I want to fiddle with this when I have time too. I was thinking you could hang a windscreen off those fins.

Roger: "do you flatten the bends on one side to get a better contact with the pot?" I see that as a definite improvement. It occurred to me, but I'd need to dial up my jig-making skills or maybe just hammer on my fins for a while to flatten one side. I agree that it would be better, but hard to do on a small scale. It's an advantage of the MSR set-up.

Why don't you insert 2 of your file folder pieces each time you make the inner fold that would touch the can? It would make this fold a bit wider and give you more surface area. If it's not quite square-ish enough you could maybe flatten with needlenose pliers.

David,
It occured to me that annealing the aluminum before trying to make the HX part would help with the folding and effort. The jig could be improved by simply feeding it through some old gears if you have some laying around.

They make a corrogated aluminum, used in may furnaces, that would work equally well, I think. Note that attaching an outer shield should be done with a low thermally conductive glue, anyway.

A cheaper corugation maker is available here: http://www2.fiskars.com/Products/Crafting/Embossing/Embossing-and-Texturing-Tools/Paper-Crimper
This can be used with heavy foil to produced 5" height HX sheets. Not as deep as yours of course. But it would save a LOT of bulk. Carfully rolling on thined JB weld should give you an adequate bond considering the surface area and application. Likely much better suited to smaller 300ml cup/pots, though.

David,

One of the factors effecting performance of your system may be your stove design. Looking at your original post, you were able to lower the fuel usage from 15 to 12 grams. 12 grams of fuel is 15 ml which would make the system on par with the a good stove. I suspect that the wattage that you are delivering may be too high for the diamter of the can.


It looks like you have a chimney style stove. Changing the outlet diamter may also increase your fuel efficiency. My 2 cents - Jon

Nick: Your Heineken can fins with foil tape has several advantages to mine: Lighter, quicker to make and more packable. I'll give it a try - I also keep foil tape around.

Shroud + Potstand. If your can + fins are close to a 3" or 4" outside diameter, you could use a piece of 3" or 4" aluminum duct work with little tab to support the pot at the desired height above your stuff. That ductwork is cheap, light, and can be clipped together and unclipped.

Eric: Yeah, a bigger spacer would give me more contact on the pot which is good. I'm debating about making some trapazoidal spacers from ash or oak so that my flat sides would mostly be formed as I work it through the jig.

James: I'm keeping your annealing tricks in my back pocket for other projects. With all the bends in it, this corrugated flashing wrapped pretty easily around the can. I thought about gears, offset with a little more spacing, but didn't have anything of the right size that was strong enough. That Fiskar crimper is a cool idea. Benefit per weight, I should be using thinner aluminum - something between flashing and heavy-duty aluminum foil - maybe beer can thickness. That would add very little weight although I wonder how it takes a crimp - it seems less ductile than flashing.

Thanks everyone for the thoughts and encouragement. I have to be on top of my game before my wife will let me convert the pasta pot, but it's going to happen. With before & after boil times documented.

Editted to respond to Jon: Thanks for the benchmark. I wondered about that as I am new to alcohol stoves. I realize which stove I use is a HUGE variable but it's not one I want to get into just now. That was one I got for $4 on eBay - it has fiberglass inside as a wick. It's my only alcohol stove and for a super-light weekend or even a picnic, I might use it. But for a week or for snow camping, I'm going with my canister stoves. Partly, I wanted to test the JB Weld in a lower temp, lower heat-rate application first. Then I may start converting the old BPing pots to something more like Jet-boil's pots. And then when I think I know something, buy and modify a Ti pot with whatever combo of bottom fins, side fins, and shroud have emerged as most promising.

Stovetop tests I've done in the kitchen keep coming in around 30% efficient for fuel heat value getting into the water in pots without fins or tight windscreens. I think with the right combination of ideas, I could double that efficiency without adding much weight at all - consider how little weight the Jet-boil fins add to their pot. You save that much fuel before boiling your first liter of water suggesting to me that the sweet spot is actually thicker or longer fins, two banks of fins, and/or an integrated shroud to maximize fin effectiveness.

Edited by DavidinKenai on 01/11/2012 10:06:53 MST.

Nicely done HX , David.

