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to the sequence. If the load cannot be brought under
control in 15 minutes, the 3rd lag compressor (com-
pressor 4) will be started. Before starting the 3rd lag
compressor, the lead and 1st lag compressor will be
maintained at full load (S V Step 75) and the 2nd lag
compressor will be unloaded to S V Step 40. If load
continues to rise, the 2nd and 3rd lag compressor will
be alternately loaded until the leaving chilled liquid
temperature is satisfied.
Compressor Loading
The micro loads and unloads individual compressors
by varying current to the Slide Valve solenoid which
controls oil flow to the slide valve. The slide valve load
solenoid applies oil pressure to the slide valve to over-
come spring pressure from an internal spring, increas-
ing capacity. The internal spring moves the slide valve
in the opposite direction against oil pressure to decrease
capacity.
Whenever chilled liquid leaving temperature is above
the Setpoint, loading current will increase to allow oil
pressure to move the slide valve to increase capacity.
Every 10 seconds, the micro will increment the slide
valve step from 1 to 10 steps according to error (devia-
tion from setpoint) and rate of change of chilled liquid.
The micro will always choose the compressor with the
lowest slide valve position to load on increasing de-
mand, provided the compressor is not pumping down,
has run at least 15 seconds, and is not in a "Limiting"
condition.
Loading Limiting
Load limiting will occur if a system safety threshold is
neared. The anticipatory capability of the micro limits
loading or unloads a system if the micro anticipates a
safety threshold will be exceeded. Under circumstance
where loading is required and one of the systems is
nearing a threshold, the micro may elect to split the
number of steps that it would normally load a compres-
sor between more than one compressor. For instance, if
system 1 were nearing its motor current unload point
and it was scheduled to load, the micro could split a
load signal of 10 steps between system 1 and 2. It could
only load SYS 1 "2" steps while loading SYS 2 "8"
Steps. Under these circumstances, the two systems will
not appear to equalize loading.
YORK INTERNATIONAL
Compressor Unloading and Shutdown Sequence
Whenever temperature is below the Setpoint, unload-
ing pulses will be sent to open the unloading port on
the control solenoid to relieve oil pressure on the slide
valve on the compressor with the highest slide valve
step. Opening of the unloading port on the control so-
lenoid allows spring pressure to move the slide valve
to decrease capacity. Every 5 seconds, the micro will
decrement the compressor with the highest slide valve
position by 1 - 10 steps according to the error (devia-
tion from Setpoint) and the rate of change of chilled
liquid temperature.
As load drops, the micro will continue to unload the
compressor with the highest slide valve step until all
compressor slide valves are at "0." At this point, the
last lag compressor will pump down and cycle off, if
chilled liquid temperature drops below "Setpoint - Con-
trol Range/2". On 3 and 4 compressor chillers, when a
lag compressor cycles off, the micro will set the re-
maining running lag compressors to Slide Valve Posi-
tions of "10" (future EPROM software will set the po-
sition to "30"). As load continues to decrease and all
lag compressors cycle off, the lead compressor will con-
tinue unload to a slide valve position of "0" and will
pump down and cycle off if the chilled liquid tempera-
ture drops below "Setpoint – Control Range."
A lag compressor may be shut down
before it is fully unloaded to avoid a
Chiller fault on a Low Water Tempera-
ture cut-out under the following con-
ditions: a) if chilled liquid temperature
falls below the low end of the Control
Range (CR) for more than 37 seconds,
b) if chilled liquid temperature drops
more than CR/4 below the low limit of
the Control Range.
The lead compressor may be shut
down before it is fully unloaded to
avoid a Chiller Fault on Low Water
Temperature under the following con-
ditions: a) if chilled liquid temperature
drops 2°F below the low limit of the
Control Range (CR), b) if chilled liq-
uid temperature drops more than CR/
2 below the low limit of the Control
Range.
FORM 201.18-NM3
8
171
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