Vertical Top Entry Burners in Forges

[kcrucible]

Never tried the rubble method, but don't forget, just putting a brick inside the forge can reduce the volume enough to significantly increase temps.

It also signifigantly increases surface area. :)

[maddog]

I hope this doesn't amount to hijacking a thread. This is a topic that interests me. I sure would like to understand this whole issue better.

I live at 7000' and my experience with induction burners is that at high temps they just dont perform as well as blown systems. If I run my forge with an induction burner at or near welding heat I always get a lot of scaling. These burners seem to run hottest when they are set lean which is not ideal for the steel. But I also see conditions where I clearly have a rich burn and the iron is still scaling rapidly in the forge. This means that there is both free oxygen and unburnt propane in the forge. Frankly, I am skeptical of the technique of judging the burn by its color and shape.

Propane is hard to mix and hard to crack. Which means that even if you have perfect mixing, it takes time for the fuel to combust completely. This is not likely to happen in the short time it spends in the flame front. Various strategies are used to overcome this, preheating the propane, precombustion, recombination and flameless combustion etc. None of these can be implemented in an induction burner.

Inducing turbulence at the end of the burner by means of a threaded flare (great looking flame btw), bends in the burner tube or refractory rubble to improve performance shows that the induction tube itself does not do the job completely. But any turbulence causes some back pressure and thus reduces the maximum power output of the burner. In the canonical burner designs, the nozzle is smooth. It is designed to reduce turbulence and preserve laminar flow which is considered necessary to get full performance from the burner and also to achieve flame stability without a flame holder, another source of back pressure.

I have found that when I finally get enough mixing and ignition surface to get an environment that's kind to the metal, the burner output has been significantly reduced. And the back pressure also affects the turndown ratio. I have to run higher pressure just to keep the gas flowing to cool the tube.

These limitations are probably more severe in my shop because of the altitude. But I am convinced that induction burners, while very useful, are not an optimal design. From an engineering design point of view its a serious weakness that you cant change the pressure of the airflow without increasing the amount of fuel you put in the system. Systems like this tend to have a rather small sweet spot and if that isnt where you need it... too bad. This is partly why carburettors have been replaced by fuel injection systems.

Grant once suggested an interesting idea of waving a piece of copper wire around in the forge chamber and watching the oxide colors to judge the balance of the burn. Unfortunately, when I try this at welding temps, I cant read the colors before the wire glows red and is about to melt.

[kcrucible]

I live at 7000' and my experience with induction burners is that at high temps they just dont perform as well as blown systems.

You and Robert Simmons have the same problem... an oxygen starved atmosphere.

But I also see conditions where I clearly have a rich burn and the iron is still scaling rapidly in the forge. This means that there is both free oxygen and unburnt propane in the forge. Frankly, I am skeptical of the technique of judging the burn by its color and shape.

Hmm. Given a scarcity of oxygen in a given volume I can see the problem of introducing the propane molecules to the oxygen so that combustion can occur.

Propane is hard to mix and hard to crack. Which means that even if you have perfect mixing, it takes time for the fuel to combust completely. This is not likely to happen in the short time it spends in the flame front. Various strategies are used to overcome this, preheating the propane, precombustion, recombination and flameless combustion etc. None of these can be implemented in an induction burner.

I'm pretty sure this problem is directly related to your oxygen content. It's a special case. The longer it takes to introduce all of the propane to the proper number of oxygen molecules the longer it takes to burn. The higher the gas velocity, the shorter amount of time that it has to combust completely. It's a problem that isn't generally addressed by the atmospheric burner community because most don't live in that environment.

I had found an article describing a home-heating adjustment for high altitude use (posted a link in the forums as a reply to Robert Simmons, but can't find it anymore.. youre welcome to look Basically they reduce the nozzle size to limit the amount of fuel to match the oxygen content. The smaller jet will be moving faster for a given quantity of propane, which will have the ability to generate a better vacume to pull in more air too (assuming sufficient intake area... I had to increase my area signifigantly from my initial build.) Note that for this, the tube dimensions stay the same, so that fuel/air speed is slower on exit, providing more time to fully combust.

