Honeywell AUTOMATIC CONTROL, Engineering Manual

Page: 88 / 518

PNEUMATIC CONTROL FUNDAMENTALS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
78
Fig. 35. Typical Switching Relay for Application.
During the cooling cycle, the 13 psi mainline pressure at the
pilot port (P) is below the minimum switching pressure of the
preset limits. The common port (C) connects to the normally
connected port (O), which is capped. The normally
disconnected port (X) is closed and removes the low-limit
controller from the system.
In a dpdt model, the common, normally connected, and
normally disconnected ports are duplicated in the second
switch section.
SNAP ACTING RELAY
The snap acting relay is a spdt switch that provides two-
position switching action from a modulating signal and has an
adjustable switching point. The switching differential is less
than 1.0 psi. The switching pressure is manually adjustable
for 3 to 15 psi operation.
Figure 36 shows a snap acting relay application. Operation
is similar to the switching relay. When the branchline pressure
from the outdoor air thermostat equals or exceeds the preset
switchover pressure, the relay connects the normally
disconnected port (X) and blocks the normally connected port
(O) to deliver main air to the normally open heating valve
and provide positive close off. When the outdoor air thermostat
pressure drops below the relay setpoint, the normally
disconnected port (X) is blocked and the normally connected
port (O) connects to the common port (C) to connect the valve
actuator to the room thermostat.
C2360
SNAP ACTING
RELAY
VAV TERMINAL UNIT
DAMPER ACTUATOR
N.O. HEATING
VALVE
DA OUTDOOR AIR
THERMOSTAT
DA ROOM
THERMOSTAT
M
P
C
X
O
B
M B
M M
C2362
SNAP ACTING
RELAY
LOCKOUT
RELAY
BLEED
OUTDOOR AIR
DAMPER ACTUATOR
MIXED AIR
TEMPERATURE
CONTROLLER
OUTDOOR AIR
THERMOSTAT
M
X
O
C
P
B
M
M
M
P1
B
M B
M B
P2
EXH
RETURN AIR
THERMOSTAT
Fig. 36. Typical Application for Snap Acting Relay.
LOCKOUT RELAY
The lockout relay is a three-port relay that closes off one
pressure signal when a second signal is higher. Figure 37 shows
a typical application in which mixed air control becomes
disabled when outdoor air temperature is higher than return
air temperature. To prevent air from being trapped in the line
between the lockout relay and the snap acting relay, a small
bleed must be present either in the pilot chamber of the snap
acting relay or in the line.
LOCKOUT
RELAY
P
1
B
P
2
M
M
THERMOSTAT
C2355
TO
ACTUATOR
OUTPUT
EXH
RESTRICTOR
RESTRICTOR
M B
X P
O C
M
M
ROOM
THERMOSTAT
DA WINTER
RA SUMMER
RESTRICTOR
SWITCHING
RELAY
CAP
LIMIT
CONTROLLER
VALVE
COIL
DISCHARGE
AIR
C2379
Fig. 37. Lockout Relay in Economizer Cycle.
Figure 38 shows the lockout relay used as a repeater. This
application provides circuit isolation by repeating the pilot
signal with a second air source.
Fig. 38. Lockout Relay as Repeater.