Pots With Heat Exchangers

[Ben H.]

It would be nice if someone did the side by side comparison of a heat exchanger pot and a non-heat exchanger pot and reported the efficiency over a range of tests.

[James Marco]

HE pots only add speed to the boiling. IFF you notice a speed or time to boil difference, then you are likely applying too much heat originally.  HE’s only capture heat if there is heat to be captured. Like I say, basically they simply artificially extend the bottom of the heated surface. I would suggest a smaller burner.

First off, I said that there MUST be a heat differential. You can think of this as a cold object and a warm object. In a perfect system, and over some time, both will become averaged in temperature…essentially this says that IR radiation is not lost in a perfect insulation environment and 100% is held as molecular vibration or what we interpret as heat by a finger feeling it.

But, physics defines radiation and heat loss to be balanced with radiation and heat gain, in actual fact. It is NOT the case than anything can be considered 100% steady state. Heating is occuring and cooling is occurring simultaneously at all times. If they are balanced, we *say* nothing is happening, ie no heating and no cooling, but this statement is not actually correct. Ambient is more correct implying that the heat balance is maintained relatively evenly.

Dan, what you are saying is that you actually have 30% more heat loss with a non-HE pot. HEAT LOSS. In a well designed system of insulated pots, you could do the same without the 30% heat loss…simply using less heat to start, or, less fuel.  You do not get a free ride, and, you cannot collect $200 because you pass “go”.  HE pots cannot work to magically increase stove output. They do not convert 1000BTU to 1300BTU by simply using a HE. Where did this extra 300BTU of heat come from???? The only place it could, from conserving the lost heat of the overly large burner you are using, effectively reducing the heat loss of the non-HE pot system by 300BTU.  At the limit of what you say, I could boil water with nothing by adding a “special” HE…nope…it don’t work that way.

For our purposes, heat will be transferred to the water as efficiently as possible. We need to add more heat (whether through molecular movement/vibration, increased motion of electron orbital speed within an atom of these molecules,  convectional transfers by direct molecular interaction, or other “not so magical influences”) than is being radiated away.  We want a hot flame to transfer it’s IR and “heat” to the water (ignoring pot materials, conductivity, etc…)  IFF our energy budget is 1000BTU/hr, then we can always simply make a smaller burner for 500BTU/hr for two hours to achieve the same total heat. HE pots only let us use larger burners than is really necessary to capture the heat, decreasing the heating time from two hours to one. They ONLY speed up the heating by letting us use a larger burner, they are NOT more efficient at heating. They are the same in efficiency.

So, from a build/design standpoint, we want smaller burners, a well insulated pot/lid, and a good ambient heat retaining system. Really, adding a HE will only increase the size of the burner. I think I remember around an 8 minute burn time at 70F was close to optimal (balanced between heat lost, heat produced, heat transferred, fuel used and time to boil.) This can also be stated roughly as HE’s only decrease time to boil IFF they have the heat available. But, as we know, producing more heat means faster fuel burning for the time of the boil. Heat quanta, environmental variables, and so on are a few that can be plotted on a graph.  Now draw a line between graphed curves for the minimum distance between them (overall) you desire in the final usage. Dealing with multivarient stuff is a pain (Linear Algebra) when we can simply see it on a well plotted set of curves.  I am just lazy I guess…

[DGoggins]

It would be nice if someone did the side by side comparison of a heat exchanger pot and a non-heat exchanger pot and reported the efficiency over a range of tests.

Well, lots of people have for sure….though I did just do about 26 boils last week. I was testing the following combos ->

1. Soto Amicus
2. Not Pictured: BRS 3000t
3. Evernew 600ml short/wide pot
4. Evernew 640ml Mug pot
5. Jetboil .8L spare cup (aluminum) with titanium riser ring

 

I was testing each stove with each pot. I was also testing low/med/high settings and its effect on boil times and efficiency. I wanted reproducability, so for consistent starting temps I started by using water from an Ice Bath, so it was near freezing, but….then figured it was actually much more efficient/easier to just take a large pitcher of water and wait for it to get to room temperature (68°).

