161
YORK INTERNATIONAL
FORM 201.18-NM4
scribed as the maximum acce 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 (44.0°F) and the allowable deviation (+ / - Range)
from this temperature is +/- 1°C (2.0°F), then the micro
will attempt to control leaving chilled liquid tempera-
tures to 6°C (42.0°F) to 8°C (46.0°F). This can be
viewed pictorially as follows:
To assure that the chilled liquid leaving temperature
stays within the Control Range, the micro will attempt
to control the leaving temperature to the actual Setpoint
temperature. This is accomplished by analyzing the
temperature error and the rate of change to determine
the amount of loading necessary to cool the chilled liq-
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
temperature through a combination of Fuzzy Logic con-
trol 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 un-
loading necessary to control to the desired chilled liq-
uid setpoint temperature. The micro also attempts to
maximize efficiency by spreading the cooling load be-
tween compressors, minimize compressor cycling, and
optimally utilize evaporator tube surface (maximize ef-
ficiency). This method of control is suitable for both
water and brine cooling. Control setpoints can be pro-
grammed into the chiller to establish the desired range
of leaving chilled liquid operating temperatures. A de-
scription of the operation 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 acceptable deviation (+ or - Range) This deviation
is simply called the “Control Range” and is best de-
7°C
(44.0°F)
SETPOINT
Temp.
(User
acceptable
leaving
chilled
liquid
operating
range)
CONTROL
RANGE
8°C
(46.0°F)
6°C
(42.0°F)
29023A
8
Summary of Contents for Millennium YCAS 0693
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