Fins on a Rockstar can

[Jon Fong / Flat Cat Gear]

"I like how you can fit a generic pot in there and I assume the windscreen wraps to fit within the pot for storage? "

Yes, and luckily, most mugs are close to the same diameter.

"Is it possible to get it snug enough that it adds stability to the pot? "

In my mind, I was going to put pop rivets at three points to support the mug.

"What kind of an air gap or air holes do you have at the bottom to allow for combustion air?"

I was using 6 1/4" diameter holes on the bottom all on the downwind side of the windscreen.

BTW, the design for the fins around the can is begging to be made by aluminum extrusion. Maybe someone could start a Kickstart project. The die itself should only run $500 to $750. Most of the cost would be in any secondary machining. Best regards – Jon

[Nick Gatel]

Okay, I am not an engineer or scientist. But there is a lot of surface area in the Dave's original fin design, which probably helps move some of the heat to the surrounding ambient air. Seems like the triangle Kevin posted might help prevent this, or even wrapping a band around Dave's first design. Also would be interesting to see the boil time if something like the cloth plumbers use when soldering copper pipes in a house to keep the wood from catching fire might be even better… then analyse decreased boil times versus additional weight. Also doesn't brass retain heat better? How much more would brass fins weigh?

[Nick Larsen]

Well, so far it's not looking good for my fins, or the black paint. But, my original stove crapped out on me, so I had to switch mid experiment. So I need to re-do my unaltered pot tests. If I'm lucky, I'll get some time to finish the experiment this weekend.
Who knew it would take so long to boil 9 pots of water? If I have time, I'll also play with the fin configurations.

[Kevin Beeden]

> Also, if by their extenstion below the pot or their outer diameter, they may also serve as a windscreen to the pot / fins.

Yes, that was part of my thinking too.

I'm now part way through extending the HX height, adding exhaust ports & blocking sections, with a view to adding Thermawrap above this, making an HX & insulated pot.

Simple aluminium flue tape could be used to surround the HX, to add a bit of stability if needed.

And I've been looking at high temperature doubled-sided tapes for bonding the HX, e.g. 3M 9640PC tape.

[edit] d'oh! whilst it's easy to add exhaust ports to the outer triangle, it's not so easy to add exhaust ports for the inner triangle. So I'll abandon that cunning plan…

ps. how to make it? tediously, I suspect… That was one of the downfalls of the SqueezeBox, which had two, opposing folds so was even harder. But I think a simple concertina fold should be possible using conventional packaging manufacturing techniques, even if I can't point you at exactly how to do it… I have a sketch with six pins in front of me; make folds around pins, shift & repeat; two pins provide alignment, and the other four are used for the four folds required per section. I've thought about moving blocks, too, hinged around the 'can centre'.

[Daniel Sandström]

Keep in mind that a real heat exchanger is constructed to be in contact with both elements. In some of the protoypes there's not much contact at all –> that would be more of a windscreen/flame tunnel.

Very nice idea though, will follow the progress. :)

[James Marco]

Both air turbulence, AND donduction are important. Here is a quickie sketch modeled after some heat exchangers I was working on for a commercial application. Same principles, really. A simple stamping process to make the actual disks (shown assembled, ie after forming the cone.) Then press fitted over a can. It might be a little awgwarg to carry fully assembled, but the disks could be slipped off and carried seperatly. Probably about 2oz, total weight. I would gues about a 50%, maybe more increase in heating the water in a can, based on the number of disks that were used, of course. This induces plenty of turbulence, captures heated air from the previous exchanger, funnels it back to the can via conduction, convection and radiation. Simple to design and manufacture, not so simple to make at home, but certainly possible. Again, no patents, please. Open and free for use to all.

[Ben C]

I see a lot of good designs for fins. I think most of them will work fine at absorbing the heat. The fins then need to transfer the heat to the can. I don't think the fins will conduct to the can very well, though, without some sort of adhesive that is conductive or has fair amount of surface area in contact with the can. I would love to hear some ideas on ways to manufacture a good contact between the fins and can. That's where I am struggling.

[Kevin Beeden]

It's certainly true that good thermal contact is necessary in order to ensure that the HX fins couple any energy they pick up to the pan, and thence to the pan contents. But, given that David used glue (and probably a fairly thin layer at that, and I'm looking at high temperature tape, these thin layers will have a failry small thermal resistance; certainly a lot better than air.

Bear in mind that heatsinks for CPUs/large FPGAs are bonded using either thermally-conductive epoxy or thermal tape. Also, have a look at the quoted thermal conductivity for the 3M 9460PC tape I linked to earlier. Hmm, on the other hand, don't; it's a terribly vague specification…

The other issue as mentioned earlier is the thermal resistance of very thin foils; whilst the fins may get got (indeed, so hot they melt), the heat cannot get to the pot due to the high thermal resistance (which is why the fins may melt; they are isolated from the 'cold' contents of the pot, which, if containing water or wet food, won't get much hotter than 100C).