Inducing turbulence at the end of the burner by means of a threaded flare (great looking flame btw), bends in the burner tube or refractory rubble to improve performance shows that the induction tube itself does not do the job completely. But any turbulence causes some back pressure and thus reduces the maximum power output of the burner.

That can be adjusted by other means as I indicated above, and the tube can be lengthened to promote mixing. Any reduction in maximum power comes via increased efficiency by getting a better burn. If that means that you need 2 burners to get enough into the chamber at that altitude, that's just the way it is.

I remember driving a "lowlands-tuned-car" over the continental divide. It was having a MUCH rougher time of it, barely crawling, than a local car that was tuned for higher altitude This is a general combustion problem, not just with regards to forges.

In the canonical burner designs, the nozzle is smooth. It is designed to reduce turbulence and preserve laminar flow which is considered necessary to get full performance from the burner and also to achieve flame stability without a flame holder, another source of back pressure.

The canonical burner design isn't designed to be used in the rockey mountains. The "rules of thumb" are incorrect for the environment.

These limitations are probably more severe in my shop because of the altitude. But I am convinced that induction burners, while very useful, are not an optimal design. From an engineering design point of view its a serious weakness that you cant change the pressure of the airflow without increasing the amount of fuel you put in the system.

Ah, but that isn't actually true. Smaller mig tips will put the same quantity of propane into the forge with higher pressure, and larger tips will put the same quantity of propane into the forge with lower pressure. Smaller tube diameters will increase pressure, larger tubes will decrease pressure. The "flare" that I use increases from 1" to 1.25" in a short distance (much faster than the 1:12 ratio generally suggested) and the air at the edges hit the threads and generate turbulance which migrates inward to lesser and lesser degrees. I like to imagine it as causing the turbulant air to be forming the proper 1:12 ratio. Sure, tuning in fixed hardware is not instantly variable, but your environment doesn't change rapidly. Once you tune the burner for your area then it doesn't need to change.

The "rules-of-thumb" were designed for sea-level to plains. The charts I've seen give a range of tip sizes for a given tube diameter to accomodate altitude no doubt. Being at sea level, I used the largest tip. If I was signifigantly inland I might use the smaller listed. If I were at high altitude I'd consider dropping back to the top end of the smaller burner.

I like the reducing flare as a way to promote mixing and generating a stable flame. You may want to try it.

[maddog]

Of course I adjust my burner for atitude. I am using a 023 tip in a 3/4" burner. It isn't practical to swap out tips in the middle of forging to run the burner in different temperature ranges. A blown burner doesnt have this problem. Yes there is less oxygen available and I still have unburnt fuel and free oxygen in my forge. Tube velocity may be higher in my case because of the smaller jet but once the mix enters the forge and expands, the speed is not much different.

Altitude only accounts for some of this. The difference in air pressure is about 20% which is considerably less than the variations between different burner designs. A design that cant tolerate a perturbation of this magnitude is "brittle".

It is pretty much an established fact in the combustion industry that you cant expect to get complete combustion of any gas in a flame front. There isnt time and propane is more difficult than most other fuel gases.

[Robert Simmons]

For high altitude I found that it works to introduce supplementary oxygen directly into the chamber via a blower rather than building a burner that is a blown burner itself. This completes combustion and almost all gasses are consumed inside the forge. You might try it. I am thinking of building a variable volume forge something like the one frosty has but I will be supplementing the burner with extra air flow to increase temperature and complete the burn.

[Nakedanvil - Grant Sarver]

For high altitude I found that it works to introduce supplementary oxygen directly into the chamber via a blower rather than building a burner that is a blown burner itself. This completes combustion and almost all gasses are consumed inside the forge. You might try it. I am thinking of building a variable volume forge something like the one frosty has but I will be supplementing the burner with extra air flow to increase temperature and complete the burn.

Many people have no problem with "standard" burners at high elevations. I think you're drawing conclusions with precious little data. If I remember, you were unable to get regular burners to work at all. "Grain of salt" time, I'd say.

