Static pressure setpoint varied by
AHU OA damper positions to maintain
minimum pressure to satisfy all operating
AHU fan OA damper positions.
6 Control program coordinates fan start-stop
and loading.
FEATURES
1. OA fan provides minimum OA for multiple AHUs, which
are usually stacked one above the other in a tall building.
Each AHU has a minimum airflow OA damper and
airflow station.
2. Duct pressure (and fan loading) optimized to satisfy AHU
with greatest demand.
3. Pressure increased during night purge and ventilation
prepurge modes to accelerate cycles.
4. OA filter reduces AHU filter burden, reduces OA fan
maintenance, reduces air flow station maintenance, and
prevents OA airflow from dropping as filter loads.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Control network, software, and programming advises OA
fan of AHU damper demands.
2. The system is designed so that coils are not subject to
freezing conditions.
SPECIFICATIONS
The OA fan shall start anytime any AHU operates in the
occupied, night purge, or prepurge mode, subject to a software
on-off-auto function.
ENGINEERING CMANUAL OF AUTOMATION CONTROL
5
4
0.21 0.25
MAXIMUM
SETPOINT
6
CONTROL
M10446
PROGRAM
The OA fan loading shall be under EPID (see Control
Fundamentals section) control with a start value of 25% and a
ramp duration of 250 seconds.
NOTE: EPID was selected (and designed specifically) for this
type of application because of the start-up nature of
VAV fans. The problem is the large PID error signal
(the difference between the actual duct pressure and
duct pressure setpoint) that exists at the start moment.
At startup the duct pressure is zero and the setpoint of
the VAV supply fans is usually over one inch of water,
at which point the proportional element of PID calls
for heavy loading, and the integral element adds to
that. Over half the time the real fan start-up load is
minimal since the building is not warm and the VAV
boxes start at or near their minimum airflow setpoint.
It is therefore very predictable that the fan will
accelerate rapidly, the duct pressure will rise rapidly
(and, with PID, will overshoot), and unsafe conditions
may occur.
Recognizing this, EPID starts the fan at a minimum
loading, senses the error, and ramps the error slowly
and linearly into PID control usually over a 30 to 90
second period (adjustable to any value). Thus the fan
loads quickly to 20 or 25%, then loads slowly
thereafter. As the error is fed into PID, the fan loads
to remove the error (and does remove it as it is
received), and the setpoint is reached in a very orderly
and controlled manner with no overshoot. During
startup, EPID control runs the motor at a speed high
enough to prevent overheating and to reach a load level
to prove operation to any monitoring BMCS; however,
the speed is lower than that necessary to meet the VAV
boxes minimum airflow demand.
With pneumatic proportional control, branch line
restrictors are often placed in the air line to the inlet
vane actuator (or to the variable fan drive transducer
line) to slow this loading (a check valve can be
installed around the restrictor such that unloading is
instantaneous). PI control, can have "integral windup",
which accumulates during times that a P error is
allowed to exist. The integral wind-up raises the P
setpoint so the fan starts with full capacity and can
overpressure the supply ducts. The restricted branch
line allows the branch signal to increase slowly.
Anytime all AHUs operating in the occupied mode have OA
dampers less than 80% open, the OA fan duct static pressure
setpoint shall be decremented at the rate of 0.1 inches of water
every 60 seconds. Anytime any AHU is operating in the
occupied mode and the OA damper is full open, the OA fan
duct static pressure setpoint shall be incremented at the rate of
0.1 inches of water every 60 seconds up to a maximum value.
The decementing and incrementing values shall be verified
during commissioning. Anytime any AHU operates in the night
purge or prepurge modes, the OA fan duct static pressure
setpoint shall be equal to the maximum setpoint.
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