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mally open contacts. This causes current to flow
through the contactor (2M) winding and energizes the
contactor. The energized contactor starts the blower
motor (3E).
Closing the circuit between terminals Yl and R com¬
pletes the control circuit for Chiller No. 1 and turns
this chiller on for single stage cooling. Chiller No. 2 is
inactive and will not operate until the temperature in
the conditioned space rises and calls for additional
cooling.
If the cooling load of the conditioned space is too
large for Chiller No. 1 to provide sufficient cooling,
the thermostat's second stage cooling bulb will close
the circuit between Y2 and R.
Closing the circuit between Y2 and R, places the con¬
trol relay (2A2) across the secondary of Chiller No. 1
transformer (1A1). The energized control relay (2A2)
completes the control circuit of Chiller No. 2 and
turns this chiller on (two stage operation).
When the thermostat is satisfied, the thermostat's
second bulb will break the circuit between terminals
R and Y2. This action de-energizes the control relay
(2A2) which opens the control circuit of Chiller No. 2
and turns off Chiller No. 2. Chiller No. 2 will remain
idle until additional cooling is required.
HIGH TEMPERATURE CUTOFF
(Model 180-479)
Refer to Figure 7. Each chiller of Model 180-479 is
equipped with a high temperature limit switch (7H1
on Chiller No. 1 and 7H3 on Chiller No. 2) located on
the front of the generator, plus a circuit breaker (2J1
on Chiller No. 1 and 2J2 on Chiller No. 2). The circuit
breaker for each chiller is located in the chiller con¬
trol box.
If the generator of either chiller becomes overheated,
its associated limit switch (7H1 or 7H3) will open,
providing a current path through heater element of
the circuit breaker (2J1 or 2J2) of the respective
chiller. The current flow through the heater element
of the circuit breaker will cause its normally closed
contacts to open and break the 24-volt circuit to the
magnetic gas valve (5B1 or 5B2). The de-energized
gas valve will turn off the gas supply to the burners of
the overheated generator. The remaining normally
operating chiller will continue to operate.
The circuit breaker must be reset manually by
pushing the red reset button that protrudes through
the rear of the chiller control box.
A secondary high temperature limit switch (7H2 on
Chiller No. 1 and 7H4 on Chiller No. 2) is connected
in series with the gas valve (5B1 on Chiller No. 1 and
5B2 on Chiller No. 2) to insure closing of the gas valve
in the event of a high temperature failure and the
limit switch (7H1 on Chiller No. 1 and 7H3 on Chiller
No. 2) does not open.
The secondary limit switch is located on the generator
header and must be reset manually.
HIGH TEMPERATURE CUTOFF
(Model 240-479)
Refer to Figure 8. Each chiller of the 240-479 is
equipped with two high temperature limit switches
(7H1 & 7H2 on Chiller No. 1, and 7H5 & 7H6 on
Chiller No. 2). A high temperature limit switch is
located on the front of each generator of each chiller.
If
either
generator
of
Chiller
No.
1
becomes
overheated, the associated high temperature limit
switch (7H1 or 7H2) will open, causing the circuit
breaker (2J1) to go into lockout position. The open
contacts of the circuit breaker de-energizes the gas
valve and shuts off the gas supply to the burners of
both Chiller No. 1 generators.
The circuit breaker
(2Jl).for
Chiller No.
1
is located
in the Chiller No. 1 control box and must be reset
manually.
Two secondary high temperature limit switches (7H3
and 7H4-one on each generator header of Chiller No.
1) are connected in series with the Chiller No. 1 gas
valve
(5B1).
If a
generator
overheats
and
its
associated high temperature limit switch (7Hl or
7H2) does not open, the secondary high temperature
limit switch (7H3 or 7H4) on the header of this
generator will open and break the circuit to the gas
valve, de-energizing the gas valve and shutting off
the gas supply to the burners of both Chiller No. 1
generators.
The high temperature cutoff circuit of Chiller No. 2
functions the same as the high temperature cutoff cir¬
cuit of Chiller No. 1. The high temperature cutoff cir¬
cuit of Chiller No. 2 is comprised of high temperature
limit switches 7H5 and 7H6, circuit breaker 2J2,
secondary high temperature limit switches 7H7 and
7H8.
If a high temperature failure should occur and disable
Chiller No. 1, Chiller No. 2 will continue to operate
and provide single stage cooling; likewise, Chiller No.
1 will provide single stage cooling if a high tem¬
perature failure should disable Chiller No. 2.
AMBISTAT PART LOAD CONTROL
(Models 180-479 and 240-479)
Refer to Figures 7 and 8. Each chiller is equipped
with an ambistat part load control (7L1 on Chiller
No. 1 and 7L2 on Chiller No. 2). The circuit consists
of a temperature sensitive switch (located in the
chiller control box) and a capillary tube. One end of
the capillary tube is connected to the switch and the
other end is inserted in the chilled water supply line.
The switch contacts open when the chilled water tem¬
perature drops below 40°F. and breaks the circuit to
the gas valve, shutting off the gas supply to the
burners.
The chiller power relay remains energized and the
condenser fan motors, solution pump and water pump
will continue to run.
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