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Vertical Top Entry Burners in Forges


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I tried to send frosty a PM but his box is full so I thought I would ask here.

On some gas forges such as Frosty's variable volume forge, that I have seen, the burners are mounted vertically with the burners facing directly at the floor of the forge. This would seem to me to have two problems first I would be worried that the Venturi burners would suffer from so much back pressure that they would have trouble operating properly and second, I would think that the heat of the forge would back up into the burner and damage the burner tube. So I am wondering how they make this work and how do they avoid those problems.

I like the concept of the variable volume forge because my static volume forge takes a hell of a lot of fuel to heat because of its thickness cast and its inability to vary its volume. Frosty's design seems easy enough to implement and would allow one to close down the forge tighter and use less fuel when working with smaller work. However, I have found that Venturi burners seem to perform much better when extra air is introduced into the chamber to assist combustion, a sort of a composite blown and Venturi burner in one.

Are there other designs of variable volume forge I should be looking at?

-- Robert

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Not really a reply concerning top burners, but regarding the extra air, etc, if I remember correctly you were inserting air via the second port because if you did it "inline" it was causing the burner to blow itself out due to the velocity?

If you're starting over anyway, I'd recommend that you try a reducer on the end as a turbulent flare. It does a heck of a job as a flame holder in my experiance.

burner_final_fire_high.jpg?w=450&h=337

The stepdown at the end of the burner tube transitioning to threads + threads at the larger output end generate a lot of turbulent flow which locks the flame in place no matter how high the velocity, and most of the combustion takes place within a few inches of the flare, which could be helpful if trying to use a really small chamber to extract all of the heat.

In fact, it's so turbulent that at very low pressures it was actually causing backpressure resulting in stuttering until I tapered the end threads up (barely changed the outermost threads, the innermost mostly gone) using a dremel with a grinding wheel. Now I have great performance over the entire range... at very low pressure (picture not shown) it's like a blue butane lighter... gentle and wavey.


The reason I mention this is that if you can simplify the burner to one tube, that will make the variable dimensional forge much simpler to impliment since you'll only have a single required point. A sidearm burner would probably be the best implimentation since you can hook up a blower (or not) to the air inlet as the case dictates.



Regarding top mount heat, given the pressure of the jet the accumulated heat should be constantly pushing AWAY from the burner and toward the exhaust. The burner inlet should be the coolest place in the forge I'd think. Making sure that you have an exhaust large enough to handle the burner would be important. I've heard some say 4x the inlet diameter(s) to avoid backpressure, some did 7x.

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Thinking about the original question though... you may want to consider a bottom-feed forge. I know that Grant was developing one.


Here's an example of what is known as a "vertical forge."

http://www.alchemyforge.net/smithy.html
http://forums.dfoggknives.com/index.php?showtopic=786


That's basically what I've done with my furnace... input on the bottom , forging ports in the side for a 3" by 5" entrance. If I need more volume, I can put spacers under the lid to lift it up and create a higher and wider entrance.

fujrnace_together.jpg?w=450&h=337

It will never get to HUGE dimensions, but I think it should handle a decent size of item. You generally only need one section of metal hot at a time... you can only work so fast. If you're not concerned with melting metal for casting, then the lower dimension can be much smaller obviously.

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I don't quite understand what you are getting at:

Take a vessel and put a port in the top and a port in the bottom; now block one port and blow into it. Then switch. Why would blowing in the top one have more pressure than blowing in the bottom one?

Now as to heat: the heat load is quite highly based on radiation from the hot forge sides If the forge was designed right that should be pretty equal everywhere save the outlet of the burners and the door/exhaust area.

Where I would worry about top vs other burners is: after you shut the forge down and you no longer have the cool gas keeping everything outside the forge cool and the chimney effect can allow heat from the inside of the forge to rise up into the burners.

Also if there is any exhaust leakage around the burner ports then the hot exhaust can rise up and be recycled by the burner increasing CO output.

I would wonder about using a cast iron fitting *inside* the forge as mine will go above cast iron melting temps.

