Setpoints Keys; General; Chilled Liquid Temperature Control - York MILLENNIUM YCAS Installation Operation & Maintenance

Micropanel
6. SETPOINTS KEYS & CHILLED LIQUID CONTROL

6.1 GENERAL

The microprocessor monitors leaving chilled liquid tem-
perature and adjusts the chiller cooling capacity to main-
tain this temperature within a programmed range. The
capacity is controlled by switching compressors on or
off, and by varying a load/unload current to each com-
pressor slide valve to adjust the capacity of the com-
pressors. The microprocessor controls chilled liquid tem-
perature through a combination of Fuzzy Logic control
and internal timers. Fuzzy Logic enables the micro to
analyze the deviation from setpoint and the rate of change
and determine the amount of loading and unloading nec-
essary to control to the desired chilled liquid setpoint
temperature. The micro also attempts to maximize effi-
ciency by spreading the cooling load between compres-
sors, minimize compressor cycling, and optimally utilize
evaporator tube surface (maximize efficiency). This
method of control is suitable for both water and brine
cooling. Control setpoints can be programmed into the
chiller to establish the desired range of leaving chilled
liquid operating temperatures. A description of the op-
eration and programming follows.

6.2 CHILLED LIQUID TEMPERATURE CONTROL

The Setpoints keys are used to program the required
chilled water liquid temperature for the application. This
is accomplished by programming the "Setpoint" and the
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acceptable deviation (+ or - Range) This deviation is
simply called the "Range" and is best described as the
maximum acceptable + and - deviation from Setpoint.
The minimum acceptable temperature is the Lower
Range and is calculated by subtracting the "-" Range
from the Setpoint. The Lower Range is the lowest ac-
ceptable leaving temperature. The highest acceptable
temperature is referred to as the Upper Range and is
calculated by adding the "+" Range to the Setpoint. The
Upper Range is the highest acceptable leaving tempera-
ture. For example, if the desired Setpoint temperature
is 7°C (44.0ºF) and the allowable deviation (+ / - Range)
from this temperature is +/- 1°C (2.0ºF), then the micro
will attempt to control leaving chilled liquid tempera-
tures to 6°C (42.0ºF) to 8°C (46.0ºF). This can be viewed
pictorially as follows:
8°C
(46.0°F)
SETPOINT
7ºC
Temp.
(44.0ºF)
6ºC
(42.0ºF)
SETPOINTS
KEYS
29023A
(User accept-
able leaving
CONTROL
chilled liquid
RANGE
operating
range)
YORK INTERNATIONAL