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liquid to the setpoint temperature. The amount of load-
ing is varied by changing the amount of signal to the
slide valve solenoid of each compressor.
7.3 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 microprocessor initiates changes may
vary according to the 'ERROR' (deviation from setpoint)
and 'RATE' (rate of change of liquid temperature).
Each time a change is made, the incremental change
may vary from 1 to 10 steps as determined
from the microprocessor.
In cases where internal limiting is in effect due to pos-
sible fault conditions, the microprocessor will load the
compressor with the lowest number of steps, alternat-
ing loading back and forth between compressors until
both are fully loaded or unloaded.
In some cases the microprocessor will be required to
make decisions regarding loading under conditions
where the 'ERROR' and 'RATE' conflict.
For example, the microprocessor may elect to unload a
compressor if the 'ERROR' is 'zero' (chilled liquid tem-
perature = setpoint), while the 'RATE' is 'negative' (liq-
uid temperature is falling). The microprocessor may also
elect to hold capacity when error is 'positive' (chilled liq-
uid temperature > setpoint) because the 'RATE' is 'nega-
tive' (liquid temperature is falling), as shown below:
Negative
RATE
Negative
Unload
Zero
Unload
Positive
Hold
FIG. 21 – SLIDE VALVE RANGE
7.4 LOAD TIMERS
Fixed timers are set to minimize undershoot and over-
shoot as a result of slide valve control.
YORK INTERNATIONAL
ERROR
Zero
Positive
Unload
Hold
Hold
Load
Load
Load
Load timers are always set at 10 seconds between
changes.
Unload timers are set at 5 seconds between changes.
7.5 SLIDE VALVE POSITION
A slide valve position (S V STEP), under the keypad
'SYSTEM # DATA' keys, of '75' indicates that the com-
pressor 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 determine slide valve position.
7.6 COMPRESSOR STARTING AND LOADING
SEQUENCE
With no compressors running, the 'DAILY SCHEDULE'
permits, all safeties and run permissives are satisfied,
the anti-recycle timers have timed out, and the leaving
liquid temperature rises above the upper limit of the
range, the lead compressor will be started:
• A full current signal is sent to the compressor slide
valve control solenoid to allow the internal spring
to push the slide valve to a minimum loading posi-
tion to assure it is fully unloaded at start.
• For the first 15 seconds of operation, or until pump
down to cutout occurs, the liquid line solenoid valve
will remain closed.
• After the initial period of 15 seconds, the micro-
processor will begin to load up the lead compres-
sor to bring the chilled liquid temperature to the
setpoint.
After 5 minutes of run time, if setpoint temperature is
not met:
• The microprocessor 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.
FORM 201.25-NM1 (302)
7
63
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