[kcrucible]

It is pretty much an established fact in the combustion industry that you cant expect to get complete combustion of any gas in a flame front. There isnt time and propane is more difficult than most other fuel gases.

It doesn't have to combust solely in the flame front, just before it leaves the forge (and ideally before the oxygen gets to your metal.)

I dunno.. it just seems to me that adding a blower onto the burner in order to increase the amount of oxygen increases the speed of the gas, which will make it even harder to combust in time.


Hmmm. I wonder if a cast heating-manifold would be a good solution? Instead of the fuel injecting into the gorge cavity, it gets pushed along a cylindrical coiled cavity lining the forge before dumping into the cavity (or even just exhausting at that point if there's not much more heat to be gained.)

In that scenario, the oxygen is static or comsumed within the chamber. You'd need a blower if the coil was long to keep things moving, unless it was a vertical forge and you could rely on chimney action.

Ideally you'd make it out of a very-conductive, but high-heat material

[Robert Simmons]

Many people have no problem with "standard" burners at high elevations. I think you're drawing conclusions with precious little data. If I remember, you were unable to get regular burners to work at all. "Grain of salt" time, I'd say.

I was able to get them working outside the forge but inside was a different matter. I could be wrong on this or many things. I am not a mechanical engineer. I am only reporting what I observed.

[Nakedanvil - Grant Sarver]

I was able to get them working outside the forge but inside was a different matter. I could be wrong on this or many things. I am not a mechanical engineer. I am only reporting what I observed.

I'm not an engineer either. By nature, we form conclusions from the observations we have. Not saying you're right or wrong, just cautioning against drawing conclusions from just one observed case. The fact that it wouldn't work in the forge indicates that there may have been more going on.

[HWooldridge]

My "flare" seems to go a great job of mixing and slowing right at the tail of the burner. So much gets burnt right then and there that there's not that much left to burn after the fact. Huge turbulance inducer via the threads at the end that could be used by pretty much any burner. (I actually ground them down somewhat to get smoother operation at low pressures.)





Because the turbulance is generated at a much larger diameter than the burner tube itself, no signifigant backpressure is created. When I tried a flame holder at burner tube diameter bad things happened.

Just out of curiosity, have you ever tried putting a 90 degree fitting on the end of the burner (so that the burner resembles a typical torch profile)? Seems to me that would induce even more turbulence.

[maddog]

It doesn't have to combust solely in the flame front, just before it leaves the forge (and ideally before the oxygen gets to your metal.)

I dunno.. it just seems to me that adding a blower onto the burner in order to increase the amount of oxygen increases the speed of the gas, which will make it even harder to combust in time.


Hmmm. I wonder if a cast heating-manifold would be a good solution? Instead of the fuel injecting into the gorge cavity, it gets pushed along a cylindrical coiled cavity lining the forge before dumping into the cavity (or even just exhausting at that point if there's not much more heat to be gained.)

In that scenario, the oxygen is static or comsumed within the chamber. You'd need a blower if the coil was long to keep things moving, unless it was a vertical forge and you could rely on chimney action.

Ideally you'd make it out of a very-conductive, but high-heat material

Yes that's the key, the combustion has to be complete before the steel is exposed to the hot gas. The main advantage of a blower in this respect is that you can push the air/fuel mix through some kind of baffle and not worry about the back pressure. Im not sure what a "combustion manifold" is, but it sounds like the same idea. Is there a drawing of this kind of setup? I have experimented with various kinds of combustion baffles, forcing the mix through a pile of refractory chips, a block of porous refractory or a labyrinth and they are all effective at improving the combustion but have other practical problems of their own.

My next mad scientist project is going to be a ribbon burner based on the Pine Ridge design. There is a very interesting thread in this forum where prburner explains his design. It looks like a very promising idea.

Forges are a particularly demanding application for burners. Industrial furnace, kilns etc have large chambers for mixing, they ramp up to temperature slowly and there is less concern about the composition of the atmosphere. Some furnaces even control their temperature by forcing excess air into the chamber for cooling. Forges by comparison are small, required to reach very high temps (higher than most other applications), need to be able to change temperature very rapidly and all this while maintaining a benign atmosphere.

