York MILLENNIUM YCUL0045SC, Installation Operation & Maintenance

Page: 95 / 172

YORK INTERNATIONAL
FORM 150.63-NM2
95
The operating sequence described below relates to op-
eration after power has been applied, such as start-up
commissioning. When a compressor starts, internal tim-
ers limit the minimum time before another compressor
can start to 1 minute.
1. For the condensing unit to run, any remote cycling
contacts must be closed, the Daily Schedule must
not be scheduling the condensing unit off, the air prov-
ing switch(es) must be closed, and a load 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 sys-
tems shut down.
UNIT OPERATING SEQUENCE
2
4. Several seconds after the compressor starts, that
system’s first condenser fan will be cycled on (out-
door air temperature > 25°F (-3.89°C)). See the sec-
tion on Operating Controls for details concerning con-
denser fan cycling.
5. After 3 minutes of compressor run time, the next com-
pressor 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 system.
Additional compressors will be started at 150 to 180
second intervals as needed to satisfy (discharge tem-
perature or suction pressure) 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 or 30 seconds, depending on the
mode of control as compared to setpoint. See the sec-
tion 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. 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 cut-
out, or for 180 seconds, whichever comes first.