TLUD - Top Lighting Up Draft Quotes

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zelph
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Re: TLUD - Top Lighting Up Draft Quotes

Postby zelph » Sun Oct 24, 2010 1:22 pm

Our discussions have begun over at HF and the renewed interest in TLUD stoves prompts me to unlock this thread and open it to the public for comments. This is an update of what has transpired recently: (I go by the name "stovemandan" over there because someone else uses my zelph name. :roll:

Terry_Dodson 10-08-2010 14:43
Stovemandan any more work on the MK1 or maybe a folding version yet? :D

Stovemandan 10-08-2010 16:11
Quote:
Originally Posted by Terry_Dodson (Post 333076)
Stovemandan any more work on the MK1 or maybe a folding version yet? :D
No good word yet on the MK1 Ultra Light version going into production. I won't be making a folding version soon. Maybe in January when I've get homebound.:D It should work as well as the round Woodgaz.

Now you got me thinking on it:confused: shucks:D maybe in December:lol:

2/3's of this thread has been copied over to bplite.com in the Wood Stove Forum

Stovemandan 10-22-2010 22:02
Quote:
turnerminator

I can't say I agree Dan, my stove is a backpacking size and weight and would not work without an outer wall. I'm not saying that my stove design is perfect, or even close to it but it works for me better than a non fan forced design because its far more powerful.
All things considered, I think that you could be right in your opinion for a natural draft backpacking size stove.

I reckon we should take this onto the tlud thread, to avoid hijacking Fourdogs thread.
If I remember right, your stove is primarily for wood pellets. The "Beaner" is also a wood pellet stove. World Stove suggests starting out with wood pellets.

I think we all agree that pre heating air in a 2" tall space is not going to happen. Once the stove is lit a column of air begins to rise and pulls the air upward all around the stove body, some of the air goes into the holes at the base of the stove as in the BB. The air then travels upward 1.75 inches and exits out the secondary air holes. That's cold air entering and without wood gas. The air disrupts the flames coming off the wood and we then see the classic air pattern that many think is wood gas entering and burning. Once the stove is lit woodgas is given off and ignites and the process begins.Only after a while do we see the secondary air entering, by then the stove is hot and does not need any preheated air. It has been proven that cold air has more oxygen than expanded hot air. The double wall is insignificant. In the case of the BB, we have seen videos of the stove being jam packed with twigs way above the holes where the secondary air is to enter. Actually over the top of the pot support. What good are the secondary air holes in that situation. Most of the time you see same situation on youtube of the double wall stoves made of 1 quart cans. Most being overloaded to the rim of the can.

The fan forced wood burners have not been successful in the 3rd world countries.

Quote:
Redoleary


I would imagine you could get additional flow rate with addational height or with a venturi or both. I don't know if the propeller thing fourdog puts in his stove increase flow when burning wood but it definitely makes a diff when burning alcohol
A friend of mine is sending me one of the Fourdog stoves to test. I think my findings will be that the propeller design is only good for alcohol to create the swirl pattern. Once the stove is loaded with wood the grate will be covered and thus prevent a swirl pattern. The air will hit against the fuel and begin to mix go directly upward. I don't see a swirl to be had at any time during the burn. As ash and charcoal are formed it will not permit the swirl.

Redoleary 10-23-2010 05:37
Quote:
Originally Posted by Stovemandan (Post 340724)
A friend of mine is sending me one of the Fourdog stoves to test. I think my findings will be that the propeller design is only good for alcohol to create the swirl pattern. Once the stove is loaded with wood the grate will be covered and thus prevent a swirl pattern. The air will hit against the fuel and begin to mix go directly upward. I don't see a swirl to be had at any time during the burn. As ash and charcoal are formed it will not permit the swirl.
I agree, the propeller just becomes the bottom grate when wood is in there, but seems to provide the right amount of primary air for a really good burn.
I think I've even tried putting the prop over the fire once it had burned down enough to fit in, I don't think that did anything either... I really don't remember. It must not have or I'd still be doing it.:) I'd have to review an older video to see if the prop made much diff in the alcohol stove... sure looks purdy tho.:thumbup1:

ktstone 10-23-2010 09:43
Quote:
Originally Posted by jaygnar (Post 178249)
Yeah, that's what people mean about the stupid use of complex terminology.
Inverted downdraft = Updraft
Whoever came up with "inverted downdraft" should be pushed down a rubber flight of stairs and spanked soundly for good measure.:D
It's almost like the people were trying to confuse others on purpose.
I too was confused early on by the silly wordsmithery!!
Kinda like "military intelligence," jumbo shrimp," and plastic glass." LOL

turnerminator 10-23-2010 10:46
Quote:
Originally Posted by Stovemandan (Post 340724)
If I remember right, your stove is primarily for wood pellets. The "Beaner" is also a wood pellet stove. World Stove suggests starting out with wood pellets.

