Trane SYS-APM001-EN manual Chiller sequencing in a double-ended decoupled system

System Configurations

When more than one chiller plant is operating, finding the right location for the differential pressure sensor can be difficult. The point of lowest pressure in the system shifts depending on which loads are using the most water. It will probably be necessary to have a number of differential pressure sensors in the distribution loop. In that instance, the control system determines the lowest differential pressure signal and uses it to determine what speed signals to send to the distribution pumps. Sending the same speed signal to both distribution pumps is simple and will prevent the pumps from “fighting” each other for the load. However, it will likely result in more chillers operating than would be required to meet the load (more on this below). There is one exception: If only one chilled-water plant has an operating chiller, the distribution pump associated with that plant should be the only one operating.

Chiller sequencing in a double-ended decoupled system

In addition to pump control, the sequencing of chillers must be integrated between the two plants. With a double-ended plant, it is possible to have one plant operating in deficit flow and the other with sufficient surplus flow to meet the needs of the loads (load/flow imbalance). In other words, a sufficient number of chillers are operating to meet the load, but the distribution pumps are not delivering the flow to where it is needed. There are two solutions to this problem:

1Adjust the speeds of the distribution pumps to move the water to where it is needed. This solution is simple in concept but difficult in application and will not be covered here. It increases complexity and should be considered carefully prior to implementation. If this is attempted, both distribution pump speeds should be changed slowly, and in opposite directions (increase one, decrease the other).

2Allow each of the plants to make start/stop decisions based on the flow in its decoupler. In this case, the needs of the loads will be met; however, more chillers may be running than would otherwise be required.

Starting chillers. When deficit flow is detected at a chilled-water plant, a chiller associated with that plant should be started. This should start chillers in the plant closest to the loads using the most water and help avoid the load/ flow imbalance described above. Deficit flow can be detected by either of the common flow detection methods. (See “Flow-based control” on page 51.) Note that this limits the operator's flexibility to preferentially start chillers in the plant further from the load (preferential start may be desirable to load the plant with the most efficient chillers first, for example).

Stopping chillers. On a single-plant decoupled system, surplus flow through the decoupler pipe is the normal operating mode. When the surplus flow exceeds the flow of one of the chillers in the plant by a fixed percentage, that chiller and its pump can be stopped and the plant will still be creating surplus flow. On double-ended plants this same logic can be used; however, a chiller should only be shut off when both plants have sufficient surplus to shut off a chiller. Only one chiller should be stopped at this time, and it should be the one in the plant with the most surplus flow. Thus, the normal operating mode for a double-ended plant is with one plant in surplus and the other operating

54 Chiller System Design and Control SYS-APM001-EN