Is a heat exchanger pot worth the weight?

Take the tall outer titanium shell of an old alcohol or wood burning stove, if you can find one, and use it as a windscreen. Mine has the added advantage of allowing one to use wood chips and metal tent stakes threaded through the slots up top to hold a pot of water. If somehow  run out of canister gas, this will work perfectly for cooking. It’s never happened however.

Ryan does not like it when I exercise my Australia sarcasm. Fair enough.
But putting the word ‘efficiency’ over a photo which shows flames going up the side of the pot, right to the top, does not seem encouraging.

Cheers

Gear Skeptic might be having a quiet laugh about it as well.

“The calculation are for a Hemishpere and not a Sphere: That is where the 1/2 came in the formula above. An HX pot will probably be more efficient with material as it is in the plane of the flame spread than the hemisphere as you don’t know how well the flames will hug the surface. My 2 cents.”

Well, I am not sure about the heat transfer through a hemispherical part of this pot design. You will find that heating the pot is only the first phase of heat transfer. The actual goal is to let the heat, warm the water so you can cook/warm up stuff in the water.

As such, a minor exercise in overall physics will show that a sphere has the least surface area for the internal volume. Rather, you can think of the water as totally surrounding the heat source (like the proverbial Dyson Sphere, but on a smaller scale.) I believe they have the mathematics of a hemispherical bottomed pot backwards.

Actually, you would want an inverted sphere around a heat source to gather as much energy as possible from the heat source. Since this is not really practical, an inverted hemisphere is used as an example.

This is constrained by having a gas (generally speaking, everything must be a gas before it can be burned) burn releasing byproducts: CO2, H2O (primarily.) These are hot gases that rise due to gravity/displacement. Soo, a perfect heat transfer would work well. Unfortunately, the trapped air in an inverted hemisphere prevents further entrance by rising gasses from the source.Simply, the heated gas puddles allowing heated gas to flow around the pot. I was getting about a 20%reduction in heat transfer when tested.

In the opposite case, where you have a hemisphere pointed down, but, this allows a smaller volume of heat to be transferred to the water. Surface area vs heat transfer is all opposed to maximum heating effect of the water.
The actual heating effect of smaller surfaces conducting the same heat to the larger internal volume (water in this case) means the outer skin needs to be hotter (10Cal/4sqin or 10Cal/10sqin.) Hence, there is greater opportunity for heat loss due to simple radiation as delta of ambient radiative heat loss. (Generally, ambient radiative heat loss means garbage in equals garbage out, or, GIGO. Heating one side increases radiative heat to the lower ambient side.)
So, if neither a inverted hemisphere nor a hemisphere work well for heat transfer to our water, then the answer lies in a plane as the mostest heat absorption. Ofcourse, allowing heated gasses to transfer the heat a little time to work means a slight ripple in the plane will work. Or, the basis for heat absorption of a heat exchanger.

You can optimize these effects (gas flow, heat flow, heat absorption, etc) by having the depth of each ripple correspond to the average heated gas flow, preventing puddling, but increasing surface area to heat absorption. Of course, the ideal distance changes with the amount of heat generated from the heat source.

In conclusion, a hemispherical heat gatherer, whether inverted or not, does not work the best for heat transfer from our average cooking stove. The compromise is a flat bottom with a resistance to gas flow along the bottom. Roughly speaking, when you can touch the top while heating water, you are doing pretty good.

Marco, I agree 100%.  That’s why it is worth watching to see if it really does have an effect or is it all spin.  The Switch is more money than I want to spend just to play with it.  It will probably take a few months to get a real (actual usage) review on the product.  My 2 cents.

OK, so here is a 125 g complete HX cooking system that is MYOG project.

Key performance is that it will boil 500 ml using under 9 g of fuel in an 8-mph wind.

It uses a modified Fire Maple G3 pot, a modified BRS 3000t and a Cheetah JetBoil Windscreen

Here is a more detailed video

That is a tiny windscreen.  Yet it seems to work good.

Great video of the completed project, Jon.

4.4 oz total: Nice!

It’s funny.  The windscreen works well on the smaller G3 pot but not very well on the G2 pot.  The openning of the pot bottom of the G2 is too large to block the wind.

Super impressive Jon, well done!

Regarding boiling 500 mL using under 9 g of fuel in an 8-mph wind, what is your starting water temperature?

All of my test are 500 ml at 68 F (20 C).  The 500ml adds a bit of margin over 2 cups (473 ml) and Internationally, few people know internalize the volume of 2 cups (I always got questions about this).  20 C also is better understood Internationally, and makes calculations easier.  I use a 5 gallon bucket as a source and at 68 F, I have to add ice cubes in the summer and hot water in the winter to adjust the temperature.