I think we all agree that pre heating air in a 2" tall space is not going to happen. Once the stove is lit a column of air begins to rise and pulls the air upward all around the stove body, some of the air goes into the holes at the base of the stove as in the BB. The air then travels upward 1.75 inches and exits out the secondary air holes. That's cold air entering and without wood gas. The air disrupts the flames coming off the wood and we then see the classic air pattern that many think is wood gas entering and burning. Once the stove is lit woodgas is given off and ignites and the process begins.Only after a while do we see the secondary air entering, by then the stove is hot and does not need any preheated air. It has been proven that cold air has more oxygen than expanded hot air. The double wall is insignificant. In the case of the BB, we have seen videos of the stove being jam packed with twigs way above the holes where the secondary air is to enter. Actually over the top of the pot support. What good are the secondary air holes in that situation. Most of the time you see same situation on youtube of the double wall stoves made of 1 quart cans. Most being overloaded to the rim of the can.

The fan forced wood burners have not been successful in the 3rd world countries.



A friend of mine is sending me one of the Fourdog stoves to test. I think my findings will be that the propeller design is only good for alcohol to create the swirl pattern. Once the stove is loaded with wood the grate will be covered and thus prevent a swirl pattern. The air will hit against the fuel and begin to mix go directly upward. I don't see a swirl to be had at any time during the burn. As ash and charcoal are formed it will not permit the swirl.
Its not as simple as hot/cold air though. The temperature of combustion is directly related to the constituents of the gas. Hotter secondary air will increase the flame temperature, providing more heat to the pot. Hotter air also reduces soot and tar.
Colder secondary air will reduce the flame temp, producing heavier tars and soot. Heavier tars have more energy than the higher temp gases, which goes a long way towards explaining why you say that a second wall is uneeded in natural draught stoves.

There is a balancing act going on, and a very complicated one too with a huge number of variables.

Will a small, very hot and clean flame heat the pot any more than a larger, dirtier, cooler one? this is the question.

As for preheating the air, it does happen in the secondary air in my stove and not a small amount either. If I empty the fuel load out half burnt, the air coming out of the top holes is incredibly hot-enough to burn my hands.
Whether this is beneficial or not I am not convinced either way, but I do like cooking over wood and having no soot on the pan.:cool:

Stovemandan 10-23-2010 15:53
Quote:
Its not as simple as hot/cold air though. The temperature of combustion is directly related to the constituents of the gas. Hotter secondary air will increase the flame temperature, providing more heat to the pot. Hotter air also reduces soot and tar.
Colder secondary air will reduce the flame temp, producing heavier tars and soot. Heavier tars have more energy than the higher temp gases, which goes a long way towards explaining why you say that a second wall is uneeded in natural draught stoves.

There is a balancing act going on, and a very complicated one too with a huge number of variables.
I agree with what you say. That information is valuable to the scientific community developing stoves for in house use in 3rd world countries.

I contend that the second wall is insignificant in backpacking stoves. The depth of the fire box of the BB is approximately 1.75 inches. Ryan Jordan of backpackinglight.com says the primary air enters the lower holes and is preheated before it enters the fire box. I have a BB and I know the air enters the lower holes and immediately goes into the underside of the fire box via the grate. That is cold air entering the fire box. The stove is filled to the lower part of the secondary air holes. The tinder is on top and is lit. The little fragile fire begins to grow and an air pattern begins to develope. Primary air is all around the top of the stove providing oxygen for the fuel to burn. A small amount begins to enter beneath the grate and go through the fuel that is vertical stacked(less resistance);) The fire is really beginning to get bigger and has heated the stove well enough to cause the air to be drawn up the wall chamber to the upper "secondary" air holes and starts flowing into the flames to produce the well known pattern that folks are led to believe is wood gas. The wood gas is in the fire box coming off the wood, it's burning, flames are rising and the incoming secondary air mixes into the flames and causes the cool looking jet effect:boggle: I love it.

