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Author Topic: By what physics principle might this cooling device work?  (Read 2438 times)

Offline chris

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A gentleman called Andrew wrote to me attaching details of a cooling device being built in Banglasdesh. It comprises fizzy drink bottles with the bottom halves cut off inserted neck-first in a grid-pattern on a panel.

The panel is then placed over a window with the cut-off ends of the bottles pointing outwards.

The claim on the website promoting the device says that "the wider ends catch the passing wind" and funnel it into the building through the necks. An alleged temperature drop of 5 degrees is achieved inside.

http://inhabitat.com/this-amazing-bangladeshi-air-cooler-is-made-from-plastic-bottles-and-uses-no-electricity/

This explanation doesn't sound right to me, so I thought I'd subject it to the rigours of the Naked Scientists forum.

My instinct is that they've got it the wrong way around. the passing breeze will cause a lower pressure in the open, wide ends of the bottles, accelerating air from the neck regions towards the outside. Effectively it will pull air out of the building won't it? This will, presumably be replaced by cooler air entering from below...?

What does everyone here think?
« Last Edit: 26/07/2016 12:39:28 by chris »


 

Offline hamdani yusuf

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Re: By why physics principle does this cooling device work?
« Reply #1 on: 26/07/2016 11:32:35 »
A gentleman called Andrew wrote to me attaching details of a cooling device being built in Banglasdesh. It comprises fizzy drink bottles with the bottom halves cut off inserted neck-first in a grid-pattern on a panel.

The panel is then placed over a window with the cut-off ends of the bottles pointing outwards.

The claim on the website promoting the device says that "the wider ends catch the passing wind" and funnel it into the building through the necks. An alleged temperature drop of 5 degrees is achieved inside.

http://inhabitat.com/this-amazing-bangladeshi-air-cooler-is-made-from-plastic-bottles-and-uses-no-electricity/

This explanation doesn't sound right to me, so I thought I'd subject it to the rigours of the Naked Scientists forum.

My instinct is that they've got it the wrong way around. the passing breeze will cause a lower pressure in the open, wide ends of the bottles, accelerating air from the neck regions towards the outside. Effectively it will pull air out of the building won't it? This will, presumably be replaced by cooler air entering from below...?

What does everyone here think?
I think so. On average the pressure inside will be lower than outside the building, so the temperature is also lower.
This hypothesis can be confirmed by observing the direction of air flow through the bottle necks. In contrast to explanation in the video, the air should flow through the bottles from inside to outside the building.
 

Offline RD

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Re: By why physics principle does this cooling device work?
« Reply #2 on: 26/07/2016 12:29:42 »
Re: By why physics principle does this cooling device work?

Superficially it looks like Bernoulli principle ...


https://en.wikipedia.org/wiki/Bernoulli%27s_principle

but I doubt that could be responsible for any cooling effect which would show on a thermometer.
The increased air-flow through the holes would increase cooling-evaporation from sweaty-skin held close to them, but the temperature of the room wouldn't be reduced.

If the device does cause cooling it could just be functioning as a blind : blocking insolation.
( they should try the holey boards on the windows without the bottles attached).
« Last Edit: 26/07/2016 12:45:28 by RD »
 

Offline puppypower

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Re: By why physics principle does this cooling device work?
« Reply #3 on: 26/07/2016 12:45:03 »
I would guess it works on the principle of the cooling sack. When water or any liquid evaporates it absorbs energy. This is why we feel cooler when we sweat. The cooling sack is a porous animal skin membrane, used for holding water. It is designed so some of the water will slowly seep through the skin, and sweat/evaporate, cooling the surface. In the desert, where this is used, the desert can get 110F, while water in the cooling sack can be 70F. It is a very clever American Indian invention; Navajo?

At Disney, they will spray a fine mist over the large crowds, who are waiting in the long lines to get on rides. The summer heat evaporates the mist, absorbing energy, cooling the crowd. This is based on a dynamic version of the cooling sack, where pumps and misting nozzles take the place of free diffusion in the cooling sack. Convection and turbulence add to the evaporation rate.

The relative humidity and the temperature differences, between the mist/cooling sack and the bulk air, will determine the rate of evaporation. These change as the water evaporates, until a steady state is reached. The rate of evaporation determines the amount of energy absorbed and how cool it will get.

The device pulls a vacuum on the water, so it evaporates easier.
 

