Unit Operating Sequence - York YCAL0019 Installation, Operation And Maintenance Manual

SECTION 6 – COMMISSIONING

UNIT OPERATING SEQUENCE

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 sched-
uling the chiller OFF, and temperature demand
must be present.
2. When power is applied to the system, the micro-
processor will start a 2 minute timer. This is the
same timer that prevents an instantaneous start af-
ter a power failure.
3. At the end of the 2 minute timer, the micropro-
cessor will check for cooling demand. If all con-
ditions allow for start, a compressor on the lead
system will start and the liquid line solenoid will
open. Coincident with the start, the anti-coinci-
dent timer will be set and begin counting down-
ward from "60" seconds to "0" seconds.
4. 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" sys-
tem. A new lead/lag assignment is made whenever
all systems shut down.
5. Several seconds after the compressor starts, that
systems first condenser fan will be cycled ON
(outdoor air temperature more than 25 °F (-4 °C)
152
or discharge pressure). See the section on Operat-
ing Controls for details concerning condenser fan
cycling.
6. After 1 minute of compressor run time, the next
compressor in sequence will start when a sys-
tem has to load. Additional compressors will be
started at 60 second intervals as needed to satisfy
temperature setpoint.
7. 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 five min-
utes. Refer to the section on Capacity Control for
a detailed explanation of system and compressor
staging.
8. As the load decreases below setpoint, the com-
pressors 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 set-
point, and control mode. See the section on Ca-
pacity Control for a detailed explanation.
9. 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
"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 pressure cutout or for 180
seconds, whichever comes first.
FORM 150.67-NM1
ISSUE DATE: 4/28/2017
JOHNSON CONTROLS