Consistency was a challenge though…because…its hard to get the exact same flame setting on either stove. And…it would have been better if I had a scientific temperature probe and cut the experiment when it reached a particular temperature. Instead I did to roiling boil…which…exactly when I stopped was probably different each time, or maybe I lifted off the lid on one experiment more vs the other, etc.

Here is a picture of all of my results if you are interested ->

https://photos.app.goo.gl/5BLkn1ZWnkEIKdeA2

It of course, is a mess to look at, but the gist of it is ->

1. In a Kitchen setting…the brs and amicus had the same efficiency. That of course would change out in the field because a BRS would need a much better windscreen than the amicus.
2. It takes about 1g of fuel to heat water from ice to 68°.
3. About a 4-5% difference in efficiency between med/high power, but 25% faster speed.
4. Ballpark averages…for 2 cups of water (not ice water), with a Jetboil it would take 4.75g to boil. The evernew short/wide pot, 6g, and with the tall/narrow mug, about 6.4g.

Which all makes sense. The HX ring is not just a marketing gimmick. And, using a short/wide pot is naturally more efficient than a tall/narrow mug.

Now…. @jamesdmarco …. I’m just giving my actual test results. I think the bottom line that you are saying is that with the results I’m getting, its working because the stove itself is just putting out way too much heat, and so if I used a “smaller stove”, the HX ring wouldn’t do me any good (like…using a starlyte in a caldera cone). Thats all good and fine…but…me, like a lot of hikers, especially with groups….want SPEED too. So we want a “hot stove”, and if a HX ring helps with that…then…great.

As for which setup I’ll be using…I guess it depends. Like Hikin’ Jim points out…on shorter trips, the less efficient evernew pots will be more weight efficient than bringing the jetboil. I actually have it all mapped out …feel free to copy this spreadsheet to your own google doc, though of course…this is mainly an unreadable mess to everyone but me ->

https://docs.google.com/spreadsheets/d/1Qc3d_j5VJss65DAWdUh6D2QA6euaM5lwsr-lDpavugM/edit?usp=sharing

Main take away I’m talking about right now…is that the evernew setup is lighter (positive numbers), and when it switches to negative numbers, the jetboil is lighter. Basically…evernew setup is lighter most of hte time until you have to bring a second canister sooner than with a jetboil setup ->

boils start end
1 2.55 2.6
2 2.5 2.65
3 2.45 2.7
4 2.45 2.75
5 2.45 2.75
14 2.1 3.1
15 2.05 3.1
16 -3.4 -2.35
21 -3.6 -2.1
22 -3.6 -2.1
23 1.8 3.35
29 1.55 3.55
30 1.55 3.6
31 -3.95 -1.75

Like…with these setups, the jetboil is lighter at 16 boil mark, but swings back to the evernew setup being lighter at 23boils.  Of course, this only works if you strip down the jetboil (don’t bring the cup, heavy lid, I cut the cozy, etc).

[David Thomas]

Heat-exchange fins do more than allow you to use a larger flame.  And they (can) do more than just increase the surface area.  Disturbing the insulating boundary layer that develops against the pot improves efficiency.  HX fins (which I consider, in part, vortex generators) can do that, more pot area won’t.

If the #1 take-home from the heat exchange classes I took was “more surface helps”, a close second was “turbulence greatly increases heat transfer”.

(the #0 lesson is counter-current HX is better than co-current, but that doesn’t apply to us because our water isn’t flowing).

[DAN-Y]

HE’s only capture heat if there is heat to be captured. Like I say, basically they simply artificially extend the bottom of the heated surface.

 

I agree.

All HX pots are not created equal. Some have more ability to transfer the heat as seen in this pot:

In a perfect world, no heat would escape up the sides of a pot. Having it insulated prevents warm air on the outside of the pot from entering to get to the cold water inside. We want equalization, right? Heat rises up and over the top of a pot also, adding heat to the lid.

