Honeywell VP525C, Инструкция по эксплуатации, Страница: 159 / 228

Pneumatic Control Combinations
70-6925 151
H
u
m
idi
st
at
s
an
d
Th
er
m
o
st
at
s
Controller
s
Sensors
Relay
s
Switch
es
Actuators
Va
lv
e
s
Ac
ces
sor
ie
s
En
gi
ne
eri
ng
Gu
ide
Cr
o
s
s
Refere
nc
e
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
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.
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
branchline 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).
EXH
X O C
E/P
RELAY
CHECK
VALVE
P/E
RELAYS
M
M B
THERMOSTAT
C2368
TO LOAD SIDE
OF POWER
SUPPLY SWITCH
WIRED TO START
ELECTRICAL
EQUIPMENT
H G
ADJUSTABLE
RESTRICTOR