York MILLENIUM YCAS Series, Installation Operation & Maintenance, Page: 131 / 156

131
YORK INTERNATIONAL
FORM 201.18-NM6
uid to the Setpoint Temperature. The amount of loading
is varied 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 possible
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 “+”
(temperature is above setpoint) because the rate of
change of chilled liquid is “-”. Below is a chart which
illustrates these conditions.
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 versus actual compressor
capacity.
Compressor Starting & Loading Sequence
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 liq-
uid line solenoid valve will remain closed. After an ini-
tial period of 15 seconds, the micro will begin to load
up the lead compressor to bring the chilled liquid tem-
perature to setpoint.
After 5 minutes of run time, if Setpoint Temperature is
not met, the micro will start the 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 occurring on the compressor with
the lowest slide valve step until the leaving chilled liq-
uid is satisfied.
ERROR
Negative Unload Unload Hold
Zero Unload Hold Load
Positive Hold Load Load
RATE
Negative Zero Positive
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