York YCWJ45EE0, Установка, эксплуатация и обслуживание

Страница: 64 / 104

64
YORK INTERNATIONAL
LOADING AND UNLOADING
RETURN WATER TEMPERATURE CONTROL
In return water temperature control, loading and unload-
ing will take place according to the difference 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.
Normal loading will occur at intervals of 60 seconds,
according to temperatures determined by the formula
below.
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 may be pre-
vented, if water temperature changes faster than the rate
sensitivity. Loading will never occur in intervals faster
than 60 sec. under any circumstances. This prevents
cycling of the compressors and loaders.
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 were ordered.
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 as-
suming loading timers and rate control software al-
lows.
The micro will unload a stage 2°F below the tempera-
ture at which it was loaded assuming unloading tim-
ers and rate control software allows.
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, a low RATE SENSITIVITY may be selected. 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 3° - 5°F/min. RATE SENSITIVITY is recom-
mended. This may be needed for small water loops. How-
ever, if problems arise where the chiller does not load or
abruptly turns a compressor off and fails to pull tempera-
ture 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.
R A T E S E N S I T I V I T Y
= 5 . 0 F / M I N .