My first try at the Garlington design was under absolute calm conditions. The flames were clean and the prettiest blue you'd ever want to see. The flames were small and fragile. They were always at a point of "flame lift off" I was expecting it to go out any minute. I used a small soup can to make the stove. I lucked out to have the twigs placed in the stove in a proper maner to allow sufficient air intake from the bottom. One line of holes around the stove approximately 3/4 way up the can.

Here is what Ryan Jordan says about the operation of the BushBuddy:

Usage Guidelines, by Ryan Jordan

Woodfire cooking is an art.

1. I never, ever, use a wind screen with the Bushbuddy Ultra. If you use it in wind, its performance will decline. It's best to locate the stove out of the wind. I have found that the very slightest breeze can improve performance by improving air delivery to the intake holes, but it's almost negligible and the negative impact that wind has upon the flame delivery to the bottom of your pot may be significant.

I put the emphasis in red.

Watch this video of a single wall stove that is top lit. It is loaded with hardwood twigs, vertical stacked. Paper is used as tinder along with split twigs. The paper ash protects the fragile flame under it that has begun. Even though a little breeze is blowing the fire proceeds into a full fledged burn without difficulty. Most all of my recent videos of the Woodgaz stove shows it being lit in a pretty good breeze.



Quote:
but I do like cooking over wood and having no soot on the pan.
That only happens with wood pellets and moose dung:D just kidding about the moose dung

mbiraman 10-23-2010 16:52
Stoveman; i'd be curious to see what results you get with softwood instead of hardwood. Those of us who live out west don't have the hardwood forests you do back east.

turnerminator 10-23-2010 17:53
I agree mostly with what you say Dan. Out in the field there is wind (especially over here) and a good stove should be able to burn in the wind, else it not much use really.

The primary air in a tlud should be relatively cold-if it isn't, the burn will be inconsistent. Its a bad design that preheats the primary air, and the bushbuddy doesn't do this either. It is cold primary air indeed.

My stove will burn blue, with real wood and no soot, but the time needed to chop the wood into nearly perfectly consistent sized pellets make this fuel inpractical to me. Its a PITA. This is one of the reasons I carry a few pellets.
I do have a bushbuddy sized fan tlud that burns vertically stacked twigs, but it still smokes a bit and soots the pot.

As an aside, have you tried a boil test with the pot raised off the mesh slightly, perhaps on a piece of wire bent up from the side of the top of the mesh? This should increase heat transfer and reduce sooting to a certain extent, due to the gases path not being impeded by the mesh as it travels over the bottom of the pot.

I like your design a lot, I think you have solved some problems with it.:)

Sailor 10-23-2010 21:38
You guys are baiting me to add my two cents...
Regards an outer wall, which I've tested non-forced air TLUD w/ and w/o, my belief is that in a hiking size stove, the outer walls benefit is mainly to channel the air flow and therefore provide greater velocity as it arrives at the secondary ports. The secondary air is heated some, but clearly not enough to make a real difference in helping combustion, as is tried in full size home heating stoves. However, that slight heating within the walls also, in addition to the channeling, helps increase the velocity just as a natural thermal. Sticking a computer fan does the same job, and unfortunately, a better job. A windscreen may decrease the forced air effect of the wind blowing and/or restrict air flow, but with my experiments both with wood and alchy stoves, I have noticed a reduction of stove burn when using various windscreens loosely fitted. So a small breeze is just a fan. Lastly, the point of the windscreen is prevent wind from disrupting the exiting flame pattern between stove and pot and to prevent the cold outside air from replacing the rising hot air from around the pot so quickly. In fact, a close, say 1/4" windscreen from top of stove to top of pot significantly increases efficiency (as long as air flow is not restricted).

I am more doubtful today and a year ago that natural draft can achieve the burn rate that forced air gets, considering how much O2 is pumped into the system with a fan. However, I'm still tinkering with shapes of the secondary air channel, including at the very top (tapering, rather than abrupt top) and secondary ports to increase venturi effect, so see what non-forced air improvements can be made.
Ok, that was three cents...Sailor
http://www.woodgaz-stove.com/

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zelph
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Re: TLUD - Top Lighting Up Draft Quotes

Postby zelph » Wed Nov 17, 2010 10:51 pm

You guys have got to read this. It comes from the horses mouth, the big guy that knows stoves, Winiarski him self.

http://www.bioenergylists.org/stoves...rinciples.html

Larry Winiarski's Rocket Stove Principles



Dear Friends,

I was typing up Larry's latest simple stove principles for Aprovecho's

newsletter, "News From Aprovecho", (two to three times a year, $30/year,

describes activity, I'm editing number 59) and thought I'd send it along.