Having folded a lot of Al for the SqueezeBox stove, I'd suggest running something along the short folded section if you want to flatten it; if the foil is soft and thin enough, you'll probably be able to flatten pretty well. I used a block of hardwood, my thumbnail, or a metal rod, depending on what came to hand at the time...

I wonder how well Nick's flue tape HX fins work, since the foil is very thin, and likely to have relatively high thermal resistance (despite the high conductivity of Al). Would be very nice to get some comparison figures. It could be that the fins are simply slowing the hot gases, thus encouraging heat transfer, rather than actually conducting heat to the pan.

I dug out a couple of HX-related threads that I remembered reading/posting to; they may have some tips on bonding, etc. and hypertext allows us to keep connected things connected...

titanium heat exchanger help

I questioned why the MSR HX had such a wide retaining strip in that thread, which goes against my immediate thoughts that arose on this thread, of surrounding the HX fins with a cylindrical 'container'*. Admittedly, this could be thin foil like the rest; the idea being to retain the hot gases within the HX. The same concept really of extending the HX below the base of the pan to act as a windshield, a random thought posted of a thread about wood stoves... Thread drift, and all that...

Al Heat exchanger + Ti Pot

* This container could extend to the ground to provide pot support and windshield. To avoid controversy, let's keep it cyclindrical...

"I wonder how well Nick's flue tape HX fins work, since the foil is very thin, and likely to have relatively high thermal resistance (despite the high conductivity of Al). Would be very nice to get some comparison figures. It could be that the fins are simply slowing the hot gases, thus encouraging heat transfer, rather than actually conducting heat to the pan."

Kevin: I was wondering that, too, since I didn't see enough aluminum thickness to move much heat to the pot. But he reports it made a measureable difference. You're probably right that reducing excess ambient air flow coming up between the windscreen and pot maybe have helped by keeping that annulular air space hotter. In which case, probably better to just reduce the windscreen diameter a bit. But what if his fins are acting like little vortex generators? That would easily explain better HX and some of them are pointing in non-vertical directions as you'd want VGs to do. I was going to do some tests on soup cans with and without VGs, and now Nick has given me a GREAT way to quickly apply VGs to a pot. A way that packs nicely because they fold out of the way.

"I questioned why the MSR HX had such a wide retaining strip in that thread, which goes against my immediate thoughts that arose on this thread, of surrounding the HX fins with a cylindrical 'container'*. Admittedly, this could be thin foil like the rest; the idea being to retain the hot gases within the HX."

Also, the retaining strip around the outside, would provide better thermal contact between the fins and the pot, if you tightened the retaining strip.

You could make this lighter by just using a piece of wire.

Jerry; yeah, the wide strip strapped the HX to the pan, but it seemed way OTT for that function. I think you replied on one of the linked threads, with your wire suggestion; same idea as a thin strap...

When the JetBoil came out, I scribbled a few alternative HX ideas. One of which was spiral fins around the outside of the pan....

Hi David

> I'm keeping your annealing tricks in my back pocket for other projects. With all the
> bends in it, this corrugated flashing wrapped pretty easily around the can.

In my experience, flashing is sold fully annealed.

Cheers

David,

Flashing sold at Home Depot is a 5182 series, H18 (full hard). You can tell becasue when you anneal it, it is pretty wimpy. Best regards - Jon

OK, mine is from Australia, so there are obviously differences. Fascinating.

Cheers

<"mine is from Australia, so there are obviously differences"

Rpger: Well, yeah. What you call roof flashing, we call a shower pan. And vis-a-versa.

>"Flashing sold at Home Depot is a 5182 series, H18 (full hard). You can tell because when you anneal it, it is pretty wimpy. Best regards"

Jon: Thanks.

Some basic questions: So it is NOT annealed off the shelf and therefore is more brittle and less easily worked? But if something IS annealed, it is generally more workable?

Are soda cans not annealed?

Tempering chocolate, I know. Annealing aluminum, not so much. Can you give me a "recipe" (metalurgy, chemical engineering and cooking are all about a certain temperature for a certain time) to anneal aluminum? And if I want it hard again, do I have to work-harden it?

