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

Pneumatic Control System Example
70-6925 155
Humidistats
and
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Engineering
Guide
Cross
Reference
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.
Fig. 76. Supply Fan Load Control.
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
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.
VSD
L
B
RA
1.5 IN. WC
RA
1.5 IN. WC
PC
2
PC
1
M
2
EP
H
B
P
2
P
1
L
BM
H
PT
SUPPLY FAN
M10300
STARTER
EAST ZONE
WEST ZONE
HSR
VSD
REFERENCE PICK-UP
RUN TO 15 FEET
ABOVE ROOF
L
B
DA
0.05 IN. WC
PC
M
2
EP
H
1
PT
TO SPACE
STATIC PRESSURE
PICK-UP
RETURN FAN
M10299
STARTER