The manufactures of these HX pots say they are 30% more efficient (not me)

Marco says, bah humbug :-) just increase pot bottom diameter and taper wall (think conical) to achieve same result.

[Jerry Adams]

If you run your stove in the sun and look at the shadow, you can see where the heat from the burner goes, sort of a poor man’s Schlieren

That shows the technique but you can barely see the image.  I need to work on my photography.  A more uniform white screen would help.  But I can see it with my eyeballs.

Especially if you run it hot, more heat will be produced and some of it will overflow around and further away from the pot and be carried away which is wasted.  If you run it slow, more of the heat will flow next to the pot where the heat can be transferred.

My attempt of a drawing what I saw with poor man Schlieren:

If you can direct more of this flow of flame output next to the pot, more heat will be absorbed into pot so it will be more efficient.  Like a Caldera Cone does that

With a Caldera Cone, the heat is directed to the pot, but it also heats up the Caldera Cone and little of that will get absorbed into the pot so that’s wasted.

Heat exchanger pots tend to direct the heat next to the pot.  The heat also heats up the exchanger, but then that is conducted to the pot so it’s not wasted.

My MYOG exchanger directs the heat next to the pot, and also conducts the heat absorbed by the exchanger.  I measure 10% improvement in efficiency.  1.3 ounces.  A strip of aluminum with a steel wire to press it against the pot.  It also provides wind protection so you don’t need a windscreen, although a bigger windscreen would be better (but weigh more).

I copied some MSR product I saw maybe 10 years ago.  I don’t think they make it anymore.  It weighed something like a pound.

[Ben H.]

HE pots only add speed to the boiling…

no

IFF you notice a speed or time to boil difference, then you are likely applying too much heat originally.  HE’s only capture heat if there is heat to be captured. Like I say, basically they simply artificially extend the bottom of the heated surface. I would suggest a smaller burner.

kind of… but mostly no

First off, I said that there MUST be a heat differential. You can think of this as a cold object and a warm object. In a perfect system, and over some time, both will become averaged in temperature…essentially this says that IR radiation is not lost in a perfect insulation environment and 100% is held as molecular vibration or what we interpret as heat by a finger feeling it.

But, physics defines radiation and heat loss to be balanced with radiation and heat gain, in actual fact. It is NOT the case than anything can be considered 100% steady state. Heating is occuring and cooling is occurring simultaneously at all times. If they are balanced, we *say* nothing is happening, ie no heating and no cooling, but this statement is not actually correct. Ambient is more correct implying that the heat balance is maintained relatively evenly.

ahhh…. okkkk

Dan, what you are saying is that you actually have 30% more heat loss with a non-HE pot. HEAT LOSS. In a well designed system of insulated pots, you could do the same without the 30% heat loss…simply using less heat to start, or, less fuel.  You do not get a free ride, and, you cannot collect $200 because you pass “go”.  HE pots cannot work to magically increase stove output. They do not convert 1000BTU to 1300BTU by simply using a HE. Where did this extra 300BTU of heat come from???? The only place it could, from conserving the lost heat of the overly large burner you are using, effectively reducing the heat loss of the non-HE pot system by 300BTU.  At the limit of what you say, I could boil water with nothing by adding a “special” HE…nope…it don’t work that way.

ummm… no.  Efficiency in this case is defined as heat absorbed divided by heat released.  You can’t absorb more than is being released, but in your example if the fuel releases 1500BTU and one pot is able to absorb 1000BTU while the other is able to absorb 1300BTU.  The 1300BTU pot is more efficient.  That is the way it works!