Offline chris

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Re: By what physics principle might this cooling device work?
« Reply #4 on: 26/07/2016 22:12:16 »
There is no water; the device consists of an array of drink bottles cut in half. I think it must be a pressure effect created by air blowing across the relatively large apertures of the cut-off bottles. This generates a lower air pressure, which draws air from the neck direction i.e. inside the house. This creates airflow, removing the warmer air trapped inside the property (I postulate).
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #5 on: 26/07/2016 23:45:36 »
There is no water; the device consists of an array of drink bottles cut in half. I think it must be a pressure effect created by air blowing across the relatively large apertures of the cut-off bottles. This generates a lower air pressure, which draws air from the neck direction i.e. inside the house. This creates airflow, removing the warmer air trapped inside the property (I postulate).

I'm not sure which exact technique is being discussed here, but it sounds similar to earth ship architecture, where interior cooling can involve rain water run off from the roof that is stored under the structure.  The idea is that the interior is cooled by creating suck in the interior living space above the water, that draws air from ground level openings across the water, cooling the air as it passes into the interior.

To say so I should know which way round the bottles are oriented, I think its neck to exterior, but I've got a bit of a mental block going tbh. :).  I know that the mass of the earth filled tyre walls retain thermal temperature and a resulting heat induction causes flow.

Here is a documentary I watched some time ago now showing the architect Michael Reynolds building with the technique.

https://m.youtube.com/watch?v=4IxUQ5MXhm0
« Last Edit: 26/07/2016 23:48:09 by timey »
 

Offline chris

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Re: By what physics principle might this cooling device work?
« Reply #6 on: 27/07/2016 09:11:18 »
I should know which way round the bottles are oriented, I think its neck to exterior, but I've got a bit of a mental block going tbh. :).  I know that the mass of the earth filled tyre walls retain thermal temperature and a resulting heat induction causes flow.

If you read the background details in my original post (above) you'll see that that bottles are arranged with the wide ends pointing outwards and the necks pointing into the living space.

I think we need some physics input from Alan and Evan here!
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #7 on: 27/07/2016 12:42:10 »
I should know which way round the bottles are oriented, I think its neck to exterior, but I've got a bit of a mental block going tbh. :).  I know that the mass of the earth filled tyre walls retain thermal temperature and a resulting heat induction causes flow.

If you read the background details in my original post (above) you'll see that that bottles are arranged with the wide ends pointing outwards and the necks pointing into the living space.

I think we need some physics input from Alan and Evan here!
Thanks Chris.  I did read your post before replying and have also now read the inhabit link provided.

Yes - as I thought, the technology is using the same physics as the earth ship architecture puts into practice, but modified for non eco housing.

The inhabit link states that this is the first non electric cooling system of its kind... Well, perhaps of its exact kind it is, but the physics of density difference between hot and cold air have been previously utilised to astonishing affect without electricity by our ancestors.  The Romans were renowned for it.

Here is a much shorter clip of people who lost their homes in the 2004 tsunami being taught how to build earth ships for affordable home replacement and sustainable living.

https://m.youtube.com/watch?v=49oE6nbwz8E

The architect of the earth ship design has had a lot of trouble in persuading planning permission for these structures world wide.  Hardly surprising when it is considered that earth ship residents, having made their initial investment, are then utility bill free.
I wonder if the non-electric air cooling unit has met with similar distribution difficulties in the face of being free from further billing...
 

Online jeffreyH

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Re: By what physics principle might this cooling device work?
« Reply #8 on: 27/07/2016 13:46:41 »
Since the atmosphere is basically a low density fluid we can use the following to make calculations of pressure differences.

http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

It all depends upon whether a change in pressure of the fluid causes a significant temperature difference. I haven't bothered to check.

Also the number of bottles may amplify the effect.
 

Offline chris

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Re: By what physics principle might this cooling device work?
« Reply #9 on: 27/07/2016 13:54:48 »
Since the atmosphere is basically a low density fluid we can use the following to make calculations of pressure differences.

http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

It all depends upon whether a change in pressure of the fluid causes a significant temperature difference. I haven't bothered to check.

Also the number of bottles may amplify the effect.

But the air would be expanding on the way OUT of the building and hence the temperature drop would be in the environment rather than in the building surely?
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #10 on: 27/07/2016 14:34:31 »
Since the atmosphere is basically a low density fluid we can use the following to make calculations of pressure differences.

http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

It all depends upon whether a change in pressure of the fluid causes a significant temperature difference. I haven't bothered to check.

Also the number of bottles may amplify the effect.

But the air would be expanding on the way OUT of the building and hence the temperature drop would be in the environment rather than in the building surely?
Cold air is denser than hot air, and subsequently cold air under any circumstances will be drawn into a hot room if there is an opening.

