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Forges 101


Mikey98118

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So, Frosty was saying the other day, that there are things missing from this thread. Naturally, I wondered what, and immediately box forges came to mind. Frosty and I have discussed brick pile forges on this forum, but not on this thread. To my mind, a brick pile forge has the advantage of being made of bricks, and therefore being as changeable as something built of Legos; both for shape and size.

Whereas, the size and shape of what I call a box forge has practical limits tied to your choice of ceramic boards and backing bricks. Comments, disagreements, and/or questions?

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The two-brick forge

The smallest of brick forges is called the two-brick forge because most of them are built from two standard insulated firebricks that have been hollowed out, and a small hole drilled in the side of one of the bricks for a flame from a small air/propane torch to enter. The exhaust gas leaves out the front of the bricks, and the heating stock enters the forge through the same hole.

    I would suggest K26 brick instead; split bricks cemented together with Metrikote, and with their inside surfaces seal coated with it, and the side hole drilled with a hole saw incrusted with carbide grit.

The Mag-Torch MT245C is an excellent propane torch with a low cost; it can be positioned at—not in—the hole opening, where it can induce the secondary air it needs, and avoid being overheated.

Note that the MT245C has a weakness; its gas jet is only a few thousandths diameter, and easily gets plugged up from tars and waxes present in propane fuel gas keep a small glass container and solvent handy to soak it in, along with a compressed air container of the kind used to clean off computer parts, etc. to blow it clear of debris with.

 

 

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Even this smallest of forges can benefit from a baffle wall in front of the exhaust opening. A simple hard clay firebrick with a center-hole drilled through it for the stock  to be passed through will divert heated gas from the blacksmith and help to keep the forge hot.

 

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    A third brick can be used as a convenient forge stand and as further insulation to keep the forge from overheating whatever surface it is placed on.

    One of the reasons foamed clay insulating bricks became the popular choice for these miniature forges is that nearly anything can be used to shape its surface; this is not true of K26 brick. You will need a grinder with grinding wheels meant for use on brick—not steel.

    The flame hole should be drilled toward the back end of the brick, to allow the flame to traverse the length of the forge before exiting out of the front exhaust exit.  

 



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                                                                                       Box Forges

 

 

The size and shape of what I call box forges are usually limited to the builder’s choice of ceramic boards and backing bricks, although these parts can easily be cemented together; thus all such size restrictions are really due to a lack of knowledge or will.

    The burner or burners can be down-facing from the top of the forge or facing horizontally from the side of the forge and positioned near to its top.

All parts on the top of the forge need to be cemented into a solid surface, to withstand the force of gravity. Ceramic boards on the side of the forge should be cemented together, but bricks on the side and bottom of the forge should be allowed to sit trapped in the forge structure, but be not cemented into a solid surface; this allows them to expand and contract during thermal cycling.

     The forge needs a rigid bottom plate to sit on; this may be metal or a double layer of cement board. A framework of angle iron is sufficient to hold the forge parts rigidly in place, although many people prefer sheet metal; if you choose sheet metal, remember to allow joints in its structure to allow movement; otherwise, it will be warped by the forge heat. You might also consider cement board trapped in angle iron, instead of sheet metal for the forge walls.  



 

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Brick pile forges

 

Brick pile forges have the advantage of being reshaped and resized, within the limit of their tops. If the top ends up larger than the rest of the forge, no harm is done; if it ends up to small it can be enlarged by cementing more brick around its borders. Additional burners can be included by use of a hole saw with carbide grit; so much for the good news.

    Use of hard clay firebricks will make for a cold forge and a large fuel bill. Foamed clay insulating bricks were never intended for use as a hot-face; they will rapidly crumble to rubble if used this way. K26 bricks are good to 2600 F, tuff mechanically, and able to withstand the rapid thermal cycling found in forges; They do need to have their hot-face sides sealed with a good high alumina castable refractory, such as Kast-O-lite 30, or with Metrikote.

