DIY Skirted pot query

[Edward John M]

Having been unable to find an UL unit I am going to make my own using titanium foil as the skirt.

Main or only use will be melting snow for drinking and cooking water and for use on either an MSR XGK-EX or the Firemaple 117T

Two small questions.

What is the best gap distance between the skirt and the pot for best efficiency?

If it happens to be 5 or 6mm than I can use small brass stand-offs used in amplifier building

What, if any, extra skirt should I allow below the bottom of the pot?

The aluminium billy I will be using [ or at least experimenting with ] has a small radius at the base which may be enough to direct the heat up into the gap but it would be easy enough to make it slightly longer and protect the foil with a small pad of CCF or similar. If I use a pad it will do secondary duty as somewhere insulated to put a pot of stew or of coffee

 

[James Marco]

Yes, quite. I had advocated that many years ago. Sort of like this:

Of course, this allows it to be used with canisters, alky, WG, and remote stoves. There were some details to be worked out, attachment between pot and skirt for example. But yes, it works as well as a cone set-up. I used it several times on an older SVEA as well as a Coleman F1, Pocketrocket, etc. It works. But the foil I was using did not hold up well. Four or five uses and it was pretty much scrap.

[Jerry Adams]

My take on this:

Strip of aluminum is 2.5 inches wide.  The accordion folds are 1/2 inch.  There are 18 channels around the stove.  #18 steel wire.  It fits inside Evernew 900 pot.  There are 4 places where I drilled small holes for the #18 wire to go through, one of them shown.  I carefully bent the wire at a length so that it applies spring pressure to keep it on the pot.  You can unclip it so that it fits inside pot.

A lot of the flame exhaust flows through the channels.  Some of the heat gets absorbed by the pot.  Some of the heat gets absorbed by the aluminum, then conducted to the pot.  Reduces fuel use about 10%.  It also goes down low enough to function as wind screen.

A previous version was more like James’.  I’ve used my current version for years and haven’t felt compelled to make a new version.  I tried one that was taller, which made a better windscreen but then didn’t fit into pot.  Maybe 3/4 inch accordion folds would be more efficient.

[Jerry Adams]

1.27 ounces

[Edward John M]

I was going to use thick Titanium foil because of the ability of Ti to take much more heat. I was asking the questions before I actually ordered the sheet from Titanium Goat because $50- is a substantial amount of money if it isn’t going to work. Exchange rate plus postage is a killer

Spacers are available in either 6mm or 9mm if I buy untapped but as the pots circumference is 530mm and the Ti foil comes in a 20″ piece I can use either if I cut the 12″ section in half.

The billy in question is 165mm tall so a 6″ depth for the foil sounds about right to me. At 355grams it isn’t lightweight but it will do for now for experimenting with. 3 litres is what I would consider acceptable for 2 people; maybe 3 persons

[William N]

This is an interesting problem. The enclosed skirt with the vent holes at the top at first seems like the perfect solution, until you think about the ‘two surfaces’ The surface of the pot and the surface of the skirt. They’re basically getting the same amount of heat.

It is better than just the pot to the open air, but what if you could also put the heat that’s in the skirt into the pot? And that’s solution #2 above. I like the wire ‘spring’ device to keep the fins against the pot. There’s still an outside and an inside.

Then there’s the Jet Boil fins method. The fins are on the bottom, the pot is insulated. What I always notice is that it’s hard to get burned from the JetBoil pot.

Now I don’t fully understand the engineering and thermodynamics of these three systems, but I have some experience with the JetBoil solution. Most of us have seen or used the Jet Boil pot. It works. Some of us have seen the JetBoil skillet, and it generally gets bad reviews (personally I like it. I backpack with eggs, veggies, olive oil, I would never use the JB skillet in my kitchen, it heats unevenly, but on the trail, it’s okay).  It too has fins on the bottom. The big difference between the skillet and the pot is that you get  better view of what the heat is doing with the skillet. The center of the skillet, in a ring a bit larger than the burner underneath is the hottest spot, farther out above the fins, a lot less heat.  To me this indicates that a really high percentage of the heat is going right into the pot just above the burner.

I also (think I) know the fins are efficient because not a lot of heat seems to be escaping, but I also know that they aren’t getting super hot because there’s no scorching above the fins. So there’s not a lot of heat this far out.

