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Micro Panel Contents
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 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 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
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.
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
pressure on the slide valve. This allows spring pressure
to move the slide valve to decrease capacity. Every 5
seconds, 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 loading.
As load drops, the micro will unload the compressor with
the high slide valve step until all compressor slide valves
are at "0" At this point, the the last lag compressor 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 Positions of "10".
As load continues to decrease, the lead compressor will
be pumped down and cycled off if the chilled liquid tem-
perature drops below "Setpoint – Control Range/2".
134
A lag compressor may be shut down
before it is fully unloaded to avoid a
Chiller fault on a Low Water Tempera-
ture cutout 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.
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.
The lead compressor may be shut
down before it is fully unloaded to
avoid a Chiller Fault on a Low Water
Temperature fault under the follow-
ing conditions: a) if chilled liquid
temperature drops 1°C (2°F) below
the low limit of the Control Range
(CR), b) if chilled liquid temperature
drops more than CR/2 below the low
limit of the Control Range.
6.3
LOCAL COOLING SETPOINTS KEY
The Local Cooling Setpoints key is used to program
the required Leaving Chilled Liquid control tempera-
tures for the application. When the key is pressed, the
following message will be displayed:
S E T P O I N T
=
R A N G E
=
+ / -
Key in the desired Chilled Liquid Setpoint and the al-
lowable deviation (Range). The micro will accept val-
ues from -12 to 21°C (10.0 - 70.0°F). For values below
4°C (40°F), Dip Switch S1, Switch #1 on the Micro-
processor Board must be properly programmed for
Brine Cooling (see Section 3.7). If unacceptable val-
6 . 7
° C
1 . 1
° C
YORK INTERNATIONAL
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