Jetboil cold weather heat exchanger

[Bob Moulder]

I have been working to come up with a heat exchanger (HX) device for a gas canister stove setup that is efficient, safe and reliable, and also offers consistent output as the fuel burns off. The goal was to devise a passive 'set-and-forget' system because the last thing I want to be doing when it's cold is refilling a canister bath or monitoring canister temperature when using reflected radiant heat from a windscreen.

First, a nod to the ideas presented in these two threads, from which my HX design is merely a further refinement:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=59148

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=59755

Below is a photo of what I came up with after a little testing, using a Snow Peak Gigapower as the test unit. The HX strip is made of some .0028" copper sheet folded over a few times to make a strip 1 inch wide. The Velcro fastening loop is of the type with the plastic fitting that permits it to be folded back over itself and cinched down very tightly. (This is important to ensure optimal contact between the HX strip and the canister surfaces.) A thin piece of silicone sheet is used as both a heat shield to protect the Velcro and to hold the HX strip more firmly in place.

.

The first tests with this setup were very promising. The coldest temperature at which I was able to test this rig was 18degF on a trip to the Catskills. The very first tests were on the deck of my condo at 23degF. However, even at these relatively modest temperatures there was a very noticeable improvement in stove performance when using the HX device. Boil times and fuel efficiency using a Primus Eta 1.8l pot were extremely impressive, right on par with MSR Reactor.

The problem with the Gigapower stove is that it is not at all resistant to wind; the slightest breeze really diminishes performance, and any sharp gust over about 10 mph can blow it out. I made a windscreen that attached to the bottom of the Primus pot which helped but was not a totally adequate solution. The burner was still buffeted rather badly by wind eddies.

.

The MSR Reactor (and now Windboiler) would be the absolute best choice for wind resistance, but using a HX strip with these burner designs is not easily workable.

So I went to REI and looked at the Jetboil models they had in stock and chose the Minimo because of its more open pot support structure that would easily accommodate the HX strip. After a little testing with the HX strip, this has become my stove of choice. It works extremely well with the HX and it is reasonably wind resistant without any modification or external screen.

Fortunately, a couple of nights ago it was 10degF (-12degC) on my deck and I was able to give the Minimo a thorough test. Even with a fresh fuel canister starting at room temperature, without the HX device the stove would boil strongly for a couple of minutes and then the performance would degrade quickly until the flame was almost dead in about 8 minutes. With the HX device it was a totally different story – the stove ran vigorously until the last 3-4 minutes when the fuel in the canister was almost totally exhausted.

Here is a VIDEO of the Minimo in operation at 10degF with the HX device.

For those worried about the operating temperature and the possible risk of explosion, here are some photos 1) showing the temperature of the HX strip where it contacts the canister, and 2) the surface of the canister about 1.5" away from the strip. (The probe temps are screen left on the thermometer, 143degF and 57degF respectively.)

.

.

[Jerry Adams]

Good ideas. Good experiments.

How much does copper strip/velcro strip weigh?

[Bob Moulder]

Sorry, I meant to include that! Total 26.4g

.

[Greg Mihalik]

Nice job, and nice science.

"The HX strip is made of some .0028" copper sheet folded over a few times to make a strip 1 inch wide."

I suspect that by using a solid strip of "dead soft" copper you could attain the same performance with less weight.

The evolution continues.

[Bob Moulder]

I suspect that by using a solid strip of "dead soft" copper you could attain the same performance with less weight.

Perhaps, Greg, but I don't think it would amount to more than a couple of grams. I was just using the material I had on hand from a previous and totally unrelated hobby use. I thought about purchasing some thicker copper bar stock from McMaster-Carr but it didn't seem worth the expense; it works great as is. The combined thickness of the folded sheet is .0225", which seems to be a 'sweet spot' with regard to heat conductivity.

But the concept is there and it would be great to see any improvements upon it if anyone wants to experiment further. For me, this is the first time I would be comfortable traveling into a really cold environment with a canister stove. I've seen "cold" stove tests on YouTube with temps in the 30's and 40's and I just had to chuckle – dude, that ain't cold! ;^)

.

edit: typo

[Jerry Adams]

Maybe if the copper was thinner, but wider, it would block wind?

[Bob Moulder]

Interesting you should mention that, Jerry, because I thought of making it wider, or folding it in a conical, then flattened, shape and putting the bigger end at the burner head or on the canister.

But the simple 1" wide strip worked so well that I decided to be satisfied with the result. I might change my mind after observing the stove performance at sub-zero temperatures, however.

I can send you a couple of sheets if you want to experiment.

BTW, I forgot to mention that the silicone sheet material came from some ice cube tray covers purchased at Bed Bath & Beyond: http://www.bedbathandbeyond.com/1/1/152372-oxo-good-grips-no-spill-ice-cube-tray.html

[Jerry Adams]

I got plenty of aluminum in various thicknesses : )

Silicone from ice cube trays? Excellent!!! Whenever I see anything I imagine how I could use it for something.

[Bob Moulder]

Aluminum would definitely require wider stock because of its reduced thermal conductivity relative to copper.

http://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html

I must admit I was surprised to learn that Cu has better thermal conductivity than Au!!!

But that's another interesting idea about having a HX that also shields the burner head from wind.

Cold and wind – the bane of gas stoves!

[Bob Gross]

"I got plenty of aluminum in various thicknesses : ) "

Beer cans!

–B.G.–

[Bob Moulder]

Ahh… whooosh! That went right over my head. ;^)

[Jerry Adams]

"Beer cans!"