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My forges are top burner. I have no problem with backpressure because my openings are sized correctly. As to damaging the burner the very simple answer is to simply close the choke. This stops the air flow and things do not get damaged. If you have seen the pics from my other post you will see that I get plenty hot. And I have a fancy paint job on the burner chokes that have not been damaged.

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Sorry, I saw the notice you'd sent me an PM but I don't think I have PM enabled. Maybe I just never figured out how to work it, I know I can't reply to one. You can indeed send me an E-mail through IFI and I'll get back that way.

I'm not sure what's going on with your set up or if you're just asking questions before investing time making a thing. If you're asking questions before making something you aren't familiar with then good for you.

Generally excess back pressure problems are a result of closing up the forge too tightly or poorly tuned naturally aspirated (NA) burners. Most home built NA burners have no venturi in them at all. A bell reducer intake such as Ron Reil uses in his EZ burners are a long way from a venturi. Mike Porter's don't have venturis either and he doesn't alude to them at all.

A "venturi" burner is a misnomer as most NA burners are based on primary pressure induction devices and venturi devices are secondary vacuum induction devices; two whole different breeds of kitty.

There are two basic types of induction burner, the linear inducer such as Ron's burners and the ejecter type such as the "Sidearm" Mike Porter's or my "T" induction burners. In all cases they induce combustion air (the secondary) with a vacuum produced by the fuel being injected at pressure (the primary) into a restricted volume, usually a tube.

A linear is defined as having the primary injector in LINE with the air intake (secondary) while an ejector is defined by having the primary jet at a 90* to the secondary (air intake port or ports)

The easy way to tell is by simply looking up the burner tube, if you're looking out the air intake or a choke plate it's a linear induction devide. If the air intake ports are on the side(s) it's an ejector.

Don't ask me why I'm not good enough at math to understand Bernouli's gas equations (principle) Not even a working handle, but an ejector induction device is about 30% more efficient than a linear inducer so it's MUCH easier to build and tune an ejector yourself you have lot's wider tolerances.

As a working handle, figure a burner's output is a result of the tube's cross section/area. Size vs output changes are exponential. Double the diameter, square the output.

A 1" burner puts out 4x the BTUs of a 1/2" burner and 2x of a 3/4" burner. Fuel use is exponential too.

An ejector will induce about 30% more secondary (air) than a linear so you don't need to be as precise building it, don't really need to flare the tube or add a flare at the output end and it won't be effected as much by breezes in the shop. Properly tuned it doesn't matter what kind of burner you use, a cu/' of propane or natural gas has just so many BTUs.

Getting the burner tuned properly and putting enough fuel into a particular volume will determine how hot it's going to get for x value of refractory specific heat or insulation.

The better the insulation value of a refractory the less fuel necessary to bring it to a given temperature. However the greater the liner's specific heat the greater it's ability to deliver BTUs via infra red radiation to a cooler object. In short while it doesn't take as much fuel to heat Kaowool to high yellow, say 2,700-3,000f a piece of cold iron will suck the heat out of it faster than heavy 3,000f fire brick.

This means if you're doing high production quantity work you may want a hard liner to reduce the heat time in the forge but if you do small quantity you may opt for the higher efficiency of insulation. Of course you can try what I did in my old pipe forge and put a thin hard refractory inner liner inside a Kaowool insulation outer liner.

What decided me on putting the "T" burners in a vertical down orientation on my variable volume forge was discovering any kind of teflon heated over about 450f or so released very toxic fumes. This means NO as in ZERO, NONE at all T tape or paste anywhere NEAR the burner. My mig contact tip jets are screwed into 1/4-28 tapped Inside of the 1/8" MPT end of a 1/8"mptx 1/4" flare fitting.

I use 1/4" copper tubing to supply propane to the burner and it runs from the burner to a manifold a safe cool distance from the forge. The steel and iron fittings of the manifold are sealed with teflon paste or T tape but none of it gets too warm to hold in your hand so no danger.