The bottle neck (oriented neck to interior) is encouraging the cooler air to be drawn in faster. The cooling system of the bottles placed in the lattice in a window aperture will more effectively cool the interior than an open window, because of this drawing effect.
 

Offline chris

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Re: By what physics principle might this cooling device work?
« Reply #11 on: 27/07/2016 14:45:49 »
The bottle neck (oriented neck to interior) is encouraging the cooler air to be drawn in faster.

Why?
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #12 on: 27/07/2016 15:53:54 »
The bottle neck (oriented neck to interior) is encouraging the cooler air to be drawn in faster.

Why?
Hotter air is lighter.  The hot air in the interior is always moving upwards creating pull.

If the tubes in the lattice were of the same circumference at the exterior as they are on the interior side (ie: no bottle neck) then the air in the tube would be of similar temperature at both sides.  Bottlenecking the interior side of the tube means that the air in the interior side of the tube is that much hotter than the air in the exterior side of the tube and flow is accelerated.

It's basically chimney physics and is called a negative pressure system.  The hotter the air is inside in comparison to the temperature of the air outside, the more draw will be occurring.
 

Offline wolfekeeper

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Re: By what physics principle might this cooling device work?
« Reply #13 on: 28/07/2016 00:22:19 »
No, no, no.

This is effectively a fan with no moving parts, powered by any gentle breezes there may be. There's no significant temperature change going on here. The air blows against the side of the building, relatively slowly, and the air speeds up through the neck of the bottles, and it produces an array of jets of air that blow away sweat from the skin, making you feel cooler.

It will cool the buildings a bit, but only by ventilation effects.
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #14 on: 28/07/2016 01:03:58 »
No, no, no.

This is effectively a fan with no moving parts, powered by any gentle breezes there may be. There's no significant temperature change going on here. The air blows against the side of the building, relatively slowly, and the air speeds up through the neck of the bottles, and it produces an array of jets of air that blow away sweat from the skin, making you feel cooler.

It will cool the buildings a bit, but only by ventilation effects.

The link provided in the original post reported a 5 degree drop in temperature.  That is a pretty significant drop and I doubt anyone in Bangladesh would block a window aperture for an effect that a fan would initiate.

If the air is caused to speed up through the necks of the bottles, (as you have said), more air is drawn in than otherwise would be.  This literally forces the hotter air rising to top of interior to find exit.  A temperature drop is inevitable.

Significant further cooling of an interior can be effected by drawing air from outside into a structure over water that is stored below it via a similar system.

Its not new technology.  The Romans were renowned for it, and I have no doubt that negative pressure systems were adopted by the Romans from more dated civilisation.
 

Offline chris

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Re: By what physics principle might this cooling device work?
« Reply #15 on: 28/07/2016 08:33:37 »
You say this is is chimney physics. But chimneys pull gas OUT of the house, not in!

I think the way this is working, if it does, is that the wide bases of the bottles see the passing breeze, which generates a pressure drop in the open bottle relative to the narrow aperture. This causes air to move from the neck region towards the cut-off end. In turn, this leads to net air movement out of the bottle / room and into the external environment. Consequently, cooler air is drawn into the property to replace the exiting warm air.

I'd be very surprised if the bottles drew air into the room. The effect of the neck would be to increase the pressure as air flowed from the cut-off base inwards, which would oppose flow.

 
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #16 on: 28/07/2016 12:29:58 »
You say this is is chimney physics. But chimneys pull gas OUT of the house, not in!

I think the way this is working, if it does, is that the wide bases of the bottles see the passing breeze, which generates a pressure drop in the open bottle relative to the narrow aperture. This causes air to move from the neck region towards the cut-off end. In turn, this leads to net air movement out of the bottle / room and into the external environment. Consequently, cooler air is drawn into the property to replace the exiting warm air.

I'd be very surprised if the bottles drew air into the room. The effect of the neck would be to increase the pressure as air flowed from the cut-off base inwards, which would oppose flow.

http://www.csia.org/homeowner-resources/how_your_chimney_really_works.aspx

Quote:
"Your House as a System

Even though you can't see it, the air in your house is constantly in motion. In general, airflow tries to flow out of your house in the upper parts and make up air tries to flow into your house in the lower parts of your house. Thinking of your house as a system makes it easy to understand the reasons for that airflow. The actual flow of air into and out of any home is influenced by a number of constantly changing factors, including: stack effect; wind loading; interior mechanical systems and fuel-burning appliances such as furnaces, fireplaces, wood stoves and water heaters.