Robert, I've heard of other people doing the same. I think Frosty once mentioned it. But for myself, I dont want to have to feed pure oxygen into my forge. Certainly not when I can use a forced air system and get good results. I use atmospheric burners as auxilaries for heating jobs around the shop that cant go into the forge or for the Raku kiln that my lady wants to set up in the back yard. I made this last burner so that I can align the hinge on a 5" leg vise.

[kcrucible]

Im not sure what a "combustion manifold" is, but it sounds like the same idea. Is there a drawing of this kind of setup?

Well, I just came up with the idea in my head. I'm sure that someone else has had the same idea and there are drawings out there.


Have you ever seen a picture of a kiln with embedded electric elements, so that the wire isn't exposed to the interior of the chamber?

Think of the same thing, but instead of nichrome wire, it's actually a coiled hollow tube through which the combustion gasses flow. Around and around, heating the chamber sides without ever actually expelling the gas into the chamber (or if it does, only at the very end after which no more oxygen should be present.) So the time for combustion is huge, and it has a LOT of time to extract the energy into the forge.


I'm thinking you could make it be taking those long skinny balloons and taping it in a coil along the inside of a refractory barrel. Insert an inner form leaving about an inch of gap and pour in non-insulative castable. Wehn it hardens, pop the balloon. Now you have a coiled chamber to pass your gas through that doesn't reach the metal at all. If you want to pop into the chamber at the end instead of outside, you can do that too to extract just a bit more heat and keep the atmosphere non-oxygenated (the normal oxygen-air gets pushed out from the fully combusted air.


Would be a bit challening to make but seems pretty cool to me. Obviously you'd want the castable and wool to be nice and thick on the OUTSIDE of the coil.

[HWooldridge]

Well, I just came up with the idea in my head. I'm sure that someone else has had the same idea and there are drawings out there.


Have you ever seen a picture of a kiln with embedded electric elements, so that the wire isn't exposed to the interior of the chamber?

Think of the same thing, but instead of nichrome wire, it's actually a coiled hollow tube through which the combustion gasses flow. Around and around, heating the chamber sides without ever actually expelling the gas into the chamber (or if it does, only at the very end after which no more oxygen should be present.) So the time for combustion is huge, and it has a LOT of time to extract the energy into the forge.


Would be a bit challening to make but seems pretty cool to me. Obviously you'd want the castable and wool to be nice and thick on the OUTSIDE of the coil.

You are basically describing a "muffle furnace" - here is a Wiki link description:

http://en.wikipedia.org/wiki/Muffle

[Nakedanvil - Grant Sarver]

There's a lengthy discussion on ribbon burner forges over here: RIBBON BURNERS

[maddog]

Well, I just came up with the idea in my head. I'm sure that someone else has had the same idea and there are drawings out there.

Yes this idea is used in furnace combustors though I dont think Ive ever heard of one being cast around a sausage balloon!. The air/fuel mix is forced through some kind of labyrinth to expose it to as much surface area as possible. Sometimes there is a parallel channel for the incoming unburnt mix which is preheated by heat exchange through the walls. I think the "muffle furnace" is a special version in which the combustion gases are kept separate from the work.

Closely related ideas are "porous matrix" combustors, where the gas is forced through a porous block of refractory and perforated refractory burners where the combustion mix is split up into a multitude of small channels, like ribbon burners. Then there are various recirculating burners which draw some of the hot combustion gas back into the unburnt mix for pre heating. P-Tube burners are a version of this idea that I find particularly interesting.

I've made a "combustion manifold" burner pretty much along the lines of kcrucible's idea. When I fired it up, it oscillated at about 50bps. It sounded like a small briggs and stratton running without a muffler. The vibration shook the refractory structure apart before it got up to heat. I realized I had no way of analysing this design for resonant frequencies and I was out of my depth.

