York YCAS 0138EB, Installation Operation & Maintenance

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155
JOHNSON CONTROLS
FORM 201.18-NM7
described 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
temperature. For example, if the desired Setpoint tem-
perature is 44.0°F (7°C) and the allowable deviation (+
/ - Range) from this temperature is +/- 2.0°F (1°C), then
the micro will attempt to control leaving chilled liquid
temperatures to 42.0°F (6°C) to 46.0°F (8°C). 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 tem-
perature error and the rate of change to determine the
amount of loading necessary to cool the chilled liquid
6. SETPOINTS KEYS & CHILLED LIQUID CONTROL
6.1 GENERAL
The microprocessor monitors leaving chilled liquid
temperature and adjusts the chiller cooling capacity to
maintain 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 compressor slide valve to adjust the capacity of
the compressors. The microprocessor controls chilled
liquid temperature 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 necessary to control to the desired chilled
liquid 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 effi -
ciency). 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 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 devi-
ation is simply called the “Control Range” and is best
44.0°F
(7°C)
SETPOINT
Temp.
(User
acceptable
leaving
chilled liquid
operating
range)
CONTROL
RANGE
46.0°F
(8°C)
42.0°F
(6°C)
29023A
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