H2O heat transfer efficiency

[tikitime]

There's a couple gadgets out there that use ultra-fine mist heads to wet down the coild of the heat exchanger on Central air units. I tried it out myself after reading this article about rooftop cooling the same way, and was pleased to see a cooldown in my house from 82 to 77 in 1 hour, something that normally takes 3 hours when i get home on Sunday nights after being out of town.

http://www.doctajstutoring.com/webdoc22.htm

picture a diy version with http://www.mrlandscaper.com/mlquarcirmis.html

[cfg83]

tikitime -

Welcome to EM!

My landscape-architect friend has been talking to me about this for my flat roof in LA. Right now I have a roof leak. If I can successfully fix it, I might give this a DIY try.

However, California is currently on track to have it's worst drought in 30 years, so I have reservations on whether I will be able to justify this solution.

CarloSW2

[tikitime]

I think the trick is to use an ultrafine spray and have some type of valving system that only comes on when the roof surface exceeds 120 degrees or so,
otherwise you could waste a lot of water. I don't know how much water you'll need, since I don't know the square footage. original article asserts it's pretty small if you run it for 2 minutes each time, typically 4 times an hour.


I've read about commercial systems that do this. Imagine cooling a 40,000 sq foot warehouse! A case study I read last week said their A/C bill was reduced from 60,000 per year down to 28,000 per year, which gave the system less than 1 year for payback.

other stuff to ponder:


stolen from roofmister.com:
Q) What are the physical properties that support roof cooling?

A) Roof cooling is based on the fact that water, as it evaporates, absorbs large quantities of heat. For example, one gallon of water at 100°F will, in evaporating, absorb approximately 8500 Btu's. In other words, the evaporation of less than 1-1/2 gallons of water over an hour’s time will absorb 12,750 plus BTUs more heat than a one ton mechanical air conditioner.


other links
http://txspace.tamu.edu/bitstream/handle/1969.1/6787/ESL-HH-85-09-14.pdf?sequence=1

[tikitime]

Please take this with a grain of salt ... lifted from MFG website
http://www.ecoology.com/physics_101.htm
.....

The amount of heat energy that gets into the body of your building is determined by the formula


Qin = U x (Troof - Tinside).

Let's look at an example of what this formula means. For this example, we will take a value of .35 for the constant, "U". On a day where the air temperature reaches 70 degrees or more, it is quite likely that the roof will see a temperature of about 165 degrees F. Assuming that you keep your building at 78 degrees at floor level, the underside of your roof will probably be about 90 degrees. The formula then becomes;



Qin = .35 x (165 - 90) = 24 BTU/Hr/Square foot



For a 40,000 square foot building, 960,000 BTU's per hour get in.

By definition, one ton of air conditioning is equal to 12,000 BTU/Hr. You then need 80 TONS of air conditioning to remove this heat energy.



According to our physics professor, and the laws of physics, one gallon of water requires [absorbs] 8,265 BTUs to evaporate - go from a liquid state to a gaseous state.



Water, properly applied to your roof, will remove most of that solar energy heating your roof. Instead of 165 degrees, your roof will be kept at a much cooler 95 to 100 degrees.



Our equation now becomes;

Qin = .35 x (100 - 90) = 3.5 BTUs/Hr/Square foot.
Everything is the same, except your roof is 90 degrees instead of 165 degrees. Now, your 40,000-foot building has

140,000 BTUs getting in, requiring 12 TONS of air instead of 80 TONS.

[cfg83]

tikitime -

We actually have a perfect "test cell" at home. We have a small flat-roofed room connected to our garage, maybe 9'x13' = 117 sqft. It would be very easy to set up the misters there. It's not usable during high-summer because of the heat. Maybe something like this could make the difference.

But, that's where the leak is, so I gotta fix that first, !

CarloSW2

[Arminius]

Thanks for posting that. Does the system deal with the humidity? When I was in Mexico the air conditioning in the resort cooled the rooms just fine, but everything felt damp. I'm not sure what kind of system they had, however.

[dremd]

I have one at my house:-)

System consist of

40 feet of 1" PVC scavenged from my well mounted on roof peak
lots of misters (I think I want to switch to drip sprinkler sprayers)
A 1" Sprinkler valve
A Sprinkler Rain switch placed on the drip line to manage run off.
A Cheap home cooling thermostat mounted on porch roof underside (cooled by sprayers)
a battery (need to replace with a wall wart)
Feeds off of my Solar hot water feed line.

I want to add some copper tubing around the hot side of my A/C Piping; allowing the water to flow through on its way to the misters/ water heater should take some load off the A/C and not matter much on the roof.

[ebacherville]

i have always though there should be a product that you can cover your roof with in the summer thats white or foil like to deflect the suns heat.. I have a 2000 sq foot home all one floor so its a HUGE roof.. if i could make it mirrored or white instead of black it would save a ton of cooling..

Maybe polished aluminum sheets or mirrored plexiglass like materal .. it would be spendy as all get out..