Implementing chilled water supply temperature reset
Chilled water reset raises the chiller's supply water temperature whenever full cooling capacity is not needed, instead of holding a fixed low set-point year round. Because a chiller works less hard to make warmer water, resetting the set-point upward in step with actual demand cuts compressor energy without compromising the cooling delivered.
What it is
Most chilled water plants are commissioned with a single low supply temperature chosen for the worst-case design day, then left there permanently. Reset replaces that fixed set-point with a control strategy that lets the supply temperature float upward when loads are light or ambient conditions are mild, and pulls it back down only when a served load actually demands it.
Why it is done
A chiller's efficiency improves as the gap it must pump heat across narrows, so producing slightly warmer chilled water for most of the year takes meaningfully less compressor power. Holding the design set-point at all times means the plant pays the worst-case penalty continuously, even though the worst case occurs only on a handful of hours.
How it is done
The loads served by the chilled water are reviewed to find which one limits how warm the supply can go — usually a dehumidification or process-temperature requirement. A reset schedule is defined, driving the set-point off outdoor conditions or, better, off the most-open control valve in the system so the temperature rises until one load is just satisfied. The strategy is commissioned with guard limits, then verified that no served load is starved.
- Identify limiting load
- Choose reset driver
- Define reset schedule
- Set guard limits
- Commission control
- Verify no load starved
What to watch for
Resetting too aggressively starves a humidity or process load that only struggles occasionally, producing complaints that get the whole strategy disabled. Driving reset off outdoor temperature alone ignores internal loads, so valve-position reset is generally more robust.
Related practices
Running a compressed-air leak survey programme
Retrofitting waste-heat recovery
Retrofitting variable-speed drives
Related topics
Setpoint · Control Loop · Specific Energy Consumption (SEC)
Common in: Pharmaceuticals · Food Processing · Chemicals · Dairy · Brewing & Beverage