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inverted evaporative cooling
2

inverted evaporative cooling

inverted evaporative cooling

(OP)
Context:
I want to cool my house by heat piping between the ground/basement space to living space of the floor above.
There is 5m vertically between ceiling of the first floor and the floor of the basement.

My plan is to make a loop 5m long (vertically) filled with water, up to, 4m and vacuum out the air with a pump (and guage), so that water will boil at 20c.
The heat input will be a heat sink placed vertically on the pipe/wall (with 4m being the mid point of the heat sink)
and a second heat sink in the basement , likely on the floor.

Reading suggests that applying heat at the top usually results in a lack fluid to at that point and hence no cycling of the working fluid.
As a apposed to applying heat at the bottom where the working fluid boils and cycles through evaporation/condensation as its fed by gravity back.



My question is: would filling the fluid almost to the top (i.e. the heat input) cycle?

(and in a useful way?)




RE: inverted evaporative cooling

Inverted heat pipe applications use internal wick structures and capillary action to get the working fluid back up to the top after it has condensed at the bottom.

If you filled the tube to the top you would just be counting on the thermal conductivity of the water to transfer the heat rather than a phase change.

RE: inverted evaporative cooling

A far more effective heat pipe would be a sheet metal duct (pipe) with a fan to move coll air upwards.

Even if you were to successfully make a heat pipe (which you will not), you still need to transfer the heat into and out of the pipe. This requires surface area and a temperature difference, both of which are small in your application.

RE: inverted evaporative cooling

I second what Hendersdc say: If you have a water filled pipe there will be a small heat transfer by natural convection through the water but not that much.

"Real" heat pipes works by evaporating a liquid (often acetone) in the bottom of the pipe and getting this to condense at the top by pumping a "cold" liquid on the outside of the tube (the liquid that you want to heat). This creates a lot of movement due to the large density difference between the vapor and the condensed liquid. I havnt heard about the "inverted" type befor but a "wick" system seems workable.

If your basement is cool i would suggest ducts to move the air instead.

RE: inverted evaporative cooling

Boiling water with a vacuum results in the water evaporating and filling the vacuum. The result being the vacuum pump has to be run almost continually in order to maintain the vacuum, which is probably not what you imagined.

Essentially, you are talking about using water as a refrigerant (R718).

Also boiling the water in the basement would have the reverse effect of what you want. Heat would be absorbed and flow from the basement up into the house where the water would condense, releasing the heat.

A closed loop system could possibly work, but an ordinary vacuum pump would be damaged by the high humidity/water passing through it. Possibly some type of screw compressor designed for such a purpose.

A valve for throttling back (maintaining) the pressure would (probably) be needed in the return line, possibly an ordinary expansion valve for refrigeration or capillary tube, though high compression would likely not be required to condense water and should be avoided if possible, something of the sort would likely be required to maintain the vacuum in a closed system, otherwise, you'd just be sending bubbles through a tank of water or something.

The heat generated could be dumped into a water tank recovering some of the expense of running the vacuum/compressor.

Something like the following. No guarantee it could be built as a DIY project, or that it would work exactly as shown. and would probably require, or could develop dangerously high pressure in the return line, (relief valves required) but refrigeration systems using water as the refrigerant do exist. Mostly for industrial purposes. I don't know of any being marketed for residential use.



On the other hand, this video (and others) makes it look very easy. Perhaps something worth experimenting with.

https://youtu.be/jn1X_I8-9h8

This presents some interesting possibilities:

https://youtu.be/hHNoHhbfFDQ


RE: inverted evaporative cooling

The video claims 1 gm/2 min = 0.03 kg/hr. In order to provide 1100W of cooling to cool a 400 sf room, you need to cool down and evaporate about 12 kg/hr, so that's a scaling up factor of about 400x, i.e., pumping capacity 400x larger

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RE: inverted evaporative cooling

The Latent Heat of vaporization for water is about 10X higher than conventional refrigerants, R11, R12, R22 = +/- 200 Kj/Kg

Water over 2000 Kj/Kg engineeringtoolbox.com

I'm not sure that I'd necessarily go by what was said in that video, as with the small mason jar as evaporator, it appeared there was quite a bit of liquid splashing around, possibly getting sucked into the line without effect.

I'm wondering if the gravity vacuum idea could be used at less of a height if a larger column of water were used. Say a 4" or 6" PVC pipe, instead of the thin tubing. Water could be returned via a pump with a pressure tank and thin capillary tube, with just enough flow to replace whatever water boils off.

I don't think the vacuum created by the larger column of water going down would serve to draw a fine capillary tube filled with water back up? That would be like, not only perpetual motion but free air conditioning to boot. Though I'm having some difficulty figuring out exactly why that would not work.



It seems to be almost a certainty that a large column of water pulling a vacuum could lift the water in a thin capillary tube.

What's wrong with this picture?

Apparently, the diameter of the tube(s) makes no difference as far as the height necessary to create the vacuum (Pascal's Principle).

A capillary tube, likewise would (probably) not lift the water higher than the column on the other side, for the same reasons, though a true capillary tube does exhibit some odd properties, I wouldn't count on it.

RE: inverted evaporative cooling

An interesting method used in vacuum cooling for food processing is to use a condenser, reducing vapor flow and drying the air before it reaches the vacuum pump.



https://www.heuchfresh.com.au/vacuum-cooling-expla...

The vacuum chilling process used is apparently several times more efficient than conventional forced air chilling.

This is however a batch process vacuum evaporating the moisture from the product itself. Perhaps it could be adapted to space cooling in one way or another.

RE: inverted evaporative cooling

you guys moved really fast into a refrigeration cycle smile

Still as a "heat pipe" stagnant water will not work.

@Tom Booth, you last picture seems a lot like a general principle for freeze drying. Although often when considering a powder or agglomerate you use a spray nozzle and/or fluidizing nad maybe low pressure to increase evaporaion/ lower temperature by evaporation.

Best regards, Morten

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