Conclusion? In all stove set-ups, I don’t think there’s a lot of heat going out the side compared to what’s hitting the bottom of the pot-most of the heat is hitting the bottom of the pot. (There are probably some numbers somewhere proving me an idiot.)  The best skirt method would do three things. 1. Insulate the side of the metal pot from radiating heat and being cooled by air movement 2. Capture the heat energy going up the side of the pot and 3. by conduction transfer any heat from the skirt to the pot.

I think the JetBoil pot (how is the MSR pot similar or different?) basically works by 1. capturing extra heat before it goes out the sides 2. Insulate the pot.  For Jerry Adams’ zig-zag skirt, I think a capture flange at the bottom, and then the zig-zags in contact with the pot would be effective. I also think that it would be best if the fins were welded or soldered to the pot. Carbon build up insulates (burnt carbon is one of the ablation methods used in heat shields for space vehicle re-entry. Is there any sciFi where in a pinch the hero makes a thick layer of hard candy to use as a heat shield?)

If you want to make a zig-zag ring for underneath the pot, rough back of the envelope seems to indicate starting with a length of foil about 3.5-4x longer than the outside circumference of the pot. You could make perfectly parallel fins if you curved the strip, but it probably won’t be necessary. (with a straight strip the zig-zag gaps will be tighter on the inside then the outside). At the top of the zig-zag I would make a short flat bit where it contacts the pot. A skirt around the outside of the fins would probably help, and also keep the whole business from turning into a Slinky toy. (Well PhotoShop refuses to open… so no picture).   Does it show that I’m supposed to be doing something else?

[Ken Thompson]

Why not a cone?

[Jerry Adams]

that makes sense, good thoughts

I’m not arguing that my idea is right or anything, just thinking about it:

At the bottom, it’s very hot.  If you weld stuff to the pot you get thermal expansion and stuff so the welds tend to fail.

Somebody said that if you can promote turbulence that helps.  If you have laminar flow, hotter air will be above the pot and not transfer heat to the pot as much.  Like a strip on the surface or maybe a vortex burner.  I don’t know…

With my zizgzag and spring, the aluminum makes good thermal contact with the pot.  Any carbon would get rubbed off.  When the water is boiling, there’s a lot of bubbles coming off right where the strip touches the pot.  There must be a lot of heat transferred.

[David Thomas]

” ‘two surfaces’ The surface of the pot and the surface of the skirt. They’re basically getting the same amount of heat.”
“Somebody said that if you can promote turbulence that helps.  If you have laminar flow, hotter air will be above the pot and not transfer heat to the pot as much. “

The inner surface – the pot – will get somewhat more heat because the water inside is such a good heat sink (whereas the windscreen’s outer surface is air in an insulating boundary later).   But rather than two smooth surfaces (outside of pot, inside of windscreen), if the inner one is rough, it will stir up the inner boundary layer and you’ll get better heat transfer to the pot.  Leaving the inner windscreen smooth would maintain more of an insulating boundary there, where you want it.  Rough could be bumps or BBs or fins.

I did some side-by-side experiments with and without little fins angled at 45 degrees near the bottom of the pot’s vertical wall.  Akin to vortex generators on an airplane wing:

It did noticeably increase heat transfer for hardly any weight at all.  Alas, they were kind of fragile and made the pot harder to pack.

I only once played with VG fins on the bottom of the pot.  I JB-welded L-angle aluminum onto the bottom of an aluminum pot.  It made a big difference – seemingly more than the added area and heat conduction through the fins would account for.  The downside was that after dozen of uses, the fins started to fall off – I suspect due to thermally expansion and contraction at the glued joints.  The fins would have gotten much hotter than the bottom of the pot which never exceeds 100C by much.  Smaller footprints of the fins or each fin having multiple, small feet might help with those thermal-expansion stresses.

[R]

David,

Can you share any dimensions of the VG fins you tried?

I wonder How things would go with a turbulence device mechanically attached. Something like a highly perforated sheet or squigly wire mesh pressed to the bottom of the pot by stiff wires hooked over the lip of the pot. I’m guessing adhesive aluminum tape wouldn’t withstand direct flame – the adhesive would probably burn off and the foil might melt.

A while back I played with a pleated foil pot, like a paper cupcake liner. The thought was greater surface area and built in fins for heat exchange.  Didn’t get as far as thermal testing.

[David Thomas]

Rene,

The VG fins I used were 0.5 cm wide and 1 cm long (away from the pot).  Since I was testing over an alcohol burner and candles, the temps were low and I just used aluminum duct tape – that really sticky stuff that is what you should seal ducts with, not the grey cloth stuff that has gotten so popular with the masses.  A 0.5 x 3 cm strip of it, folded over on itself and leaving two sticky tabs to adhere to the pot worked at those comparatively low temperatures.

