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Chapter 4 Applications
42 CNT-APG002-EN
Determining the staging points
This section describes how to find the points at which stages are turned
on and off.
Start with these guidelines:
• To avoid having a stage turn off at the lowest extreme, always have at
least one stage on at 10% of the output range. Turn that stage off
when the control value is less than 10%. Due to hysteresis (the pro-
gramming of equipment to react in a different way depending on
whether the control value is increasing or decreasing), this stage may
be on only when the output is decreasing.
• To avoid having a stage turn on at the PID maximum value, have all
stages on at 90% of the output range.
• To reduce equipment cycling, stages should overlap.
• As a starting point, assume that the overlap range is the same for all
stages. You can adjust the staging points later to optimize the system.
To determine the staging points:
1. Use the following formula to find the overlap range:
(assuming the system has three fans).
2. To create overlap, the first stage should turn on at the lowest extreme
plus 2 times the overlap range and turn off at the lowest extreme, or:
For a three-fan system, the first stage should turn on at 50% and turn
off at less than 10%.
3. For each subsequent stage, the on and off points are described by:
Although not discussed in this section, equipment minimum-on and -off
times become a factor as more stages are added to the system. The higher
stages may be on for shorter periods of time. System behavior may
become erratic if a stage control is dominated by minimum-on and -off
times instead of the calculated control value.
Overlap range
highest extreme lowest extreme–
stage count 1+
--------------------------------------------------------------------------------------------=
Overlap range
90% 10%–
stage count 1+
-----------------------------------------
80%
31+
-------------20%===
Stage 1
On: control value 10% 2 overlap range×()+50%=≥
Off: control value 10%<
=
Stage n
On: control value 10% n 1+()overlap range×+≥
Off: control value 10% n 1–()overlap range×+<
=