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Roof Mist Cooling System Design

05/25/2019 9:07 AM

Good morning,

I am looking for information (heat transfer equations) regarding the design of a roof mist cooling system / evaporative cooling system.

See attached link:

https://home.howstuffworks.com/home-improvement/construction/green/10-ways-cool-roof8.htm

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#1

Re: Roof mist cooling system design

05/25/2019 9:18 AM

Why not mock up and do some real world testing? I've done a little with $10 clearance bag of misting components.

Results indicate roi is favorable and further research is justified.

...math can't spritz a box quite like a spritzer.. that might make for an interesting summer challenge.??. thanks for the idea.

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#2

Re: Roof mist cooling system design

05/25/2019 9:44 AM

The cooling effect will vary according to relative humidity...

..."The rule is you cannot cool air below the wet bulb temperature. With 90 deg.F and 50% RH the WB is 74.5 deg.F

But in practical achieving wet bulb temperature is also very difficult.So as a general rule evaporative coolers are considered to be operating at 80% efficiency. So now the outlet temperature you achieve is :

DBout= DBin- [0.8(DBin-WBin)]

=90 - [0.8(90-74.5)] = 90-12.4 = 77.6 deg.F

Now the process of evaporative cooling is Isoenthalpic. (but the amount of sensible and latent heat capacities change before and after the process)
To check water content, take a psychrometric chart and plot the initial and final conditions. For the second point follow the enthalpy line and see where it cuts the 77.6 deg.F DB line. Now take the moisture content from the right side of the chart. The difference will give you how much water is being evaporated in the process.

For your process it is 0.1ml/cu.ft of air.

You can download good psychrometric chart from here.

http://www.heatpipe.com/ProductsServices/forengineers/heat_pipe_psychrometrics.htm

https://knowledge.autodesk.com/support/revit-products/getting-started/caas/simplecontent/content/psychrometric-charts-part-1.html

https://www.eng-tips.com/viewthread.cfm?qid=39597

https://diy.stackexchange.com/questions/847/would-putting-a-sprinkler-on-my-roof-help-cool-my-home

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#4
In reply to #2

Re: Roof mist cooling system design

05/25/2019 7:29 PM

If you depend on evaporation for cooling, the wet bulb temperature is your limit, but I would think if the water was colder than that, that it would cool your house to a colder temperature.

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#5
In reply to #4

Re: Roof mist cooling system design

05/25/2019 11:40 PM

Colder water doesn't evaporate as fast...would have to work it out, but it might be a wash....

https://www.newair.com/blogs/learn/the-secrets-to-keeping-cool-with-evaporative-coolers

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#8
In reply to #5

Re: Roof mist cooling system design

05/26/2019 6:27 PM

If you have cold water (below the wet bulb temperature) in contact with an unsaturated atmosphere, it will condense water vapor from the atmosphere, warming up due to heat of condensation until it reaches the wet bulb temperature.

If the water is above the wet bulb temperature, there will be evaporation, cooling down until it reaches the wet bulb temperature. In either case, the water temperature is driven toward the wet bulb temperature.

If water is in contact with the roof, there will be heat transfer depending on the temperatures of the water and roof. If the water is above the wet bulb temperature, heat of vaporization will aid cooling the roof, whereas with water below the wet bulb temperature, condensation will hinder cooling the roof.

Heat Flow

Water below wet bulb temperature: roof --> water <-- atmosphere

Water above wet bulb temperature: roof --> water --> atmosphere

A lot depends on air circulation and the effectiveness of heat transfer to the roof. But as long as the water is below the roof temperature, it will cool the roof. If the water temperature is colder than the wet bulb temperature, it would be advantageous to not have the water exposed to the atmosphere.

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#3

Re: Roof mist cooling system design

05/25/2019 2:31 PM

The average house has around 2500 square feet of surface area.That's a lot of area. So that is about 2500 pounds of water per 24 hours or 301 gallons.That will absorb a lot of BTU's. in 24 hours. I remember one summer when our A/C went out and parts were on back order. Everyone laughed when I put a sprinkler on my roof and set it to spray,but I can tell you it did make a big difference in the inside temperature. My water was from a well,so it was around 55 degrees F. The water running down the gutter spouts was near scalding at first,but it soon settled down to just hot to the touch.Misting sounds like a good idea to me if your roof can endure constant water and if the mold and algae don't take over.

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#6

Re: Roof mist cooling system design

05/26/2019 5:40 AM

Here's a thought how about upping the roof insulation?, you pay for it once and no need to use gallons of water.

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#7

Re: Roof mist cooling system design

05/26/2019 7:25 AM

Roof misting systems do have some effect and can be significant in some situations. We had "sprinklers" on the roof of a factory that war around 2 acres. The impact was that the cooler roof meant that radiant heat onto the workers was based on the roof temperature rather than the ambient temperature of the roofing material. The roof temperature was significantly higher than the ambient air temperature.

Essentially the roof temp approached the wet bulb temperature that others have highlighted. So where roof components were originally above 55 Deg C, this dropped to closer to 30 Deg C on a day with 40 Deg C ambient air.

A/C for the workspace was never an option due to injection moulding machines and other processes in place (like spray painting) that exhausted significant volumes of air to outside.

The airflow at the building entrances was noticeably "hot" but seemed to be less so once within the general workspace.

