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BANGKOK 21 April 2019 19:42
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sweatalot

sprinkler on the roof instead of a/c

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Thanks for your input - I am still waiting if somebody has experience with a construction like this.

It is for sure that this water cooling would have an effect. The question is only how much?

Why will it have an effect?

Some posters mentioned the evaporation effect. Water comes on a hot roof. It uses the heat energy to evaporate.

(means changing fluid water into steam) For evaporating energy is needed which is taken from the hot roof which lowers the temperature.

So far physical facts.

It could be calculated but this would be difficult because you would need a lot of facts like conductivity of the roof material, the windows, heat capacity of the walls etc. So experimenting will be easier.

Inside temperature will not be lower than outside temperature. Agreed. But have you ever come into a car which has been parked in the sunshine? You would be lucky if the inside temperature would be like outside.

If you use A/C to get the temperature under outside temperature your electric bill would be significantly lower.

Our room size is about 60 sqm with a pitched area and 6 Thai windows with as much as no isolation. The ceiling is about 3,50 m

Airconditioning would be quite expensive and not very effective unless we get new windows.

Water bill? I mentioned that we have our own well (130 m deep) So costs would be only for the electricity used for pumping

There are solar powered water pumps available, I have three small ones though I doubt the ones I have could pump water from a well to your roof. Maybe there are higher watt available but I think the same as other posters, doubt it would make much difference.
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I lived in a place with swamp coolers before, very effective in low humidity situations, less ideal here.

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So, do you have a ceiling under this roof that you might want to water? If so, there is no way you would notice any significant delta in comfort unless you just BELIEVE. It sounds like your construction is a hot box and that's just the way it's going to be. Even A/C might not help that much. Suggest you try shading and fans instead.

Where's Dr. physics Naam for things like this?

He uses roof fans.

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Just another thought if you decide to go with your idea put up some upvc guttering up and running it back into your well.

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Why not just try drying your clothes inside with a fan blowing on them and see the effect.

Indian households hang woven Mats of Vertigris in the windows which they spray with water to cool down and keep insects out the house.

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A research paper from Osaka University (.pdf)

Effect of the Evaporative Cooling Techniques by Spraying Mist Water on Reducing Urban Heat Flux and Saving Energy in Apartment House

Conclusion

Verification tests and numerical simulations were conducted in order to investigate the effect ofmisting technologies on reducing urban heat flux and saving energy. An apartment house was used as the test building in an investigation of three types of evaporative cooling techniques: "Rooftop spraying", "Veranda spraying" and "Spraying to the outdoor unit of room air conditioner". "Rooftop spraying" was intended to improve the thermal environment of the top floor by spraying water droplets onto the roof surface. "Veranda spraying" was spraying a fine mist of water droplets from the veranda such that cooled outside air would enter the room using the natural draft. "Spraying to the outdoor unit of room air conditioner" was spraying water droplets in the air inlet of an outdoor unit, thereby lowering the temperature of supply air and heat exchange fins and thus improving air-conditioning efficiency. We confirmed that misting technologies had the effect of reducing surface temperature, air-conditioning usage time, improving air-conditioning efficiency and reducing the cooling energy consumption through the verification tests. And also, through the numerical simulations, we confirmed that the introduction of misting technologies had the effect of saving energy consumption for cooling by over 80%. As for an urban heat flux, numerical simulations also confirmed the effect of reducing by over 60%.

On rooftop spraying:

The indoor environment showed a fall of 1.2°C at 120 cm above floor level, while a larger temperature lowering effect was obtained closer to the ceiling

As I've said, you can't write it off - it all depends on where the heat comes from. But conversely, it's just no substitute for AC - which is the question poised by the topic title :)

The other part that was interesting was the evaporative cooling of the AC compressor:

The average reduction in energy consumption during the test period was 36%.

Up until this year, Saijo Denki sold a lineup of AC's that had integrated evaporative coolers in the outdoor unit. I have a couple of them. When humidity is low, the air coming out of the outdoor unit actually feels cool. As humidity rises, it feels just as warm as any other AC though.

So what that tells me, is these tests results from Japan were done during fairly low humidity.

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2fishin2 Just because you haven't see it doesn't mean it won't work.

It's common in parts of Thailand with little humidity. I normally see it used on open sided tin roof restaurant areas and on river boat restaurants. In my experience it makes a difference and does cool down the area underneath.

Hardheaded.....like I said if it was so effective, there would be a device and it would be a product on the market. Standard Air conditioners have been around a longgggg time.

Just because some podunk "tin roof restaurant" sprays water on their roof doesnt make it a viable alternative. More than likely that restaurant is an outdoor one with no air conditioners and only fans and ventilation for cooling. Your example doesnt "hold water" pun intended.

The topic of the thread is "sprinkler on the roof instead of a/c". Pretty easy answer AC would be much better than water to cool the OPs room. The effect of spraying water would be minimal compared to a proper AC.

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Why it doesn't work could be explained this way. Over simplified too. Because the heat load on the roof is greater than the cooling effect from the water, you can never get the attic air cool enough to make a noticeable difference in the room. If you put fans in the attic it would do more good than the water would.