Reflecting on the Rocket I might point out a interesting point: no secondary

air. I've tried adding heated secondary air into the top end of the internal

chimney above the combustion chamber but haven't noticed an improvement in

amount of smoke or in fuel efficiency. I ended up thinking that enough

primary air is left at the top of the combustion zone anyway. Adding air may

just reduce temperatures.
I'll test this further with better equipment.

The Rocket stove is trying to create supportive conditions for complete

initial combustion which seems to pretty much work when the right amount of

fuel is introduced. The added draft created by the insulated chimney above

the fire pulls in lots of air, which like a fan, makes a hotter, vigorous

burn.

----------------------------------------------------------------------

Now I have a question for you, do you think Dr. Winiarski would feel the same way about our DIY "woodgas stoves"

Do you think he would say don't introduce secondary air above the burn chamber?

Did you see him say introducing air may reduce temperatures. That's the big guy's opinion.

I say the double wall is not necessary. I think Larry would agree with me No need to introduce warm secondary air for combustion.
__________________
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Re: TLUD - Top Lighting Up Draft Quotes

Postby sudden » Wed Nov 17, 2010 11:28 pm

"when the right amount of fuel is introduced"

Did he elaborate on this point as well?

Sounds like a very complex topic. Right amount? right type? right diameter? right moisture content?

fwiw, I think you already proved the double wall is not necessary. The added insulation of a rocket stove is different because it keeps the chimney temps up. Wonder what happens when you make a double wall with no holes? Insulation maybe? Cleaner burn? Also have to consider the chimney/combustion chamber length. Gotta give it a chance to burn.
"People are not persuaded by what we say, but rather by what they understand."

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Re: TLUD - Top Lighting Up Draft Quotes

Postby cadyak » Thu Nov 18, 2010 9:48 am

Anyone catch the guy making the woodgas powered car on History Channel a couple of days ago?
The show is called "Apocalypse, Pa"
The guy is a nut but a very determined one.

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zelph
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Re: TLUD - Top Lighting Up Draft Quotes

Postby zelph » Thu Nov 18, 2010 9:54 am

When the Dr title gets attached to a name you can be sure to expect a lot of indepth, highly complex way to do things. There are skeptics out there that perpetuate the idea of a second wall is necessary in a TLUD stove and also having to be insulated. In our world of backpacking size stoves those two building factors are not necessary. We hear time and time again about the weight factor having an effect on our gear. I stumbled across the site yesterday that gave the above info from Winiarski, thought it would be nice to post it. We learn by repetition, at least I do :D

I saw a video yester that showed a DIY insulated can used in making the stove. Only 3" from burning fuel to top of stove. Will 3" heat the stove/flu well enough to make a difference in the performance? I don't think so. Insulating will do well for large commercial units, not little backpacking stoves.

cadyak, I missed that, maybe I can get it on the net latter today. :D
http://www.woodgaz-stove.com/

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Re: TLUD - Top Lighting Up Draft Quotes

Postby zelph » Mon Jan 03, 2011 2:16 pm

I just wanted to store this information in this thread for future use. It's pertinant to single wall v. double wall and the need for secondary air for combustion. It's a good read for TLUD experts.


Theory of Design of a Functional Secondary Air System

The JUCA design does not depend on air-starvation (air-tight) operation so the problems of excessive creosote production and carbon monoxide production do not represent the great problem existing in most wood burners on the market. For this reason, it was unnecessary for us to try to arrange a functioning secondary air system even when that was the current rage. We quietly said for years that it was unlikely that any of the secondary air systems worked very well, if at all. Independent researchers eventually showed that we were right all along. With this preface, we herein will give the reasoning why all existing secondary air systems don't work well; and the design considerations necessary to make a system that does work as intended (again, for AIRTIGHT products where it could be important).

See other sheets of ours for a description of the overall theory of airtight products and that of products like the JUCA (Sheets 128, 120, 310, 314 and others).

