water mist to improve AC function?
July 4, 2009 11:55 AM   Subscribe

Misting the AC compressor to increase efficiency?

So, my home happens to have central AC - a rarity here in the US Pacific Northwest. There are usually only a week or two of days hot enough to use it every year. We are in one of those weeks now. I understand how AC works and understand how swamp coolers work.

I was wondering about combining the best of both to increase the efficiency of my central AC - My thought was to mist cool water onto the radiator fins of the compressor so that cool water absorbs heat and draws it away through evaporation. This would undoubtedly draw away more heat than just the convection / conduction through air contact with which the machine was designed.

Setting aside the minor complexity of building a mister with mist heads around the radiator which is triggered by the AC power flowing to the fan does anyone know if this would be a BAD idea?
Seems to me that if it makes AC cool better and thus cool the house faster then someone in the industry would already have marketed this as an add-on or upgrade if it was really useful.

This leads me to surmise that either the benefits are negligible OR the constant wetness / evaporation / deposit of minerals from the evaporated water is a problem over time.

Haven't experimented yet, other than just spraying the yard hose on the compressor and noting the dramatically greater subjective experience of heat exchange from it.

I'm just wondering if anyone has data on this question that would save me the time and expense of experimenting and collecting the data myself.
posted by BrooksCooper to Home & Garden (6 answers total) 1 user marked this as a favorite
This does work, but the minerals left behind by the water will quickly gunk up the condenser. You might have better luck spraying water onto some swamp cooler media instead, and having the cooler air flow through the condenser afterwards. Then you can just replace the media when it gets full of minerals.

This is a little bit like how air conditioning in some large buildings works. The condenser rejects heat into water instead of air, and that water is pumped up to a cooling tower where airflow and evaporation reject the heat into the atmosphere. It's more efficient because the water returning from the cooling tower is cooler than the outside air.
posted by FishBike at 12:23 PM on July 4, 2009

This is probably not done for reasons of maintenance but I can also imagine it wouldn't be popular. Areas that are brutally hot are often the same ones that are in constant drought so using water just to make your AC more efficient (to save electricty) seems like a bad trade off.
posted by chairface at 2:07 PM on July 4, 2009

Some years ago, an HVAC professional suggested that we do this to our AC on very hot days, to boost the cooling. But he cautioned us severely not to do it unless the ambient temperature was over 90 degrees, because otherwise the water would be likely to freeze and damage the AC.

I have absolutely no expertise when it comes to these matters -- I figure that's what checks were made for -- so I can't evaluate the veracity of the advice, but perhaps you can.
posted by DrGail at 3:36 PM on July 4, 2009

Some window units are designed so the fan spatters water onto the fins of the condenser, but this is water that has been condensed from the air on the cold side of the AC, so it is effectively distilled and mineral free.
posted by Good Brain at 7:15 PM on July 4, 2009

I did that the other day at my house, actually. I went out with the hose and sprayed down the condensor. In South Carolina, it's been over 90 for months, now. It worked a little bit, but I imagine that more prolonged spray might have boosted the performance some more.
Rather than rigging something up to spray the condensor with water, it would be much easier to build something that would shade it from direct sunlight, a major factor in inefficient cooling performance.

Water cooled A/C condensors are very efficient and are in use on large, non residential structures. The condensors are submerged, rather than misted.

Already in production are kits used on drag racing cars that mist down a radiator or intercooler to lower the temperature of the vehicle before or during a race. Their pumps will run on a 12 volt DC circuit, but you probably could sub in an aquarium pump that runs on household current.
posted by Jon-o at 8:09 PM on July 4, 2009

"This leads me to surmise that either the benefits are negligible OR the constant wetness / evaporation / deposit of minerals from the evaporated water is a problem over time..."

Actually, it does work - works well - and appears to be moving in the direction of commercial acceptance.

A quick survey of the contenders and detractors: wetting the coils with untreated water is not recommended. The best idea is to cool the air around the condenser coils. Spray mist away from the coils, allow most of the water to evaporate, use a chemical water treatment that disperses calcium - e.g., polyacrylic acid (PAA).

I'll direct you to two university studies on the concept - the most recent comes from Hong Kong Polytechnic University by KT Chan (et al, with no commercial sponsor). The other report is from Tulane University by RG Watts (et al, with a commercial sponsor). Both reports appear to reach the same conclusion - that ac misting is effective in reducing the energy consumption of air cooled condensing units (air conditioners).

I purchased a product from the commercial sponsor of the Tulane U study. It has cut my electrical usage for AC by about 20% (they claim 'up to 30%).

Links: Hong Kong Polytechnic Study
The Tulane University Study
posted by Spinoff55 at 7:08 AM on June 24, 2010

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