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Cellular concrete blocks

 

If you have a local source of lightweight insulating cellular concrete blocks, it can make a convenient structural choice for outer walls and subfloors so long as sufficient inner insulating layers separate them from the higher heats of forge interiors.

    However, you need to avoid the kind of cellular concrete that uses plastic based insulation in its formula; it will outgas toxic fumes.
-----------------

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Burner positioning

The most successful commercial box forge features one, two, and three burner models, with the burners on top, and facing vertically down; this position allows fairly narrow forges, with sufficient distance for the flame to mostly combust before hitting the work pieces on their floors; an excellent use of forge space.

Most amateurs don't leave sufficient hight for the flame to combust before impinging heavily on the work, and instead make the forge too wide, with their burners too far forward. All of these problems can be ameliorated by facing the burner or burners on the side of the forge and just below the overhead.

We spend a lot pf time promoting "proper" burner positioning in "proper" forge shapes (tunnel, D, and oval) that can produce a "proper" swirl of forge gases. But the fact of the matter is that high power burners with hard flames tend to create swirl whether the forge shape is round or square; that still leaves burner positioning to do badly, so don't dispare :)

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  • 2 weeks later...

Hi, I'm not sure if this is the place to post a new question, but I've got a relatively simple one.

I want to build a cuboid shaped forge,  about1 foot on each side, but narrower than tall. I have a guy who's going to assemble the metal parts for free, except the gas equipment. I plan to line it with ceramic blanket, sealed with Plibrico Plicast 31 in place of satanite. Satanite is unavailable in Singapore, and prohibitively expensive to  get from America or Australia.

My question is, for such a setup, can I just stick these in the top of the forge, or is there some pitfall I'm missing? If I understand correctly, I need a holder, which is just a ring with 3 screws to hold the burner in place, and the tip of the burner should be somewhere in the ceramic layer.

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I recommend Frosty's "T" burners in this kind of forge, because it is the only really hot flame that is also soft; as in SHORT. So read everything you can about his burners, and listen to what he tells you about how to mount them and tune them.

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Input flame and internal atmosphere in the forge

One 3/4” naturally aspirated burner, which is capable of making a neutral flame, will heat 350 cubic inches of open interior volume to welding temperature in a properly insulated forge (2” thick layer of ceramic fiber or the equivalent insulation in some other form). Add an additional 70 cubic inches for a burner capable of making a neutral flame in a single flame envelope (little to no secondary flame). Such a flame from a burner with a burner entrance port that is set up to control secondary air from being induced into the forge by its flame, and you can add another 35 cubic inches, for a total of 455 cubic inches. Addition of the proper sealing and heat reflective coating will raise forge temperature still further and allow lower fuel gas pressure to be used to gain yellow heat.

    A decent 3/4” inch burner will sufficiently heat a forge built from a five-gallon propane cylinder. Such a design in a 1/2” burner is more than sufficient to heat a forge built from an empty one-gallon non-refillable Freon or helium cylinder. A decent 3/8” burner is more than sufficient to heat a coffee-can forge; a really hot 1/4” burner will heat it sufficiently for welding.

    So why not use cubic volumes to describe all these burner sizes? The listing for coffee-can forges should give you a clue about how much the numbers will vary according to burner design. On the other hand, naturally aspirated burners all have very long turn-down ranges. If you are anxious about using a hot enough burner for your forge, use the next larger burner size, and turn it down. If you personally need to get the burner size just right later on, it’s easy to change the burner out for a smaller one at that time.

   

    When looking at the flame—from a really hot burner—in a cold forge, It will look much as it does out in the open air, but within moments it will lengthen and become smoother in outline, as the forge starts to superheat; it will also lighten in hue to blue-white. There will be very little to no secondary flame within the forge, even while it is cold; lesser burners will make more complicated flame envelopes, but this is the ideal; this holds true for multi-flame burners, just as it does for single flame burners.