Conduction is one heat transfer mechanism. Another is radiation. If we can trap the hot gases within the HX so that their transit time is longer, they have more time to radiate energy to the pot (and the HX). It may be that by the time the gases have reached the HX, they're no longer incandescent, so the radiation may be much reduced.

[David Thomas]

>"Another is radiation. If we can trap the hot gases within the HX so that their transit time is longer, they have more time to radiate energy to the pot (and the HX). It may be that by the time the gases have reached the HX, they're no longer incandescent, so the radiation may be much reduced."

Kevin, You're right that radiation is another heat transfer mechanism but only solid objects (and incandescing gases, as you note) radiant heat (in the form of visible or IR photons). Invisible gases don't radiant heat nor absorb IR photons. Hence, solid objects cool markedly below ambient air temperatures on a clear, windless night – they radiate heat into deep space. Look for dew/frost on the lawn, your car and other objects and consider their "view angles" of clear sky versus solid objects like trees and your house. It can give you a good sense of how radiant heat transfer does and doesn't come into play.

I'm going to try a different geometry of fins next with an aim to

1) increase contact area (fin base to can)
2) reduce weight of fins
3) sllow for easy fabrication*
4) maybe include an integral shroud this time

Editted to add footnote: *by easy fabrication, my goal would be something you could make with a pocketknife, using materials from a trash can plus maybe a tube of epoxy from the store.

[Kevin Beeden]

The atmosphere is thin, but it is there, and it radiates back at us. Take an IR thermometer and point it at the sky. It won't read 3K, which is the usual assumption for a black body radiating into 'deep space'. On the OM thread that mustn't be mentioned, someone (Len Novak) tried that experiment, and posted results; near the horizon, it read about -20C during the day. As elevation increased, the meter went out-of-range. It may be that the meter was 'seeing itself', but this NASA link suggests not. This link also looks useful, but I'd need to digest it.

All matter radiates, regardless of its state (solid, liquid, gas, plasma), provided it's above absolute zero. Gases don't have to be incandescent to radiate. The question is: is this radiation is at a thermally-useful wavelength? I'd have to dig a bit deeper to get the answer to that… but my guess is that it probably isn't, or is vanishingly small.

Anyway, back to the HX…

Talking of IR instruments, it would be instructive to use an IR camera on an HX to see what is going on with the temperatures; should be able to see the effect of high thermal resistance in the fins. I'm sure we have one at work. I wonder if I can borrow it for a weekend…

[David Thomas]

Kevin,

You're right in what you post and maybe I should have included all the caveats in my previous post. Yes, water vapor in the atmosphere absorbs and re-emits IR, that's why all the IR telescopes, especially, are on Mauna Kea to be above 95% of the atmosphere's moisture.

I just walked outside with my IR non-contact thermormeter and pointed it at the sky. -74F (-59C) was the reading. It is a clear, cold day, with the sun very low in the sky, NOAA/FAA (at Kenai Airport (ENA to your travel agent, PAEN within Alaska), 3 miles away) reports:

Wind Calm
Visibility 10 mile(s)
Sky conditions clear
Temperature -18.0 F (-27.8 C)
Dew Point -23.1 F (-30.6 C)
Relative Humidity 76%
Pressure (altimeter) 30.55 in. Hg (1034 hPa)

I'm 55 feet above sea level, 80 feet at low tide (big tidal exchanges here).

I noted a 25-30 foot thick inversion layer (based on where the wood smoke settles out) when driving the kids to school, I'd guess that the air at 50 feet is -5F or so. And drier (in absolute humidity) than at ground level.

You're right at ALL matter radiants, but for scientists – those who seek the answer to Life, The Universe and Everything.

And you're right to question if it is a thermally-useful wavelength. As an engineer and especially as a shade-tree stove mechanic, a clear sky doesn't radiate anything of significance to my projects. Whether my sky is -74F or -20C, with that T^4 dependency it isn't a factor compared to 100C pots, 200C gases and 500C stove parts.

Radiation to space (or, as I think of it, the imbalance of radiation up to clear skies versus down to us) IS important for BPers in a lot of realms. It creates cold spots in vallies. It causes dew and frost on tents and plants. It absolutely makes us sleep colder if we're out in the open. And it can drive local weather through katabatic winds.

But you've got me thinking. My simplistic approximation that clear air doesn't radiant significant heat IS true for clear skies in my climate but what about in stove exahust gases which are much hotter AND wetter than any atmospheric air? If there is a bunch of IR bouncing around within a windscreen then you'd want the pot to be black and the windscreen to be bare metal.