Homes built in the past 25 years, and older homes that have been renovated, have been made more airtight. This makes it much more difficult for makeup air to enter the home. As the saying goes, “hot air rises”, and so does the warm air in your home.

When the warm air rises to the upper areas it's called the stack effect. That trapped air creates a pressurized area and forces its way out - through even very small openings such as recessed light fixtures and window frames. At the same time replacement air is trying to enter in the lower part of the building to make up for the escaping air.

Somewhere in your house, amid all this airflow, is something called the Neutral Pressure Plane (NPP). Above this theoretical plane, the air pressure is slightly positive compared to the outdoor air pressure and is trying to force its way out of the house. Below the plane, it is slightly negative and the house is trying to draw air in. The location of the NPP can constantly change in response to changing conditions."

Quote:
' When a fireplace chimney is full of hot air, it actually pulls air through the firebox. This pulling effect is called draft and it corresponds to the amount of pressure in a water hose - the only difference is that the air pressure is negative and the water pressure is positive (think of using a straw to drink with instead of to blow bubbles). Thus, a chimney is called a negative pressure system. Increasing the draft in your chimney is like opening the faucet wider on the hose. The simplest way to increase the draft in your chimney is to burn the fire hotter - hotter air is lighter, so it has more pull."

A chimney also draws air in from exterior, as well as being the route out for the hotter gas. But fire technology is a lot more complicated than what's going on here with the bottle necks.

Necks are oriented interior side.
Air is hotter on interior side.
Cooler air from outside flows into the tube.
The difference between aperture size of exterior to the reduced interior bottle neck ensures that the difference in temperature between the air outside and the air inside is maintained and cooler air is drawn into the interior at a more accelerated rate than it would otherwise be.
The density of the cooler air being drawn in creates a pressurised area of hotter air at the top of the interior, and the hotter air is forced out as more cooler air is drawn in.
« Last Edit: 28/07/2016 12:46:11 by timey »
 

Offline dlorde

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Re: By what physics principle might this cooling device work?
« Reply #17 on: 28/07/2016 13:42:12 »
In time-honoured tradition, I did an experiment, putting the cut off end of a plastic bottle at various angles to the path path of air from a fan at its slowest speed, and moving a thin strip of tissue paper across the neck of the bottle. When the bottle was at less than about 80 degrees to the fan (i.e. the bottle was angled in the direction of the airflow, the paper was blown away from the neck of the bottle. When the bottle was more than about 80 degrees to the fan (i.e. the air was blowing perpendicularly across the open end) the paper was sucked towards the neck of the bottle.

So by those results, unless the wind is blowing along the wall the bottles are sticking out of, the effect will generally be to funnel air into the room. When the wind is parallel to the wall, it is likely to draw air out of the room. More research is warranted.
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #18 on: 28/07/2016 15:42:35 »
In time-honoured tradition, I did an experiment, putting the cut off end of a plastic bottle at various angles to the path path of air from a fan at its slowest speed, and moving a thin strip of tissue paper across the neck of the bottle. When the bottle was at less than about 80 degrees to the fan (i.e. the bottle was angled in the direction of the airflow, the paper was blown away from the neck of the bottle. When the bottle was more than about 80 degrees to the fan (i.e. the air was blowing perpendicularly across the open end) the paper was sucked towards the neck of the bottle.

So by those results, unless the wind is blowing along the wall the bottles are sticking out of, the effect will generally be to funnel air into the room. When the wind is parallel to the wall, it is likely to draw air out of the room. More research is warranted.

You seem to miss the fact that the air inside being hotter causes draw.

Having actually spent a considerable amount of time in India, I know that anyone with any money has air con.

The link said that 25 000 of these units have already been installed.  I imagine that this unit may be being passed off to the middle classes as a cheaper, greener option for cooling smaller room sizes...

But where I see this technology really being utilised is by much poorer people who can simply cut the bottoms off discarded drinks bottles and lodge them in a drinks crate.  Where the many members of a family occupy a very small space, made out of poorly insulated materials, fixing this array into an aperture in the side of their one room interior will make a considerable difference.  Much more difference than just the regular opening of a window, and furthermore they can make it themselves out of rubbish.

This is but a really crude utilisation of a negative pressure system.  I previously mentioned, both negative and positive pressure systems have been utilised to precision sophistication in architecture that predates the Romans, for both cooling and heating systems, all without electricity.  And yes, that architecture also included a knowledge of how to create and utilise any exterior up and down draft via choice of, and thickness of building material, and angle to prevailing wind...
(in fact what appears to be electricity batteries bigger than a transit van have been dug up in Baghdad, thought to be from Babylonian era, but that's another story)

If Evan or Alan had said any of the above, I doubt anyone would find question with it...

