Pneumatic-Electric Relay Control; Pneumatic Recycling Control - Honeywell AUTOMATIC CONTROL SI Edition 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
35-49 kPa
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 49 kPa. Relay 2 starts
Fan 2 when the controller branchline pressure reaches 84 kPa.
On a decrease in branchline pressure, Relay 2 stops Fan 2 at
70 kPa branchline pressure, and Relay 1 stops Fan 1 at 35 kPa
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 49-63 kPa
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 63 kPa, the cooling tower fan is
WIRED TO
START FAN 2
P/E RELAYS C N.C. N.O.
SET
70-84 kPa
C4225
HIGH SPEED
LOW AUXILIARY
FAN STARTER
P/E RELAY 2
SET 80-94 kPa
SENSOR IN
DA
CONDENSER
CONTROLLER
WATER
B M S
M
C4227
started on low speed by Relay 1 which makes common to
normally open. As a further rise in temperature increases the
branchline pressure to 94 kPa, 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 80 and 49 kPa
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
E/P
CHECK
RELAY
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).
ENGINEERING MANUAL OF AUTOMATIC CONTROL
88
TO LOAD SIDE
OF POWER WIRED TO START
SUPPLY SWITCH
ELECTRICAL EQUIPMENT
H G
P/E
X O C
RELAYS
C2368
EXH