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Monday, December 12, 2011

DIY Alcohol Stoves -- Basic Design Principles

In a pinch, you can use just about anything to make an alcohol "stove" out of.  Really, there are only two criteria: Your stove has to be able to hold a liquid (the alcohol in other words), and your stove can't be made of a flammable material.

What's that? Oh, you want a good stove?  Yeah, well that's just a bit more complicated.  No worries.  Let's talk about a few basics of alcohol stove design.

There are many, many types of alcohol burners, but for the purposes of this post, I'm going to limit the discussion to open type burners made from aluminum cans.  Open burners are relatively easy to make, effective, and widely used, so it's not like we're limiting ourselves too much.
Various alcohol stoves prior to testing
OK, so what constitutes a good stove?  Everyone is going to look at this a little differently, but below are my critiera.  See also the Standards for DIY Alcohol Stoves in the appendix of this post.
1.  Lightweight.  Hey, I gotta carry this thing, right?  And if I'm going to give up the convenience of gas, then I better be getting something out of it -- like a lighter pack.
2.  Efficient.  So, what good is a light stove if it eats fuel like an old 450 V8 Chevrolet engine with a four barrel carburetor?  Who cares if the stove is light if I have to carry four times the fuel?!  The danged thing needs to save  me weight, remember?  I want a stove that will do the job with a minimum amount of fuel. 
3.  Effective.  Yeah, it has to be light.  Yeah, it has to be efficient.  But it has to do the job.  A stove that won't cook doesn't save me anything.  It's got to work. Not negotiable. 
4.  Minimum "fiddle factor."  If I need a degree in mechanical engineering and a Rube Goldberg mindset just to run the thing, then it's not the stove for me.

With me so far?  Sure, we can think of more criteria, but those are the basics.  Let's go with those and see what we can come up with.

Preparing to test another set of alcohol stoves
You can make an alcohol stove out of most anything, but aluminum drink cans are the most commonly used item for DIY alcohol stoves.  They're light, they're easy to get, and there's a free drink inside if you buy the can!  :)

Now here's where it gets tricky.  There are 10,000 ways to slice and dice an aluminum can to make a stove.  There are tons of sites out there on the net that list detailed templates.  I may post something like that at some point, but that's not my purpose today.  What I'd like to do is take a step back and look at a slightly bigger picture in terms of DIY alcohol stove design:  What are some of the basic principles involved?  
Testing various open type burners
OK, so any fool can lop the top off an aluminum can, splash in some alcohol, and light it on fire.  The chances of getting an efficient stove that way are pretty much zero.  For example, take a look at this photo:
An alcohol stove made from 23.5 fluid ounce/695ml drink cans
See those towering, bright yellow flames?  This stove puts out a lot of heat, but it burns through a lot of alcohol quickly.  Can you say "inefficient?"  The first lesson of alcohol stove design is this:  fast stoves are fuel hogs.  The corollary to that is that slow stoves are more efficient.

So, what's the problem with the above stove?  Poor materials?  Poor execution?  Not really.  It's a design issue.  It just burns too darned fast.  So how can we slow it down and make it more efficient?

It's hard to tell in the photo above, but that's a bigger than average stove.  It's made out of 23.5fl oz/695 ml sized cans.  What if we downsized a little bit?  Take a look at this stove. It's made out of 7.5fl oz/222ml sized cans.
An alcohol stove made from 7.5 fluid ounce/222ml drink cans
Much better!  Much smaller flames and a lot less yellow.  So, lesson two:  The width of the burner matters.
So is that the answer?  Just use a small can?  Take a look at this photo:
Various types of alcohol stoves being tested.
Notice that there are three stoves made from the smaller cans.  The two shorter small stoves have bright yellow flames (inefficient), but notice that the taller one in the center of the back row.  Its flames are a nice pale blue -- you can barely see them.  So, lesson three is that the height of the burner is important.

Now, did you notice the brass burner in the photo above?  That's no DIY burner; that's a Trangia burner.  Trangia has been making alcohol burners since at least 1951, and they're pretty darned good burners.  Can we learn anything by looking at a Trangia burner?  Let's compare a Trangia burner to the stove we called inefficient in the first photo above.
An alcohol stove made from 23.5fl oz/695 ml drink cans (left) and a Trangia burner (right)
Well, now what the heck?  The two stoves have roughly the same width at the base, but the Trangia is pretty efficient, and the other stove, well, isn't.  What's the difference?  Take a look at the opening in the Trangia burner.  Notice that it's not full width.  The opening is smaller than that the width of the entire burner.  Lesson four:  Yes, width matters, but the width of the opening is even more important than the width of the entire burner.

OK, now take a look at this photo:
Alcohol stoves of varying designs.
The two stoves in the back row are the proven 12-10 stove from Trail Designs. Note how the opening is smaller than the width of the cans -- just like the Trangia. Hey, maybe we really are on to something here!