But people should realise, I do actually either already have the knowledge of, or research what the content of my posts contain, or state it as a supposition.
 

Offline dlorde

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Re: By what physics principle might this cooling device work?
« Reply #19 on: 28/07/2016 22:46:16 »
So by those results, unless the wind is blowing along the wall the bottles are sticking out of, the effect will generally be to funnel air into the room. When the wind is parallel to the wall, it is likely to draw air out of the room. More research is warranted.
You seem to miss the fact that the air inside being hotter causes draw.
I was aware of that suggestion, but as I said, I was referring only to the results of my own experiment which didn't include a temperature differential. 
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #20 on: 29/07/2016 00:56:19 »
So by those results, unless the wind is blowing along the wall the bottles are sticking out of, the effect will generally be to funnel air into the room. When the wind is parallel to the wall, it is likely to draw air out of the room. More research is warranted.
You seem to miss the fact that the air inside being hotter causes draw.
I was aware of that suggestion, but as I said, I was referring only to the results of my own experiment which didn't include a temperature differential.
Ok, perhaps I was a little over touchy.  Please accept my apology.  I didn't mean to be unfriendly.

You could create a relevant experiment quite easily by making a box structure out of corrugated iron, or similar, and inserting a cut off bottle (glass) into side of structure, neck oriented to interior.  Make a chimney out of a smaller diameter metal pipe in the top.  Ideally an air flow meter placed at bottle neck and chimney pipe would register the flow, but if you add a smoke source near intake, you would get visuals.
Heat other side of box with Bunsen burner...
 

Offline Bored chemist

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Re: By what physics principle might this cooling device work?
« Reply #21 on: 29/07/2016 18:26:38 »
There is no evidence cited that these things actually work.
It could be that you are trying to explain something which doesn't actually exist.

The claim is that replacing a window with one of these drops the temperature by some degrees.
What effect did opening the window have?
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #22 on: 29/07/2016 19:17:37 »
There is no evidence cited that these things actually work.
It could be that you are trying to explain something which doesn't actually exist.

The claim is that replacing a window with one of these drops the temperature by some degrees.
What effect did opening the window have?
The link provided in the original post said that 25 000 of these particular units have already been installed in Bangladesh.

Do you know how hot it gets in Bangladesh?  Do you realise that some days there is no breeze factor at all?

Do you imagine that a person under those circumstances would block a window aperture with one of these apparatus if it did not work better than just having an open window?

Are you not aware that far more sophisticated negative (and positive) pressure systems have been utilised by our distant ancestors, without the assistance of electricity, to cool, (and heat), their architectural designs?

If Dlorde were to conduct the experiment suggested, he could simply remove the bottle neck from side of box structure and then log the difference in speed of airflow.
 

Offline timey

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Re: By what physics principle might this cooling device work?
« Reply #23 on: 29/07/2016 19:37:41 »
There is no evidence cited that these things actually work.
It could be that you are trying to explain something which doesn't actually exist.

The claim is that replacing a window with one of these drops the temperature by some degrees.
What effect did opening the window have?
The link provided in the original post said that 25 000 of these particular units have already been installed in Bangladesh.

Do you know how hot it gets in Bangladesh?  Do you realise that some days there is no breeze factor at all?

Do you imagine that a person under those circumstances would block a window aperture with one of these apparatus if it did not work better than just having an open window?

Are you not aware that far more sophisticated negative (and positive) pressure systems have been utilised by our distant ancestors, without the assistance of electricity, to cool, (and heat), their architectural designs?

If Dlorde were to conduct the experiment suggested, he could simply remove the bottle neck from side of box structure and then log the difference in speed of airflow.
Anyway... I wasn't describing anything.  The poster asked what the physics of the design are...

The physics are a very crude utilisation of a negative pressure system, as I already said.  Not much point in posting if nobody believes me though, is there?

I'll leave it for the pro's...
 

Offline ProjectSailor

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Re: By what physics principle might this cooling device work?
« Reply #24 on: 01/08/2016 15:28:59 »
Sounds like a Hilsch(?forgive spelling) Vortex to me.

https://en.wikipedia.org/wiki/Vortex_tube

A crude one, that really would be useful in industry if it actually worked as described.   
 

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Re: By what physics principle might this cooling device work?
« Reply #24 on: 01/08/2016 15:28:59 »

 

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