YORK INTERNATIONAL
32
FORM 201.25-NM1 (302)
Unit Operation
UNIT OPERATION
General Description
The units are designed to work independently, or in
conjunction with other equipment via a YORK ISN
building management system or other automated con-
trol system. When operating, the unit controls monitor
the chilled liquid system temperature at the unit and
take the appropriate action to maintain this tempera-
ture within desired limits. This action will involve run-
ning one or both 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 via the water
cooled condenser.
The following sections give an overview of the opera-
tion of the unit. For detailed information, reference
should be made to the MBCS Operating Instructions
for the unit.
START-UP
Check the main power supplies to the unit are ‘ON’,
all refrigerant service valves are open (anti-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 that system 1 and 2
switches on the microprocessor circuit board are in the
‘ON’ position.
Press the ‘STATUS’ key on the keypad and then switch
the unit ‘ON/OFF’ toggle 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 cutouts 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.
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 tempera-
ture 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.
When a compressor is running the controller monitors
oil pressure, motor current, and various other system pa-
rameters such as discharge pressure, chilled liquid tem-
perature, etc. Should any problems occur, the control
system will immediately take appropriate action and dis-
play the nature of the fault (see MBCS Section).
SHUTDOWN
The unit can be stopped at any time by switching the
unit ‘ON/OFF’ toggle switch just below the keypad to
the ‘OFF’ position. The compressor heaters will ener-
gize to prevent refrigerant condensing in the compres-
sor rotors and to prevent the compressor oil becoming
saturated with refrigerant.
To prevent damage to the unit the con-
trol supply to the compressor heaters
should not be switched off, even when
the unit is not required to run.
If mains power must be switched off, (for extended main-
tenance or a shutdown period), the compressor suction,
discharge and liquid line service valves on both systems
should be closed (clockwise) and if there is a possibility
of liquid freezing due to low ambient temperatures, the
cooler and condenser should be drained. The valves should
be opened, the cooler and condenser refilled and the power
must be switched on
for at least 8 hours before the unit
is restarted!
SECTION 6
UNIT OPERATION
