High-Pressure Selector Relay; Low-Pressure Selector Relay; Load Analyzer Relay - Honeywell AUTOMATIC CONTROL Engineering Manual

HIGH-PRESSURE SELECTOR RELAY

The high-pressure selector relay is a three-port relay that
transmits the higher of two input signals to the output branch.
The high sensitivity of the relay allows it to be used in sensor
lines with an accuracy of 2 to 3 degrees F.
The application shown in Figure 39 uses pressures from two
zones and a high-pressure selector relay to determine control.
A separate thermostat controls each zone damper. The
thermostat that calls for the most cooling (highest branchline
pressure) controls the cooling valve through the high-pressure
selector relay.
DA ZONE
THERMOSTAT
M B
M
N.C. ZONE
DAMPER
2-7 PSI
DAMPER
ACTUATOR
HIGH-PRESSURE
SELECTOR RELAY
8-12 PSI N.C.
COOLING COIL
VALVE
Fig. 39. Typical Application for
High-Pressure Selector Relay.

LOW-PRESSURE SELECTOR RELAY

The low-pressure selector relay is a three-port relay that
selects the lower of two input pressure signals and acts as a
repeater for the lower of the two inputs. The relay requires an
external restrictor on the input to the branch port. Figure 40
shows a low-pressure selector relay controlling the heating coil
valve from the thermostat that calls for the most heat.
DA ZONE
THERMOSTAT
M B
M
N.C. ZONE
DAMPER
P 1
P 2
B
2-7 PSI
DAMPER
ACTUATOR
C2363
PNEUMATIC CONTROL FUNDAMENTALS
DA ZONE
THERMOSTAT
M B
M
N.O. ZONE
DAMPER
8-12 PSI DAMPER
ACTUATOR
EXH
RESTRICTOR
2-7 PSI N.O.
HEATING COILVALVE
Fig. 40. Typical Application for
Low-Pressure Selector Relay.

LOAD ANALYZER RELAY

The load analyzer relay is a bleed-type, diaphragm-logic
pressure selector. The relay selects the highest and lowest
branch pressure from multiple inputs to operate final control
elements (Fig. 41). The relay contains multiple diaphragms
and control nozzles. Each input pressure connects to two
diaphragms.
M B
M
1
HIGHEST
M
M
LOWEST
8-12 PSI
N.C. COOLING
VALVE
2-7 PSI
N.O. HEATING
VALVE
Fig. 41. Load Analyzer Relay in
Multizone Air Unit Application.
In Figure 41, the load analyzer relay selects the lowest
pressure signal from the thermostat in the coldest zone and
transmits that signal to a normally open heating valve. The
relay transmits the highest pressure signal from the thermostat
in the warmest zone to a normally closed cooling valve.
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ENGINEERING MANUAL OF AUTOMATIC CONTROL
DA ZONE
THERMOSTAT
M B
M
N.O. ZONE
DAMPER
P 1
P 2
B
8-12 PSI
DAMPER
LOW-PRESSURE
ACTUATOR
SELECTOR RELAY
C2364
DA ZONE THERMOSTATS
M B M B
M B
M M
M
TO
ZONE
DAMPERS
2 3
N
LOAD ANALYZER RELAY
C2369