Supply Fan Control Sequence; Return Fan Control Sequence - Honeywell Pneumatic Controls Engineering Manual

Supply Fan Control Sequence

Any time the Supply Fan (Fig. 76) runs, the pressure controller
with the greatest demand, Static Pressure Controller PC1 or
PC2, operates the Electronic-Pressure Transducer PT. The
controller used is determined by High Pressure Selector Relay
HSR. Transducer PT controls the Supply Fan Variable Speed
Drive (VSD) to maintain duct static pressure. The pick-up
probes for Static Pressure Controllers PC1 and PC2 are located
at the end of the east and west zone ducts.
SUPPLY FAN
VSD
STARTER
RA
1.5 IN. WC
HSR
B
PT
Fig. 76. Supply Fan Load Control.
EAST ZONE
WEST ZONE
L H
RA
L
H 1.5 IN. WC
B M
PC
1
B M PC
2
P 1
2
EP
P 2
M10300
Pneumatic Control System Example

Return Fan Control Sequence

Static Pressure Controller PC (Fig. 77) controls the return fan
variable speed drive to maintain space static pressure. The
pick-up probe is located in the space
RETURN FAN
REFERENCE PICK-UP
VSD
RUN TO 15 FEET
ABOVE ROOF
STARTER
1
PT
Fig. 77. Return Fan Load Control.
NOTES:
1. Because of varying exhaust between occupied and
warm-up modes, space static pressure control of
the return fan is selected. Return fan tracking from
supply fan airflow is acceptable but is complex if
varying exhaust is worked into the control scheme.
2. Exercise care in selecting location of the inside
pick-up and in selection of the pressure controller.
Location of the reference pick-up above the roof is
recommended by ASHRAE.
3. To prevent unnecessary hunting by the return fan at
start-up, the supply fan control signal should be
slow loading such that the supply fan goes from
zero or a minimum to maximum load over three
minutes. Shut down should not be restricted.
70-6925
TO SPACE
STATIC PRESSURE
PICK-UP
DA
L H
0.05 IN. WC
B M PC
2
EP
M10299
155