035-19569-101 (0109)
6-4
GB
6.4 Chiller Operation
After the compressor reaches operating speed, the pre-
rotation vanes will begin to open under the control of the
microprocessor which senses the leaving chilled liquid
temperature. The unit capacity will vary to maintain the
leaving CHILLED LIQUID TEMPERATURE SETPOINT.
The pre-rotation vanes are modulated by an actuator
under the control of the microprocessor. The vane control
routine employs proportional plus derivative (rate) control
action. A drop in chilled liquid temperature will cause
the actuator to close the pre-rotation vanes to decrease
chiller capacity. When the chilled liquid temperature
rises, the actuator will open the pre-rotation vanes to
increase the capacity of the chiller.
However, the current draw (A) of the compressor motor
cannot exceed the setting of the % CURRENT LIMIT at
any time during the unit operation. The microprocessor
40% to 100% three-phase peak current limit software
function, plus the 3-phase 100% solid state overload
current limiter (CM-2), on Electro-Mechanical Starter
applications, or the Solid State Starter current limit
function will override the temperature control function
and prevent the pre-rotation vanes from opening beyond
the % CURRENT LIMIT setting.
If the load continues to decrease, after the pre-rotation
vanes are entirely closed, the chiller will be shut down by
the Leaving Chilled Liquid – Low Temperature Control.
6.4.1 Condenser Water Temperature Control
The unit is designed to use less power by taking
advantage of lower than design temperatures that are
naturally produced by cooling towers throughout the
operating year. Exact control of condenser water such
as a cooling tower bypass, is not necessary for most
installations. The chiller requires only that the minimum
condenser water temperature be no lower than the value
determined by referring to the formula below:
Where:
ECWT = Entering Condenser Water Temperature
LCWT = Leaving Chilled Water Temperature
C RANGE = Condensing Water Temperature Range at
the given load condition
At start-up, the entering condenser water temperature
may be as much as 14 °C colder than the return
chilled water temperature. Cooling tower fan cycling
will normally provide adequate control of the entering
condenser water temperature on most installations.
6.5 Operating Log Sheet
An accurate record of system operating conditions
(temperatures and pressures) recorded at regular
intervals throughout each 24 hour operating period
should be kept. An optional status printer is available
for this purpose using automatic data logging after
programming the DATA LOGGER function.
The record of readings serves as a valuable reference
for operating the system. Readings taken when a system
is newly installed will establish normal conditions with
which to compare later readings.
For example, an increase in condenser water temperature
difference (leaving condenser water temperature minus
entering condenser water temperature) may be an
indication of dirty condenser tubes.
6.6 Maintenance and Service
If the system is malfunctioning or the unit is stopped by
one of the safety controls, consult Section 8. After repair
or adjustment the compressor cannot be started or the
particular fault continues to affect the performance of the
unit, please call the nearest York Offi ce. Failure to report
constant troubles could damage the unit and increase
the cost of repairs.
6.6.1 Normal and Safety System Shutdowns
Normal and safety system shutdowns have been built into
the unit to protect it from damage during certain operating
conditions. The system will be stopped automatically
by controls that respond to high temperatures, low
temperatures, and low and high pressures, etc.
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