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Staging cooling-tower fans
CNT-APG002-EN 43
®
Example 1: Two-stage fan system
The staging points are calculated as follows:
1. Calculate the overlap range.
2. Calculate the first stage control points.
3. Calculate the second stage control points.
The staging points are illustrated in Figure 31. You should not have to
adjust the extremes at 10% and 90%, but you may need to adjust the mid-
dle staging points to optimize the system.
Figure 31: Two-stage fan on and off points
Overlap range
80%
stage count 1+
-----------------------------------------
80%
21+
-------------26.7%===
Stage 1
On: control value 10% 2 26.7× %()+63%≈≥
Off: control value 10%<
=
Stage 2
On: control value 10% 3 26.7× %()+90%=≥
Off: control value 10% 1 26.7× %()+36%≈<
=
Control value (%)
Fan 2
Fan 1
No. of
fans
2
1
0
0 10 20 30 40 50 60 70 80 90 100