Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

Page: 140 / 512

ENGINEERING MANUAL OF AUTOMATIC CONTROL
ELECTRONIC CONTROL FUNDAMENTALS
130
TYPICAL SYSTEM APPLICATION
Figure 22 shows a typical air handling system controlled by
two electronic controllers, C1 and C2; sequencer S;
multicompensator M; temperature sensors T1 through T4;
modulating hot and chilled water valves V1 and V2; and
outdoor, return, and exhaust air damper actuators. The control
sequence is as follows:
— Controller C1 provides outdoor compensated, summer-
winter control of space temperature for a heating/cooling
system which requires PI control with a low limit. Sensor
T4 provides the compensation signal through
multicompensator M which allows one outdoor
temperature sensor to provide a common input to several
controllers. Controller C1 modulates the hot and
chilled water valves V1 and V2 in sequence to
maintain space temperature measured by sensor T1 at
a preselected setpoint. Sequencer S allows sequencing
the two valve actuators from a single controller. Low-
limit sensor T2 assumes control when the discharge
air temperature drops to the control range of the low
limit setpoint. A minimum discharge air temperature
is maintained regardless of space temperature.
When the outdoor temperature is below the selected reset
changeover point set on C1, the controller is in the winter
compensation mode. As the outdoor air temperature falls,
the space temperature setpoint is raised. When the outdoor
temperature is above the reset changeover point, the
controller is in the summer compensation mode. As the
outdoor temperature rises, the space temperature setpoint
is raised.
— Controller C2 provides PI mixed air temperature control
with economizer operation. When the outdoor temperature
measured by sensor T4 is below the setting of the
economizer startpoint setting, the controller provides
proportional control of the dampers to maintain mixed
air temperature measured by sensor T3 at the selected
setpoint. When the outdoor air temperature is above the
economizer startpoint setting, the controller closes the
outdoor air dampers to a preset minimum.
Fig. 22. Typical Application with Electronic Controllers.
C3086
EXHAUST
AIR
OUTDOOR
AIR
T3
HOT
WATER
V1
CHILLED
WATER
V2
SUPPLY
FAN
T2
T4
T1
C2
C1
S
M
MIXED
AIR
RETURN
AIR