Honeywell AUTOMATIC CONTROL, Engineering Manual

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PNEUMATIC CONTROL FUNDAMENTALS
81 ENGINEERING MANUAL OF AUTOMATIC CONTROL
Figure 45 shows an averaging relay in a typical application
with two thermostat signals as inputs. The average of the
thermostat signals controls a valve or damper actuator.
Fig. 45. Averaging Relay Application.
RATIO RELAY
The ratio relay is a four-port, non bleed relay that produces
a modulating pressure output proportional to the thermostat
or controller branchline output. Ratio relays can be used to
control two or three pneumatic valves or damper actuators in
sequence from a single thermostat. The ratio relay has a fixed
input pressure range of either 3 or 5 psi for a 10 psi output
range and an adjustable start point. For example, in a ratio
relay with a 5 psi range set for a 7 psi start, as the input pressure
varies from 7 to 12 psi (start point plus range), the output
pressure will vary from 3 to 13 psi.
In Figure 46, three 3 psi span ratio relays are set for 3 to 6,
6 to 9, and 9 to 12 psi inputs, respectively. The thermostat
signal through the relays proportions in sequence the three
valves or actuators that have identical 3 to 13 psi springs.
PNEUMATIC POTENTIOMETER
The pneumatic potentiometer is a three-port, adjustable
linear restrictor used in control systems to sum two input signal
values, average two input pressures, or as an adjustable flow
restriction. The potentiometer is a linear, restricted air passage
between two input ports. The pressure at the adjustable output
port is a value based on the inputs at the two end connections
and the location of the wiper between them.
Figure 47 shows a pneumatic potentiometer providing an
average of two input signals. The wiper is set at mid-scale for
averaging or off-center for a weighted average. It can be used
this way to average two air velocity transmitter signals from
ducts with different areas by positioning the wiper according
to the ratio of the duct areas. This outputs a signal proportional
to the airflow.
SIGNALS
FROM
SENSORS
WEIGHTED
AVERAGE
SIGNAL
AVERAGE
SIGNAL
C2374
PNEUMATIC POTENTIOMETER
INPUT 1
OUTPUT INPUT 2
Fig. 47. Pneumatic Potentiometer as Averaging Relay.
Figure 48 shows a pneumatic potentiometer as an adjustable
airflow restrictor.
Fig. 48. Pneumatic Potentiometer as
THERMOSTAT 2
THERMOSTAT 1
AVERAGING
RELAY
C2345
M B M B
P2
B
P1
TO
ACTUATOR
M
Fig. 46. Ratio Relays in Sequencing Control Application.
Adjustable Airflow Restrictor.
TO
CONTROLLED
DEVICE
C2372
PNEUMATIC POTENTIOMETER
INPUT 1
OUTPUT INPUT 2
M CAP
C2370
RATIO RELAY 1
3-6 PSI
HEATING
VALVE
3–13 PSI
MIXING
DAMPERS
3–13 PSI
COOLING
VALVE
3–13 PSI
DA ZONE
THERMOSTAT
M B
EXH
M
RATIO RELAY 3
9-12 PSI
EXH
M
M
M
E
B
P
RATIO RELAY 2
6-9 PSI
EXH
M
M
E
B
P
M
E
B
P