I am impressed by the effectiveness of that tiny windscreen.

Cheers

It has always been my belief that the most underlooked value of a Heat Exchange pot has been it’s partial function as a windblock/shield.  Yes, a small windscreen placed at the right spot can make a huge difference.  In this case it is quite impressive given that this is using the BRS 3000t, probably one of the most sensative stove in the windy conditions.

Up to moderate wind speeds my Stash pot’s Hx ring is an effective wind block but it’s worth carrying an UL shield for high wind.

Jon,

Can a 4oz fuel cannister and the stove pack away in the cut down pot with the lid on?

How do you find the edge for drinking hot beverages directly out of the cut pot?   Too sharp?

I wonder if a straight cut edge is hotter or cooler than a lip edge.  On the stash, I need to let tea cool for 5 mins.    Your Soto Thermostack lid recommendation is perfect but seems to be unobtanium up here

This is cool!

As Jon mentions there’s lots of improvement waiting in HX pots. I’m spoiled by the low conductivity of Ti and not sure I can go back to burning lips and fingers again with aluminum. With a Ti pot I don’t need pot grabbers.

I wonder what the performance to weight difference is between this system and an equivalently sized non-HX  Ti pot with a Windmaster and no mini screen? I seem to remember Jon getting about similar boil times in 8mph

David D – In my testing, the Stash was pretty rock solid up to a 4-mph wind, after that it dropped off.  As cut, a 4 oz fuel canister and BRS will not fit.  This was cut to the bottom of the rivet hole and was meant to match the TOAKS 550.  If it were me, I would make the cut a little higher to store the stove and not worry about the hole (or you could plug it as well).  A little extra storage space would be good.  The cut edge is not too sharp, but it will be hot.  I purchased the Thermostack directly from Soto.

George H – It’s funny, the G3 (600 ml) and the G2 (750) weigh the same.  The wall thickness on the G3 is thicker for some reason.  Long term, I suspect that HX pots could come down to a 20% premium over a non-HX mug.  15 years ago, the Snow Peak Trek 700 seemed to be the mug of choice, now the TOAKS 550 is very popular due to the weight/volume advantage.  HX mugs will probably go through a similar shakedown.  Generically speaking, a non HX mug with a good windscreen will double the fuel consumption in calm conditions: 7 g in calm to 14 g in 8-mph wind, this seems to be pretty consistent.  The exception to the case may be the Soto WindMaster as I have been able to boil 500 ml in a mug with no windscreen but consumed over 18 g of fuel at 8-mph .  A cool feature that I haven’t mentioned is that the Soto WindMaster gets better in the wind with large diameter pot.  The larger diameter has a long enough distance between the edge of the pot and the burner head so that the wind is deflected away from the burner head and does not seem to impact the flame profile.  Hope that makes sense..

Thanks Jon.  Soto doesn’t ship to Canada but there’s always the Toaks lid option.

If I give it a go, I’ll try out a more full coverage windscreen like this, 10g, cut to size and slid under the sleeve.  More coverage for shifting wind but might take some tweaking to avoid CO production

Jon,any thoughts on other stoves that might work well with the G3 setup? Something like fms300t or amicus/windmaster?

Like any regular mug, an HX mug will get a performance improvement with a windscreen. For reasons that I do not fully understand, smaller burner heads (BRS, Fire Maple 300t, Fire Maple Hornet II) tend to be more efficient with HX mugs in calm conditions (compared to the Soto WindMaster for example). In higher winds, those same stoves are more sensitive to the wind. I happen to like the Fire Maple Polaris myself. My 2 cents.

Just released a Part 2 video on the MYOG HX pot.  The reported weight of the HX mug was off by the weight of the shroud.  Good news is all the performance testing is valid and was done before the shroud was removed.  I also moved the cut line up in order to better store a fuel canister, the modified BRS stove and make allowance for the Soto Thermostack lid.  The weight of the trimmed HX mug moved from 66 g to 88 g and the volume increased from 560 ml to 630 ml.  Also, I added data on the impact of pack weight vs boils needed at 2 different wind speed.

Nice.  The video states that the 560mL with shroud is 88g, how much does the 630mL with shroud weigh?  Grazis tuto

In the video, I cut the stove about 0.2″ above the bottom edge of the lower rivet.  This increased the capacity to 635 ml and the weight (including the shroud) was 88 grams.  Sorry if that was not clear.

Jon