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Gear Workshop The Gear Workshop forum is for the discussion of homemade backpacking gear, gear modifications, and repairs.


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  #1  
Old 07-10-2013, 11:02 PM
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GGervin GGervin is offline
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Homemade heat exchangers - beyond the MSR

I have an MSR heat exchanger. I don't happen to have MSR stoves or pots, I just found the heat exchanger on clearance somewhere, and bought it because it was "the right thing to do." (Moral arguments for buying things are always wonderful, if specious...)

I bought it to increase the efficiency of my Coleman Xtreme in snow. (Melting snow to make water takes up an awful lot of fuel, so this is a welcome help.) I use a GSI pot of smaller diameter than anything MSR makes, so agaist MDR's advice, I cut the heat exchanger down, and adapted it to my GSI. Works great for me.

For the other 3 seasons, I carry a much smaller diameter Coleman stainless steel cook pot in my emergency kit/possibles bag. It's become my main boiling pot. But it's very heat inefficient - it's maybe 2/3 taller than it is wide. Because it's so tall, I'm certain I waste fuel when I use it. Also, my Gigabyte stove doesn't hold the pot in place securely, the pot slides too easily. The pot is about 1/2" less in diameter than the standard Gigabyte windscreen. I won't cut the MSR down further, since I still need it for my Xtreme use in snow. My intended solution is to make my own heat exchanger which a) funnels otherwise lost heat up the sides of the pot, and b) keeps the pot from slipping off the stove when it's not completely level (which is most of the time, as all of you know from your own experience with stoves).

I don't want to re-invent the wheel, so I'd like to know if anyone has tried to make their own stove heat exchanger. What materials did you use, and what did you come up with? This question is probably pretty far "out there," so I can only hope someone can answer...

Last edited by GGervin : 07-15-2013 at 01:19 AM.
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  #2  
Old 07-11-2013, 12:26 AM
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Reality Reality is offline
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I'd be interested in seeing some photos of your setup with the heat exchanger, if it's not too much trouble.

Thanks for posting this, GGervin. Interesting topic.

Reality
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  #3  
Old 07-13-2013, 08:03 PM
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GGervin GGervin is offline
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This is going to get a little long. It introduces the idea that fuel efficiency and stove heating efficiency both have to do with stove/pot/windscreen systems, not just the stove itself. I am looking to take a presently inefficient system and improve it considerably. I want to do it by designing a homemade heat exchanger specific to my stove and pot. I don't think I'm on a pipe dream.

Here is the background of my quest:

Years ago I started packing with a GAZ Bleuet 270 that came with a standard windscreen:



Its performance was decent due to the large burner and the windscreen’s ability to reflect a lot of heat upward. But I found Sigg made a special version of the Tourist Elite cookset which came with a windscreen tailor-made for the 270:



You can see how closely the Sigg windscreen nestles to the GAZ windscreen. This windscreen also fit the pot like a glove, and dramatically increased the efficiency of the cookset. The pot fit inside it with no more than 1/4" diameter to spare (the pot pictured is an MSR Blacklite pot, which is the same size as the Tourist's):



The new windscreen simply took a good design and made it truly impressive. Previously, the heat was directed just sort of mostly up by a big burner and small reflector, but now it was almost all directed up by a big, contained reflector that funneled heat up the sides of the pot as well. That significantly decreased wasted heat, and dramatically increased the surface area of the pot that got heat from the stove. By modern standards, this set-up is heavy and very bulky. That helped to doom the otherwise excellent 270/Sigg system. But I've never seen a windscreen that improved a stove/pot combo like this one did.

The pot in the set was designed to be relatively short with wide diameter, and that also helped. Generally, any wide short pot will heat water faster than a tall narrow one.

The efficiency of the custom designed Sigg windscreen didn’t disappear with the Bluet 270. MSR invented a heat exchanger which is kind of like the Sigg Tourist windscreen on steroids. The Sigg windscreen attached to the stove. The MSR heat exchanger attaches to the pot by means of a hook and thumbscrew, so it adjusts to some degree to wide diameter pots. It projects below the pot maybe ½”. That lets it gather heat like the enclosed Sigg did. The MSR then focuses heat on the walls of the pot by means of heat channels (you can see those rib–like channels in the photo). MSR says it increases heating efficiency by 25%, and I’m sure that’s correct. But the MSR h.e. is heavy and very bulky (nearly as bad as the old 270/Sigg combo). You have to need it to want to carry it. It’s also designed to work with wide short pots - and there are a lot of pots on the market now that won't fit it. So the MSR's a great idea, but in my opinion, limited in application.

I adapted an MSR h.e. to use with my Coleman Xtreme and a GSI pot that's narrower than the Blacklite by cutting the MSR h.e. down. I'll add photos and details of that later in this thread. For now, I'll note the cut down MSR works well for that specific pot, but it's still fairly heavy and bulky (and won't work on my Blacklite anymore). In snow where melting water with minimum fuel is needed, the MSR h.e. is desirable, but I never carry it in the other 3 seasons.

