kubiack Posted September 4, 2012 Share Posted September 4, 2012 I’m working a new design for my gas forge and have a question. Once your forge is up to temp how hot does the shell get with 2” of ceramic wool insulation? Quote Link to comment Share on other sites More sharing options...
Dave Hammer Posted September 4, 2012 Share Posted September 4, 2012 There is no single answer... The outer shell temperature will vary, based upon a number of circumstances. When the dragon breath escapes, does it touch any metal? Does your thermal blanket have a refractory over it(reflective surface)? How long do you run your forge? How thick is your shell? How hot does your burn chamber get (and stay) when you consider it "up to temp"?. What is the weight of your thermal blanket? In general, even if your forge is made reasonably well, it will probably still be way too hot to touch after it has been in use for a while (at least on most surfaces). If you are wondering if you can burn yourself if you touch it (YES!!!). If you are wondering if it is hot enough to start other stuff on fire...., it shouldn't be if there is a few inches between it and any combustible surfaces (with the exception of within the range of the dragon breath). If you have your forge either "sitting on fire brick" or "elevated a few inches" (with metal legs), it should be reasonably safe. How far the dragon breath comes out depends upon how you manage your propane (pressure and volume) and air (if there is a blower involved). If the dragon breath touches metal, that metal will eventually get red hot. The longer your forge is being heated, the warmer the outer shell will get (thermal blanket is an insulator, but it will take in heat and convect that heat to the shell). After a short forging session, I could still touch my shell (not in the area where the dragon breath comes out). After a session of over a half hour or so, it gets too hot to touch almost everywhere. I have measured the temperature of my shell (even away from the openings) to be over 700 degrees when my forge has been in use for much longer periods of time. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted September 4, 2012 Share Posted September 4, 2012 I rightly don't know as in the 15 years I have been using it I never measured it. I would assume that 8 hours at 2500 degF would result in a different temp than 2 hours at 1500 degF Can you tell us what you are trying to accomplish and perhaps we could suggest different ways to get there? Quote Link to comment Share on other sites More sharing options...
kubiack Posted September 4, 2012 Author Share Posted September 4, 2012 I’m just pondering new ways to skin the cat. I would like to build a larger gas forge than what I have now. My current forge I built with light weight firebricks contained in a steel frame and an atmospheric burner. It works very well but the size is just to limiting for most of my work. My idea was to try and capture some of the lost energy by heating the incoming air by passing it around the forge shell. I have two pipe cutoffs in the scrap pile and an extra blower. The idea is to mount a 10” pipe inside a 12” pipe and enclose the gap at the ends. The air would enter at the bottom of the forge and travel up between the pipes to the top, while being heated by the inner pipe, where it would exit into the burner. I don’t spend enough time in the shop for it to make any real impact on fuel savings it is mostly an exercise in the process of design and testing. I already have the materials and the construction would not be much more work than a single shell. A few questions to ponder: How hot does the forge shell get? Would the air have enough time in the shell to get hot? Would cooling the forge shell rob heat from the forge which could be used in heating steel? Will the transfer of heat from the interior of the forge to the shell keep up with the amount of heat removed by the air? Quote Link to comment Share on other sites More sharing options...
Frosty Posted September 4, 2012 Share Posted September 4, 2012 Do a web search for "recuperative wall furnace." You say a second blower, are you saying your using a blown burner? This wouldn't be what's called an "atmospheric", it'd be a gun burner though the industry has another name. I'd like to give you some answers but your question is just too vague, as in a general description of what you THINK might work. It's not something that'll get you solid answers, let alone useful data. You'd do better sitting at a coffee shop with folk brainstorming over napkin sketches. I'm not kidding, lots of really high tech has been designed in coffee shops over napkins. Another thing to think about is commercially most don't heat the combustion air, they heat the propane. See hot air balloon burner. I don't know why this works better but industry wouldn't do it that way if it didn't. Frosty the Lucky. Quote Link to comment Share on other sites More sharing options...
kubiack Posted September 5, 2012 Author Share Posted September 5, 2012 Well Frosty I thought I was being pretty specific. I said the forge I am using now is an atmospheric burner and the one I am thinking building will use a blower. The forge will consist of two pipes one mounted inside the other with an inch gap between the two. Air will be feed in from the blower through a hole in the bottom on the outer pipe and forced around the hot inner pipe to another hole in the top side of the outer pipe where it will exit the shell and enter the burner. Other than the second outer pipe it will be pretty much like any other blown forge with a steel outer shell and two inches of ceramic blanket on the inside I have used one of the Sandia Recuperative forges but was looking for an easier way to accomplish the same thing. My original question was about the temperate of peoples forge shell who use ceramic blanket for the insulation and I was asked to give more details as to what I was trying to accomplish. Quote Link to comment Share on other sites More sharing options...