For our purposes, heat will be transferred to the water as efficiently as possible. We need to add more heat (whether through molecular movement/vibration, increased motion of electron orbital speed within an atom of these molecules,  convectional transfers by direct molecular interaction, or other “not so magical influences”) than is being radiated away.  We want a hot flame to transfer it’s IR and “heat” to the water (ignoring pot materials, conductivity, etc…)  IFF our energy budget is 1000BTU/hr, then we can always simply make a smaller burner for 500BTU/hr for two hours to achieve the same total heat. HE pots only let us use larger burners than is really necessary to capture the heat, decreasing the heating time from two hours to one. They ONLY speed up the heating by letting us use a larger burner, they are NOT more efficient at heating. They are the same in efficiency.

absolutely not

So, from a build/design standpoint, we want smaller burners, a well insulated pot/lid, and a good ambient heat retaining system. Really, adding a HE will only increase the size of the burner. I think I remember around an 8 minute burn time at 70F was close to optimal (balanced between heat lost, heat produced, heat transferred, fuel used and time to boil.) This can also be stated roughly as HE’s only decrease time to boil IFF they have the heat available. But, as we know, producing more heat means faster fuel burning for the time of the boil.

wrong again

Heat quanta, environmental variables, and so on are a few that can be plotted on a graph.  Now draw a line between graphed curves for the minimum distance between them (overall) you desire in the final usage. Dealing with multivarient stuff is a pain (Linear Algebra) when we can simply see it on a well plotted set of curves.  I am just lazy I guess…

mostly gibberish, but okayyyy

I am not trying to be mean, but this post is all over the map and I don’t want it to confuse the community.

 

[David Thomas]

Jerry, we’ve had one of those MSR “heat exchangers” for 25+ years, although I rarely used it once I had pots with welded-on fins. They’re still available for $26-$30.

I’d estimate more of the benefit is as a windscreen and by channeling air and creating more turbulence along the pot’s sides.

[Jerry Adams]

yeah!  that’s it

one thing I notice is that inside the pot, opposite where the fins contact the side of the pot, the water boils, so heat must be flowing from the heat exchanger material to the pot

[James Marco]

Ben, as one example, HE pots also radiate more heat than a plain pot. Increasing radiation in any pot will decrease overall system heat transfer requiring more heat/fuel. It is an ongoing process in ALL cases, with a HE or without one.

The amount of mass more than the mass of the water you are heating will make a difference. The entire mass of the pot needs to be heated as well as the water.  It is lost heat. As an example, a gold pot (with a density of about 7 times that of aluminum) will absorb extra heat, removing heat from the system, despite the improved conductance of the gold. Heat exchangers are additional mass on a pot.

In your example, a pot that collects 1000BTU at max efficiency should not be 50% over driven with a 1500BTU burner to start with. If you overdrive the maximum heat transfer capability of any pot, you waste heat and/or fuel, while decreasing time to boil. (I understand that a certain percentage of heat is lost during combustion, anyway.)  Using 50% less heat/fuel sounds more “efficient” in my book. You are simply stating the corollary to HE pots. Heat exchangers only collect EXCESS heat from a burner.  Don’t produce the *excess* heat and their effectiveness decreases and will actually invert near the limit.

The big point is that HE pots are *not* overall more efficient with fuel than non-HE pots. But, using larger burners and HE pots will decrease time to boil if that is what you are optimizing for.

And, on a plot, fuel used compared to time to boil is rather easy to figure out.  Adding a third variable, say pot heat transfer (heat exchange for example since ALL pots do this, anyway,) complicates matters since these would cross at different points when compared with either of the other variables.

This brings up the “gibberish” relating to efficiency.  We are plotting more than 2 variables. Adding another variable introduces a third dimension to the solution. Adding ambient temperature to the problem applies another dimension (say the amount of IR radiation.) And so on… Adding another independent variable introduces another dimension of the problem. Efficiency is only a direct 2 dimensional comparison between two variables and cannot be used to define the result, rather a curve of possible results will be produced depending on what variable we wish to solve for and under what conditions. Again, it will NOT be the same point on a graph…well, usually.

Ben, we are not considering the same things (time vs fuel consumption vs heat transfer vs weight.) I believe you are too limited in your viewpoint since nothing you said convinced me otherwise, rather it was based on incomplete analysis of my rather poor and possibly confusing explanation.

Jerry, do you use one all the time? Why not?