    You need to remember that there are at least two different flames going on within the average gas forge; the flame being input by the burner, and the possible output flame leaving the forge via the exhaust opening. When smiths discuss terms like dragon's breath it is the exhaust flame they are speaking of, which is a very different animal than the incoming flames from a burner. Not that both flames aren't equally important, but they need to be treated separately for clarity.

    So, if we are speaking about the burner flame, straight blue from a total primary combustion envelope is desirable, but many older burner designs have a white inner flame ahead of a blue secondary flame, followed by a darker larger and less substantial appearing tertiary  flame of  "secondary combustion"; by that I refer to the combustion of byproducts of the primary flame envelope, which is something of a fiction in this case, for the white inner flame IS actually is the primary flame envelope in this case, and the blue flame is the secondary flame envelope here, so that what is often considered as the secondary flame envelope is actually the third envelope. How to resolve this; just don't go there. Buy or build a good enough burner to see no white in the flame, and then tune it up well enough to have very little secondary flame.

    The next question tends to be "how dark a blue?" Different fuels give off different hues, and lean flames are always darker blue than neutral flames in any given fuel. In fact one burner could be run so lean that the primary flame turned purple from the amount of red that excess superheated oxygen could be included in it. On the other hand, any slightest tinge of green in the flame is an unmistakable sign that it is way too fuel rich; such a flame will be pumping out dangerous amounts of carbon monoxide. The simplest way to judge a neutral flame is that it’s blue is a lighter hue, and it has very little to no secondary flame; any darkening beyond that is from too much oxygen; it is so called lean flame.

    You can also get thin yellow and red streaks in a perfectly tuned burner's flame, due to breakdown products of oxidation from some alloys of stainless steel, mild steel, or cast iron flame retention nozzles. Flame nozzles of #304 stainless can put on quite a show that way; it's harmless. #316 stainless makes fewer streaks and last longer.

    Air/butane flames from so called "blue flame" air-fuel torches are darker blue than from air/propane flames, yet butane pocket lighters started out being set up to make soft yellow flames.

     But isn't the exhaust flame just the tail end of the burner's flame alter all? Yes, it can be just that in a forge that is just loping along, but in a forge turned up into yellow to white heat ranges...NO. In fact the goal is no output flame at all; just clear super-heated flue gases. If you have a forge and burner capable of this kind of performance everything else about the exhaust changes too.

    With the average forge, a small amount of blue exhaust flame is considered normal. In our example of a really hot forge, if you keep turning up the input flame beyond the forge's ability to completely burn it internally, you still won't get blue exhaust flames; some of the forge’s yellow-white “atmosphere” will overflow out of the exhaust, and complete combustion within a few short inches.

    What changed? The forge itself is changing the combustion equation by super-heating the byproducts of the primary combustion envelope. How is this possible, since immediately after combustion, flame temperatures naturally decline? Radiant energy input from the incandescent forge surfaces is being bounced back and forth through the gases.

    If the forge is orange-hot you could consider heat losses in the byproducts to be multiplying faster than radiant energy is being added to them. In yellow to white-hot forges, losses are being subtracted while radiant energy is multiplying gains. It isn't possible to understand internal combustion processes in a modern forge as just a chemical process, because of heat gain from radiant surfaces; such a forge is more oven than furnace.

Burner placement: In most forges the burner is placed at the top. And its flame is flame is aimed directly at the floor or is aimed at it on an angle; the point of this is for the flame to impinge on the toughest surface that can be provided so that other surfaces can be saved from flame impingement. High (purity) alumina kiln shelves are the most effective way to provide a forge floor; this has been the position of choice for decades, but with better wall materials available at reasonable prices, this design limit is eliminated

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Exhaust size and shape

As in so many other matters, NO exhaust size or shape is ever perfect. Therefore V-A-RI-A-B-L-E is the optimal size and shape; all other dimensions can be outright wrong, but never just right. This is one of the many reasons for controlling exhaust with an external baffle wall beyond a larger than needed shell opening; thus allowing the least heat loss through radiation, while maintaining perfect backpressure in the forge.

