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The operating sequence described below relates to op-
eration on a hot water start after power has been ap-
plied, such as start-up commissioning. When a com-
pressor starts, internal timers limit the minimum time
before another compressor can start to 1 minute.
1. For the chiller system to run, the Flow Switch must
be closed, any remote cycling contacts must be
closed, the Daily Schedule must not be scheduling
the chiller off, and temperature demand must be
present.
2. When power is applied to the system, the micropro-
cessor will start a 2 minute timer. This is the same
timer that prevents an instantaneous start after a
power failure.
3. At the end of the 2 minute timer, the microprocessor
will check for cooling demand. If all conditions allow
for start, the first compressor on the lead system will
start and the liquid line solenoid will open. The com-
pressor with the least run time in that system will be
the first to start. Coincident with the start, the anti-
coincident timer will be set and begin counting down-
ward from "60" seconds to "0" seconds.
If the unit is programmed for Auto Lead/Lag, the sys-
tem with the shortest average run-time of the com-
pressors will be assigned as the "lead" system. A new
lead/lag assignment is made whenever all systems
shut down.
4. Several seconds after the compressor starts, that
systems first condenser fan will be cycled on (out-
YORK INTERNATIONAL
UNIT OPERATING SEQUENCE
door air temperature > 25°F (-4°C)). See the section
on Operating Controls for details concerning con-
denser fan cycling.
5. After 1 minutes of compressor run time, the next
compressor in sequence will start when a system
has to load. This compressor will be the one with the
least run time that is currently not running in that sys-
tem. Additional compressors will be started at 60
second intervals as needed to satisfy temperature
setpoint.
6. If demand requires, the lag system will cycle on with
the same timing sequences as the lead system. Re-
fer to the section on Capacity Control for a detailed
explanation of system and compressor staging.
7. As the load decreases below setpoint, the compres-
sors will be shut down in sequence. This will occur at
intervals of either 60, 30, or 20 seconds based on
water temperature as compared to setpoint, and con-
trol mode. See the section on Capacity Control for a
detailed explanation.
8. When the last compressor in a "system" (two or three
compressors per system), is to be cycled off, the sys-
tem will initiate a pump-down. Each "system" has a
pump-down feature upon shut-off. On a non-safety,
non-unit switch shutdown, the LLSV will be turned
off, and the last compressor will be allowed to run
until the suction pressure falls below the suction pres-
sure cutout or for 180 seconds, which ever comes
first.
FORM 150.62-NM2
1
51
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