Heat from the forge, either chimney effect on shut down or exhaust from another burner running doesn't get the burners hot enough to damage either the brass fitting or the copper tube. I've never seen a burner get red outside the forge lid. To prevent excess heat loss through a shut down burner in the same chamber I just block the intakes with a little Kaowool.

I have a couple on the output end of the 3/4" burners on my VV forge. They act as a rudimentary flare and let me dip them in kaolin clay slip to make them last longer before burning up.

A cast iron flare will just melt if your burner's even close to tuned properly. Mike Porter's been getting up to 3,200f from his Generation 5 burners. If I turn the psi up mine will melt steel to a bubbling puddle if I don't keep an eye on it.

Originally I planned on mounting the burners horizontally a couple inches off the floor but changed my mind. Horizontal burners take up more room in the shop but there are benefits I find attractive and may play with.

Vertical up is a definite possibility one of the local guys is exerpimenting with and I'm paying close attention to how well they work.

Please keep us updated on any experiments you try, I'll sponge the results just as happily as I'll pass on what I've learned.

Frosty the Lucky.

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Frosty,

Thanks for taking the time to reply. I have a forge but it is just too big for the small work I am doing as a beginner and thus I am thinking of your plan. I will have to check out your burners and see if I can build one in my shop. I tried a number of atmospheric burners and had trouble keeping them lit in the forge and then when lit they weren't producing complete combustion, instead producing long lazy yellow flames. I also tried a couple of directly blown burners but have had problems with those as well because my flame kept leaping off the burner and flaming out. So I went to a composite after one day accidentally directing my improvised blower (an old hair dryer) into the forge and saw the forge suddenly roar to life.

I dont know how the vertical up burners would work. I would worry about dripping molten flux into the burners but perhaps it could work. I dont suppose you have an opinion on 90 degree directional burners like the two in THIS pdf. The other problem I woud wonder about is back pressure with the burners shooting right at the floor only 3 or 4 inches away. Clearly you resolved that with your tuned burner. I would love to know how.

Anyway, I also was wondering how you cut the holes for your burners. I heard that you ditched the soft brick on top in favor of pleated kaowool packed tight. How tight did you have it packed? How thick is the resulting pleated kaowool and how did you punch holes for the burners through that? Also I noticed you use only a single layer of soft brick for the bottom insulation? Is that enough to make the forge not heat up massively? Also what thickness metal did you use for the frame?

I am thinking of a design like yours, especially the scissor jack adjuster is just brilliant. I thought the bottom frame could be a set of soft brick with a set of hard brick on top which will hold heat and help out with durability. Also I was thinking I could put a ring of soft brick on side around the hard brick for more insulation. Then I had thought of doing the roof as you did of course. I was trying to decide between making the right wall 3" of kaowool with burners sticking through it or putting them vertical as you did. Vertical has its benefits of course as the heat is right on the material but I was wondering about the danger of overheating the burner. A quasi melted brass tub breaking would be depressing to say the least. Currently my atmospheric burner doesnt get that hot at all. It may still be worth it to add a blower tube to introduce more air for special circumstances, what do you think? Finally I thought of coating all the kaowool with ITC-100 to complete the structure.

Anyway, I was wondering what you think of the idea.

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I built burners to Frosty's specifications, and have good performance from them. I do not even let the flare into the forge, instead it is in the insulation, and only enough to protect the shell. I also packed a small strip of kaowool around the flare to prevent flow around it.

I am using one burner in a vertical down configuration. My forge is sized around a common stove firebrick, so about 5x9 for a base, and is a "mailbox" configuration with 1 inch of wool under the brick and on the back wall. I have 2 inches on the sides and top. I use a lose brick as a door. I melt steel easily if I am not paying attention.

The volume is about 150 cubic inches, and I get about 8 hours on a single exchange grill bottle (15# propane), for me that is 4-6 sessions as I rarely get more than 2 hours in the forge at a time.


Don't use the muffler putty.


I originally built a much larger 2 burner forge and could not afford to feed it as it would suck 15# of propane down in less than 3 hours.

