48
YORK INTERNATIONAL
The RATE SENSITIVITY is a means of “overriding” the
loading/unloading timers when water temperatures are
in the RATE CONTROL RANGE or the CONTROL
RANGE. This allows the micro to react to abrupt down-
ward changes in leaving or return water temperatures.
The ability to respond to “rate of change” variations in
water temperatures gives the micro “anticipation” capa-
bilities to reduce the possibility of “overshoot” in leaving
water temperature.
In demand limiting applications, to avoid cycling, or to
avoid overshoot, RATE SENSITIVITY should be low. This
allows the micro to go into rate control to prevent loading
or cause unloading whenever water temperatures drop
faster than the programmed RATE SENSITIVITY. Rate
Control can go into effect whenever water temperatures
are in RATE CONTROL RANGE or the CONTROL
RANGE. For these applications, a 35.0°F/min. RATE
SENSITIVITY is recommended. This may be needed for
small water loops. However, if problems arise where the
chiller does not load and pull temperature down, select
5.0°F/min.
NOTE: Too small of a RATE SENSITIVITY Selection
may prevent loading due to varying flows or if
the water system allows a slug of cold water to
enter which falsely fools the micro into thinking
the RATE SENSITIVITY has been exceeded,
preventing loading and allowing leaving water
temperature to rise above the desired tempera-
ture.
For normal comfort cooling, batch, or process applica-
tions, select a high RATE SENSITIVITY of 5.0°F/min.
Before Rate Control can go into effect, the water tem-
perature would have to change at a very high rate to
exceed the RATE SENSITIVITY value programmed. This
will assure normal loading will occur at the fastest pos-
sible speed. In most applications, 5.0°F/min. is sug-
gested. If unsure of a RATE SENSITIVITY selection,
use 5.0°F/min.
To program the RATE SENSITIVITY, first press the PRO-
GRAM key. Repetitively press the ENTER key until the
display below appears.
Key in the desired value and press the ENTER key. The
new value will be entered into memory and the display
will advance to the next user programmable limit.
The micro will accept a range of programmable values
between 0.5-5.0°F/min.
LOADING AND UNLOADING
RETURN WATER TEMPERATURE CONTROL
In return water temperature control, loading and unload-
ing will take place according to the deference between
the leaving water temperature setpoint and the actual
return water temperature. By programming the CONTROL
RANGE equal to the actual temperature drop across the
evaporator fully loaded, the microprocessor will be able
to maintain the desired leaving water temperature by
controlling off of the return water temperature (RWT).
Simply, the micro will know that the chiller should be
fully loaded when the RWT is equal to the SE
CONTROL RANGE. As the RWT drops, the micro will
unload the chiller which reduces the capacity (tempera-
ture drop across the evaporator). This maintains the leav-
ing water temperature at the desired setpoint.
Loading may be prevented due to the rate control cir-
cuitry. Anytime the return water temperature falls within
the CONTROL RANGE or the RATE CONTROL RANGE,
RATE CONTROL is in effect and loading and unloading
may be prevented, if water temperature changes faster
than the rate sensitivity. Loading and unloading will never
occur in intervals faster than 30 sec. under any circum-
stances. This prevents cycling of the compressors and
loaders. For the first 3 minutes after start-up, loading
cannot occur in intervals less than 1 minute per step.
Unloading occurs on temperature drop at temperatures
determined by the formula below. Internal timers have no
effect on unloading.
The micro is capable of providing 5, 7, or 10 steps of
loading/unloading although 10 steps is not possible with
the type compressors utilized. The chiller MUST be pro-
grammed for the number of steps which are standard
See page 68. Otherwise, improper operation and water
temperature control problems will result.
The temperature between stages of loading/unloading is
equal to the CONTROL RANGE divided by the number
of stages. For example:
CONTROL RANGE = 10°F
Number of Stages = 5
10°F
÷
5 = 2°F between stages
In this example, the micro will add a stage of
loading each time the return water temperature
rises 2°F assuming loading timers and rate con-
trol software allows.
The micro will unload a stage 2°F below the
temperature at which it was loaded assuming
unloading timers and rate control software allows.
R A T E
S E N S I T I V I T Y
=
5 . 0
F / M I N .