Now, take a look at the third stove from the left in the front row. Note how it too has an opening smaller than its width.  It's a little hard to tell from that photo, but that's a very tall stove.  Recall from lesson three that the height of the burner is important. So, let's fire that burner up and see how it works.
Testing a tall stove with a small opening.
Well, we've certainly gotten rid of those nasty yellow flames!  But this thing is a PITA to get started, and the flame is so small that it barely heats water.  It might be fine for simmering, but it won't boil water.  After 20 minutes, my water still hadn't boiled, so I cancelled the test.  This model just isn't practical.

Height makes for a more efficient stove, but too tall and your water never boils.  There is a relationship between height and opening size, so lesson five:  For a practical stove, the optimal ratio between height and opening size is approximately 1:1.

What would a stove with a 1:1 opening size to height ratio look like?  About like this:
An alcohol stove where the opening size is approximately equal to the height of the stove
So, great theory, but does it work in practice?  As a matter of fact that ratio works pretty well.
A 1:1 ratio alcohol stove in use
Yeah, there's a little yellow in the flame, but that's not bad when you compare the size and amount of yellow to the stove in the first photo in this post.  And the flame is a nice middle of the road size.  It's not so small as to be useless, yet it's not so large as to be wasteful.  In my tests with 500ml of approximately 45F/7C water, I was able to bring the water to a boil with about 1/2 fluid ounce (approx. 15ml) of green denatured alcohol.  In terms of efficiency, that's pretty good. 

So review:
1.  Fast stoves are fuel hogs; slow stoves are more efficient.
2.  The width of the burner matters
3.  The height of the burner matters
4.  The width of the opening is even more important than the width of the burner
5.  For a practical stove, the ratio of the width of the opening to the height of the burner is about 1:1.

OK, there you have it, some basic design considerations for making a practical, efficient open type alcohol burner.  There's more to it, such as exactly which template you want to follow, but in the interests of keeping this post shorter than the Encyclopedia Britannica or the New York City phone directory, I'm going to end things here for today.

Thanks for joining me on another Adventure in Stoving,

HJ

Appendix:  Standards for DIY Alcohol Stoves
Below are some numbers for you to benchmark your stoves against.  These are some fairly high standards.  Do not be discouraged if your first couple of stoves don't hit all the numbers. Of course, these numbers are just my thoughts on the subject.  Ultimately, your DIY stove must please you.

Weight:  A lightweight burner should weigh one ounce (28g) or less.  For example, the 12-10 stove from Trail Designs weighs 0.6oz (16g).

Efficiency:  An efficient stove should be able to boil 500ml of 20C water using 15ml of alcohol or less in a plain pot (no heat exchanger in other words) with a lid at 1 Atmosphere (1013 mBar) of pressure.  Use of a windscreen is permitted (recommended actually).  You may vary the height of the pot above the burner as you see fit.  About 2cm to 3cm seems to work well in my experience.  YMMV.

With respect to to the above efficiency standard, atmospheric pressure is hard for most of us to control.  Cut yourself some slack on the atmospheric pressure, but if you live at high altitude on the Tibetan Plateau, just don't crow too much when your stove meets the rest of the criteria.

Effectiveness:   Your burner should consistently be able to bring the water to a boil in 10 minutes or less in a wide variety of conditions, e.g. differing air temperatures, wind velocities (within reason), altitudes, etc.  Stoves that only work on your work bench at home do not qualify.  Your stove should be able to work under typical, real world outdoor conditions.  Generally, alcohol stoves work better when the air temperature is at least 40F/5C (that's actually pretty chilly for most alcohol stoves).  If your stove continues to work well below 40F/5C, then you've got a real winner.

Minimum "fiddle factor:"  Your stove should not require priming and should be able to be fueled from a normal flip top squeeze type bottle in the field without a separate funnel or any other external device.  Your stove should light with a common match in one try, a normal Bic type lighter in about one or two tries, or a typical fire steel within about three tries.  The point being that it shouldn't be a major hassle to get your stove to light.

Related posts and articles:

AcknowledgmentsI am indebted to many people who shared with me ideas about alcohol stove design.  I'm sure I'll forget someone important, but when a debt of thanks is owed, better to remember to thank a few than to thank no one at all.  My thanks to:  James M, James N, Jon F, Kevin B, Tony B, Turkoker, and Zelph.  Particular thanks to Will L. who patiently did the majority of the actual fabrication of the stoves (hey, I'm a blogger not a stove manufacturer.  I'm really more of an "ideas" guy, see?).  :)

31 comments:

  1. Fascinating stuff, I've found similar effects, however there are many other factors that can influence stove performance.

    Is there an inner wall in the final stove?

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  2. Hi, Jim,

    You're right, there are a TON of other factors including but not limited to wind, ground temperature, air temperature, altitude, etc.