David,

The most common aluminum materials are 1000, 3000 & 5000 series. These aluminums are not heat treated. With these series, the hardness is due to mechanical stress (strain hardening). You can anneal them in an oven and it is usually a time / temperature chart (google it). You can soften section of them by applying local heat. For your folds, using a soldering iron to heat the bend section may work fine (easy to try anyway). One hint is if the aluminum changes color to a brown shade (that’s the protective finish burning). Once you soften the aluminum, you can only harden them again by straining them. BTW, I find the full hard aluminum much easier to work with than soft aluminum. It is easyier to cut, file and punch. Dead soft aluminum is worse than wet paper.

In general practice, windscreens made from roof flashing will all end up being dead soft. The reason is that most windscreens get pretty hot and over time, the time / temperature curves mentioned above kick in. If you tweak your current fins, I am willing to bet that they are softer than when you first started.

Cans are a similar story. I believe that they are a highly strained aluminum. Cat food cans are incredibly stiff. Then again, you heat them up and they get soft as well.

The only exception is if you use 6000 or 7000 series aluminum. These alumni can be heat treated. It’s a long process and probably too expensive to pursue. Best regards - Jon

Thank you, Jon. That's very helpful in knowing what I can do, can't, and what to expect over time.

Yeah, there are specific formuli for various metals, alloys, hardnesses, ductility, etc. I think the model people also do the same by putting several flat sheets from cans in an oven and simply run a clean cycle. This will take the work hardening (from forming the can) out of it and return it to very soft maliable heavy duty foil. As was mentioned, the same will happen once it is on the pot, anyway. It is MUCH easier to work with, folding, bending, even stretching, than leaving any work hardened tensions in the metal.

The same thing happens when I make the bottoms on the ExPots. I anneal them, then retool the bottom with a half dozen deeper ripples. Well worth the 15% increase in performance and it can be used on ANY stove, not just a caldera cone. The bottom again becomes harder due to the tooling work applied, it does not stay soft.

This is also why bending something till it breaks usually works. The compression/tension of bending causes the metal to become brittle. After three or more back and forth, it breaks, but the actual edge will be quite hard. Copper can be pounded till it achieves a good metalic density. It makes a fair to good knife, actually. But, like any tooled metal, heating it will reset the molecules, softening it again. I think the Egytians used copper stone cutting tools for statuary made by working raw copper. I believe forming sheets, as they did to make old tin or copper ceilings also causes the stretched metal to be stronger than unworked metal. Drawing metal works because the finer wire, after pulling, is stronger than the origonal. Heat again, is used to anneal it, or reset the molecules. Anyway, there are a LOT of examples. (I made a few celing tile forms for a restoration project when I was in my 20's and spent some time looking stuff up, BW: before web. We used copper sheets and painted them.)

For just a couple pieces of roof flashing, rolling is the same as drawing only limited to a couple dimensions, I would run them over a stove burner till they start browning. Once they have reached that heat, they should be quite wimpy. You can go further, but you risk melting it. Try a couple scraps. It only takes a couple minutes, likely less, over a methane or propane blow torch flame. MAP gas will be too hot, more than likely.

> Are soda cans not annealed?

Wooh, no! Most of the stuff you buy that's finished product won't be annealed; it will be work hardened as per the last machining step. Unless it specifically needs some heat treatment/annealing for the final application (the cap may be treated to allow the opening to tear as designed).

Soda cans are produced by a pretty fascinating 'drawing and ironing' process. Some nice seminar slides on Aluminium here. Hmmm... that doesn't look right; I have a saved version called 'aluminium_cans.ppt'. But here's a nice Scientific American article about cans.

If you play about with can sidewall and stoves long enough, you soon find out how soft it is when it's annealed. For instance, if you make something like this out of Al cans:



the side walls soften so much that the thing collapses... It practically turns to putty.

> For just a couple pieces of roof flashing, rolling is the same as drawing only limited to a couple dimensions, I would run them over a stove burner till they start browning.

I agree with Jon & James; get some scraps and play about with your stove, and you'll soon get the hang of how to achieve the 'Goldilocks' softness...

Of course, the other issue for a foil HX is that you may anneal them in the burner flame... It's a question of how hot they get, which in turn will depend on burner power (strictly, the 'waste' heat passing through the HX), length & thickness of the fins (i.e. the thermal resistance which will determine the temperature gradient from tip to pot).

You may end up with floppy fins...