Section 6 Operation; General Description; Start-Up; Normal Running And Cycling - York MILLENNIUM YCAS Installation Operation & Maintenance

Operation

GENERAL DESCRIPTION

The units are designed to work independently, or in con-
junction with other equipment via a YORK ISN build-
ing management system or other automated control sys-
tem. When operating, the unit controls monitor the chilled
liquid system temperature at the unit and take the ap-
propriate action to maintain this temperature within de-
sired limits. This action will involve running one or more
compressors at a suitable load step to match the cooling
effect of the refrigerating systems to the heat load on
the liquid system. The heat removed from the chilled
liquid is then rejected from the air cooled condenser
coils.
The following sections give an overview of the opera-
tion of the unit. For detailed information, reference should
be made to the Chiller Control Panel Programming and
Data Access Operating Instructions for the unit (page
97).

START-UP

Check the main power supplies to the unit are 'ON', all
refrigerant service valves are open (counter-clockwise
one turn short of fully open) and chilled liquid flow has
been established (unless the unit chilled liquid pump start
control is being used, in which case just ensure the pump
supply is on). Ensure only the correct system switches
(SYS 1-2) on the microprocessor circuit board are in
the 'ON' position.
Press the 'STATUS' key on the keypad and then switch
the unit ON/OFF switch below the keypad to the ON
position.
The controller will perform a pre-check to ensure that
the daily/holiday schedule and any remote interlocks will
allow the unit to run, all safety cut-outs are satisfied and
that cooling load is required (i.e. that the chilled liquid
temperature is outside the set limits). Any problems
found by the pre-check will be displayed if present. If
no problems are present and cooling duty is required the
lead compressor will start.
The display will show the anti-coincidence timer status
for the lag compressor, followed by 'NO COOL LOAD'
until it is called to operate by the control system.
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OPERATION

NORMAL RUNNING AND CYCLING

Once the unit has been started, all operations are fully
automatic. After an initial period at minimum capacity
on the lead compressor, the control system will adjust
the unit load depending on the chilled liquid temperature
and rate of temperature change. If high heat load is
present, the controller will increase the capacity of the
lead compressor and/or start-up the other compressor.
If very little heat load is present, the lead compressor
will continue at minimum capacity or may simply stop
again to avoid overcooling the liquid. If the latter is the
case, one compressor will restart automatically should
the liquid temperature rise again.
Once a compressor is running, discharge pressure rises
as refrigerant is pumped into the air cooled condenser
coils. This pressure is controlled by stages of fans to
ensure maximum unit efficiency while maintaining suf-
ficient pressure for correct operation of the condensers
and expansion valves.
When a compressor is running the controller monitors
oil pressure, motor current, and various other system
parameters such as discharge pressure, chilled liquid
temperature, etc. Should any problems occur, the con-
trol system will immediately take appropriate action and
display the nature of the fault (Section 2, Page 108).

SHUTDOWN

The unit can be stopped at any time by switching the
unit ON/OFF switch just below the keypad to the OFF
position. The compressor and oil separator heaters will
energize to prevent refrigerant condensing in the com-
pressor rotors and to prevent the compressor oil becom-
ing saturated with refrigerant. If ambient temperatures
are low, the cooler heater mats will also energize to pre-
vent the possibility of liquid freezing in the vessels. The
mains power to the unit should not normally be switched
OFF, even when the unit is not required to run.
If main power must be switched OFF, (for extended
maintenance or a shutdown period), the compressor
suction, discharge and motor cooling service stop valves
should be closed (clockwise) and if there is a possibility
of liquid freezing due to low ambient temperatures, the
coolers should be drained. Valves should be opened and
power must be switched on for at least 8 Hours (36
Hours if ambient temperature is over 30°C [86ºF]) be-
fore the unit is restarted.
YORK INTERNATIONAL