    Yes, there is an inner wall in the final version.

    HJ

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  3. man you're blog has done taken off. Nice job HJ!

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  4. Hi, Zé,

    Thanks, bro. Yeah, people are actually reading it. Well, what do you know? ;)

    HJ

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  5. I think "Fast stoves are fuel hogs; slow stoves are more efficient" is a bit too simple.

    A faster stove will actually be more efficient. Imagine a very slow stove that heats the pot slower than it loses heat to the environment, this is super inefficient. The shorter the time we are cooking for the less time there is for heat to be lost to the environment.

    The problem with what you call fast stoves is that they aren't getting enough oxygen so some of the fuel isn't burned, this is what's happening when we see yellow flames. (There is also the problem that if a flame is too large it goes up the side of the pot.)

    The most efficient stove will be a stove that burns at the optimum oxygen to fuel ratio, where all the fuel is being burned properly and we are using all the oxygen we can get our hands on. If the burner has an efficient air to fuel ratio then the more powerful the better as long as it's going to the pot.

    Also any ideas on why the height of the stove makes a difference? I know it can affect the pressure in jet style stoves, where shorter gives more pressure hence bigger jets.

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  6. My measure of heating efficiency is "how much fuel does it take to boil two cups of water." Yes, it is more complex than that, but "how much fuel does it take to boil two cups of water" IS the bottom line.

    Consider this: You have two alcohol stoves. Both *burn* with perfect efficiency, but stove number one burns faster (more heat output per minute) than stove number two. Which stove will require more fuel to boil two cups of water? Stove number one will require more fuel.

    Why? Because only a certain amount of heat can be transferred from the flame to the water per minute.

    Don't get me wrong. Burn efficiency does matter, but overall heating efficiency is the real bottom line, and in terms of overall heating efficiency, a slower stove is generally more efficient unless you get one that is so slow that heat loss to the environment is greater than the heat output of the stove. For most alcohol stoves, burning that slowly just isn't an issue, not even close.

    The height of the burner affects how much air is getting into the burner. A greater height reduces air supply which in turn makes the burner burn more slowly.

    HJ

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  7. Here's an example of two stoves where the faster is more efficient.

    Say we have a pot that can accept 100J of energy per minute and loses 10J per minute. Our water needs 400J to boil. We have a fast stove that gives off 100J of heat per minute, and a slow stove that gives off 50J of heat per minute. And 1ml of fuel contains 10J.

    time to boil (minutes) = (energy required) / (energy in - energy lost)
    fuel used (ml) = total stove output / 10

    using the equations the fuel used by the fast stove would be
    time to boil = 400 / (100 - 10) = 4.44 minutes
    fuel used = 4.44 * 100 / 10 = 44.4ml

    and the fuel used by the slow stove would be
    time to boil = 400 / (50 - 10) = 10 minutes
    fuel used = 10 * 50 / 10 = 50ml

    All the numbers where made up and it is a very simplified example, but I hope you can see why I think a fast stove can be better than a slow stove. The more efficient stove is the one that can output as much heat as the pot can take. If the stove had given off more heat than the stove could take it would also be less efficient.

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  8. Ah. I see what you're saying. And you're right. There are times when a faster stove can be more efficient than a slower stove. I have played with DIY stoves that were so slow that they never even brought the water to a boil. They would go through their entire fuel supply and produce no boil at all. Not efficient.

    However, as a practical matter, relatively few stoves are so slow that this is an issue. As a general, practical rule, slower stoves are *typically* more efficient. Not always, but let's say 95% of the time.

    Clearly, though, you are correct and there are definitely times when a faster stove will be more efficient than a slower stove.

    HJ

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  9. Hi mate,

    Nice roundup of the open alcohol stoves. Any thoughts (or links) on the closed alcohol stoves that are visible in the second picture?

    I'm looking at making some of the stoves mentioned, would be great to read some thoughts on those closed ones.

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  10. Rogier,

    I don't yet have enough testing and investigation to put up a full length post on this type of stove (closed jet/pressurized). However, a couple of observations:
    1. This type always requires priming. See http://adventuresinstoving.blogspot.com/2011/12/priming-and-alcohol-stoves.html for best technique.
    2. This type is actually slightly easier to make than open type burners.
    3. Don't screw the thumb screw all the way down. You'll get a better seal at 3/4 in than all the way in.
    4. This type is a little harder to fuel. A tube added to the pour spout of your fuel bottle goes a long way toward ease of fueling. See the first photo in this post: http://adventuresinstoving.blogspot.com/2011/12/methanol-safe-handling.html

    That's all I've got at this juncture.