No

Roof flashing that I bought for windscreens

Beer cans are worth a nickel

Gold has better conductance than Al or copper

Actually, you have to factor in density also. Copper 3.3 times heavier, 70% more conductance – conductance per weight aluminum weighs half as much to get same conductance

[Bob Gross]

I can pretty well guess that I won't be using gold. Copper is a possibility, but I would need to find the right piece of scrap to start from. I might have some copper pipe fittings, and if I cut them up and beat them flat, I might have something. Beer cans, however, are ubiquitous. Aluminum roof flashing I have.

Somebody suggested that I use copper strip sold as a slug and snail barrier. That is really thin copper foil with adhesive on the back. I think it might be too thin.

I already have a good supply of thin sheet silicone.

–B.G.–

[Jerry Adams]

You can buy strips of copper to put on roof to prevent moss

(or just throw pennies on your roof – do this while drinking beer out of aluminum cans)

[Bob Gross]

I don't think that there is much copper in a penny anymore.

If I stick pennies into my armpits, will that prevent moss from growing?

–B.G.–

[Roger Caffin]

> Aluminum would definitely require wider stock because of its reduced thermal
> conductivity relative to copper.
If I may …
For my Winter Stove production I found that a strip of 1.2 mm thick aluminium was much better than wider and thinner. It lost less heat on the way down.

> Cold and wind – the bane of gas stoves!
Not inside a tent. No problems there.

Cheers

[Bob Moulder]

a strip of 1.2 mm thick aluminium was much better than wider and thinner. It lost less heat on the way down.

Roger, I know that you (and many others on BPL) know a helluva lot more about stoves and the art of designing stoves than I do, so I defer to your expertise.

My only word of caution – perhaps warranted, perhaps not – is for anyone making one of these to do his/her own temperature tests to make sure the canister does not overheat, especially when using other materials and dimensions. I did not test with other materials.

It might matter a lot or it might not matter at all – I don't know. All I can say is that I have run the combo above for the duration of several full 230g canister burns with zero overheating.

[Bob Moulder]

> Cold and wind – the bane of gas stoves!
Not inside a tent. No problems there.

Up until about this time last year I was strictly a liquid-fuel (naphtha) stove user… Whisperlite, XGK… which I really wouldn't want to operate in a tent. I had only a couple of o-ring problems over the years, but inside a tent they could have proved disastrous. And of course the CO problem…

Now that I see canister stoves as a viable option, that might well change. However, whenever possible I like to be outdoors while operating a stove simply because it's less cramped. I'm not comfortable sitting cross-legged or hunched over or lying on my side propped up with an elbow. Refer to "Old Guys Backpacking" for more on this.

[Jerry Adams]

"My only word of caution – perhaps warranted, perhaps not – is for anyone making one of these to do his/her own temperature tests to make sure the canister does not overheat"

Definitely. Put finger on canister and if it's hot, turn stove off. Maybe okay if it's warm on top but cold on the side where the butane is inside?

"For my Winter Stove production I found that a strip of 1.2 mm thick aluminium was much better than wider and thinner. It lost less heat on the way down."

Good point. Typical flashing is maybe half that, so might not be the best choice. I used some aluminum I have that's maybe 0.8 mm, but maybe thicker would have been better?

[Stuart R]

I think the original design of folded thin copper sheet has an advantage in that it should more easily bend to the radius of the canister, thus ensuring a good area of contact. Also, the tip is less likely to melt.

[Bob Moulder]

The thin copper sheet is malleable and does indeed conform readily to the canister radius and makes optimal contact with the canister when the Velcro strap is cinched down tightly.

I saw the contact area as a problem with other iterations of this concept.

One of the first HX examples I found is the one pictured below. It is obvious that even the tiniest gap has a huge negative effect on heat transfer. Flattening the wire to increase surface area does no good if canister and wire do not make solid contact with all available surface area.

.

[David Thomas]

>"It is obvious that even the tiniest gap has a huge negative effect on heat transfer. "

They make stuff for that. Heat sink compound. $5 for two tubes at Radio Shack. It is designed to fill the gap between two metal surface to move as much heat as possible from one metal object to another.

Get the tightest fit you can, and then use that goo in between the two surfaces.

[Roger Caffin]

> make sure the canister does not overheat
Absolutley.
The touch test is golden. You go 'ouch' just about 40 C. The canisters are required by DoT regs to have zero failures at 50 C.

Yeah, thinner material does conform better, but maybe you can do a bit of hammering at home first?

Cheers

[Roger Caffin]

Hi David

Heat Sink compound – usually white.
Generally known in the electronics world as TWS: Travelling White S***
It gets EVERYWHERE!
Your pack, tent, SB, clothing – all your gear would come home white.
Speaks from vast experience …

Cheers

[jimmer ultralight]

contain the same non drying slurry like heat slink paste I use on various welding and spot anealing projects,than yes ,it would be a little messy in camp..

If so, maybe you could "mold" the surface of the metal bar to the canister using a bit of our old freind,JB Weld?

It is heat resistent enough and has enough steel particles in it the bar should still conduct well if it were aplied VERY thin, It would perfectly "bed" the heat strip to the cannister side with no mess.

I would bend the metal bar to as close to tbe cannister contour as possible and then sand the contact area on the strip and degrease it well. Then,rub some paste wax on the side of an empty cannister of the apropriate size for a molding area.Put a nice layer of JBW on the strip and tape it down on the cannister firmly with duct tape..

24 hours later, a perfect fit. And if you waxed the cannister properly, removeable as well..;)

[Author]

Posts