York YCAS0130 - Manual (Page 46)

46 YORK INTERNATIONAL
FORM 201.19-NM1 (204)
Op er a tion
GENERAL DESCRIPTION
The units are designed to work independently, or in
con junc tion with other equipment via a YORK ISN
build ing management system or other automated con-
trol sys tem. When operating, the unit controls monitor
the chilled liquid system temperature at the unit and
take the ap pro pri ate action to maintain this temperature
with in de sired limits. This action will involve running
one or more compressors at a suitable load step to match
the cool ing 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
con dens er coils.
The following sections give an overview of the op er a tion
of the unit. For detailed information, reference should
be made to the Chiller Control Panel Pro gram ming and
Data Access Op er at ing In struc tions for the unit (pages
122 - 192).
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 fl ow has
been es tab lished (unless the unit chilled liquid pump
start con trol 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 cir cuit 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 sat is fi ed and
that cooling load is required (i.e. that the chilled liquid
temperature is outside the set limits). Any prob lems
found by the pre-check will be displayed if present. If
no prob lems are present and cooling duty is required the
lead compressor will start.
The display will show the anti-coincidence timer sta tus
for the lag compressor, followed by ‘NO COOL LOAD’
until it is called to operate by the control sys tem.
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 tem per a ture
and rate of temperature change. If high heat load is pres-
ent, 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 ris es
as refrigerant is pumped into the air cooled con dens er
coils. This pressure is controlled by stages of fans to
ensure maximum unit effi ciency while main tain ing suf-
fi cient pressure for correct operation of the con dens ers
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 8).
SHUTDOWN
The unit can be stopped at any time by switching the
UNIT ON/OFF switch just below the keypad to the
OFF po si tion. The compressor heat er will en er gize to
prevent refrigerant condensing in the com pres sor rotors.
If ambient tem per a tures are low, the evaporator heater
mats will also en er gize to pre vent the possibility of liq-
uid freezing in the ves sels. The mains pow er to the unit
should not nor mal ly be switched OFF, even when the
unit is not re quired to run.
The system switches (S2-S5) on the microboard can be
used to cycle a system OFF. An automatic pumpdown
will occur using the system switches.
If mains power must be switched OFF, (for extended
main te nance or a shutdown period), the compressor suc-
tion, discharge and motor cooling service stop valves
should be closed (clockwise) and if there is a pos si bil i ty
of liq uid freezing due to low ambient temperatures, the
evaporators 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 86°F [30°C])
be fore the unit is restarted.
OPERATION