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SYS-APM001-EN Chiller System Design and Control 53
Figure 36. Double-ended decoupled system
One of the benefits of decoupled water systems is that they are simple to
control. The distribution pump flow is determined by a pressure transducer
located at the furthest load. Flow in the decoupler indicates when to start and
stop chillers and the chiller pumps are turned on and off with the chillers.
Much of this simplicity is lost when multiple chiller plants are connected to
the same system. The system shown in the figure above is a fairly simple
example, but even so it can be used to show the difficulty of controlling these
systems. The following sections point out some of the complications.
Pump control in a double-ended decoupled system
Chiller pump control in a double-ended decoupled system remains
unchanged; the chiller pump is started when the chiller is enabled. On a
single-plant decoupled system, the distribution pump's speed is modulated
based on a pressure sensor located at the end of the loop (point of lowest
pressure) to maintain sufficient pressure drop across all the loads.
Chiller 2
Chiller 1
Chiller 3
Check Valves
New Plant
Loads
Existing Bypass Line
New Bypass Line
Production
Distribution
Existing Plant
Production
Distribution
Chiller 4
Other chiller plant/distribution loop
arrangements are possible, but if used,
they should be reviewed to make certain
they will be free from hydronic
problems.