[DAN-Y]

yeah!  that’s it

one thing I notice is that inside the pot, opposite where the fins contact the side of the pot, the water boils, so heat must be flowing from the heat exchanger material to the pot.

 

Yes but, there are folks out there that want to insulate the sides of the pot with neoprene cozy’s ;-)

That would be so cool to see the water boiling on the inside walls of the pot. I may purchase the exchanger just to see it do it’s thing.

I once created a stove that focused the heat in a 1 inch area on the bottom of the pot. Once the water heated to boiling, it only boiled in the center of the pot where the flame was focused. Can you imagine a 1 inch diameter column of large bubbles coming up the center of the pot? It was called the “stiletto” stove and can be found on whiteblaze.net (the stove history channel) :-) The burner portion of the stove was a tealite candle container modified to perform like a Starlyte burner.

https://whiteblaze.net/forum/showthread.php/23364-Stiletto-alcohol-burner?highlight=venturi

 

[Jerry Adams]

By the way, good analysis James, I pretty much follow you

One thing is the heat exchanger fins don’t increase IR much.  They block the IR from the pot, that is the outer pot surface and inner heat exchanger surface radiate IR to each other so there’s no over-all loss.  The IR radiation from the outer surface of the exchanger fins is about the same as from the outer pot surface would have been without the exchanger.

If your heat exchanger was low emissivity aluminum rather than higher emissivity pot then IR radiation could be less.  You want the pot outer surface emissivity to be high to absorb heat from the hot exhaust gasses, but the heat exchanger outer surface is on the other side of the hot exhaust gasses so it can be low emmisivity.

Efficiency, of importance to lightweight backpacker, is ounces of butane per day.  Which can be translated to ounces of butane to heat a quantity of water a specific temperature range at the ambient conditions typical or worst case for a backpacking trip.

A big part of inefficiency is how much of the exhaust gasses just get blown away.  If it’s windy, that can be the worst.

I use my MYOG all the time.  Mostly it’s a windscreen.  Trade-off towards being lightweight and it fits in my pot but not a real good windscreen.  I measure efficiency improvement of 10% compared to just a windscreen.  If it’s very windy I have to have sheltered location.

[Jerry Adams]

Dan, it seems like for more efficiency you want a bigger pot surface area.

The hot exhaust gasses hit the pot at one place and start transferring heat.  As the exhaust gasses flow along the pot, they keep transferring heat and getting cooler.  At the end of the surface, the exhaust gasses will flow off into the air and be lost, hopefully at that point they’ll be cool.  If the sides of the pot are absorbing heat from exhaust gasses it will be more efficient.

Old style inefficient natural gas house heaters have masonry chimneys to carry away the hot exhaust gasses.  New, high efficiency heaters can have PVC pipe to carry away the exhaust gasses that are no longer very hot.

 

[Mario Caceres]

I like the wide and short regular TI pots.  (No HE for me).   I use the 0.9L evernew for solo trips and the 1.3L evernew when I go with my girlfriend.  This is what I do on the field.  On the Bottom a Vari-Vent windscreen from Trail designs, I like that I can adjust the air intake based on wind conditions, the stove I’m using and the kind of heat I need.  (i.e boiling water vs simmering or baking).  Also I like that I can adjust the exhaust by making the gap wider or narrower (again based on the amount of heat I’m using/need).  I use the 4″ tall screen as it easily fit inside my smallest pot (0.9l evernew).  When I’m using a stove that raises the pot too much above the screen (i.e my kovia spider) I have a small piece of aluminum that allows me to extend the Vari-vent so the walls of my pot are always covered (Except for where the handles go).  When I’m using my alcohol stove this extension is not needed.

[Eric Blumensaadt]

Mario,

Titanium pots & skillets do not spread heat well (center hot spots) when compared to anodized aluminum cookware. Plus the expense and weight are just not worth it – IMHO, that is.

Everything else you’re doing with wind screens, lids and cozys looks correct for heat conservation.

