period two
Chilled-Water System Design
notes
chillers piped in parallel
Dedicated Pumps
off
| | 42°F |
| | [5.6°C] |
| | on |
| 54°F | 60% to70% |
| of system flow |
| [12.2°C] |
coil starvedfor flow | Figure 42 |
If separate, dedicated pumps are used with each chiller, a pump-and-chiller pair can be turned on and off together as the cooling load varies. This solves the temperature mixing problem that occurred in the previous, single-pump configuration, but it presents a new problem in a system that uses a constant- flow method of terminal control.
Below 50-percent load, only one chiller and one pump are operating. The total water flow in the system decreases significantly, typically 60 to 70 percent of full system flow. Ideally, at this part-load flow rate, all of the coils will receive less water, regardless of their actual need. Typically, however, some coils receive full water flow and others receive little or no water. In either case, heavily-loaded coils will usually be “starved” for flow. Examples of spaces with constant heavy loads that may suffer include computer rooms, conference rooms, photocopy rooms, and rooms with high solar loads.
