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Common water insoluble powders to use as heatpipe wick. Answered

I'm wanting to make a heatpipe which works against gravity and so am looking for commonly available materials to use as a wick. Like a sugarcube on a spoon soaking up coffee. Except that since I'll probably be using water as my working fluid, the wick will need to be insoluble in water, as well as heat proof to a couple hundred degrees.

A few things I've thought of to try are flour, talcum powder, chalk dust and sand.

Thoughts?

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foreverdisturbed (author)2011-03-10

If you use any thing other than Copper or graphite inside your heat pipe it will offgas which means you will loose your vacuume and the pipe will cease to work.

1 watt of power is equal to 1 degree celcius of heat.

300 watts is more power than a heat pipe can handle against gravity but, not too much for a pool boiler or thermosiphon.

My background is 18 years working in electronics cooling for Thermacore Inc.

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user

Year to the day.

I've ended up going with a thermosiphon, it's not too much hassle in the end having the tank above the heater.

What's a pool boiler?

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A pool boiler is just anouther name for a Thermosiphon since a Thermosiphon will only work with a large amount of fluid compared to a heat pipe which sould only have as much fluid as the wick will hold.

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lemonie (author)2010-03-11

What about glass-fibres? E.g. of the insulation kind.

L

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SolarFlower_org (author)lemonie2010-03-11

Fibreglass apparently works good, carbon fibre too, but both have availability and -forming into 2-3 meter long rope- issues.

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lemonie (author)SolarFlower_org2010-03-11

Can you get it to wick over that distance (uphill)?

L

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SolarFlower_org (author)lemonie2010-03-11

Kiteman: good idea. I'll try that.

Lemonie: possibly not. Doing some more research it looks difficult to wick above a meter vertical, tho maybe I could just shorten the height required. otherwise I might have to look into loop heat pipes and other crafty things which will no doubt be vastly more hassle to construct.

This is all only if my light pipe doesn't work, which is hanging in the bathroom releasing its gasses and waiting to be capped and tried out.

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lemonie (author)SolarFlower_org2010-03-11

Which end is the heat acting upon?

L

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SolarFlower_org (author)lemonie2010-03-11

Yeah that's the thing. If it was hot end low there wouldn't be a problem, but it aint, and there is.


Isn't it just capillary action? I haven't seen mention of anything else in the literature...

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lemonie (author)SolarFlower_org2010-03-11

The heat modifies things - this isn't a candle - so what is happening at the top and bottom?

L

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SolarFlower_org (author)lemonie2010-03-12

What I'd like to do is to have as the main pipe body a slightly flexible crush proof plastic hose, something which can take a vacuum, then cap each end with short lengths of copper tube, also capped. Inside is the working fluid and the wick, which would probably just be a tube filled with the wick material running the full length. It wouldn't need to be attached to the outer pipe.

The high end receives about 300 W of concentrated solar, the bottom end is plugged into some application; probably be a sealed reservoir of water for steam power or purification.

I could use a steel pipe for the body, but would like some flexibility for ease of setup.  Also being able to hotswap applications would be nice. And the collector / concentrator will be turning with the sun, so that will need to be accommodated somehow.

The pipe will need to go from 1-1.5 meters up to the same distance out, so about 2m in length.

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user

I would say rethink the design, and put what you are heating above your solar concentrator on the south side. it will simplify the heat pipe design, and improve it's efficiency. you could also do a solar refrigerator design, where you have a huge reservoir of calcium chloride and ammonia at the heated end to reabsorb the fluid when it is dark out. the pipe will have to go up before it goes down to keep the salt in the collector, and then go to what you are heating to condense the fluid, and then continue to a holding reservoir where it will evaporate during the night and keep your icebox cold. dual purpose!

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lemonie (author)SolarFlower_org2010-03-12

You are moving heat down? Or did I read that wrong? Have you got a diagram?

