York MILLENNIUM YCAS Installation Operation & Maintenance page 129

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 Setpoint tem-
perature. 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 to
the Setpoint Temperature. The amount of loading is var-
ied by changing the amount of signal to the slide valve
solenoid of each compressor.
Slide Valve Control
The slide valve of each compressor can be moved 75
steps, where "0" equals minimum capacity and fully
loaded equals 75 steps. The amount of movement that
occurs when the micro initiates changes may vary ac-
cording to the error or deviation from setpoint and the
rate of change of chilled liquid temperature. Each time
a change is made, the incremental change may vary
from 1 to 10 steps as determined from the micro. In
cases where internal limiting is not in effect due to pos-
sible fault conditions, the micro will load the compressor
with the lowest number of steps, alternating loading back
and forth between compressors until both are fully loaded
or unloaded.
In some cases the micro will be required to make deci-
sions regarding loading under conditions where the "er-
ror" and "rate" conflict. For example, the micro may
elect to unload a compressor if the error is "0" (tem-
perature is at setpoint), while the rate of change of chilled
liquid temperature is negative (falling). The micro may
also elect to hold capacity when error is "+" ( tempera-
ture is above setpoint) because the rate of change of
chilled liquid is "-". Below is a chart which illustrates
these conditions.
ERROR
Negative Zero Positive
RATE
Negative Unload
Zero Unload
Positive Hold
YORK INTERNATIONAL
Unload Hold
Hold Load
Load Load
Load Timers
Fixed timers are set to minimize undershoot and over-
shoot as a result of slide valve control.
• Load Timers are always set at 10 seconds between
changes.
• Unload timers are set at 5 seconds between
changes.
Slide Valve Position
A slide valve position (S V STEP), under the keypad
system keys, of 75 indicates that the compressor is fully
loaded. However due to the non-exact movement of
the mechanism, a position less than 75, possibly 60, could
also mean that the compressor is fully loaded. Keep this
potential indicator error in mind when attempting to de-
termine slide valve position.
Compressor Starting & Loading Sequence For
2-, 3-, and 4-Compressor YCAS Chillers
If no compressors are running, the Daily Schedule per-
mits, all safeties and run permissives are satisfied, the
anti-recycle timers have timed out, and the leaving liq-
uid temperature rises above the upper limit of the Con-
trol Range, the lead compressor will be started. A full
current signal is sent to the compressor slide valve con-
trol solenoid to allow the internal spring to push the slide
valve to a minimum loading position to assure it is fully
unloaded at start. For the first 15 seconds of operation,
or until pumpdown to cutout occurs, the liquid line sole-
noid valve will remain closed. After an initial period of
15 seconds, the micro will begin to load up the lead com-
pressor to bring the chilled liquid temperature to setpoint.
After 5 minutes of run time, if Setpoint Temperature is
not met, the micro will start the 1st lag compressor. This
is not dependent on slide valve position which after 5
minutes will be fully loaded at a S V Step of "75". The
lead compressor will be reduced in capacity to a slide
valve step of 40. The lag compressor will then be loaded
until it also reaches a slide valve step of 40 while the lead
compressor is maintained at a constant load. At this point
the compressors will be alternately loaded with loading
always occuring on the compressor with the lowest slide
valve step until the leaving chilled liquid is satisfied.
FORM 201.18-NM2
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129