Next I tried forcing the air through a pile of refractory chips. Much like the ceramic chip forge idea. This worked great, it burnt white hot and very clean. But at that temp, the chips welded together and then began to slump into a fused lump of something that looked like refractory vomit. I tried different refractories like mizzou plus, greencast plus. Sooner or later the same thing happened.

The last thing I tried was making perforated blocks, much like a ribbon burner though I hadn't heard of those at the time. The blocks held together well enough and burned great but the back pressure required to force the air through the block found its way through small cracks in the refractory housing that held the block and the plenum chamber. Once this happens, the crack continues to grow and flames start appearing at various points around the back of the burner.

The ribbon burner design which is now being used by some smiths in their forges looks like it could solve all these problems. A particularly nice version is the Pine Ridge Burner. In this design, the housing is made of steel and kept cool by a deep barrier of refractory and by air flow through the narrow steel tubes.

I really have no idea what the hell I am doing, but I love messing with this stuff.

PS: Grant thats a very interesting link. I am reading it carefully. Thanks

[kcrucible]

Quick reply. Been thinking a bit more about this. The Vertical Forge design moves the burner far away from the metal, providing more time to mix and combust. More surface area to absorb/reradiate the heat along the gas path keeps the sides from doing your rubble-melt bit. A rough wall surface introduces additional drag on its way to the metal/exhaust so provides more time too.

[Frosty]

Wow, this thread has come a long way since last I checked.

I'm not going to try replying to individual points, there are just too many. Outside of what experimenting I've done, most of my info comes from commercial induction devices a good friend of mine was researching some years ago. He gave me a box of papers, etc. etc. and I probably still have it somewhere. Also, when the net went public there was a lot more free info and I used to spend a lot of time on the IBM Patent Server, now a pay per view service if it's even still there.

I was starting to play with building an ejector burner when I got to talking to Ron Reil. We brainstormed the things a lot. I pointed out a patent I'd found on the IBM server of a farrier's forge from the 20's or 30's with a naturally aspirated propane burner. Ron based his burners on that one which was very much like Ron't EZ burner. Ron spent a LOT of time experimenting to get it working the way it should.

Ron and I haven't talked about burners for probably 14 years or more. Too many people were E-mailing him with questions he'd answered in depth on his web pages and FAQs. Ron doesn't suffer the lazy well so if a person hadn't even bothered to read the info he sure as heck ain't gonna waste time talking to em. Of course the guys who copped attitude and started bad mouthing him on various sites, lists, etc. didn't do any good so he just stopped talking to people about the things.

Mike Porter doesn't talk about burners to folk anymore either to my knowledge though he can be approached. He did his thing, wrote his book and is more interested in applications.

Enough history, I didn't invent any of this I just like tinkering and talking. A few interesting things have happened over the years though. I stopped messing with ejectors when Ron started messing with linears so I did too. It wasn't till I got tired of trying to get one tuned well that I went back to messing with ejectors. Ron was still experimenting with varied results when I built my first ejector and discovered how much easier they were.

I hadn't talked to Mike Porter till well after he'd written his book and he, Ron and Rex wren't talking to each other anymore. anywho, Mike's burners work nearly as well as a commercial ejector type purpose built propane burner. His design requires some darned precision work though, still they're really good devices.

Okay, some basics. Adjustability. A properly made and tuned induction device induces the same ratio no matter what the altitude. That is for one designed to induce a gas using a high pressure gas as primary. Steam through the jet will induce a higher ratio of intake air, or whatever gas.

A commercially made linear inducer designed for gasses maxes out around 19:1, it will induce 19 parts of air for every part of primary gas. Or 19 parts of air to every part of propane. A neutral mix of air and propane is 17.5:1 so you don't need one quite as efficient as a max capacity commercial linear inducer.

Getting a home built up to a 17.5:1 ratio on the other hand takes some finicky work but once you get it it doesn't matter what your altitude is. The primary pressure being injected into the tube causes a vacuum behind it which induces the secondary, air for a burner. The less dense the atmosphere the less energy it takes to induce so it draws more air for a no difference situation. Properly made they'll work without further tuning from sea level till the air is so thin nothing will burn, somewhere over 30,000'.