Best example of this, is a fire. Ever notice how much water is required to put out fire? Then cool the it down so you can find hot spots? Even after the flames are out the stored heat in the wood is so great it still takes thousands of gallons to cool. Same physics are in motion in cooling the house but not as severe. Thermal heat transfer problems can be very complex even in a simple house problem like this.

Try attic fans first. One at each end of the house. You want one blowing in from the shaded side and an exhaust fan on the sunny side.

Exhaust fans function much better here than swamp coolers.

It is not unreasonable to think that a swamp cooler in a room and a dehumidifier at the far end of the room could have some beneficial affect on the air temperature

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This would be a swamp cooler on an epic scale. That being said, the water (especially since we are in a drought) and the electricity, and I have to say the probable marginal cooling is not in my estimation a good bet.

Buy yourself a cheap AC and be done with it. You'll be cooler and without a what I can only imagine a whole lot of mess of water cascading from your roof 24/7

AND!... dont forget the mould everywhere, and what nasties comes from it. Dont waste your money chasing an idea.

I agree with others here....just buy an A/C.

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Why it doesn't work could be explained this way. Over simplified too. Because the heat load on the roof is greater than the cooling effect from the water, you can never get the attic air cool enough to make a noticeable difference in the room. If you put fans in the attic it would do more good than the water would.

Best example of this, is a fire. Ever notice how much water is required to put out fire? Then cool the it down so you can find hot spots? Even after the flames are out the stored heat in the wood is so great it still takes thousands of gallons to cool. Same physics are in motion in cooling the house but not as severe. Thermal heat transfer problems can be very complex even in a simple house problem like this.

Try attic fans first. One at each end of the house. You want one blowing in from the shaded side and an exhaust fan on the sunny side.

Exhaust fans function much better here than swamp coolers.

It is not unreasonable to think that a swamp cooler in a room and a dehumidifier at the far end of the room could have some beneficial affect on the air temperature

They are totally different concepts, an exhaust fan is not going to cool your house, it will stop the roof from heating it up a little and from retaining the heat but a better idea would be to put a roof over your roof, if direct sunlight doesn't even touch your roof then it won't aid in heating your house up.

An evaporative cooler will actively reduce the temperature and humidify the area, very useful for very hot and dry climates.

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Why it doesn't work could be explained this way. Over simplified too. Because the heat load on the roof is greater than the cooling effect from the water, you can never get the attic air cool enough to make a noticeable difference in the room. If you put fans in the attic it would do more good than the water would.

Best example of this, is a fire. Ever notice how much water is required to put out fire? Then cool the it down so you can find hot spots? Even after the flames are out the stored heat in the wood is so great it still takes thousands of gallons to cool. Same physics are in motion in cooling the house but not as severe. Thermal heat transfer problems can be very complex even in a simple house problem like this.

Try attic fans first. One at each end of the house. You want one blowing in from the shaded side and an exhaust fan on the sunny side.

Exhaust fans function much better here than swamp coolers.

It is not unreasonable to think that a swamp cooler in a room and a dehumidifier at the far end of the room could have some beneficial affect on the air temperature

They are totally different concepts, an exhaust fan is not going to cool your house, it will stop the roof from heating it up a little and from retaining the heat but a better idea would be to put a roof over your roof, if direct sunlight doesn't even touch your roof then it won't aid in heating your house up.

An evaporative cooler will actively reduce the temperature and humidify the area, very useful for very hot and dry climates.

Exhaust fans get rid of the hot air which rises to fill the space below the roof. They also keep air circulating. Hot air goes up and out. Cooler air from the floor level moves up.

Yes, shading your roof is a good thing as is having a light colored roof.

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Thanks for your input - I am still waiting if somebody has experience with a construction like this.

It is for sure that this water cooling would have an effect. The question is only how much?

Why will it have an effect?

Some posters mentioned the evaporation effect. Water comes on a hot roof. It uses the heat energy to evaporate.

(means changing fluid water into steam) For evaporating energy is needed which is taken from the hot roof which lowers the temperature.

So far physical facts.

It could be calculated but this would be difficult because you would need a lot of facts like conductivity of the roof material, the windows, heat capacity of the walls etc. So experimenting will be easier.

Inside temperature will not be lower than outside temperature. Agreed. But have you ever come into a car which has been parked in the sunshine? You would be lucky if the inside temperature would be like outside.

If you use A/C to get the temperature under outside temperature your electric bill would be significantly lower.

Our room size is about 60 sqm with a pitched area and 6 Thai windows with as much as no isolation. The ceiling is about 3,50 m

Airconditioning would be quite expensive and not very effective unless we get new windows.

Water bill? I mentioned that we have our own well (130 m deep) So costs would be only for the electricity used for pumping

Why will it have an effect? Because Evaporation is an endothermic process, in that heat is absorbed during evaporation. The processes of evaporation and condensation take 7.5 times as much energy as melting or freezing. Change of state of H2O water to H2O gas can take place at any temperature between 0 and 100 degrees Celsius and in your case steam is not involved because Steam is a term for the gaseous phase of water, which is formed when water boils, and yours won't boil. ;)

So the inside air temperature can be lower than the outside air temperatures.

What you haven't told us is how low you would like the temperature to be.

Another factor is that AC will also lower the humidity. Lower humidity ( if it's not under 40%) is more comfortable.

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