I suspect though that evaporative cooling into the space would be more effective and efficient since ALL the impact is directed inside the space, while with sprays/misting you are essentially also cooling the environmental envelope surrounding the building to get the same effect.

That would not have been suitable for the factory since the humidified air created real problems with spray painting process known as "bloom" where the evaporating solvents would enable moisture condensation into the drying paint.

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#9

Re: Roof Mist Cooling System Design

05/27/2019 11:16 AM

The topic has been around a long time, but is still valid for today. See: https://oaktrust.library.tamu.edu/handle/1969.1/93948 which was published in 1982 with a similar topic by the same author in 1980.

Related topics to explore are roof coating color selection and ventilation of attic spaces below the roof but above the attic insulation.

--JMM

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#10

Re: Roof Mist Cooling System Design

05/27/2019 8:25 PM

As I live near a rainforest and have a lot of water. I just run an oscillating sprinkler on the roof. The resulting water running off the sides is sometimes uncomfortably hot.

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#11

Re: Roof Mist Cooling System Design

05/28/2019 6:03 AM

...then look in Perry, "The Chemical Engineer's Handbook", any edition, for that is where they will be found, among other places.

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#12

Re: Roof Mist Cooling System Design

05/28/2019 6:05 AM

What happens to the minerals left behind after the water has evaporated?

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#13

Re: Roof Mist Cooling System Design

05/28/2019 9:18 AM

Do bear in mind that one cannot achieve lower temperatures than the wet-bulb temperature of the air that is passing the <...Roof Mist Cooling System...> at any particular moment without using a heat pump or refrigeration system of some sort.

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#14

Re: Roof Mist Cooling System Design

05/28/2019 3:00 PM

Last summer (5 months ago) we installed sprinklers on the tin roof of one of the buildings at the factory I work at.

The idea was to analyze how it worked in order to make it extensive to the rest of the buildings.

It is true that its efficiency depends on many factors (like air moisture, etc), but IT WORKS GREAT!

In the following months we will extend the system to the rest of the plant

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#15

Re: Roof Mist Cooling System Design

05/31/2019 3:35 AM

Executive Summary, Perspective, and Context

Water phase change concepts:
Freezing is a warming process.
Melting is a cooling process.
Condensing is a warming process.
Evaporating is a cooling process.

Quantitative values for water:
It takes 79 kcal/liter of heat to melt 0 degree Centigrade ice.
It takes 1 kcal/liter of heat to warm water one degree C.
It takes 100 kcal/liter of heat to warm water from 0 to 100 degrees C.
It takes 533 kcal/liter of heat to evaporate 100 degree Centigrade water.

Conclusion: it takes 179 kcal/liter of heat to melt ice and warm the
water to just below boiling.
It takes 533 kcal/liter of heat to evaporate already hot water.

Observation: Evaporation of water absorbs a boatload of heat relative
to warming water and melting ice.

Bottom Line: If you can cause water to evaporate and apply the cooling
from that process to something you want to cool, you can have a very
significant effect. This is why water can cool incandescent steel to
human friendly temperatures in very short timeframes. In the case of
a glowing rod of steel thrust into a bucket of water, the liquid water
makes good thermal contact with the steel and the water vapor bubbles
carry away the water vapor very efficiently. Humans in a building are
a bit tougher to cool with evaporation than glowing underwater steel
for three glaring reasons:
1. The humans are not in direct contact with the water and thus depend
on thermal conductivity of the roof and air circulation to transfer
their heat into the water.
2. The water is not rapidly driven to evaporate because the working
temperatures are not above its boiling point.
3. The water vapor forms above the water and is not carried away from
the objects being cooled by a profound density difference.

Sweating only suffers from point 2 but misting a roof labors against
all three disadvantages. Due to the 533 number it still might do a
lot of good if one can contrive ways of reducing the impact of the
obstacles identified. A common strategy is to use some efficient
refrigeration cycle to concentrate heat directly into the water at
some working temperature higher than human skin temperature. You
can observe this approach in industrial scale evaporative cooling
towers which also serve as condensers for HVAC refrigerators.

So, evaporating water is an excellent technology for carrying away
unwanted heat and it depends on the specific flows and temps details.
The other responses detail some of those flows and temps without
refrigeration enhancement. I just wanted to place your strategy
into the context that I perceive as a wider scope of the technology
than you expressed in your question. Water evaporation is at the
heart of modern, practical, big building cooling but it is used in
combination with refrigeration as a way to increase the working
temperature to cause the water to rapidly evaporate. This moves
the technology a great distance along its curves toward excellent
effectiveness by spending some energy dollars. Some enlightened
people are aware that this is a spectrum and that their thermostats
are a dial which allows them to choose where, along a wide range,
they want to operate.

Perhaps some experts here know whether there
are opportunities to improve current practices by studying HVAC
system operations from, say, the point of view of the rate of
water evaporation and cooling tower temperatures for various
outdoor temperature and humiditiy profiles. It strikes me that
one might be able to gain some operational latitude by including
an insulated tank of water as a thermal mass to take advantage of
diurnal periods of opportunity to operate more efficiently. Cool,
dry, evenings might, for example, allow stockpiling of an ice
slurry reservoir to offset reduced evaporative cooling capacity
during more hot and humid hours of some days. Here, in Austin,
we have a lot of hot, humid mornings, even hotter but less humid
afternoons, and rapid radiative cooling of the dry air after the
sun sets.
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