Since the lack of enough oxygen in the fire's vicinity is the cause for the incomplete combustion in an air-starvation burner, it would be advantageous to supply air to it later on to complete the combustion. The hitch is that the primary reaction that needs to occur (carbon monoxide plus oxygen gives carbon dioxide) will only occur above about 1200°F. If it didn't happen while it was still in the flame tips, we may have trouble keeping it hot enough for the reaction to go.

Let's consider an example. The actual flame temperature of a wood fire can range from about 900°F to 2500°F. An "average" fire will commonly be around 1900°F. Almost instantly on leaving the flame tip the smoke mixes with other air or smoke, quickly reducing the temperature. The amount of this temperature reduction is dependent on many variables, some of which are not yet fully understood. For argument's sake, let's say it is at 1400°F. In order to permit substantial secondary combustion to occur, it will be necessary to supply a decent amount of secondary combustion air, generally on the order of the primary air supply.

This is necessary so that the statistical probability of CO molecules being able to "bump into" O2 in the hot zone is high, preferably at least 90%. The molecules will only be in this environment a very short time, but we want the great majority of them to have the opportunity to combine with the oxygen atoms. These conditions are mandatory to ensure substantial and consistent secondary combustion over a wide range of firing conditions.

Some currently available products do seem to be able to support secondary combustion SOME OF THE TIME and TO A LIMITED EXTENT. Under optimal conditions maybe 1/3 of the available fuel is recovered. Under most other conditions, less. The amount of air supplied is too small to allow high probability of the CO and O2 reacting. Just do a molal analysis to see the lop-sided proportion of many CO to few O2 molecules. Actually if pure oxygen was fed, it would work fairly well. Air being 80% Nitrogen just reduces the probabilities of reaction.

And it represents more material that must be pre-heated so as not to chill the smoke to below 1200°F. Getting back to our example, if we mix our 1400°F smoke with an equal amount of room temperature secondary air, the resultant temperature of the mixture will be less than 800°F, far less than the necessary 1200°F. No reaction. Poor efficiency. A lot of creosote. A lot of pollution. Bad. You can probably see that you are going to need a source of secondary combustion air at about 1000°F or higher under these conditions. A pre-heater will be necessary to boost the room air to 1000°F.

Unfortunately, there are some conditions of low fire (severely held back) where the smoke itself is under 1200°F within inches of the logs. In that case secondary combustion is almost out of the question. It is ironic that in the situation of a severely suffocated fire that most needs the effect of secondary combustion, it is most difficult to obtain. When the fire is burning relatively freely (and therefore cleanly), that is when it is easiest to initiate secondary combustion.

Again let's get back to the example at hand. We need to pre-heat air to 1000°F. It will be necessary to use a heat exchanger to do this. Some current products have a 6" long tube to pre-heat the air as it passes through. We'll see that this isn't even close to enough exchanger surface. Assuming that the stove consumes 35 CFM of primary air for the fire, we will also need 35 CFM of secondary air as described above. To heat 35 CFM from 70°F to 1000°F will take about 35 x (1000-70) x 0.24 / 28 * 60 or approximately 17000 Btu/hr. The 0.24 is the air's specific heat; the 1/28 is the air's specific volume at the mean temperature; 60 is the number of minutes in an hour.

When we are talking about a unit that is only going to develop 20,000 or 30,000 Btu/hr, you can see that we are going to have to use more than half of the capability for pre-heating. The secondary combustion might add 25% to the output (maybe 7000 Btu/hr) but you use 17000 to do it. A losing proposition. Except for the safety considerations, it would be foolish to consider.

Conventional heat exchange analysis (see other sheets in the 300 series) will give the necessary areas of heat exchange for this pre-heater. We will avoid the math here. A two stage boost heater is most logical and effective here, where the first stage heats the air to (600°F in our example). The necessary area in a 700°F part of the stove for this exchanger is 1.6 sq. ft. The air then passes to the second exchanger to be heated further (to 1000°F) in a hotter part of the firebox right over the flame tips. The necessary area of this exchanger is 1.5 sq. ft., assuming the smoke temp is 1300°F in that part of the firebox.

If the supply tube is 2" in diameter, the first exchanger must be nearly 9 feet long (wrapped around inside the firebox) and then the second will also be about 9 feet long. The secondary combustion air supply would have to pass through a total of 18 feet of specially located heat exchanger to ensure good secondary combustion. There would not be much room left in the firebox in most stoves for any exchangers for USEFUL heat. And remember, even then there are conditions when secondary combustion still won't occur. Is there any wonder why currently available products with a stub tube pre-heater don't work?