Phil

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The burners will work rightside up, upside down, or any angle in between. Static pressure is static pressure, it is equal in all directions within any subsystem. You don't get a gust of wind upward into your attic do you? A gale going up the stairs from the basement?

As for "heat rising," well, that's a common misunderstanding. Cold air displaces hot air resulting in hot air baloons rising. Heat will move from where there is more heat to where there is less heat. (Unless forced to do otherwise.)

Any boyancy of the flame is negligable compared to the pressure in the flare or tube.

I hope this eliminates more confusion than it causes.

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Many thanks to everyone who commented in this thread - it's always good to read about other folks' experiences and developments. Several people have put a great deal of time and development into this subject; I appreciate the willingness to share the knowledge.

I built a couple of two-burner Sandia forges when the plans first came out several years ago. I built them to the drawing specifications and used the recommended materials but neither forge would get up to welding temps. I called Robb Gunter and discussed it over the phone but we were never able to get beyond beyond "bright yellow" - plenty good for forging but no fire welding. That burner design seemed to be easily affected by breezes and tuning problems.

However, it was built so I used this design for several years and worked around the forge welding problem by using coal when necessary. Eventually, I got tired of tuning so I removed the venturi burners and hung a blower on the back but left the top mounted burners in place. It still won't weld but works fine for what I normally do so I've lived with it since then.

One thing I did learn is that although the 2600 Insulboard I used limits the design to a square or angular shape, it allows easy fabrication and is very reflective. The top mounted burners are simply straight 3/4" tubes and dump into the box but I found over time that it was better to allow the flame to hit the reflective surface first because the forge seemed to recover faster when steel was placed inside. That means I currently have only a strip of fire brick at the front of the forge for the material to rest on but the flames are hitting the refractory surface and creating the radiant heat. The Insulboard is eventually eaten up with slag but is easy to replace and will normally last for a couple years of regular use.

If I were to build another square forge, I would pipe the burners in from the side and have the flame pass over the top of the work and use fire brick for the floor. I also think my 3/4 burners are undersized for this particular box so would either build a three burner or increase the tube diameter.

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Hollis, I also built a Sandia forge years ago. A group of us made four together. Three worked similarly to your yours- OK but not great; one of the four for some reason worked much better but it still wouldn't weld. I modified mine with new burners using a venturi throat and a low pressure blower passing air through a different type of heat exchnger to try and still get a recuperative effect. It worked pretty well but when I wore it out and needed a new one I took a slightly different approach. I always liked the rigid board insulation so I worked with that but I wanted to get rid of the flat top. I made a box similar in shape to the Sandia but with a simple pitched roof so the burners would be mounted at an angle and I could have a little more volume too. I had originally planned on buying a couple of T Rexs to put in it and did buy one to try out. Since then though I've developed my own burner that I like better so that's what I'm using now. Two of these will take the new unit to forge welding temperatures. I brought the unit in the attached pictures to Abana Memphis this year where Steve Parker and Phil Cox worked out of it. I beleive Phil did some welding in it in his demo though I never got to see it- he did ask if it was OK to use flux in it. I use a kiln shelf in the botom for that but I'll be switching to a castable floor plate soon.

For what it's worth I als made an up firing furnace for Shelly Thomas to use for twisting her large pipes and it burned finein that orientation; the burners don't particularly care which way they are facing but you can get junk down the up facing tubes if you don't plan a way to protect them so if anyone plans to make such an arrangement be sure the burners are accessable for cleaning.

