FORM 150.44-NM4
47
YORK INTERNATIONAL
RANGE or the CONTROL RANGE. This allows the mi-
cro to react to abrupt downward changes in leaving or
return water temperatures. The ability to respond to
rate of change variations in water temperatures gives
the micro anticipation capabilities to reduce the possi-
bility of overshoot in leaving water temperature.
In demand limiting applications, to avoid cycling, or to
avoid overshoot, a low RATE SENSITIVITY may be se-
lected. 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 when-
ever water temperatures are in RATE CONTROL
RANGE or the CONTROL RANGE. For these applica-
tions, a 35°F/min. RATE SENSITIVITY is recom-
mended. 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 wa-
ter system allows a slug of cold water to enter
which falsely fools the micro into thinking the
RATE SENSITIVITY has been exceeded, pre-
venting loading and allowing leaving water tem-
perature to rise above the desired temperature.
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. In most ap-
plications, 5.0°F/min. is suggested.
If unsure of a RATE
SENSITIVITY selection, use 5.0°F/min.
To program the RATE SENSITIVITY, first press the
PROGRAM 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 difference between
the leaving water temperature setpoint and the actual re-
turn 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 con-
trolling 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 (temperature drop
across the evaporator). This maintains the leaving 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
uploading will never occur in intervals faster than 30
sec. under any circumstances. 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
programmed for the number of steps which were or-
dered. Otherwise, improper operation and water tem-
perature control problems will result.
The temperature between stages of loading/unloading
is equal to the CONTROL RANGE divided by the num-
ber 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 allows.
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.
Listed in Fig. 15 and 16 are the loading and unloading
sequences for 5 and 7 steps of control. A typical setpoint
of 45°F with a cooling range of 10°F is shown for ex-
ample purposes. The chiller will be completely loaded
at a return water temperature of 55°F and will be com-
pletely off at a temperature of 45°F, thus maintaining a
leaving water temperature of 45°F. As mentioned be-
fore, loading/unloading timers as well as rate control
software must be satisfied before loading/unloading will
occur. This reduces the possibility of cycling.
R A T E
S E N S I T I V I T Y
=
5 . 0
F / M I N .