Ron lives at 6,200' and uses the same burners he made when he lived in Boise and demoes with from there to San Fransisco to New Mexico.

The burner flare was an interesting misunderstanding. The entire tube from the throat to the outlet should be tapered 1:12 ratio, not just a couple inches at the outlet end. That just flaring the end works says a lot about how simple and insensitive these things are.

What I like about ejector induction type burners is they're a lot more efficient. A commercial one maxes out around 29:1 so a person has a whole lot more leeway building one. Being such robust inducers also means they're a lot less sensitive to back pressure, external breezes and such. Have you ever seen the big vacuum tanker trucks with the big flex hose cleaning storm drains? They're used Ejector type inducers to make vacuum for about 20 years now and they're not terribly large ones either, maybe 3-5" outputs.

I don't use cast iron in the forge at all, so what I use for a sort of flare is a thread protector, they're like a coupler but made by simply tapping some steel pipe instead of cast iron like a proper coupler. Using a bell reducer as a flare is more a flame holder and if your burner is even close to well tuned it'll melt right off.

How a flare works to improve efficiency and as a flame holder is pretty straight forward. When you increase the diameter of the tube's ID the flowing mix needs to expand to fill the increased volume so the pressure behind it drops improving the draw and because the flow has to slow down it's velocity drops blow the rate of combustion so the flame front stays closer to the outlet. It has nothing to do with turbulence, turbulence in the flare is a bad thing and will degrade efficiency even if it does improve mixing some. It doesn't balance out.

The 12:1 ratio is the maximum rate you can change the cross section of flow without causing turbulence. A water nozzle contracts at less than 12:1 and an air nozzle at just a hair under 12:1. Same for making it bigger, expand it more than 12:1 and the turbulence will start degrading the flow.

Mike Porter and Ron both were very much into laminar flow so their designs reflect the preference. In a linear inducer a laminar flow is almost a must. In an ejector it's really difficult to get a laminar flow but it doesn't seem to matter.

I prefer a controlled non-laminar flow to aid mixing so my air intakes are opposite each other while Mike's are staggered. The Sidearm has only one intake and has as non-laminar a flow as you'll find but works nicely.

Refractories. If you're doing a lot of forge welding a high phosphate or phosphate bonded refractory is pretty proof against hot bases like borax. At more than 2,000f Borax is more a caustic than just a base. Pyramid Super A S (Air Set) was a high phosphate refractory with a working range up to nearly 4,500f in an amonia atmosphere. Puff a little 4,000f amonia at a car and it'll go through it like Capt. Kirk shot it with a photon torpedo. Unfortunately Pyramid has been out of business for quite a while and unless someplace has some under a bench somewhere Super A S is unavailable.

There are however other high phosphate or phosphate bonded refractories but you'll have to find them. Please let us know if you do!

An alumina refractory is a lot more resistent to hot flux too. Anything that's silica based is pretty vulnerable, silica is base soluable.

ITC-100 is kaolin clay and zirconia flour though I don't know the ratio. Kaolin is an alumina ceramic good to around 4,000f and zirconis oxide won't even fire under 3,000f or better and doesn't give a fig about hot caustics. It's also a wonderful IR reflecter so more of the heat stays inside the forge chamber instead of heating the liner and shell.

One of our local guys just finished a vertical up fired quanset shaped forge and I'm just waiting to hear how it works or give it a try myself. It isn't as tall as Grant's and I think his burners are mounted at a tangent with vault roof. Once I get pics or hopefully get to check it out I'll get back with a report.

I'm about fried right now and have probably been rambling anyway. I've set my browser to check this thread so hopefully I'll be able to stay on top of it.

Frosty the Lucky.

[pkrankow]

WOW. Thanks Frosty. That is a whole lot of information in one go. Thanks.
Phil

[maddog]

Great post Frosty. How do you set up a choke on your T burner?

[pkrankow]

Great post Frosty. How do you set up a choke on your T burner?

He mentioned previously (last year) using a piece of tape as a temporary choke.

Phil