    One thing backyard casters and blacksmiths both worry over is how large to make the exhaust openings on their equipment. Too small and you have high back pressure killing burner performance; too large and you can't get enough heat to stay in the equipment interior to do your work. Of course, the closer to the "right" opening size your equipment has the stronger the forge or furnace can be built. Just don't get suckered into confusing the right size for the perfect size. As long as burner output can by varied (turn-down range), there can't be any such thing as a perfect exhaust opening size. The right size is what is needed to accommodate the burner's highest output (the highest you are willing to take it to). If you want the best performance at lower gas pressures, they can easily be provided by forge doors that have built-in variable baffle walls. How to do this, unless you already have a r-e-l-i-a-b-l-e figure to start from? You need to make up something with an exhaust hole on it that can be varied in size; like hard firebrick, or with several round kiln shelves with different openings to fit stock through.

    Even should you get the best possible performance from your burner with a given exhaust opening, you are likely to be fighting very poor fuel economy. Try separating exhaust losses from radiant heat losses by putting up a movable barrier of brick, or a changeable high alumina kiln shelf with a center opening. Keep the bricks or shelf at a small distance from the opening, to allow exhaust gases to move up and out, just beyond the baffle, while bouncing radiation off of a re-emissive (heat reflective) coating, and back into your forge. You can move a hinged and latched door holding the baffle wall, closer and farther away from the forge opening, depending on how forcefully you are running the burner, while keeping the stock opening only as large as is needed to move parts through.

    This arrangement helps to slow the flow of expended gas in the forge interior, as it heads toward the exhaust opening; and then speed the gas up through the opening; another highly desirable trade-off, but how exactly is this trick done? As the gas exits through the restricted area of the exhaust opening, its flow speeds up due to buoyancy, much as a river's water speeds up as it approaches the falls. But isn't any opening subject to buoyancy? Yes, but the less its restriction to flow the less flow speed is slowed down in the forge interior, and the less it is sped up at the exit.

    So, you are gaining hang time for the heated gas in the forge, and recuperative savings from bounce back of radiant energy; a win-win situation. A baffle wall helps divert heated gas upward faster than a simple exhaust opening can; it also minimalizes infrared and visible light from impacting your eyes and skin. Both factors improve your health and comfort.

    Is there any improvement to be made over firebrick as a “baffle wall? Yes, but only after you use the forge enough to know what size and shape opening you normally favor: even then you may want to keep the brick on hand for occasional use with unusual parts.

    High alumina kiln shelves are seven times more insulating than hard fire brick; it is also tougher at forge temperatures, which is an important consideration for something you will end up shoving parts back and forth through.

    Using alternate kiln shelves, with different part openings is okay, but building an elaborate system of moving kiln shelf parts to ape the ability of bricks to change their openings comes under the heading of "gilding the Lilly." The additional energy savings it provides probably isn't worth the effort.

    It doesn't take long to bring a typical 3/4" thick high alumina kiln shelf to match every other surface temperature you will find in the rest of the forge. On the other hand, that sure wouldn't hold true if someone is using hard half bricks for the forge floor. Also, you want to coat the floor with one of the re-emissive coating formulas. A modified zirconia silicate of 95% and Veegum ((or bentonite) 5%; this mixture makes a tough re-emissive coating for wear surfaces. There are other choices, but none of them are as economical or easily purchased.

