Air Cooled or Water Cooled? An Owner-Driven Choice.
When selecting an air conditioning system for a new building, a choice that has to be made early in the process is whether the system should reject heat to the atmosphere by evaporating water (water cooled) or by radiating heat from coils, as is done in a car’s radiator (air cooled). This blog series isn’t intended to discuss everything that goes into that decision, but some of the highlights will be discussed, with the intent of giving a building owner a good background to approach the discussion with their AC design professional.
This question is explored in a 4 part series that provides an overview to the various considerations in a building owner’s choice between air cooled and water cooled air conditioning equipment. The four discussion topics are:
- Where does each typically fit?
- Maintenance implications.
- Utility cost differences.
There is also a related question regarding the use of water to save energy at a time when California has to make decisions in prioritizing the use of water. For some related thoughts on that, please see my monthly column on pages 4 & 5 of the Southern California chapter of ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) newsletter: http://www.ashrae-socal.org/pdf/SolAir-Feb-2014.pdf.
The first thing to recognize is that there are some economies of scale involved. It is hard to justify the added first cost of water cooled for small systems; for the sake of discussion, let’s call that threshold at about 100 tons. And in larger systems, the combination of first cost and energy consumption comes down pretty heavily in favor of water cooled (again, for discussion purposes, let’s call that threshold at 500 tons). So below 100 tons the default selection is air cooled, and only if you were highly motivated to save energy would a water cooled system enter the discussion. Likewise, above 500 tons the default selection would be water cooled, and only if there were extenuating circumstances would you probably choose to go air cooled.
Why wouldn’t every owner choose water cooled if it is more efficient, even if it is more expensive from a first cost standpoint? And when we say that one system is more efficient than another, how does that translate in a practical sense to an owner’s electric utility bill?
For more on that, move to Part 2 of this series.