Mechanically attached VG fins is an intriguing idea.  A band of Ti foil, with 3 sides of each tab cut so that the tab folds on the fourth side like a hinge would then not need any adhesive and could be removed and stored in the pot for transport.  Since its purpose isn’t as HX fins, it would haven’t to be tightly held to the pot.  A slight air gap between the band and the pot wouldn’t impair its function as VGs.

[R]

Just hit me that this could be a part of the stove.  No need to attach it to the pot.

Vortex Generation Pot Stand.

The race is on, stovers!

[Jerry Adams]

I thought it was David that talked about turbulence : )

Roger has a number of articles about vortex stove.  He sold some.  I think there are other sellers.

Does a BRS 3000 have a vortex burner that creates turbulent flame output?

[R]

I’m pretty sure Roger employed vortex generation to facilitate air-fuel mixing in the interest of combustion. The BRS3000 does produce a spiral-ish flame pattern, but I’m not sure what for.

I’m proposing something like a perforated, corrugated surface for the pot to rest on as an alternative to attaching vortex generators directly to the pot.

[James Marco]

Rene, Yeah, that has been tried. Anything similar to that actually interferes with the combustion process and produces a large amount of CO. Rather inefficient since more energy can be had from the molecule by adding adding a second oxygen. A good example of this is the Reactor or Windburner.

The BRS-3000 isn’t really a vortex stove. The FMS-300 is, sort-of. Roger’s stoves are modeled on the older vortex burners like the Primus, Optimus, MSR-XGK, etc. Basically any stove is a vortex burner to some degree, but, to me, it means that the majority of air needed for combustion is from the outside air, not pre-mixed before combustion as with most canister stoves. That is, the flame rises directly from the jet. There is no mixing stage.

Mechanically, this is just a tube above the jet where air and gas are mixed about the same as a Bunsen Burner in a lab. Getting back to the FMS-300, there are small holes in the tube allowing a small amount of air to mix, mostly for low heat settings and ignition. The majority of the air needed for combustion is mixed by air entering the flame region and mixing more or less simultaneously with the combustion process. But, it still retains the mixing stage, though not as the primary air input at medium and high heats. Roger drew up a good diagram of this in one of his articles. Without a pot over the burner, the FMS-300 can be inefficient and will burn with a yellow to red tipped flame, almost straight up. With a pot, it will cause some vortex to mix a large amount of air into the flame, pretty much burning everything cleanly. Too large of a pot, can cause too much air to be input for the flame size, actually cooling the flame a bit. I know that during testing, I was getting about 5gm per 1/2L average. But, the timing could change quite a bit. Between 6-10min really didn’t show any statistical difference in fuel consumption. Because everything else was the same, that assumption was supported.

[R]

James,

Can you point me at any info about the attempts you’re referring to?  Everything I can find on BPL is either a stove design focused on gas mixing and combustion or a heat exchanger.  The two MSR stoves you mention use radiant heating and a heat exchanger.  I haven’t seen any mention of vortex generators to disrupt the cool boundary layer at the surface of the pot to mix in hot exhaust gases and increase thermal transfer.

David,

Do you have any pics or a thread on your past efforts?

 

[Jerry Adams]

” I haven’t seen any mention of vortex generators to disrupt the cool boundary layer at the surface”

That may have just been my speculation.

I think there has been at least one thread that talked about putting something down stream from the burner to cause turbulence to get more heat transferred

If that happens, then it would make sense that a burner with a more laminar flow output would be less efficient than a burner with a more turbulent flow output.

[Edward John M]

I’ve not done any further work on this myself because the house renovations are falling behind. Something Roger has mentioned in a different thread tho is that the volume of the hot combustion  gases are huge compared to the separate ingredients of fuel and oxygen because of thermal expansion and they need to be given somewhere to go.

This is why my question on optimum airgap

I know skirted pots are far more efficient as I have used such all my working life, but in commercial kitchens they are called either jacketed boilers or Soyer stoves after the inventor.

This leads me to another query related to the stove used, a small stove such as the FMS300 generates a lot less heat/uses less fuel than the bigger FMS117 which uses less fuel than the MSR Expedition. So even if I assume a bigger pot is used with the bigger stove which is my intention does the gap between the skirt and the pot increase or does the optimum gap stay the same?

