period four
Chiller-Plant Control
notes
constant-volume pumping system
Chilled-Water Set Point Control
54°F
off[12.2°C]
| 37°F | 45.5°F |
on | [2.8°C] | [7.5°C] |
| |
54°F 
[12.2°C]
loweringoperating chiller set point helpsregaincontrol
Figure 103
Constant-flow chilled-water systems frequently require individual chiller set- point control. Its purpose is to help maintain the system supply-water temperature by compensating for the bypass of return water through non-operating chillers.
The chiller-plant control system adjusts the individual set points for the operating chiller to “overcool” the water before it mixes with the higher- temperature water that bypasses through the non-operating chiller. The result is that the chilled water supplied to the system is as close as possible to the desired temperature. There are limits to the amount of overcooling. Depending on the design of the chilled-water system, one of two situations may exist. Either the chiller may not have been selected to produce cold-enough water, or the temperature required may be below the freezing point of the water being cooled. In either case, the control system must be intelligent enough to limit overcooling in order to prevent damage to the chiller.
Additionally, the control system must know when to turn another chiller on to meet the system chilled-water-temperature set point. Turning an additional chiller on may be required to meet the system demand for flow, even though the operating chiller may not be fully loaded.
