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Micropanel
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.
7.7 COMPRESSOR LOADING
The microprocessor loads and unloads individual com-
pressors 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 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 microprocessor will increment
the slide valve step from 1 to 10 according to error
(deviation from setpoint) and rate of change of chilled
liquid.
The microprocessor will always choose the compres-
sor with the lowest slide valve position to load on in-
creasing demand, provided the compressor is not pump-
ing down, has run at least 15 seconds, and is not in a
'Limiting' condition.
7.8 LOADING LIMITING
Load limiting will occur if a system safety threshold is
neared. The microprocessor limits loading or unloads
a system if the microprocessor anticipates a safety
threshold will be exceeded.
Under circumstance where loading is required and one
of the systems is nearing a threshold, the microproces-
sor may elect to split the number of steps that it would
normally load a compressor between more than one
compressor.
For instance, if system 1 were nearing its motor current
unload point and it was scheduled to load, the micropro-
cessor could split a load signal of 10 steps between sys-
tems 1 and 2. It could only load system 1 '2' steps while
loading system 2 '8' steps. Under these circumstances,
the two systems will not appear to equalize loading.
64
7.9 COMPRESSOR UNLOADING AND SHUTDOWN
SEQUENCE
Whenever temperature is below the setpoint the slide
valve current is decreased to relieve oil pressure on the
slide valve.
This allows spring pressure to move the slide valve to
decrease capacity. Every 5 seconds, the microprocessor
will decrement the slide valve position by 1 -10 steps ac-
cording to the 'ERROR' (deviation from setpoint) and
the 'RATE' (rate of change of chilled liquid temperature).
Unloading will occur in the reverse sequence as load-
ing. As load drops, the microprocessor will unload the
compressor with the highest slide valve step until all
compressor slide valves are at '0'.
At this point, the lag compressor will pump down and
cycle off, if chilled liquid temperature drops below
'SETPOINT – CONTROL RANGE/2'. When the lag
compressor cycles off, the microprocessor will set the
lead compressor at slide valve position 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 – CONTROL
RANGE/2'.
7.10 LOCAL COOLING SETPOINTS KEY
The 'LOCAL COOLING SETPOINTS' key is used to
program the required leaving chilled liquid control tem-
perature and range 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, the micropro-
cessor will accept values from 10.0°F to 70°F (-12.2 to
21.1°C).
For values below 40°F (4.4°C), Dip
Switch S1-1 on the AMB board must
be programmed for glycol (Brine)
cooling.
FORM 201.25-NM1 (302)
4 4 . 0
° F
2 . 0
° F
YORK INTERNATIONAL
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