post-8-025563000 1284830165_thumb.jpg

post-8-070979200 1284830180_thumb.jpg

post-8-031017000 1284830212_thumb.jpg

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Hollis, I also built a Sandia forge years ago. A group of us made four together. Three worked similarly to your yours- OK but not great; one of the four for some reason worked much better but it still wouldn't weld. I modified mine with new burners using a venturi throat and a low pressure blower passing air through a different type of heat exchnger to try and still get a recuperative effect. It worked pretty well but when I wore it out and needed a new one I took a slightly different approach. I always liked the rigid board insulation so I worked with that but I wanted to get rid of the flat top. I made a box similar in shape to the Sandia but with a simple pitched roof so the burners would be mounted at an angle and I could have a little more volume too. I had originally planned on buying a couple of T Rexs to put in it and did buy one to try out. Since then though I've developed my own burner that I like better so that's what I'm using now. Two of these will take the new unit to forge welding temperatures. I brought the unit in the attached pictures to Abana Memphis this year where Steve Parker and Phil Cox worked out of it. I beleive Phil did some welding in it in his demo though I never got to see it- he did ask if it was OK to use flux in it. I use a kiln shelf in the botom for that but I'll be switching to a castable floor plate soon.


Steve,

That's a heckuva good idea. Allows the forge to get some swirl to the chamber but still allows use of the board refractory. When I build my next one (which will be probably be this winter after it cools off a bit) I was also considering using 2" thick board, or double it as you did. Are you using 3/4 or 1" burner pipes?

That's also a mighty handsome box you made out of stainless - should last a while... :)

BTW, I used a rammable product from AP Green for the floor plates. I will have to check my records and see if I can locate the grade but it was a heavy gray cement. I welded 1/2 strips onto a piece of plate and then hammered the material into the form. It needed to dry about a week before being fired and then the firing rate had to be slow or it would spall from internal steam. However, it was almost indestructible after the initial high fire and cool down cycle. I still have pieces of it laying around the shop - and IIRC, the Sandia forge is probably 18-20 years old.

Thx, H
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My experiments with burners mounted under the forge feeding up through the floor is that crud falls in the hole and welds itself in place. If flux is involved its even worse. I have tried the ceramic chips idea but with every refractory I have tried, the chips stick together and to the side of the cavity they sit in. I think refractory material is designed to become soft and a bit gummy at high temps so it can relieve the mechanical stress from thermal expansion.

Welding heat. I find that covering the floor of a forge with refractory rubble can significantly increase the temperature in a gas forge.

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Vertical up burner, just fired up. Get to a good heat in 3 minutes, Runs on a 5 gallon bottle all day without frosting up. Shown without bricking in front:

post-8656-073346900 1284845855_thumb.jpg post-8656-031058200 1284860006_thumb.jpg

Run this one for years. Guess you could put a dam around the burner, but it's never been a problem.

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Steve,

BTW, I used a rammable product from AP Green for the floor plates. I will have to check my records and see if I can locate the grade but it was a heavy gray cement. I welded 1/2 strips onto a piece of plate and then hammered the material into the form. It needed to dry about a week before being fired and then the firing rate had to be slow or it would spall from internal steam. However, it was almost indestructible after the initial high fire and cool down cycle. I still have pieces of it laying around the shop - and IIRC, the Sandia forge is probably 18-20 years old.

Thx, H


H,

That sounds like AP Green's Pyramid Super Airset which they stopped producing somet time ago. That was fantastic stuff much like you described. Could you dig out the name? I'd love to get some.

Thx

Steve,

Thats a beautiful forge!
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Welding heat. I find that covering the floor of a forge with refractory rubble can significantly increase the temperature in a gas forge.


Do you suppose this is from limiting direct contact to the potentially colder floor, or having a layer that recovers heat faster or allows hot gases to circulate under the part? Or all or something else?

Interesting concept

Phil
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Hollis, My burners are made from 1" OD DOM tubing rather than pipe but they would be roughly equivelent to 3/4 pipe burners. I used the doubl elayer of 1" as I already had it rather than buying 2" board. The doors on this unit are stainless but the body was made of plain steel sheet since the first two were just protoypes and test units. I'm just about to do a short run of them in all stainless for some orders. I'd love to have one of the all stainless ones to take to Quad States but I won't so I'll probably bring this one along.