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Floors

If you don’t plan to do much forge welding, laying down a Kast-O-lite 30 floor over ceramic fiber insulation (or K26 insulating brick) will be tough and more insulating than kiln shelves over a Kaowool “pillow”. For occasional welding, a layer of the kind of kitty litter that is made from pure bentonite clay bits can be used to shield the floor from welding flux, but frequent welding is certain to slop flux onto a forge floor; thus a kiln shelf, that is trapped in slightly oversized slots in the forge shell, will pay for itself with the ability to be slid out and replaced, once its top is befouled with glue like flux.

 

Insulation

It only takes a moment's comparison between heat lost through an exhaust opening with heat lost through forge walls to make it clear that just insulating the forge, in order to lower heat loss is a waste of time. You are insulating the forge to super-heat its internal surfaces into high levels of incandescence; at least into yellow, and hopefully into white-hot ranges. An efficient forge is a radiant oven. The burner flame is primarily used to create radiant heat transfer; not for heating stock directly; get that straight in your mind, or give up all hope of knowing what you're doing in forge design. Why? Because every choice you make about refractories, kiln shelves, and ceramic fiber products needs to reflect the need to superheat the forge interior without gutting those materials.

    For many years ceramic fiber insulation in walls and under the floor has consisted of two one-inch thick layers inside curved forge walls, and one-inch layers of ceramic board, with a further 1” layer of ceramic blanket between the board and shell, in box forges.

    K26 insulating firebricks have become a  tougher alternative to ceramic board in box forges and a better alternative to ceramic blanket under floors in tunnel forges; they are use rated to 2600 F, and available from eBay and other online sources; shipping charges are small because the bricks are very light.

     There are several kinds of refractories used for hard firebricks, but only one kind was sold for insulating firebricks, until recently: that was the pinkish to yellowish bricks made by including a foaming agent in clay refractory to make lightweight bricks use rated to 2300 F that you see used all too often in old gas forges, and electric pottery kilns. To call them friable is to completely understate their unsound nature; calling them future rubble is more to the point. Such bricks are meant to be used as secondary insulation in things like electric pottery kilns and pizza ovens.  Both of these tools tend to heat up and cool down slowly during very long thermal cycling; just the opposite of a gas forge.

 

    Nothing stops you from using 2600 F alumina insulating refractory (which is NOT made with a foaming agent and cures into a harder brick) in a simple wooden form to build your own insulating firebricks; ones that will last better. So, why aren't people doing so? The foamed bricks are cheap and easy to procure; they made a brave showing...at first.

    While the strength and durability of various 2300 insulating refractories to make bricks from do vary widely, all of them have a good insulation value in an environment that is at or above 2000 F, but that of a K26 brick equals that of ceramic fiber blanket products. On the other hand, the K26 bricks can provide structural integrity, while the blanket can be shaped into curved forms and then rigidized into featherweight insulation.

    Even the cheapest grade of ceramic fiber blanket doesn't melt below 3200 F. Product temperature ratings come from the level of heat the fiber products will withstand without massive shrinkage; this should illustrate the importance of locking the individual fibers in more secure positions by rigidizing; it also demystifies the seemingly magic protection given by a relatively thin sealing coat of high-temperature refractory, capped by a heat reflecting coating.

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Hot faces

Ceramic fiber products need both rigidizer and finish coatings to do well in today's gas forge; this is because better burner and forge designs create much higher internal temperatures than were common in the past. Rigidizer is especially important if you want your insulation to last. On the other hand, between using 2600 F insulation and rigidizer, you can toughen the secondary insulation layer in your forges enough so that it should stand up well to the heat that will leak past a high emissive coating (AKA heat reflector) and thin hot-face layer (typically Kast-O-lite 3000 or a high-temperature sealant like Metrikote or Plistix). Rigidizer also helps mechanically support a thin coating.

(A) You don't want to use thick ceramic fiber layers; instead of a single 2" thick layer, use two 1" thick layers. Ceramic fiber blanket will easily part into thinner layers via delamination, if you already purchased 2” thick blanket.

(B) Rigidize each layer after installation, and heat cure it, before installing the next layer.