So yet again, while the theoretical discussion is very interesting, the answer is really found with a can of black spray paint.

[Ben C]

I like your idea David. I like the thought of a thin material for the fin but agree there will be limitations on heat transfer with a thin fin. I also worry that a thin material on the fin will melt. Maybe a slower stove would help.

[Stuart R]

I love the digression. Equally applicable to a "Why do I get condensation on my tarp" thread. Maybe it's been done already.

[Nick Larsen]

The results are in! Black bottoms and fins work. Since this thread was getting long and theoretical I started another thread with my test results.

[Kevin Beeden]

> My simplistic approximation that clear air doesn't radiant significant heat IS true for clear skies in my climate but what about in stove exahust gases which are much hotter AND wetter than any atmospheric air?

Hi David,

Yeah, that was what I was really trying to get at, and the sky temperature was a digression… As you say, the sky temperature has a noticeable effect on us. On the OM thread, the discussion was about the merit of using an insulating pan lid to reduce radiative losses to the sky; in my case, a piece of Thermawrap.

It's interesting that the figures on the NASA page show 34F for clear sky, and 65F for cloud; much warmer than your sky (but maybe it was measured in Florida, in summer, rather than sunny winter Alaska…). Also the big difference that cloud cover makes.

For the 'cool' exhaust gases, yes, it's likely that they're still hot enough to radiate useful heat. I seem to remember measuring 200C in a random test of an alcohol stove of some sort. I compared it with the JetBoil, which was a lot cooler (so cool, I could hold my hand right next to the HX exits).

[Jon Fong / Flat Cat Gear]

Keep in mind that radiant heat transfer is proportional to T^4 of the absolute temperature difference between two bodies (Tbody^4 – Tcan^4) and that T is in Kelvin or Rankin. There is also a whole bunch of emissivity factors as well as angle of view to take into account. In some of my experiments, I have measure exhaust gas temperatures in the 350 F to 400 F range. At the same time, the windscreen itself only measured 120 F. My 2 cents – Jon

[David Thomas]

>"It's interesting that the figures on the NASA page show 34F for clear sky, and 65F for cloud"

Kevin, I noticed that too and figured with a cummulus cloud like that, the ground temps are probably 80F and RH in the clear air was 80-90%. Which puts the absolute humidity at . . .reaches for psychrometric chart . . . 0.023 pounds water per pound dry air.

Versus my sky today (-30F dew point) has 0.00066 pounds water per pound dry air. 350 times less. So my sky is a LOT more transparent to IR than even a clear, warm day.

Here's your free rule of thumb for the day:

Air can hold twice as much moisture when 22F (12C) degrees warmer.

Example: clouds at 0F versus clouds at 66F: both 100%RH but three steps of 22F = 2 x 2 x 2 = 8 the moisture in the warm cloud. Hence the folk wisdom that sub-zero temps make it "too cold to snow" (much).

My sky is transparent to IR because it's so fricking cold. But anytime at 10,000 feet, you've left 30% of the air mass below you, and (guestimate) 80% of the atmospheric moisture. And if it's clear and cold, you'd have even less moisture above you than I do with my (-75F by IR) skies.

I also noticed I could put my hand right above the fins on the Rockstar can. In addition to the fuel savings, a minor convenience is there less of a burn hazard above the pot. That's an advantage that Jetboil leverages to use their neoprene cosy. Maybe for us, HX fins let us wrap a pot with a multipurposed sock or scarf instead of special-purpose fiberglass cord?

[Ben C]

Also not sure the radiation heat transfer is going to vary significantly by virtue of the fins. A windscreen is going to keep the hot gases in similarly.

[Kevin Beeden]

Nice analysis, David, thanks. And a good explanation of the difference between a hot, humid Florida(?) and cold, dry Alaskan skies.

I'd concluded the same about the neoprene JetBoil cosy, too… I'm revising the exhaust port idea mentioned earlier, still thinking of the HX/thermawrap insulated pot. A conic section is also involved, although a tiny one to act as a baffle above the ports to force hot gases out of the ports, and away from the insulation.

[Kevin Beeden]

Here's the worked-up SketchUp model of the insulated, vented HX can system.

The darker band around the centre is meant to represent a strip of flue tape, to hold things together, and provide some protection for the aluminised bubblewrap (Thermawrap) insulation (the white cylinder) above the HX.

The exploded view at the right shows the structure of the HX, and the conic 'gas deflection baffle'. The latter is intended to guide the hot gases out of the vent ports, especially from the inner triangles of the HX.

Again, very much a concept drawing…

Oh, and, using the recent black paint idea, I'd probably paint the inner vanes black, but leave the outer segments shiny, to reflect heat back to the vanes, and attempt to reduce the emissivity of the outer surface…

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