Changeover Control For Two-Pressure Supply System; Compensated Control System; Electric-Pneumatic Relay Control - Honeywell AUTOMATIC CONTROL Engineering Manual

CHANGEOVER CONTROL FOR TWO-
PRESSURE SUPPLY SYSTEM
Figure 65 shows a manual switch used for changeover from
13 to 18 psi in the mains. Either heating/cooling or day/night
control systems can use this arrangement. In Position 1, the
switch supplies main pressure to the pilot chamber in the PRV.
The PRV then provides 18 psi (night or heating) main air
pressure to the control system.
HIGH PRESSURE
GAGE
FROM
COMPRESSOR
TWO-PRESSURE
REDUCING VALVE
FILTER
Fig. 65. Two-Pressure Main Supply System
with Manual Changeover.
In Position 2, the manual switch exhausts the pilot chamber
in the PRV. The PRV then provides 13 psi (day or cooling) to
the system.
Figure 66 shows a two-pressure system with automatic
changeover commonly used in day/night control. A switch in
a seven-day time clock and an E/P relay provide the
changeover. When the E/P relay energizes (day cycle), the pilot
chamber in the PRV exhausts and controls at 13 psi. When the
electric-pneumatic relay de-energizes, the pilot chamber
receives full main pressure and the PRV provides 18 psi air.
EXH
GAUGE
FROM
COMPRESSOR
TWO-PRESSURE
REDUCING VALVE
Fig. 66. Two-Pressure Main Supply System with
Automatic Changeover.

COMPENSATED CONTROL SYSTEM

In a typical compensated control system (Fig. 67), a dual-
input controller increases or decreases the temperature of the
supply water as the outdoor temperature varies. In this
application, the dual-input controller resets the water
temperature setpoint as a function of the outdoor temperature
MANUAL
SWITCH
CAP
1 3
EXH
2 4
MAIN PRESSURE
GAUGE
MAIN
AIR
C2375
THERMOSTAT
OR
TIME CLOCK
E/P RELAY
C
ELECTRIC
X
POWER
O
MAIN
AIR
X = NORMALLY DISCONNECTED
O = NORMALLY CONNECTED
C2376
PNEUMATIC CONTROL FUNDAMENTALS
according to a preset schedule. The system then provides the
scheduled water temperature to the convectors, fan-coil units,
or other heat exchangers in the system.
M
VALVE
B M
CONTROLLER
Fig. 67. Compensated Supply Water System
Using Dual-Input Controller.

ELECTRIC-PNEUMATIC RELAY CONTROL

Figure 68 shows one use of an E/P relay in a pneumatic
control circuit. The E/P relay connects to a fan circuit and
energizes when the fan is running and de-energizes when the
fan turns off, allowing the outdoor air damper to close
automatically when the fan turns off. The relay closes off the
controller branch line, exhausts the branch line going to the
damper actuator, and allows the damper to go to its normal
(closed) position. Figure 69 shows an E/P relay application
that shuts down an entire control system.
MIXED AIR
SENSOR
M
B M S
DA CONTROLLER
Fig. 68. Simple E/P Relay Combination.
SYSTEM
INTERLOCK
VOLTAGE
E/P RELAY
X O C
M
EXH
Fig. 69. E/P Relay Combination for System Shutdown.
87
ENGINEERING MANUAL OF AUTOMATIC CONTROL
HOT WATER SUPPLY
TEMPERATURE
SENSOR
S 1
OUTDOOR AIR
S 2 TEMPERATURE
SENSOR
C2356
FAN
VOLTAGE
E/P
RELAY
N.C. DAMPER
X O C
N.C. DAMPER
EXH
ACTUATOR
C2361
THERMOSTAT
M B
N.C.
VALVE
C2358