(C) Form the burner openings before rigidizing each layer. Remember to leave burner openings just a little oversize so that they can be finish coated with a hot-face layer.

(D) Dispense the rigidizer from a used cleaner bottle with a spritzer top unto fairly horizontal surfaces, and heat with a burner to set the ceramic fibers in position, before rotating curved surfaces to position further areas for the same treatment.

(E) Silica rigidizer is colloidal silica (just fumed silica, which remains suspended in water) and common everyday food coloring (to allow you to visually judge how far it is penetrating into the fiber); this water born product is easiest to dispense by spritzing. You can always pay through the nose for premixed rigidizer from a pottery supply if you prefer; I buy mine through eBay and get free delivery.

 

Hot faces can be as minimal as re-emissive coatings over ceramic board or rigidized blanket, or as thick as a 1/2” layer of Kast-O-lite 30 refractory. But a seal coating or 5/32" to 3/16” layer of homemade zirconium silicate tile is the superior option, thanks to Tony Hansen’s famous Zircopax formulas on digitalfire.com.

 

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Thanks Mike, this last post is very useful. I'm finding that too much information tends to confuse people who just want to make an efficient safe forge. You and I live for the intricate data that makes this stuff work but most people just want to know enough to make it work.

Nice job of cutting it to the bones for folk. I'm going to tag this last one for future data requests.

Frosty the Lucky.

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The shell

This is where most people start their figuring from; it does more than hold the forge materials to together; it is more important as a handy place to mount burner ports, exhaust doors, and legs on. Furthermore, nothing stops you from using refractory materials to help brace a very thin shell from the inside. The shell needs to be thick enough to provide an easy surface to mount parts too, at a minimum. Between pop rivets and silver soldering, tin cans will work just fine. Of course, a few extra thousands in the shell wall can save you a lot of work (you can employ sheet metal screws or welding), and make the structure more rigid. A 1/4" wall pipe is way too thick; think 3/32" to 5/32” wall thickness; stove pipe is a lot thinner and some guys use it quite happily with sheet metal screws or pop rivets.

 

Note: Aluminum isn’t a good shell choice because it loses its temper at 400 F, becoming quite weak. Most forge shells reach 400 F during normal use.

 

    You can hinge one end of a tunnel or oval forge, or one wall of a box forge, in order to get increased control of parts handling or to make construction and repairs easier. You can hinge the top and bottom halves of a forge together so that it can be used as a tunnel and modified clamshell design.

    Forge shape is a matter of opinion and we all have one; some of us have way too many (yes, I plead guilty as charged). The most popular shape around is the tube forge; probably because so many people have built one, that they have become the proverbial "well-worn path"; they are also out of date for any but knife maker forges. Oval shapes have been around for more than twenty years, and are finally catching on because they are a real improvement on tube forges.

    I suspect that oval forges started out as a way to get more use out of forges than the tube shape could give. But, as burners have become hotter, the added room before your flame impinges on a ceramic fiber wall has become increasingly important. Most people face their burners down at an angle so that they impinge on a high alumina kiln shelf floor; these are cheap, easily replaced, and very tough; none of which can be said of ceramic fiber materials. The floor area in an oval forge will end up at least one-third wider than in a tube forge...

    The slickest forge design that I have ever seen is an oval mini forge built from half a car muffler. Larger oval forges require sheet metal work.

Note: advanced materials, such as homemade tile made from zirconium silicate and Veegum T can allow forge burners to be aimed upward along the forge ceiling.

    I like oval forges because they are a strong shape, and therefore more portable, than a “D” shape forge, which should be mounted on a table. There is no denying that the “D” shape is a much easier and cheaper forge to build; its floor is completely flat, making it natural to employ K26 tough and highly insulating firebrick to make it. The top and sides can be shaped by bending sheet metal over a propane cylinder. Rigidized ceramic wool will stay in place under the arched portion of its shell.