Dodge Posted September 5, 2012 Share Posted September 5, 2012 Both of my forges are made using ceramic blanket insulation (Inswool to be specific) and have different outside temperature conditions. Both depend on how long and how hot I run them. I would imagine it even depends on the outside temperature. (Not as hot in the winter) I don't measure these temps but they do get pretty hot if run long enough. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted September 5, 2012 Share Posted September 5, 2012 Somehow your description doesn't seem simpler than the Sandia forge to me...And balloon burners heat the propane to vapourize it, like the old gasoline torches had the preheat coils to vapourize the gasoline. Preheating combustion air is well known in industry and the "Hot Blast" systems for smelting iron was a major step forward in both achieving higher temps and saving energy in the industrial revolution. The nice thing about the Sandia method is that it takes heat only from the exhaust stream and doesn't take any of it from the inside of the forge. It also doesn't need any time for the forge to heat up and start heating the outer shell. However let me see if I have this right: starting with the work space you then have 2" insulation, then a metal shell, then an one inch gap, then a second shell. The two shells are used to contain the combustion air being pushed through the gap by a fan. Right? This system will work, won't cool the inside of the forge, HOWEVER there will not be much heating as the shell temp never gets too high---under 451 degF as I can never light paper on my shell, and the dwell time is not that long---a burner uses a lot of air. The Sandia forge uses the much hotter exhaust gasses (probably 3 to 4 *times* as hot as the shell temp) to get better heat transfer. How about doing both? Start with the shell and then route through the exhaust? Quote Link to comment Share on other sites More sharing options...
kubiack Posted September 5, 2012 Author Share Posted September 5, 2012 Hi Thomas, Yes you got the idea. I agree the exhaust gasses would be the best place to reclaim any heat. I like the Sandia forge design but it involves a lot of cutting and bending sheet metal to make the shell. Without the tools and little sheet metal experience I think it would be more work than what I had in mind. Plus I already have the pipe cutoffs. The tube in the Sandia forge where the air and propane are heated only passes through the exhaust gases for maybe 1.5 or 2 inches so it does not have much time to heat up. The ignition temp for propane is about 900 degF so it is not getting the mix any hotter than that or it would ignite inside the burner and eventual destroy it. I would be curious to know the temperature of the mix after it passes through the heated portion of the tube. I recently watched a demo where the demonstrator had one of the Pieh Victory forges. The forge had an open front and back which were closed off with firebricks as needed. While watching him work through his project I came to like the size and versatility of the forge. The forge had only one layer of 8# kaowool and hard firebricks for the ends and he was able to weld in it. I would like to use this configuration but it does not lend itself well to capturing the heat from the exhaust gasses. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted September 5, 2012 Share Posted September 5, 2012 I'd "steal" the method of heating and forget the actual forge set up. The method is not based on the basic shape of the forge and it's use of expensive solid refractory panels. So refribbing it to work with an easier built system would work. As for the amount of heating as I recall the transfer of heat is proportional to the temperature difference and so a short area with high temps may work out better than a larger area with lower temps. Anyway I'll ask Robb next time I see him about the temps he was getting on the "output" side. Our local ABANA affiliate, SWABA, has often held our Christmas meeting at Robb's, Chad's and Brad's blacksmithing School. Quote Link to comment Share on other sites More sharing options...
