period four
Chiller-Plant Control
notes
Chilled Water Reset
▲Pros
◆Reduces chiller energy
◆Can work in constant- flow systems
▲Cons
◆Increases pump energy in variable-flow systems
◆Can cause loss of space humidity control
◆Complicates chiller sequencing control
Figure 105
As previously stated, as the chilled-water temperature set point is reset upwards, the chiller will use less energy. In constant-flow systems, this chilled-water reset strategy is fairly simple to implement and can be controlled based on the drop in return-water temperature.
In a variable-flow system, however, as the chilled-water temperature increases, the pumping energy also increases. While the COP of the chiller is approximately 6.5, the COP of the pump is about 0.65. Often the increase in pump energy will be more than the amount of chiller energy saved, especially because the chiller will often operate at part-load conditions. Another potential problem with resetting the chilled-water temperature upward is that space humidity control can be compromised if the water gets too warm. Finally, the chiller-plant control system must account for the changing supply-water temperature.
ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.3) requires the use of chilled-water temperature reset in systems larger than 25 tons [88 kW]. It does, however, exclude variable-flow systems and systems where space humidity control will be compromised.
In Period Three, the concept of designing for reduced chilled-water temperature and flow rates was briefly discussed. Some engineers feel that designing the system for low flow rates and a lower supply-water temperature, thus minimizing pump energy use, might be a better answer than attempting to reset the temperature upward.