Home Page of JUCA

http://mb-soft.com/juca/index.html


When one goes some where they can do one of three things;
-Leave it the same
-Leave it better
-Leave it worse
Also remember no matter where you go you have left a trace some where by how you got there and the gear you use.

fourdog
http://www.woodgaz-stove.com/

sudden
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Re: TLUD - Top Lighting Up Draft Quotes

Postby sudden » Mon Jan 03, 2011 11:47 pm

So when will we see an alcohol afterburner on a woodgas stove? :ugeek:
"People are not persuaded by what we say, but rather by what they understand."

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zelph
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Re: TLUD - Top Lighting Up Draft Quotes

Postby zelph » Thu Jan 06, 2011 5:37 pm

sudden wrote:So when will we see an alcohol afterburner on a woodgas stove? :ugeek:


We have to watch what the trends are.

Recently "fourdog" said his stove and the bushbuddy are not TLUD's.

He says they are "air controlled" stoves.

The above article says secondary ignition won't occur until the temperature is very high (?) I would have to read it to get exact. A temperature that will no occur going up the inner wall of a backpacking size stove.

Everyone that makes a DIY or commercial double wall stove says the air is heated in the double wall to have secondary combustion occur.

Once a fire gets going really well in a stove there is almost complete combustion of fuel. You can see it burning with very little smoke.

Fourdog is now changing his mind about TLUD's

TLUD's have always been identified as being double walled stoves with holes in the upper inside that deliver hot air to have a secondary burn occur.

I have disagreed with that ever since I saw them talking about the DIY quart can stoves on backpackinglight.com.

When I get a chance I'll pick apart the article above to point out the finer parts. That information up there came about in the designing of home heating wood burners. They gave up the idea of introducing secondary air because it does not work.

There is another article written by the designer of the Volcano stove that says he tried to introduce secondary air but thought it to be detrimental to the burning of gasses.
http://www.woodgaz-stove.com/

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zelph
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Re: TLUD - Top Lighting Up Draft Quotes

Postby zelph » Sat Apr 16, 2011 10:58 pm

I'm adding a copy of some info from a recent thread. It's relevant to this thread.

Take a look at these photos and see the flame pattern caused by the air coming in through the top holes.

The pattern looks like what thousands of stove makers say is woodgas brought up between the double wall and pushed into the center of the stove and ignited to make for the most efficient wood burning stoves. It's said by those that sell the Bushbuddy and Bushcooker and whole bunch more out there. They claim 3rd world technology has created these wonderful new stoves.

Well, you can see in the photos that this little stove is a single wall. :D

The second and third photo show how the pot support leg(one piece) is sandwiched over the top edge and then welded.

ImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImageImage

And some additional reading for the TLUD Engineers ;)

I just wanted to store this information in this thread for future use. It's pertinant to single wall v. double wall and the need for secondary air for combustion. It's a good read for TLUD experts.


"Theory of Design of a Functional Secondary Air System"


The JUCA design does not depend on air-starvation (air-tight) operation so the problems of excessive creosote production and carbon monoxide production do not represent the great problem existing in most wood burners on the market. For this reason, it was unnecessary for us to try to arrange a functioning secondary air system even when that was the current rage. We quietly said for years that it was unlikely that any of the secondary air systems worked very well, if at all. Independent researchers eventually showed that we were right all along. With this preface, we herein will give the reasoning why all existing secondary air systems don't work well; and the design considerations necessary to make a system that does work as intended (again, for AIRTIGHT products where it could be important).

See other sheets of ours for a description of the overall theory of airtight products and that of products like the JUCA (Sheets 128, 120, 310, 314 and others).

Since the lack of enough oxygen in the fire's vicinity is the cause for the incomplete combustion in an air-starvation burner, it would be advantageous to supply air to it later on to complete the combustion. The hitch is that the primary reaction that needs to occur (carbon monoxide plus oxygen gives carbon dioxide) will only occur above about 1200°F. If it didn't happen while it was still in the flame tips, we may have trouble keeping it hot enough for the reaction to go.

