FORM 150.66-NM1 (708)
43
JOHNSON CONTROLS
5
5
The operating sequence described below relates to
operation on a hot water start after power has been
applied, such as start-up commissioning. When a
compressor 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 microproces-
sor will check for cooling demand. If all conditions
allow for start, a compressor on the lead system will
start and the liquid line solenoid will open or the
EEV will begin to operate (EEV equipped chillers
only). Coincident with the start, the anti-coincident
timer will be set and begin counting downward
from “60” seconds to “0” seconds.
If the unit is programmed for Auto Lead/Lag, the
system with the shortest average run-time of the
compressors 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 fi rst condenser fan will be cycled on (out-
door air temperature > 25°F (-4°C) or discharge
pressure).
See the section on Operating Controls
for details concerning condenser fan cycling.
5. After 1 minute of compressor run time, the next
compressor in sequence will start when a system
has to load. Additional compressors will be started
at 60 second intervals as needed to satisfy tempera-
ture setpoint.
6. If demand requires, the lag system will cycle
on with the same timing sequences as the lead
system after the lead system has run for fi ve min-
utes.
Refer 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
control 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 system will initiate a pump-down. Each “sys-
tem” has a pump-down feature upon shut-off. On
a non-safety, non-unit switch shutdown, the LLSV
will be turned off or the EEV will close (EEV
equipped chillers only), and the last compressor
will be allowed to run until the suction pressure
falls below the suction pressure cutout or for 180
seconds, whichever comes fi rst.
UNIT OPERATING SEQUENCE