Pneumatic Control Combinations; General; Three-Way Mixing Valve Piping - Honeywell Pneumatic Controls Engineering Manual

output (e.g., to provide an adjustable minimum position of an
outdoor air damper). Branchline pressure from the controller to
other devices connected to the controller is not affected.
Figure 56 shows the switch functioning as a minimum
positioning switch. The damper will not close beyond the
minimum setting of the positioning switch. As the controller
signal increases above the switch setting, the switch positions
the damper according to the controller signal.
OA DAMPER
ACTUATOR
DA MIXED AIR
TEMPERATURE
M B
CONTROLLER
M
Fig. 56. Typical Three-Port Minimum
Position Switch Application.
Manual switches are generally panel mounted with a dial plate
or nameplate on the front of the panel which shows the switch
position. Gages are sometimes furnished to indicate the main
and branch pressures to the switch.
PNEUMATIC CONTROL
COMBINATIONS

General

A complete control system requires combinations of several
controls. Figure 57 shows a basic control combination of a
thermostat and one or more control valves. A normally open
control valve assembly is selected when the valve must open if
the air supply fails. A normally open control valve requires a
direct-acting thermostat in the heating application shown in
Figure 56. Cooling applications may use normally closed valves
and a direct-acting thermostat. The thermostat in Figure 56 has
a 5 degree throttling range (output varies from 3 to 13 psi of the
5 degree range) and the valves have an 8 to 12 psi spring
range, then the valve will modulate from open to closed on a 2
degree rise in temperature at the thermostat.
4psi
------------- -
10psi
MINIMUM
POSITION
SWITCH
B
P M
M
VALVE
C2348
5F = 2F

Pneumatic Control Combinations

DA
THERMOSTAT
M B
M
N.O. HEATING COIL VALVES
Fig. 57. Thermostat and One or
More Normally Open Valves.
A normally open or a normally closed valve may be combined
with a direct-acting or a reverse-acting thermostat, depending
on the requirements and the conditions in the controlled space.
Applications that require several valves controlled in unison
(e.g., multiple hot water radiation units in a large open area)
have two constraints:
— All valves that perform the same function must be of the
same normal position (all normally open or all normally
closed).
— The controller must be located where the condition it
measures is uniformly affected by changes in position of the
multiple valves. If not, the application requires more than
one controller.
A direct- or reverse-acting signal to a three-way mixing or
diverting valve must be selected carefully. Figure 58 shows that
the piping configuration determines the signal required.
DIRECT-
ACTING
SIGNAL
THREE-WAY
MIXING VALVE
HOT WATER
SUPPLY
HOT WATER COIL
Fig. 58. Three-Way Mixing Valve Piping
with Direct Actuators.
70-6925
TO OTHER
VALVES
C2349
REVERSE-
ACTING
SIGNAL
THREE-WAY
MIXING VALVE
HOT WATER
HOT WATER
RETURN RETURN
HOT WATER
SUPPLY
HOT WATER COIL
C2377
147