Pneumatic-Electric Relay Control; Pneumatic Recycling Control - Honeywell AUTOMATIC CONTROL Engineering Manual

PNEUMATIC CONTROL FUNDAMENTALS

PNEUMATIC-ELECTRIC RELAY CONTROL

A P/E relay provides the interlock when a pneumatic
controller actuates electric equipment. The relays can be set
for any desired pressure. Figure 70 shows two P/E relays
sequenced to start two fans, one at a time, as the fans are needed.
WIRED TO
START FAN 1
THERMOSTAT
C N.C. N.O.
SET
M B
5-7 PSI
M
Fig. 70. P/E Relays Controlling Fans in Sequence.
On a rise in temperature, Relay 1 puts Fan 1 in operation as
the thermostat branchline pressure reaches 7 psi. Relay 2 starts
Fan 2 when the controller branchline pressure reaches 12 psi.
On a decrease in branchline pressure, Relay 2 stops Fan 2 at
10 psi branchline pressure, and Relay 1 stops Fan 1 at 5 psi
branchline pressure.
Figure 71 shows two spdt P/E relays starting and stopping a
two-speed fan to control condenser water temperature.
COOLING TOWER
FAN STARTER
CONTROL VOLTAGE
OVERLOAD
LOW SPEED
HIGH AUXILIARY
C N.C. N.O.
C N.C. N.O.
P/E RELAY 1
SET 7-9 PSI
Fig. 71. Two-Speed Fan Operated by P/E Relays.
Voltage is applied to the common contact of Relay 1 from
the normally closed contact of Relay 2. When the controller
branchline pressure rises to 9 psi, the cooling tower fan is
started on low speed by Relay 1 which makes common to
normally open. As a further rise in temperature increases the
ENGINEERING MANUAL OF AUTOMATIC CONTROL
WIRED TO
START FAN 2
P/E RELAYS C N.C. N.O.
SET
10-12 PSI
C2359
HIGH SPEED
LOW AUXILIARY
FAN STARTER
P/E RELAY 2
SET 12-14 PSI
SENSOR IN
DA
CONDENSER
CONTROLLER
WATER
B M S
M
C2367
branchline pressure to 14 psi, Relay 2 breaks the normally
closed circuit and makes the normally open circuit, removing
voltage from Relay 1, shutting down the low speed, and
energizing the high speed. On a decrease in temperature, the
sequence reverses and the changes occur at 12 and 7 psi
respectively.

PNEUMATIC RECYCLING CONTROL

E/P and P/E relays can combine to perform a variety of logic
functions. On a circuit with multiple electrically operated
devices, recycling control can start the devices in sequence to
prevent the circuit from being overloaded. If power fails,
recycling the system from its starting point prevents the circuit
overload that could occur if all electric equipment restarts
simultaneously when power resumes.
Figure 72 shows a pneumatic-electric system that recycles
equipment when power fails.
THERMOSTAT
M B
M
CHECK
VALVE
ADJUSTABLE
RESTRICTOR
Fig. 72. Recycling System for Power Failure.
When power is applied, the E/P relay operates to close the
exhaust and connect the thermostat through an adjustable
restrictor to the P/E relays. The electrical equipment starts in
sequence determined by the P/E relay settings, the adjustable
restrictor, and the branchline pressure from the thermostat. The
adjustable restrictor provides a gradual buildup of branchline
pressure to the P/E relays for an adjustable delay between
startups. On power failure, the E/P relay cuts off the thermostat
branch line to the two P/E relays and bleeds them off through
its exhaust port, shutting down the electrical equipment. The
check valve allows the thermostat to shed the controlled loads
as rapidly as needed (without the delay imposed by the
restrictor).
88
TO LOAD SIDE
OF POWER WIRED TO START
SUPPLY SWITCH
ELECTRICAL EQUIPMENT
H G
E/P
RELAY
P/E
X O C
RELAYS
C2368
EXH