Maddog and Phil, I've heard folks say that the increased surface area of the rubble provides more reflective surface for the IR. I don't know if it's true. I do know that one of the high reflectivity coatings on the refractory cetainly helps and can give some forges the boost they neeed to weld. (ITC 100) for example

Grant, I like the little forge. I think you've shared that before. Is it naturally aspirated or use a blown burner? For Shelly's I used two of my naturally aspirated burners in a similar way shooting up at the edge of half a kaowool lined 20lb cylinder propane. The cylinder was pivoted at one edge to open for loading since she was working with long lengths of 3"+ tubing. I don't have access to a pic right now. I really liked working out of Nathan Roberstons similar forge and plan on playing around some more with the configuration. Of course you're going to obsolete them all if you can get that induction forge cost much lower (grin) .Steve G

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Refractory rubble: It's my guess that the the broken surfaces cause turbulence and promote mixing, the hot sharp edges provide ignition points and the large surface area increases radiative transfer.

I said "cover the floor" but a small pile of refractory chips placed in the right part of the forge can be enough. I should add that the pieces of refractory will tend to stick together and to the forge. If you drop flux on them they will weld together. If you are fussy about that, break them loose while they are still hot.

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Refractory rubble: It's my guess that the the broken surfaces cause turbulence and promote mixing, the hot sharp edges provide ignition points and the large surface area increases radiative transfer.



The flame should already be properly mixed and ignited. But I think the last point is probably the correct one... additional surface area.

Massive increase in surface area to transfer more heat from the air to the insulation rather than being expelled to the outside as exhaust.

Massive increase in surface area to radiate the heat back into the cavity.
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The flame should already be properly mixed and ignited. But I think the last point is probably the correct one... additional surface area.



It's my opinion (just that and no more since I have no scientific data to support it) that mixing propane properly is a problem with induction burners and they never do that good a job. When I use a blown burner, I notice improved performance if I put a mixer in line before the burner. Frosty and others have mentioned that their forges seem to do better when they introduce the gas into the squirrel cage in the blower. Another issue is that propane is hard to crack and even when well mixed it takes time to burn completely. I'd love to hear comments on this topic.
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It's my opinion (just that and no more since I have no scientific data to support it) that mixing propane properly is a problem with induction burners and they never do that good a job. When I use a blown burner, I notice improved performance if I put a mixer in line before the burner. Frosty and others have mentioned that their forges seem to do better when they introduce the gas into the squirrel cage in the blower. Another issue is that propane is hard to crack and even when well mixed it takes time to burn completely. I'd love to hear comments on this topic.


I have also been told that proper mixing is difficult with propane. I'd also wager few of us are able to measure the ideal stoichiometric mix in an empirical way so we do it the old fashioned way and visually look for proper flame characteristics. One of the problems with the first Sandia design was that the burners were deemed too short to mix properly and early versions had a sheet metal spiral placed inside the tube to facilitate mixing. Most people removed them because no real benefit could be determined and I personally suspected a better solution would have been to lengthen the pipe and allow more time for better mixing. Some forges on the market around that same time had more bends added to make the fuel mix better so the inlet pipe made a 180 before going into the firebox. OTOH, I believe one must balance the number of chokes in order to keep the pressure up or burnback becomes a possibility.

The attached pics show what I did to convert a Sandia style forge many years ago. This forge will not weld but it's pretty close at an incandescent heat and will occasionally make a piece spark if it's right under the burner, which tells me that a better fuel mix and/or improved insulation would allow consistent welding temps. Note that I introduced the fuel after the burner but before the manifold, which I felt would allow better mixing. The needle valve also helps control mix and I do adjust it depending on atmospheric conditions because a dry hot summer day or a nice cool damp day causes a noticeable difference.

post-27-080801200 1285033393_thumb.jpg

post-27-043449700 1285033436_thumb.jpg

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It's my opinion (just that and no more since I have no scientific data to support it) that mixing propane properly is a problem with induction burners and they never do that good a job. When I use a blown burner, I notice improved performance if I put a mixer in line before the burner. Frosty and others have mentioned that their forges seem to do better when they introduce the gas into the squirrel cage in the blower. Another issue is that propane is hard to crack and even when well mixed it takes time to burn completely. I'd love to hear comments on this topic.



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.)


burner_final_fire_high.jpg?w=450&h=337


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.
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