Page 46 OM 780-2
How Demand Limiting Works
Demand limiting control requires a capacity limit value, which must come from an outside source. You can choose one of
three possible sources with the Demand Limiting Type variable:
• External (analog signal)
• BACnet
• Modbus
After receiving the capacity limit from the selected source, the CSM generates the System Demand Limiting Load Limit
(Load Limit screen). If the value of this variable is less than the capacity limit produced by the soft load or load-balancing
functions, the CSM sends it to every chiller in the system.
Screw and Scroll Chillers
Since screw and scroll chillers control their capacity in stages, the System Demand Limiting Load Limit cannot be used
directly as it is in centrifugal chillers. Instead, each screw or scroll chiller’s unit controller converts the percent-load
capacity limit into a maximum-stage capacity limit. The step functions that screw chiller unit controllers use to do this are
shown in Figure 14.
Figure 14. Screw Chiller Demand Limiting
0
2
5
8
40 50 60 70 100
Capacity Limit (Compressor Stages)
80
Capacity Limit (% Load)
7
6
4
3
1
4-stage chillers
9
12
11
10
90
6-stage chillers
8-stage chillers
12-stage chillers
a0143
Demand Limiting from an External Signal
If the Demand Limiting Type variable is set to “External,” the CSM uses an external voltage or current signal as the source
of the System Demand Limiting Load Limit. An analog signal (0–10 Vdc, or 0–20 mA) must be connected to UI-11 on the
CSM’s onboard I/O panel. For more information, see the Field Wiring section of IM 781. The capacity limit is calculated
according to the function shown in Figure 15.
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