Manual Switch Control - Honeywell AUTOMATIC CONTROL Engineering Manual

PNEUMATIC CONTROL FUNDAMENTALS
Low-limit control applications typically use a direct-acting
primary controller and a normally open control valve. The
direct-acting, low-limit controller can lower the branchline
pressure regardless of the demands of the room controller, thus
opening the valve to prevent the discharge air temperature from
dropping below the limit controller setpoint. Whenever the low-
limit discharge air sensor takes control, however, the return
air sensor will not control. When the low-limit discharge air
sensor takes control, the space temperature increases and the
return air sensor will be unable to control it.
A similar combination can be used for a high-limit heating
control system without the selector relay in Figure 61. The
limit controller output is piped into the exhaust port of the
primary controller, which allows the limit controller to limit
the bleed-down of the primary controller branch line.
PRIMARY
CONTROLLER (DA)
B S M
M
PRIMARY
SENSOR
N.O.
VALVE
HEATING COIL
Fig. 61. High-Limit Control (Heating Application).
Bleed-type, low-limit controllers can be used with pilot-
bleed thermostats (Fig. 62). A restrictor installed between
the thermostat and the low-limit controller, allows the low
limit controller to bleed the branch line and open the valve.
The restrictor allows the limit controller to bleed air from
the valve actuator faster than the thermostat can supply it,
thus overriding the thermostat.
DA
THERMOSTAT
M B
M
Fig. 62. Bleed-Type, Low-Limit Control System.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
LOW-LIMIT
CONTROLLER (DA)
B S M
B S M
M
LIMIT
SENSOR
DISCHARGE
AIR
C2381
DA
LOW-LIMIT
CONTROLLER
N.O. VALVE
C2350

MANUAL SWITCH CONTROL

Common applications for a diverting switch include on/off/
automatic control for a heating or a cooling valve, open/closed
control for a damper, and changeover control for a two-pressure
air supply system. Typical applications for a proportional
switch include manual positioning, remote control point
adjustment, and minimum damper positioning.
Figure 63 shows an application for the two-position manual
switch. In Position 1, the switch places the thermostat in
control of Valve 1 and opens Valve 2 by bleeding Valve 2 to
zero through Port 1. When turned to Position 2, the switch
places the thermostat in control of Valve 2 and Valve 1 opens.
THERMOSTAT
M B
3
M
EXH
1
TWO-POSITION
SWITCH
NOTE:
POSITION 1: PORTS 3 AND 2, 1 AND 4 CONNECTED
POSITION 2: PORTS 3 AND 4, 1 AND 2 CONNECTED
Fig. 63. Application for Two-Position Manual Switch.
Figure 64 shows an application of the three-position switch
and a proportioning manual positioning switch.
DAMPER
ACTUATOR
DA
THERMOSTAT
M B
M
THREE-POSITION
SWITCH
NOTE: POSITION 1: AUTO—PORTS 2 AND 4 CONNECTED
POSITION 2: CLOSED—PORTS 2 AND 3 CONNECTED
POSITION 3: MANUAL—PORTS 2 AND 1 CONNECTED
Fig. 64. Application for Three-Position Switch and
Manual Positioning Switch.
In Position 1, the three-position switch places the thermostat
in control of the damper. Position 2 closes the damper by
bleeding air pressure to zero through Port 3. Position 3 allows
the manual positioning switch to control the damper.
86
4
2
N.O. VALVE 2
N.O. VALVE 1
C2351
MANUAL
POSITIONING
SWITCH
B M
M
2 1
E
4 3
EXH
EXH
C2352