    Box forges don’t really need a solid shell; steel angle and thread stock will do just fine for their construction. Many people prefer sheet metal construction; in that case, it is important to remember not to attempt welding them completely. You need to have some movable parts in every direction, by placing them at opposite corners, to keep the form from being distorted during thermal cycling.

     Size: Bigger is never better. Once a forge is outmatched by the workload it becomes a part-time tool. But, an outsize forge quickly becomes an embarrassment collecting dust in a corner. The reason why is fuel consumption. Shape is mostly a question of personal preference. You can always tell how personal by how loudly this view is denied.

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Burner Ports

Once you build a burner you'll want to install it in your forge or furnace, which brings us to burner ports. Some people just drill a hole in the steel shell and form a hole in the refractory, but this doesn't provide support for the burner or any way to fine-tune its aim, so most of us attach a short length of larger pipe or heavy wall tube to the outside of the shell, and use six thumb screws, in two rows of three screws each, to trap and aim the burner.

     Now let's discuss control of secondary air and cooling of the burner. Even single combustion wave burners can benefit from external cooling air if the burners penetrate extra thick insulating layers (more than 2") or are the burners are very small 1/4" or less because internal cooling from the cold incoming fuel gas could be overcome during long heats, under these conditions.

    Most burners have at least primary and secondary flame envelopes, so some builders deliberately leave their burner ports unsealed, because secondary air induction (now powered by the flame) is needed for complete combustion. Unfortunately, this nearly always leads to an overabundance of a good thing, because the flame becomes an even more powerful induction "motor" than a burner's gas stream makes. It takes energy to heat air, so extra secondary air becomes a drag on performance within the equipment; leading to as much as 20% heat reduction. Fortunately, we don't have an if/or choice to make. It is just as easy to control incoming air through the burner port as incoming air through the burner.

    Simply mount a washer brazed to a short thick tube, drilled and threaded for a thumb screw, on the burner; once the burner is installed, it can be slid up against the portal tube's end to seal the port against heating from chimney effects after shutdown, and slid closer or farther from the post for secondary air control during operation.  Is this more work? Obviously, but you should expend the additional effort.

Frosty came up with an excellent method of mounting his burners directly of brick or refractory surfaces; he threads a pipe nipple into a floor plate (?); a simple and effective solution.

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Burner Position: Why comes before where

The first question asked about burner positioning should be why; not where; their positions are always derivative.  NOT primarily for best circulation of hot gases; that is a secondary concern. Impingement comes first. The targeted point where a burner's flame strikes, must be physically tough and thermally up to wear and tear. If your forge insulation is only protected by by rigidizer and a thin seal coat, the flame needs to impinge on a high alumina kiln shelf or an exceptionally tough cast refractory floor (like Kast-O-lite 30). On the other hand, if the forge's hot-face is a 1/2" or thicker layer of Kast-O-lite, wall impingement is no longer a big issue; it's a minor issue, to be balanced against other concerns, and aided by re-emissive coatings; when and if needed.

Only secondarily, comes circulation; fortunately, this takes nowhere near as much encouragement as commonly supposed. It is a strong natural tendency for the fame to circulate within most forge shapes, including box forges. In fact, the only burner position that would effectively interfere with the circulation of hot gases within a forge would be to aim the flame directly toward the exhaust opening!

Note: Positioning burners near the front of a forge makes a close second to the previous example of bad planning.

So, we see that circulation is a weak secondary concern, which should be balanced against other factors, such as how far the flame can go before impinging on first surfaces. With a hot-face wall that is only a thin coating, you must aim the flame to impinge on the cast refractory or kiln shelve floor. You would still want to avoid your workpieces, but would also want it to strike as far from the wall it will bounce toward at possible. In that case, I prefer to aim the flame at the near edge of the floor, with only enough angle to assure that it will bounce toward the far edge of the floor, and continue up the wall.

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