    So many stoves, so little time. :)

    HJ

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  11. I bought one of Zelph's Starlyte stoves. It seems pretty decent. I fired it up with 10mL. of 90% Denatured Alcohol and 10% Yellow Heet. It took off right away with a blue flame that had a barely luminous tip. There is no simple way to simmer with it, but I recall seeing something about simmering in some of his posts. I think that I would prefer it without the integral pot stand, but that's the way it comes. The stove flame is easily blown out with a quick breath, so a wind screen will be a neccesity. The windscreen that he includes with the stove is very nice. It's corrugated aluminum that rolls up to a compact package. Overall, I'm quite pleased with it. I'll have to boil some water and see how it works.

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  12. Hi, Bill,

    The Starlyte has a very good reputation. Let me know how it goes if you get a chance to give it a spin.

    HJ

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  13. I've got all of the stuff to build one and I did just that over the weekend. I used glass wool as the filler and the thing actually worked. This little stove has some real advantages as a lightweight, compact way to boil water, but it has some drawbacks. Since the alcohol is absorbed by the filler, you can't pour it out if you don't use all of it. With the integral pot stand, you can't easily cap the stove to keep the fuel from evaporating. You will need to bag the stove or find a canister with a tight fitting lid to store it in. The canister would be additional weight to carry. I think that the best use of this type of stove would be for a day hike. Just something to boil water for coffee or noodles. With a 2 Oz. fuel bottle, it would still almost fit in your shirt pocket.

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  14. Hi, Bill,

    Yes! Alcohol stoves make great little shirt pocket stoves if you're just out on a short day hike or what have you.

    The trick with fuel is to learn how much fuel your stove takes to do the normal tasks of cooking that you do. You then bring a little measuring cup and measure out just that amount of fuel. You let the stove burn itself out, and voila! no leftover alcohol.

    HJ

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  15. That's why I fueled it with 10 mL. I got right at 10 minutes of pretty constant flame and then about 3 minutes of diminishing flame before it went out. It burns about 1 mL per minute. Now I just need to find out how much water it will boil.

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  16. Yeah, definitely play with it. You'll just have to find out what "X" amount of alcohol can do in your particular stove.

    HJ

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  17. Hi, I notices that what matters is not the shape or ratio itself, but instead, the amount of heat that feeds back the stove for the alcohol to volatilize. And thats why you have to pay atention to materials and hole/jets placing... :)
    Excelent article!!

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  18. Hmm. Well, that's a little different from my experience, but if it's working for you, great.

    HJ

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  19. I agree with Emiliano as far as height goes at least. Having flames too close to the alcohol will cause it to burn hotter - too far and it will not burn hot enough. You need to find the sweet spot where the thermal feedback is just right. Of course, it will vary some with the conditions so a person ideally should test at close to same temp they expect, of course.

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  20. Interesting thoughts. That hasn't been my experience. My observation is that it has a lot more to do with airflow than the distance from the fuel to the flame. YMMV.

    HJ

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  21. Airflow obviously is important also. I'm referring to having the same hole configurations but the holes are at different heights. Having them closer to the fuel burns hotter.

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  22. Ah! I see what you mean. Yes, varying the holes will cause different air flow patterns and alter the thermal feedback of the system, both of which will affect burn characteristics.

    HJ

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  23. you know, it could be that the height of the stove matters, because within the inner wall (mixing chamber?) is where the fuel and air mixing takes place. Having more or less room in the mixing chamber may affect combustion at the jet, by making the combustion mixture more or less efficient.

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    Replies
    1. There are a variety of factors (it gets pretty complex), but yes the volume of the "chamber" affects the mix which in turn effects combustion.

      HJ

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  24. Jim,

    Just wanted to drop you a note and say thanks for all the great information. It really helped when I made my first stove.

    http://www.rickacheson.blogspot.com/2012/12/my-first-diy-alcohol-stove.html

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    Replies
    1. Hi, Rick,

      I'm glad you found the post helpful.

      Take care,

      HJ

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  25. I almost burned the barn down. was poring heet in my stove that I thought was burnt out. it shot flaming heet on the table and wall about 5' away. the flames went out without setting anything on fire.

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    1. Whoa! I'm glad that you didn't set anything on fire. Refilling a stove that is supposedly out is a very common accident. Supposedly a pinch of salt in the alcohol will help you see the flame better, but I haven't tried it yet. Thanks.

      HJ

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  26. Nice article. Very interesting.

    For my own use, I have found a 4 jet Groove Stove from a 7.5oz (2" diameter) soda can with the entire center cut out and about 1" high works best for me and my mini Trangia pot and a 2.5" stand . I get 2 cups of water boiled in under 5 mins with a shade over 0.5oz fuel used. I like my tea hot and I like it fast.

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    Replies
    1. Ian, 2 cups in < 5 mins with ~0.5 fl oz is really good (I assume fluid ounces). What fuel are you using? What was the starting temp of the water? What was the ambient temperature?

      HJ

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    ReplyDelete