Let's consider an example. The actual flame temperature of a wood fire can range from about 900°F to 2500°F. An "average" fire will commonly be around 1900°F. Almost instantly on leaving the flame tip the smoke mixes with other air or smoke, quickly reducing the temperature. The amount of this temperature reduction is dependent on many variables, some of which are not yet fully understood. For argument's sake, let's say it is at 1400°F. In order to permit substantial secondary combustion to occur, it will be necessary to supply a decent amount of secondary combustion air, generally on the order of the primary air supply.

This is necessary so that the statistical probability of CO molecules being able to "bump into" O2 in the hot zone is high, preferably at least 90%. The molecules will only be in this environment a very short time, but we want the great majority of them to have the opportunity to combine with the oxygen atoms. These conditions are mandatory to ensure substantial and consistent secondary combustion over a wide range of firing conditions.

Some currently available products do seem to be able to support secondary combustion SOME OF THE TIME and TO A LIMITED EXTENT. Under optimal conditions maybe 1/3 of the available fuel is recovered. Under most other conditions, less. The amount of air supplied is too small to allow high probability of the CO and O2 reacting. Just do a molal analysis to see the lop-sided proportion of many CO to few O2 molecules. Actually if pure oxygen was fed, it would work fairly well. Air being 80% Nitrogen just reduces the probabilities of reaction.

And it represents more material that must be pre-heated so as not to chill the smoke to below 1200°F. Getting back to our example, if we mix our 1400°F smoke with an equal amount of room temperature secondary air, the resultant temperature of the mixture will be less than 800°F, far less than the necessary 1200°F. No reaction. Poor efficiency. A lot of creosote. A lot of pollution. Bad. You can probably see that you are going to need a source of secondary combustion air at about 1000°F or higher under these conditions. A pre-heater will be necessary to boost the room air to 1000°F.

Unfortunately, there are some conditions of low fire (severely held back) where the smoke itself is under 1200°F within inches of the logs. In that case secondary combustion is almost out of the question. It is ironic that in the situation of a severely suffocated fire that most needs the effect of secondary combustion, it is most difficult to obtain. When the fire is burning relatively freely (and therefore cleanly), that is when it is easiest to initiate secondary combustion.

Again let's get back to the example at hand. We need to pre-heat air to 1000°F. It will be necessary to use a heat exchanger to do this. Some current products have a 6" long tube to pre-heat the air as it passes through. We'll see that this isn't even close to enough exchanger surface. Assuming that the stove consumes 35 CFM of primary air for the fire, we will also need 35 CFM of secondary air as described above. To heat 35 CFM from 70°F to 1000°F will take about 35 x (1000-70) x 0.24 / 28 * 60 or approximately 17000 Btu/hr. The 0.24 is the air's specific heat; the 1/28 is the air's specific volume at the mean temperature; 60 is the number of minutes in an hour.

When we are talking about a unit that is only going to develop 20,000 or 30,000 Btu/hr, you can see that we are going to have to use more than half of the capability for pre-heating. The secondary combustion might add 25% to the output (maybe 7000 Btu/hr) but you use 17000 to do it. A losing proposition. Except for the safety considerations, it would be foolish to consider.

Conventional heat exchange analysis (see other sheets in the 300 series) will give the necessary areas of heat exchange for this pre-heater. We will avoid the math here. A two stage boost heater is most logical and effective here, where the first stage heats the air to (600°F in our example). The necessary area in a 700°F part of the stove for this exchanger is 1.6 sq. ft. The air then passes to the second exchanger to be heated further (to 1000°F) in a hotter part of the firebox right over the flame tips. The necessary area of this exchanger is 1.5 sq. ft., assuming the smoke temp is 1300°F in that part of the firebox.

If the supply tube is 2" in diameter, the first exchanger must be nearly 9 feet long (wrapped around inside the firebox) and then the second will also be about 9 feet long. The secondary combustion air supply would have to pass through a total of 18 feet of specially located heat exchanger to ensure good secondary combustion. There would not be much room left in the firebox in most stoves for any exchangers for USEFUL heat. And remember, even then there are conditions when secondary combustion still won't occur. Is there any wonder why currently available products with a stub tube pre-heater don't work?

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zelph
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Re: TLUD - Top Lighting Up Draft Quotes

Postby zelph » Tue Jun 26, 2012 9:55 am

I bumped this thread up for EnglishStu. there is some posts in it made by someone in england that uses wood pellets. I'll search it to see if I can find his name.
http://www.woodgaz-stove.com/


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