Mixing Damper Control Sequence; Discharge Air Temperature Control Sequence; Off/Failure Mode Control Sequence - Honeywell Pneumatic Controls Engineering Manual

Mixing Damper Control Sequence

Any time the AHU (Fig. 79) runs in the occupied mode with
Electric-Pneumatic Relay 1EP energized, Outside Air airflow
Controller P-F modulates the outside air damper toward open
and the return air damper toward closed (or vice versa) in
unison to maintain design outside air at 4000 cfm.
4-11 PSI
N.O.
OUTSIDE
AIR
P-F
L H
RA
B M
1
EP

Discharge Air Temperature Control Sequence

Any time the AHU (Fig. 79) operates in the non-warm-up mode,
Switching Relay SR-4 operates to allow the normal Discharge
Air Temperature Controller TC-3 to modulate the hot water
valve closed (through Switching Relay SR-1, Fig. 77) and the
chilled water valve open in sequence, on a rising cooling load,
to maintain the Temperature Controller TC-3 setpoint. Controller
TC-3 is a PI (proportional plus integral) controller.
NOTE: In this constant 4000 cfm outside air system, if the
return air is 72F and the outside air is -5F, the mixed
air temperature will drop below 55F if the AHU airflow
drops below 52 percent of the design airflow.
4-11 PSI
N.C.
N.O.
6-11 PSI
M
FROM WARM-UP
CONTROL
(AIR DURING
COLD RETURN)
SR-5
AIR WHEN
2
P
O
FAN ON EP
C X
Fig. 79. Mixing Damper and Discharge Air Temperature Control.
Pneumatic Control System Example
NOTE: These dampers can control in sequence also, but uni-
son control positions the damper blades better for mix-
ing which is helpful during freezing periods. If the
outside air is provided from an outside air shaft with an
outside air fan, an outside air filter is helpful to keep
the flow sensing element/pick-up clean and effective.
Electric-Pneumatic Relay 1EP starts the outside air
system.
SUPPLY FAN
TS-1
TO HEATING COIL
CONTROL AND TO
CHILLER PLANT
CONTROL
X
B S M
SR-4
P
O
Z
C X
M
M

Off/Failure Mode Control Sequence

If compressed air fails, both control valves open, the outside air
damper closes, and the return air damper opens.
When the fan is off, Switching Relay SR-3 (Fig. 78) positions to
close the hot water valve, Switching Relay SR-5 (Fig. 79)
positions to close the chilled water valve, the outside air damper
closes, and the return air damper opens.
70-6925
TO WARM-UP
Y
CONTROLS
2
EP
DA
55F
TC-3
PI
3-8 PSI HW GOES OPEN TO CLOSED
9-14 PSI CHW GOES CLOSED TO OPEN
P=O TO 20
B=20 TO 0
RR
B
M
6-11 PSI CHW
P E OPEN TO CLOSED
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