There is so mych physics here that I think I have to take a break from this thread because my head is full. :o)

 

 

[Mario Caceres]

Good point Eric, aluminum is supposed to transfer heat better than Ti.  So perhaps for this setup, I would be better off with an aluminum pot and a ti windscreen.  I do like quite a bit the adjustability of the air intake holes of the Vari-Vent Windscreen, which is possible because the aluminum foil the windscreen is made off is very malleable.  I’ve never worked with Titanium foil.  I wonder if the stuff that Titanium Goat sells is malleable, so i can make myself my own Vari-Vent windscreen out of titanium foil.

[David Thomas]

Jerry:  Did you put the folds in your windscreen yourself?  How?

Or can anyone think of a sheet metal product that is pre-crinkled / corrugated on that scale?  I’m debating about throwing every HX trick at a cheap pot and comparing to the unmodified version.

[David Thomas]

Regarding the energy required to heat up the HX fins – it’s a pretty small contribution.

A liter of water = 1000 grams.  If you could somehow heat it from 0C to 100C with no pot and no losses, you’d need 397 BTUs.

A 6-ounce, 170-gram, 1.3-liter aluminum pot needs an additional 15 BTUs (4% more).

A 5-ounce, 141-gram, 1.3-liter titanium pot needs an additional 7 BTUs (2% more).

If the HX fins add an ounce, even two ounces, they’d only add 1% to the BTUs needed.

Water is heavy.  Our pots are light.  And water’s heat capacity is 1.0 versus 0.2-0.25 for most solids (titanium is even lower).

[David Thomas]

GGG X has to include a water-heating efficiency contest.

[Mario Caceres]

David / Jerry,  I wonder which is better, to have corrugated channels or just one uniform gap around the pot.  I would think the heat will rise quicker thru the corrugated channels and also there is less volume of “hot air” in contact with the pot (Specially on the MSR version).

If uniform gap between pot and Windscreen is used, I’ve read a 0.25 inch gap is about right, but of course this would depend on how much heat you have in the system (and you want to vent out so doen’t choke the stove).   I have no technical background on this stuff, so just making an observation from what I think.

[Jerry Adams]

upside down view

2.5 x 21 inch piece of aluminum, flashing, maybe 0.01 inch thick

mark lines every 1/2 inch, fold over a wooden block with sharp edge

drill a total of 8 holes, 1/16th inch.  Feed through #18 galvanized wire.  This keeps the wire aligned on the aluminum.  Bend the ends so they loop on each other.  You have to adjust the length so there’s a friction fit on the pot.  This keeps the aluminum assembly on the pot while using.  If it’s tighter maybe there’s better conductivity to the pot.

You can see burn marks inside the pot opposite where the fins touch:

[Jerry Adams]

Mario, if you have a uniform gap then the outside material will get hot and most of the heat will be lost (although some will be convected/radiated back to the pot)

If you have corrugated, then there are places the outside material touches the pot, and heat is conducted better to the pot

[Jerry Adams]

Mario, my folds are 1/2 inch.  So each channel is triangular about 1/2 inch.  So that averages to about 1/4 inch.  That is consistent with your uniform 1/4 inch gap.  I had a different size before that didn’t seem to work as good, but I forget whether that was even bigger or smaller.

With my “home made Schlieren” (shadow in full sunlight) I can see that when I run stove on high, some of the exhaust gasses back up and flow out under the bottom rather than up through the channels, better on low.  If the channels were bigger, like maybe folds at 1 inch, then more of the exhaust gas would flow through the channels?  But maybe smaller channels are better so exhaust gasses are closer to the pot and exchanger material so there’s better transmission of heat?

[Mario Caceres]

Cool.  Thanks Jerry.  Another reason to use titanium for windscreen.  I notice you painted the inside of your ring black, I assume this helps bouncing heat toward the pot.

[Jerry Adams]

I painted the inside of the heat exchanger black to better absorb heat from the exhaust gasses, but I have no idea if that makes any difference

Occasionally I polish the outside to reduce emmisivity so it radiates heat less, that would help but maybe negligible

[Author]

Posts