Homemade Evaporator
August 3, 2006 12:23 PM Subscribe
What are the disadvantages of making your own water-cooled evaporative air conditioner?
My A/C has the compressor and heat exchanger in a large outside unit. I'm thinking about getting some 1/2" irrigation hose and a few small mister attachments to mist down the sides of the unit where the air is drawn in to the coils. That way, the air flowing across the coils would be cooled by evaporation and therefore the unit would not have to work as hard to exchange heat. Other than the (very small) amount of water used, what would be the drawbacks of this system? It's so simple that I don't understand why it isn't being marketed. So far, the only thing I can think of is that scale might build up on the coils.
My A/C has the compressor and heat exchanger in a large outside unit. I'm thinking about getting some 1/2" irrigation hose and a few small mister attachments to mist down the sides of the unit where the air is drawn in to the coils. That way, the air flowing across the coils would be cooled by evaporation and therefore the unit would not have to work as hard to exchange heat. Other than the (very small) amount of water used, what would be the drawbacks of this system? It's so simple that I don't understand why it isn't being marketed. So far, the only thing I can think of is that scale might build up on the coils.
I wonder why the coil isn't made of alumium or inox iron..probably different radiation properties ? Also buildup of minerals on the coil would almost certainly change the properties of the system and make it worse, so I guess demineralized water would work best...but at what cost, I can't tell.
posted by elpapacito at 12:44 PM on August 3, 2006
posted by elpapacito at 12:44 PM on August 3, 2006
Are you talking about putting water on the outside condenser coils (the warm ones)? If so, I think many air conditioners already do this (this is why window ac units don't have holes drilled in the bottom for drainage-- the warm water is thrown back onto the coils for additional evaporative cooling)?
posted by justkevin at 12:44 PM on August 3, 2006
posted by justkevin at 12:44 PM on August 3, 2006
- I think your idea of "very small amount of water" may need some calibration.
- Cooling by evaporation is much more effective when you are in a low humidity environment—there needs to be capacity in the air to absorb that sprayed water as water vapour. If not, it won't evaporate as readily. How much water you need would vary depending on the atmospheric humidity, and unless you had a control system to predict the right water flow, you could have substantial amounts of water not being evaporated, but pooling and flowing all over the place and making a mess.
- The heat exchanger has already been sized to operate in air, so it's not really working hard, it's working normally. Adding evaporative cooling would allow you to use a smaller heat exchanger (provided you could count on evaporative cooling, see 2), but will be of less benefit for the OEM heat exchanger, and you don't really need a smaller heat exchanger anyway.
- Air heat exchangers are designed differently from water heat exchangers, so you would also want to avoid getting too much sprayed water on the system, as it would mess up the flow patterns of the air and the convective cooling.
posted by cardboard at 12:50 PM on August 3, 2006
It takes constant attention (twice a day or more). Leave it alone and mould takes over, much of which is poisonous.
It doesn't work very well.
posted by KRS at 1:43 PM on August 3, 2006
It doesn't work very well.
posted by KRS at 1:43 PM on August 3, 2006
When I was a lad, I was standing in the back yard one hot summer afternoon, hose in hand, listening to our air conditioning compressor labor. "This thing can stand rain, right? And a water spray would help cool it off, even with the humidity as high as it is, right?"
The unit died a day later. It was old, so I don't know if spraying it for thirty seconds with the hose killed it, but I don't know for sure what did kill it, so beware...
posted by oats at 5:45 PM on August 3, 2006
The unit died a day later. It was old, so I don't know if spraying it for thirty seconds with the hose killed it, but I don't know for sure what did kill it, so beware...
posted by oats at 5:45 PM on August 3, 2006
Response by poster: I think my description was a little unclear. The coils are not exposed, they are encased in a sheet metal enclosure with ribbed openings to allow air flow. The misters would run water down the sides of that enclosure and not on the coils themselves. The water cools the metal, plus it will cool the air (via evaporation) that is pulled over the coils. Some water would get on the coils, of course, but not much more than would happen in rainfall since the top of the unit is also ribbed where the fan blows out the hot air.
The house is in north Texas, so the humidity is generally low enough. The heat exchanger would still be exchanging heat in air, but the hope is that the air would be much cooler and therefore make the exchange more efficient. Since the gas inside the coils would be cooled more rapidly from the cooler air (vs. hot air), the compressor would not have to work as hard and therefore take less energy.
My neighbor does something similar, except he has a mister on top of his unit. When the fan kicks on, it blows water all over the surrounding area. His well kicks on maybe once a day with just the mister running, so there really isn't much water being used. I'm looking for something more efficient.
My other neighbor has a mister hose running along the ridgeline of his house (kind of like they do with big chicken coops). When he turns it on, the temperature in his attic drops as much as 20 degrees in two hours on a 100 degree day.
Anyway, thanks for the inputs. You've given me some new angles to consider.
posted by forrest at 7:09 PM on August 3, 2006
The house is in north Texas, so the humidity is generally low enough. The heat exchanger would still be exchanging heat in air, but the hope is that the air would be much cooler and therefore make the exchange more efficient. Since the gas inside the coils would be cooled more rapidly from the cooler air (vs. hot air), the compressor would not have to work as hard and therefore take less energy.
My neighbor does something similar, except he has a mister on top of his unit. When the fan kicks on, it blows water all over the surrounding area. His well kicks on maybe once a day with just the mister running, so there really isn't much water being used. I'm looking for something more efficient.
My other neighbor has a mister hose running along the ridgeline of his house (kind of like they do with big chicken coops). When he turns it on, the temperature in his attic drops as much as 20 degrees in two hours on a 100 degree day.
Anyway, thanks for the inputs. You've given me some new angles to consider.
posted by forrest at 7:09 PM on August 3, 2006
forrest writes "My other neighbor has a mister hose running along the ridgeline of his house (kind of like they do with big chicken coops). When he turns it on, the temperature in his attic drops as much as 20 degrees in two hours on a 100 degree day."
That is a very interesting notion. Is it a common strategy in hot, dry climates? I'm wondering about the hidden costs but it seems like it might be a relatively green way to stay cool.
posted by Songdog at 1:39 PM on August 4, 2006
That is a very interesting notion. Is it a common strategy in hot, dry climates? I'm wondering about the hidden costs but it seems like it might be a relatively green way to stay cool.
posted by Songdog at 1:39 PM on August 4, 2006
This thread is closed to new comments.
The coil is solid copper; approximately three times the thickness (i.e. 0.032" vs. 0.012") of that used in air-cooled condensers, and the copper is coated to prevent corrosion. Magnesium anodes are included to treat the water and reduce coil corrosion.
A float maintains approximately 3.5 inches of water in the bottom of the condenser. Every 8 hours of run time, a timer causes a purge pump to pump all water from the bottom of the unit (approximately 5-8 gallons). The float valve then causes more water to flow until a level of about 3.5 inches is restored.
This system is not just an air cooled a/c with water sprayed on it; the coils have been specially adapted to resist corrosion from water, the water is treated to reduce its corrosive effect, and the coil thickness has been altered to work optimally with water coiling and reduce the risk of a hole appearing from corrosion. I'm wondering if the coils are not at least partially submerged in the 3.5" of water in the bottom. from the description, it seems like the coils are constantly being doused with water, which is not at all the same as lightly misting them. Your a/c is designed to withstand the elements to some extent, but I'm not sure that constantly spraying water into the innards an a/c desiged to be air cooled is a good idea. Not a professional opinion, but...
posted by Derive the Hamiltonian of... at 12:39 PM on August 3, 2006