Frosty Posted September 6, 2012 Share Posted September 6, 2012 Okay, I misread or was tired when I read your first post, my bad. Your idea will do some good but as Thomas says 2" of ceramic wool isn't going to let the shell get very hot, I put my coffee cup on mine to stay hot. Thomas it seems modern burners are heating the propane more often than the combustion air. Yes, it's to promote mixing, propane vapor acts more like a mist of liquid particles than a gas. Yes guys I know it IS a gas but it doesn't act quite like one. Propane and air don't mix well without serious help, it separates readily and settles in low spots. Sure it's still a flammable mixture but not uniformly mixed at all. Torches like the All States oxy propane torch is designed to mix air fuel with all kinds of special tools, there's a swirl strip the full length of the handle and that's after an initial mixer then there's the final mixers in the tip. This is one reason propane conversion tips are so inefficient and use upwards of 2-3x the oxy. They just pass lots of unburned oxy through. The iron/steel furnaces use methane primarily don't they? Which you preheat is a world different. Anyway, using and reusing generated heat as much as possible is a good thing, you paid for the propane you deserve the most from it. The only problem you might run into preheating the combustion air is overdoing it, if the air approaches 2,000f too closely the air:fuel mix precombust in the burner. This is a BAD thing and it's surprisingly easy to get the combustion air to and over 2,000f. The sound is AWESOME and a perfect illustration for why you should have 1/4 turn ball valves on the propane line, you really WANT to be able to shut it off quickly. <grin> Frosty the Lucky. Quote Link to comment Share on other sites More sharing options...
kubiack Posted September 6, 2012 Author Share Posted September 6, 2012 Hey Frosty, That is good information on the mixing of propane and air and something I was not aware of. I have a big propane rosebud for a Smith Equipment torch and it can really suck down a bottle of oxygen. I suppose this is one of the reasons a gas forge creates more fire scale than coal, there extra oxygen that will not mix with the propane. How well does natural gas mix with oxygen. I have a pretty large service run to the shop and am planning on going with a blown system so it would not be any trouble to use natural gas for the fuel. It would really be more convenient as I would not have to fill bottles. Would you get less fire scale using natural gas as the fuel instead of propane? I have done some research in the past but did not find much on natural as forges. About the only thing I saw was Johnson forges but nothing homemade. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted September 6, 2012 Share Posted September 6, 2012 Frosty please review what I wrote; I was trying to say that Balloon burners heat the propane to make it a gas as the draw rate is too fast to let it change on their own in the tank. Like the old gasoline torches or camp stoves had a generator section to gassify the liquid fuel. Natural gas mixes much better with air than propane does. Think of this issue as propane being like chocolate syrup and natural gas being like water---when you add them to milk one needs a lot more stirring to get an even mixture. If you have a proper burner design *Neither* gives more scaling than the other. If you do not have an optimized design then the one that's on the lean side will produce more scaling no matter if it's propane or NG. My propane forge can be adjusted from extremely oxidizing to not oxidizing at all. At "not oxidizing at all" it produces less scale than a coal or charcoal forge. Quote Link to comment Share on other sites More sharing options...
kubiack Posted September 6, 2012 Author Share Posted September 6, 2012 The burner in my forge is based on the ones in Michael Proter's book and looks very similar to the ones on the Chile Forges. It is plenty hot and seems pretty adjustable but it produces slightly more scale than a coal file, perhaps I have been running it just a bit too lean. My comment was mostly made on what Frosty said about a propane torch passing through lots of unburned oxygen because it would not mix with the propane and assuming the same would reasonably hold try for a forge burner. I can see that burning the mix inside a semi-closed container would allow more of the oxygen to be consumed in the reaction but some of it would have a change to react with the steel and cause scale. Could you explain how the torch and forge burner are different? Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted September 6, 2012 Share Posted September 6, 2012 Depends on how your forge burner is designed. Mine are designed to have extra turbulence in the gas flow to mix the Air better (so abrupt bends and other things to produce more turbulence.) Most torches like burning lean as you get more heat with less fuel than a rich burn provides. With a torch you may not care about reducing vs oxidizing---though gas welding will! Perhaps I should state it differently "There is no such thing as a propane forge burner: there are HUNDREDS if not THOUSANDS of different designs. Making a blanket statement based on one or even a few designs is like saying "no car has 4 doors because my sports car has only 2 doors" (the dreaded problem of "n" in experiments). So *many* propane forges tend towards oxidizing (and some just because the user doesn't tweak the air/gas mix after it comes up to temp) *Some* are not very oxidizing at all (and some because the user doesn't tweak the air/gas mix to optimize burn) If I were designing such I might go towards lean to avoid CO liability issues. Quote Link to comment Share on other sites More sharing options...
ZIG Posted October 6, 2012 Share Posted October 6, 2012 This was a good read. Learned somethings. Got a few ideas,too. Wish I was still running the boiler shop at work. I could've made anything I wanted. I will build my own gas forge next time. And use some of the ideas from This post. Quote Link to comment Share on other sites More sharing options...
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