130
YORK INTERNATIONAL
On 3-compressor chillers, if the two compressors cannot
bring the load under control within 10 minutes, the micro
will maintain the lead compressor fully loaded at S V
Step 75, unload the first lag compressor to Step 40, and
bring on the 2nd lag compressor. The second lag com-
pressor will load until it reaches Step 40. From this point
the compressors will be alternately loaded, always load-
ing the compressor with the least step of loading first.
On 4-compressor chillers, loading will work the same
as a 3-compressor chiller with another compressor added
to the sequence. If the load cannot be brought under
control in 15 minutes, the 3rd lag compressor (compres-
sor 4) will be started. Before starting the 3rd lag com-
pressor, the lead and 1st lag compressor will be main-
tained at full load( S V Step 75) and the 2nd lag com-
pressor will be unloaded to S V Step 40. If load contin-
ues to rise, the 2nd and 3rd lag compressor will be
alternately loaded until the leaving chilled liquid tem-
perature is satisfied.
Compressor Loading
The micro loads and unloads individual compressors by
varying current to the Slide Valve solenoid which con-
trols oil flow to the slide valve. The slide valve load
solenoid applies oil pressure to the slide valve which
increases capacity. An internal spring moves the slide
valve in the opposite direction 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 according to error (deviation
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 demand,
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 near-
ing a threshold, the micro may elect to split the number
of steps that it would normally load a compressor be-
tween more than one compressor. For instance, if sys-
tem 1 were nearing its motor current unload point and it
was scheduled to load, the micro could split a load sig-
nal 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.
Compressor Unloading and Shutdown Sequence
On 2-system chillers, whenever temperature is below
the Setpoint, unloading pulses will be sent to open the
unloading port on the control solenoid to relieve oil pres-
sure on the slide valve. This allows spring pressure to
move the slide valve to decrease capacity. Every 5 sec-
onds, the micro decrement the slide valve position by 1
-10 steps according to the error (deviation from Setpoint)
and the rate of change of chilled liquid temperature.
Unloading will occur in the reverse sequence as load-
ing. As load drops, the micro will unload the compres-
sor with the high slide valve step until all compressor
slide valves are at "0" At this point, the the last lag com-
pressor will pump down and cycle off, if chilled liquid
temperature drops below "Setpoint - Control Range/2".
When a lag compressor cycles off, the micro will set
the remaining running compressors at Slide Valve Posi-
tions of "10". As load continues to decrease, the lead
compressor will be pumped down and cycled off if the
chilled liquid temperature drops below "Setpoint - Con-
trol Range/2.
On 3-compressor chillers, the process is the same as
for 2-compressor chillers except that when the mini-
mum level of equalized loading is reached, the lead and
first lag compressor are maintained at constant load and
only the second lag compressor continues to unload. If
the second lag compressor shuts down according to de-
creasing load and demand continues to fall, then the
first lag compressor will begin to unload and if neces-
sary shut down.
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 tempera-
ture falls below the low end of the
Control Range (CR) for more than 37
seconds, b) if chilled liquid tempera-
ture drops more than CR/4 below the
low limit of the Control Range.
If demand continues to fall and the lead compressor is
the only compressor running, it will continue to unload
and shut down when leaving liquid temperature drops
below the low limit of the Control Range.
Micropanel
Summary of Contents for MILLENNIUM YCAS
Page 53: ...53 YORK INTERNATIONAL FORM 201 18 NM2 WIRING DIAGRAM ACROSS THE LINE START 7...
Page 54: ...54 YORK INTERNATIONAL FIG 12 CONTINUED ELEMENTARY DIAGRAM Technical Data...
Page 59: ...59 YORK INTERNATIONAL FORM 201 18 NM2 WIRING DIAGRAM WYE DELTA START LD03229 FIG 15 CONT D 7...
Page 60: ...60 YORK INTERNATIONAL FIG 15 CONTINUED ELEMENTARY DIAGRAM Technical Data...
Page 62: ...62 YORK INTERNATIONAL FIG 16 POWER PANEL FRONT INSIDE VIEW WYE DELTA START Technical Data...
Page 68: ...68 YORK INTERNATIONAL FIG 24 DETAIL B Technical Data...
