York YCAS Series, Installation Operation & Maintenance

Page: 170 / 216

170 YORK INTERNATIONAL
FORM 201.19-NM8 (804)
Micro Panel Contents
The minimum acceptable temperature is the Lower
Range Limit and is calculated by subtracting the “-”
Range from the Setpoint. The Lower Range Limit is
the lowest ac cept able leaving temperature. The highest
acceptable temperature is referred to as the Upper Range
Limit and is calculated by adding the “+” Range to the
Setpoint. The Upper Range Limit is the highest accept-
able leaving tem per a ture. For example, if the desired
Setpoint temperature 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 tem per a tures 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 con trol the leaving temperature to the actual Set point
tem per a ture. This is accomplished by analyzing the tem-
per a ture error and the rate of change to determine the
amount of loading necessary to cool the chilled liq uid
to the Setpoint Temperature. The amount of load ing is
var ied by changing the amount of DC Voltage signal to
the slide valve solenoid of each compressor. Voltage
increases with load (0 - approximately 9VDC at full
load).
6. SETPOINTS KEYS & CHILLED LIQUID CONTROL
6.1 GENERAL
The microprocessor monitors leaving chilled liquid
tem per a ture 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 voltage to
each com pres sor slide valve to adjust the capacity of
the com pres sors. The microprocessor controls chilled
liquid tem per a ture 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 load ing nec es sary to control to the desired chilled
liq uid setpoint temperature. The micro also attempts to
max i mize ef fi cien cy by spreading the cooling load be-
tween com pres sors, minimize compressor cycling, and
op ti mal ly uti lize evaporator tube surface (maximize
ef fi cien cy). This method of control is suitable for both
wa ter and glycol cooling. Control setpoints can be pro-
grammed into the chiller to establish the desired range
of leaving chilled liquid operating temperatures. A de-
scrip tion of the op er a tion 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 devia-
tion is simply called the “Control Range” and is best
de scribed as the maximum acce and - deviation
from Setpoint.
44.0°F
(7°C)
SETPOINT
Temp.
(User ac-
cept able
leaving
chilled liq-
uid operat-
ing range)
CON TROL
RANGE
46.0°F
(8°C)
42.0°F
(6°C)
29023A