High-Pressure Selector Relay; Low-Pressure Selector Relay; Load Analyzer Relay; Capacity Relay - Honeywell AUTOMATIC CONTROL SI Edition 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 1 to 1.5 degrees C.
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
15-50 kPa
DAMPER
ACTUATOR
HIGH-PRESSURE
SELECTOR RELAY
55-85 kPa
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.O. ZONE
DAMPER
55-85 kPa DAMPER
ACTUATOR
EXH
RESTRICTOR
15-50 kPa N.O.
HEATING COILVALVE
Fig. 40. Typical Application for
Low-Pressure Selector Relay.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
DA ZONE
THERMOSTAT
M B
M
N.C. ZONE
DAMPER
P 1
P 2
B
15-50 kPa
DAMPER
ACTUATOR
M15147
DA ZONE
THERMOSTAT
M B
M
N.O. ZONE
DAMPER
P 1
P 2
B
55-85 kPa
LOW-PRESSURE DAMPER
ACTUATOR
SELECTOR RELAY
M15148
PNEUMATIC CONTROL FUNDAMENTALS

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
55-85 kPa
N.C. COOLING
VALVE
15-50 kPa
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.

CAPACITY RELAY

The capacity relay is a direct-acting relay that isolates an
input and repeats the input pressure with a higher capacity
output. Figure 42 shows a capacity relay enabling a single bleed-
type thermostat to operate multiple damper actuators quickly
by increasing the output capacity of the thermostat.
79
DA ZONE THERMOSTATS
M B M B M B
M M M
DAMPERS
2 3
N
LOAD ANALYZER RELAY
M15149
TO
ZONE