Trane SYS-APM001-EN manual System Configurations

System Configurations

rate changes (Table 14). Selecting chillers with these characteristics improves the likelihood of stable, uninterrupted operation.

Estimate the expected flow-rate changes and make sure that the chillers you select can adapt to them. For example, one of the newest unit controllers on the market can reliably maintain the desired chilled water temperature with a flow-rate reduction of 50 percent per minute. Figure 38 shows the response of a chiller equipped with this controller in a more extreme situation. The flow dropped 67 percent in 30 seconds, with limited effect on the leaving chilled water temperature. Another, less robust chiller controller permits flow-rate changes of less than 2 percent per minute and would need 25 minutes to adapt to a flow-rate reduction of 50 percent. Fluctuations of 2 percent or more are typical, even during normal system operation. Attempting to limit flow-rate changes to this extent while starting or stopping a chiller is impractical, if not impossible.

When comparing prospective chillers, consider the transient-flow tolerance of the unit controllers. Then work closely with the chiller manufacturer to devise a flow-transition sequence that accounts for the unique operating characteristics of both the chiller and the application. Transient flow rate control is discussed in more detail on page 65.

Figure 38. Example of chiller control responsiveness to flow-rate reduction*

*Data represents a Trane AdaptiView™ and CH530 chiller controller with flow compensation.

Select for nearly equal pressure drops across all chiller evaporators. A VPF design loads and unloads the chiller(s) based primarily on the rate of water flow through the evaporator. If a difference in size or type of evaporator gives one chiller a lower pressure drop than the others in the plant, that chiller will receive a higher rate of water flow and a correspondingly greater load.

Dissimilar pressure drops can make it difficult to provide stable plant operation. Table 15 demonstrates this effect in a two-chiller system (similar to the one shown in Figure 37). In this case, more water flows through Chiller 1’s

60 Chiller System Design and Control SYS-APM001-EN