© 2017 Emerson Climate Technologies, Inc.
9
AE21-1355 R4
0.8685e-0.0257x. Warning: this curve fit is only valid for
temperature range of 100 to 160°C (212-320°F).
Multiple Compressor Application
In multiple compressor application is important to select
the right compressor to be the digital. This is achieved
by using a simple rule that is explained bellow.
Rule:
For optimum suction pressure control, the
following guideline is recommended in the selection of
Discus digital and fixed compressors, per suction
header:
1. - D > F1
2. - F2 < D+F1
3. - F3< D+F1+F2
….
N.-
FN<D+F1+2+….FN-1
In the above equations:
D is digital Discus capacity or horse power.
F1…FN are the standard Discus fixed capacity
compressor.
In essence,
1. Digital compressor must have a higher capacity than
the first fixed capacity compressor.
2. The second fixed capacity compressor must have a
smaller capacity than the sum of digital and first fixed
capacity compressor.
3 -N- And so on.
In conclusion, the digital compressor selected should be
the smallest compressor capacity that still covers all the
gaps between steps to ensure the most efficient system
control.
A designer using this rule must keep in mind the digital
range, for example on a 4D you have continuous
capacity from 50 to 100%, therefore when selecting a
4D digital compressor you should have a fixed
compressor behind it that is 50%-100% of the full load
of the digital compressor.
Example1.
You have selected a 4DAXF47KL-TSK.
This compressor capacity is about 47,000 Btu per hour,
50% capacity is about 23,500. Then, D=47,000 BTU/h
As the first fixed capacity compressor, you must choose
a compressor with a capacity between 23,500 BTU/h to
47,000 BTU/h.
3DA3F28KL-TFD is a compressor with a full capacity
about 28,000 Btu/h. F1=28,000 BTU/h
47,000 BTU/h >28,000 BTU/h
D>F1
Therefore, 3DA3F28KL-TFD could be selected as the
first fixed capacity compressor and 4DAXF47KL-TSK as
the Discus digital compressor.
Note:
For best results, the digital compressor needs to
be the lead compressor. It must be the first compressor
on and last compressor off in multiple compressor
applications.
Example 2
Select 3 fixed capacity compressor of 7.5 HP and a
Discus digital compressor.
The Discus digital compressor capacity must be higher
than 7.5 HP, then a compressor capacity of 10 Hp is
selected.
For this case F1=7.5 HP and D=10 HP
Therefore, F1+D=7.5HP+10 HP=17.5 HP
The second compressor capacity is 7.5 HP.
Therefore, F2=7.5 HP<F1+D
The third compressor capacity is 7.5 HP.
Then, F1+F2+D=7.5 HP+7.5HP+10 HP=25 HP
Therefore, F3<25 HP
The table below shows System output capacity:
