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Selection of the LAG COMPRESSOR START POINT %
under the PROGRAM key, allows the user to select the
loading point of the lead compressor at which the lag
compressor starts. Once the lag compressor starts,
equalized load sharing will begin.
Selection of the DIFFERENTIAL OFF % determines the
range of unloading over which load sharing will occur.
Two Control Schemes may be selected. Load Sharing,
programming of the start point %, and differential off %
will first be discussed in CONTROL SCHEME 1 utilizing
% FLA Motor Current and then in CONTROL SCHEME
2 utilizing % Slide Valve. Each one should be reviewed
before a selection is made. Both control schemes should
work well for most applications.
For simplicity of programming to eliminate manual pro-
gramming required in each of the two Control Schemes,
the "DEFAULT" programming points (page 58) may be
selected. This selects LOAD SHARING BASED ON %
FLA as the control means for load sharing with a LAG
COMPRESSOR START POINT % of 70% FLA. It also
selects a LAG DIFFERENTIAL OFF % of 50%. These
control points may be programmed individually under the
PROGRAM key without selecting all "Default" values as
mentioned on page 96.
CONTROL SCHEME 1
Loading And Unloading Utilizing % FLA Control
The LAG COMPRESSOR START POINT % (FLA), is
programmable in the Program mode. By selecting a LAG
COMPRESSOR START POINT % (FLA), the user is
actually selecting the % FLA at which the lead compres-
sor is allowed to load, before the lag compressor is brought
on. This % is programmable from 40 - 99%.
The micro will start the lead compressor whenever the
chilled liquid temperature is above the High Limit of the
CONTROL RANGE (CR) and the 3 minute "Start Timer"
has timed out. The 3 minute "Start Timer" is the minimum
amount of time that the compressor must remain off af-
ter a cycling shutdown (Anti-Recycle Timer Timed Out)
or after power is first applied. Once started, the micro
will pulse the slide valve to load the lead compressor to
bring the chilled liquid within the Neutral Zone.
If temperature is not brought down to within the Neutral
Zone, the lag compressor will be brought on by the micro
after the lead compressor loads up to the programmed
LAG COMPRESSOR START POINT % as determined
and programmed by the user.
If due to system conditions, the lead compressor does
not reach the programmed %, even though the lead com-
pressor may be fully loaded, the lag compressor will start
100
regardless of the programmed Start % after 5 minutes of
operation. This will occur whenever the lead compressor
cannot bring the LWT to within 2.0°F of the high end of
the CONTROL RANGE (CR). Once the lag compressor
is started, equalized loading and unloading of the two
compressors will occur based on % FLA.
The 5 minute default for lag compressor start assures
that the lag compressor will start and load in situations
where the lead compressor % FLA does not reach the
programmed LAG COMPRESSOR START POINT %.
These situations may be common because as ambients
drop, actual % FLA at full load will drop at 100% slide
valve. Actual operating % FLA for a compressor that is
fully loaded may often be 65 - 85% of the motor FLA at
ambients below 95°F.
As mentioned above, after the lead compressor starts
and loads up to the programmed 40 - 99%, if tempera-
ture is still above the CONTROL RANGE (CR), the lag
compressor will come on. It will then be brought up to a
% FLA equal to the lead compressor provided demand
dictates. At this point, the loading of both compressors
will be adjusted up or down to maintain capacity and
equalize loading.
NOTE: Under most operating conditions, viewing %
FLA of each compressor will show a differ-
ence in the FLA of each compressor which at
times may be significant. This condition will
exist even though equalized loading/unload-
ing should be observed. This is due to the
micro constantly pulsing slide valves of each
compressor to control temperature and dif-
ferences in operating conditions of the two
compressors, such as discharge pressure
which causes each compressor's slide valve
to move a different amount with a pulse of a
specific duration. Other system pressures and
mechanical tolerances within compressors
will also contribute to differences. Regard-
less of the differences in % FLA, the micro
will still assure that leaving chilled liquid tem-
perature is properly controlled.
Also, in some cases, the micro will not be programmed
to attempt to control equalized unloading over the entire
loading range which will affect the unloading sequence
slightly. Refer to page 102, "Selecting The LAG COM-
PRESSOR DIFFERENTIAL OFF %" for details.
Approximate loading time for each compressor on a hot
water start is about 5 minutes per compressor at worst
case conditions. If the load drops, both compressors will
each unload equally to a point as determined by the LAG
COMPRESSOR DIFFERENTIAL OFF %. When further
capacity reduction is required, the lag compressor will
continue to unload, and shutdown while the lead com-
pressor maintains its % FLA loading point. When fully
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