Reversing Relay; Positive-Positioning Relay; Averaging Relay - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

PNEUMATIC CONTROL FUNDAMENTALS
BLEED-TYPE
THERMOSTAT
DAMPER
ACTUATOR
M
M
Fig. 42. Typical Capacity Relay Application.

REVERSING RELAY

The reversing relay is a modulating relay with an output that
decreases at a one-to-one ratio as the input signal increases.
Figure 43 shows a reversing relay application. A falling
temperature at the direct-acting thermostat causes the branchline
pressure to decrease. The reversing relay branch pressure
increases and opens the normally closed heating valve.
DA ROOM
THERMOSTAT
M B
M
M
Fig. 43. Reversing Relay Application.

POSITIVE-POSITIONING RELAY

The positive-positioning relay (Fig. 44) mounts directly on a
valve or damper actuator. The relay positions the valve or
P E EXH
M B
CAPACITY
DAMPER
RELAY
ACTUATOR
DAMPER
ACTUATOR
C2365
B
M
E
P
EXH
REVERSING
RELAY
N.C. HEATING
VALVE
C2354
damper precisely according to the branchline pressure from a
thermostat or other controller, regardless of the load variations
affecting the valve stem or damper shaft. The relay is typically
used for large actuators for sequencing, or in applications
requiring precise control.
ROOM
THERMOSTAT
M B
P
POSITIVE-
POSITIONING
M
M
RELAY
B
DAMPER
FEEDBACK
ACTUATOR
SPRING
DAMPER
Fig. 44. Positive-Positioning Relay
with Damper and Valve Actuators.
When the relay is connected to an actuator, the feedback
spring produces a force proportional to the actual valve or
damper position. The relay positions the actuator in proportion
to the branchline input. If the connected load attempts to
unbalance the required valve stem position, the relay either
exhausts or applies main pressure to the actuator to correct the
condition. If the valve or damper sticks or the load prevents
proper positioning, the relay can apply the pressure required
(up to full main pressure) or down to zero to correct the
condition.
The positive-positioning relay also permits sequenced
operation of multiple control valves or dampers from a single
thermostat or controller. For example, a normally open heating
valve and a normally closed outdoor air damper could be
controlled from a single thermostat piloting relays on two
actuators. Relays typically have a 20, 35, or 70 kPa input
pressure span and an adjustable start pressure. As the space
temperature rises into the low end of the thermostat throttling
range, the heating valve positioner starts to close the valve.

AVERAGING RELAY

The averaging relay is a direct-acting, three-port relay used in
applications that require the average of two input pressures to
supply a controller input or to operate a controlled device directly.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
80
ROOM
THERMOSTAT
M B
P
POSITIVE-
POSITIONING
M
M
RELAY
B
INTERNAL
VALVE
FEEDBACK
ACTUATOR
SPRING
VALVE
C2366