[R]

One way to approach that is to compare fuel consumption rate and gap area.  If you measure the two in a system with accepted performance, maintain the same ratio in the new system.

Note that’s consumption rate, not just total consumption for a boil.

[Jerry Adams]

“Why not a cone?”

Like a Caldera cone

[Jerry Adams]

to compare different stoves or configurations or spacing, measure number of grams of fuel to raise a given volume of water 100 degree C.  Measure the temperature increase and scale the actual fuel used by that amount.  Do it for different configurations to see the effect.

You also have to have the stove turned up to the same level for different cases.  I have measured a 10% improvement if I turn the stove down to double the boil time.  You don’t want this to be an uncontrolled variable.

Ryan’s proposal talks about all this.

[Edward John M]

I’m not sure I understand the reference to “Ryans proposal” what have I missed?

Also my version of the MSR Expedition really only has one flame setting, it seems to be flat out or off no matter what I do to the fuel control knob. Naturally I can use a tall windscreen the height of which-ever pot we are using but I wanted a more elegant solution if possible. Also if I assume we use a large and wider pot for snow melting that will sit on top of a cooking pot to capture any excess heat that a pot with a short skirt would work better in conjunction with a shorter circular windscreen around the stove.

I have assembled something similar to the Yukon boiler in Monty Alfords book but the gap between the billy and the paint tin really is too small to be effective which explains his need to drlll a hundred holes

[R]

Jerry may be referring to Ryan Jordan’s stove testing protocol, Stove Bench: https://backpackinglight.com/stovebench/

If you’re talking about using the same stove with a larger pot (sorry if I missed that), then you really just need to maintain the same gap area: pi * (windscreen-radius ^ 2 – pot-radius ^ 2).  A larger pot means you can use a narrower gap.

This is specific to ensuring adequate exhaust flow.  This has nothing to do w/ efficiency, thermal transfer, boil time or anything else.  This just gives the minimum gap required to allow adequate exhaust flow.

If the number seems impractically small, you may want to go for the cone with holes approach.  In that case, you can’t use the area of the holes since they’ll be very close to the pot (assuming near the top of the cone).  A good approximation will be the diameter of the holes times the distance the cone is from the pot at the location of the hole – consider that the effective exhaust cross section for one hole, then work out how many you need.

[Edward John M]

Of course and it’s obvious when explained like that Thanks Rene, annular area arithmetic.

I know Roger has explained in previous posts about combustion gas volume and the figure of *600 is stuck in my head for some reason. OK time to search some more.

 

[James Marco]

Rene, mostly these were off hand tests to determine the suitability of the FMS-300. I didn’t bother keeping good records back then because I was done with most testing other than for my use. I think I sent preliminary results to Roger on the first set of tests when I got the stove. He might have the first set done with the 5-1/8″ pot. I also used a 4″, a 5.125″ heat exchanger pot, a 6.xx”(maybe a quarter, don’t remember off hand), and a 7.75″ pot. Of course, I just used the standard height setting. I would also note that very wide pots (like 8″ and above) can trap a lot of wind.

The heat shield/exhaust vents on the pots act exactly like turbulence inducers as it forces the hot gasses to bend around the bottom of the pot. I believe it encloses the lower 20% of the pot in the skirt.

Actually the Reactor needs to be lit. It will actually produce a flame if allowed to heat enough. The mesh/filler are there to break up the flame and air pattern producing a less laminar flame. This is exactly what you suggested “I’m proposing something like a perforated, corrugated surface for the pot to rest on as an alternative to attaching vortex generators directly to the pot.”

The Windburner is about the same but with out the lower skirt. There, simple air friction will act as anti-laminar devices since they are quite close together, as does the JetBoil’s.

All are quite heavy. The lightest I could come up with is a simple wrinkled aluminum wind screen/pot lid made from a single large piece of aluminum foil. Punching a bunch of holes near the top with a stake/spoon handle allowed exhaust gasses to escape. The lower edge should be about down to the control lever on canister stoves. The wrinkling induces random turbulence in the air flow, delaying it. Yes, it worked fairly effectively, but was painful to carry. I could get about 600-650ml to boiling (32F to 210F) with the same 4.5-5gm fuel. But after a few test runs, it was split in a couple places and pretty much beyond using. And it was always difficult to carry. Never took it any further because things started to get beyond my self imposed 7oz limit for simple stove/pot/lid/windscreen/spoon cook kit. Instead, I started experimenting with insulation on the pot/windscreen and lower flame settings.

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