L

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SolarFlower_org (author)lemonie2010-03-12

Yeah,one end is plugged into the concentrator and receives the heat, this is about a meter+ up.
The other end is plugged into an application on the ground, where it drops the heat.

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lemonie (author)SolarFlower_org2010-03-13

To move heat downwards you'd probably be best with a solid copper-pole (or similar), I can't see liquid doing this without a pump.

L

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Kiteman (author)lemonie2010-03-11

The height a capillary can lift water to is ≈ 0.14cm/radius

⇒ To lift 2m, the gap between fibres needs to be roughly 0.01mm

If he packs the rope tightly, that's probably do-able.

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SolarFlower_org (author)Kiteman2010-03-11

Thinking about it, the pipe will need to be about 2 meters in length, but the vertical height is only around a meter.

As a side issue; how do I figure the heat range of my pipe? If I could use ethanol as a fluid it would make other wicks available, but the max temperature for eths is apparently about 130 ºC.
I'm going to be putting about 300 W through the pipe, ideally; what temperature does that equate to?
I guess it comes down to how much of the energy is leaving, ie the pipe's efficiency, and what equilibrium that reaches?

Also, what effect would using a lower heat capacity fluid have?

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Kiteman (author)SolarFlower_org2010-03-11
I've just looked at what a heatpipe actually is

If you are pumping heat uphill, you may not need a wick at all.

The temperature isn't an issue, because you need to evacuate the tube to the point where the pressure inside your pipe is below the vapour pressure of the liquid you use.  You will have removed the oxygen, so heating the pipe over alcohol's flash point will not cause a fire.  In fact, alcohol may be good, since it has a greater vapour pressure than water, so it is easier to get a low enough vacuum in the tube.

Wikipedia says:
The vast majority of heat pipes for low temperature applications use some combination of ammonia (213–373 K), alcohol (methanol (283–403 K) or ethanol (273–403 K)) or water (303–473 K) as working fluid. Since the heat pipe contains a vacuum, the working fluid will boil and hence take up latent heat at well below its boiling point at atmospheric pressure. Water, for instance, will boil at just above 273 K (0 degrees Celsius) and so can start to effectively transfer latent heat at this low temperature.


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SolarFlower_org (author)Kiteman2010-03-12

Uphill pipes don't need wicks, but I'm heading downhill. The fluid needs to return to the top to be revaporised.

Minimum temperature isn't an issue, but maximum is. What happens if it gets too hot for the fluid I don't know, but I bet it's unpleasant.

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Kiteman (author)SolarFlower_org2010-03-12

Ahem, I said; "You will have removed the oxygen, so heating the pipe over alcohol's flash point will not cause a fire. "

Looking at the wikipedia page, you're going to need a way of packing the wick into the tube whilst leaving a void down the middle.  Tricky.

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SolarFlower_org (author)Kiteman2010-03-12

The table on this very instructive article:
http://www.cheresources.com/htpipes.shtml
has the maximum viable temperature for ethanol as a working fluid at 130º, water 200.

But I don't know what determines these temperatures, I don't know how to figure the temp I'll be working at, and I don't know what happens if it gets too hot.
As you say, not an explosion, maybe it just doesn't work good.

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lemonie (author)Kiteman2010-03-11

This isn't just capillary action though, I don't know what it is exactly.

L

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NachoMahma (author)2010-03-12

.  Why not just make your "wick" out of a substance that conducts heat well (eg, Copper, sintered for large areas)? Use a piece(s) of round Copper bar and do away with the fluid.

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user

Heat pipes are much more efficient than just conductors, they employ phase change and coduction.

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Kiteman (author)2010-03-11

Look for fibreglass rope seal.

It is used in various thicknesses as a heat-proof gasket around stove doors, or behind fireplaces when they are fixed flat against a wall.

Randomly-googled example


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Tool Using Animal (author)2010-03-11

Cross flour off your list.

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I guess at that temperature it would tend to make bread, yes.

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