The point of all this is something you just don’t hear from retailers or marketing hype: stove efficiency has to do with several factors, not just which stove you buy. How much pot surface area gets heated is a huge issue. Pot geometry, burner design, and whether there is a reflector or a windscreen all affect that. You have to look at it as a system. (The JetBoil is designed around this "system" idea, so it isn't completely foreign - just not as common as it should be.) It's possible to take an efficient stove with a decent reflector and a good windscreen, add a tall narrow pot that only poorly fits the windscreen and stove burner, and make it less efficient. The right windscreen/h.e. combo can in turn increase the efficiency of the stove by making the "wrong" pot fit it better.

Enter my Gigabyte stove and new pot:



I got the Gigabyte because it's a well designed light and compact stove with little carbon monoxide output, and it uses the standard Lindahl valve (which my old GAZ system doesn't). So it's a great 3 season replacement for the Xtreme. Note the windscreen has a lot in common with the old GAZ standard windscreen - which, of course, tempts me to think a Sigg-like attachment could be invented that would take this stove to a new level of efficiency.

But my new pot is a stainless steel one that's very tall and narrow. I like it for two reasons. For one, it fits in my "possibles" bag, which is my emergency/survival kit. That's the only thing the really tall narrow shape has going for it. The second thing I like is that it's stainless steel. I learned on other threads in this forum and elsewhere that emergency campfires can actually get hot enough to melt aluminum. The usual recommendation for survival kits is a stainless pot, so I've got this one. I want to keep using it. It works fine over the Gigabyte as is, but you can see from the photo there's a serious mismatch between the windscreen and pot diameters:



Something else you can't see in this photo is that if the stove is on a slight incline, it's easy for the pot to slide off the stove. The old GAZ 270 had the same problem, but the Sigg windscreen solved it. More food for thought.

After all that, maybe my quest now makes sense. I want to make my own heat exchanger which will better match the diameter of the pot shown above to the diameter of the windscreen. That should direct heat up the sides of the tall pot and offset the inefficiency of the pot geometry. I don't want the h.e. to extend below the pot, so there is no chance of the fuel tank overheating (I will cover that issue later). Finally, I'd like the design of the h.e. to help hold the pot on the stove, so it's less likely it will slip off when it's on an incline (which is almost always).

---------
Fast forward a couple of days...
---------

I’ve now made an initial attempt at a homemade heat exchanger. Here's the first generation:





You can see the new h.e. now fits the pot snugly to the windscreen so it's less likely to slip. It doesn't project below the pot so it can't concentrate heat near the gas tank. It also creates heat "chimneys" that force heat that would otherwise be lost up the sides of the tall pot. All that worked well, and did in fact improve the efficiency of the stove/pot/windscreen system substantially... well... actually too much...



You can see what didn't work well - wrong material and/or thickness for the amount of heat that got trapped by the exchanger design. I chose inexpensive, easily worked aluminum flashing (maybe .0125” thick) to make it from. After about a minute, the aluminum flashing just above the wind ring started glowing, and after another 20 seconds or so, the aluminum actually became transparent. Clearly time to abort that experiment... Water got to 150 degrees in that time, but never boiled.

I was a little iffy on the durability of the aluminum flashing when I got it. (Kept thinking in the back of my mind that I’d heard aluminum pots can melt over a campfire, and this is going to be near fairly intense heat…) I wasn't completely surprised when it proved insufficent. But I was surprised to discover aluminum can become transparent when too hot...

Next up on my experiment will be either different material, or re-designing the h.e. to go outside the windscreen (to increase distance between heat source and h.e.).

I do hope if anyone has made similar experiments, they will share them and I can learn quicker.



Last edited by GGervin : 07-15-2013 at 01:16 AM. Reason: Automerged Doublepost
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  #4  
Old 07-14-2013, 06:41 PM
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arl arl is offline
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The melting point of Aluminum is 1220 degrees Celsius.

Propane burns at somewhere near 1990 degrees Celsius.

If your system is efficient enough I can see some issues with Aluminum holding up. Aluminum is a strange metal at times, I can see where you might get a hot spot and structural failure.
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  #5  
Old 07-15-2013, 12:31 AM
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GGervin GGervin is offline
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After the fact, I saw those numbers too. I will say I did some measurements during the experiment. I have a Harbor Freight Tools infra-red thermometer (as you all know, Harbor Freight Tools is the industry standard in all scientific endeavor, right?). Most of the stove system (stove parts above the tank, burner, etc.) never exceeded 110 degrees F. The aluminum flashing got to 400 degrees F. but no more. That shouldn't be enough to melt. Interesting...

I think the aluminum flashing is probably an alloy, and that probably affected its heating properties. No real advertizing dishonesty, since no one marketed this stuff for backpacking stove heat exchanger design. But an interesting lesson learned.

I also suspect the thinness of the material affected durability. Heating is affected by surface area plus mass, so same surface area with less mass (= thin rather than thick) requires less heat energy before trouble sets in. I'm guessing the right aluminum alloy of the right thickness might still work, but it will be harder to bend into shape... Also heavier and bulkier to pack. Then again, stainless steel or a modified design might solve the problems. We'll just have to see where my musings lead.

Thanks, Arl, for your input. Sure would like to hear others with knowledge or experience weigh in, too.

By the way, I'm using MSR fuel, which is primarily Butane with just enough Propane to work well in cold weather. Haven't looked up Butane burn temps yet, but I wonder if it might be significantly less than pure Propane?

Last edited by GGervin : 07-15-2013 at 12:42 AM.
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