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Geoff Keyes

Blown burners, the care and feeding

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What is a good range of CFM for a blower? I have found blowers that look similar to what you are showing but I don't know if they will produce enough air.

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50-100 cfm is about right. The blower I'm using is rated 100 cfm, and I choke it down about 75%. Surplus center doesn't seem to have much right now, but that is where I've found them before. A bathroom exhaust fan will work.

 

Geoff

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Thank you. I wondering if the blowers off a furnace that help clear out the fumes would work. This is not the squirrel cage blower that powers the whole ductwork. I will check on cfm of that one. I can get a whole bunch of those.

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Kind of a dumb question, but the only "needle valves" the local hardware store has are little ones like what is used on supply lines for a refrigerator's icemaker. Is that what you're talking about? If not, where do you find the right kind?

 

Thanks

 

Dave Armour

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Geoff, thank you so much for all this info.

 

Question, what do you do differently for your heat treat furnace? Is it just the valve and control or do you have a thermal coupler?

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I have more volume, so it doesn't heat as efficiently (though it will still get hotter than I want). I also use a thermocouple to measure the temperature, since my eye calibration is pretty poor. You could build a version that used a PID controller and solenoid valves to regulate the temperature, but I've never made a system like that. I'm probably going to build an electric HT oven instead, it seems simpler to me.

 

Geoff

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Exactly. The size of the burner and the size of forge need to balance to get maximum efficient burn. So on a 1.5" burner, for forging we say (as a rule of thumb, ROT) you want about 350 ci. If you wanted a melting furnace (say 3000 F), you'd have to increase the burner size (to get more gas and air into the forge) or decrease the size of the forge. I'm guessing that if you decreased the forge by 20-25%, that would do it.

 

Same thing for the HT forge. You'd like a forge that would run naturally in the 1400-1600 F range. By naturally I mean that all systems have a "sweet spot" where all of the inputs are in balance. With that same 1.5" burner a bigger case, like a 55 gallon drum, makes the sweet spot about where you want it to be. I'm just naturally lazy, I don't like have to fight with my tools.

 

Geoff

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I'm finally getting around to building a propane forge. My question is do you start with 2 inch fitting and reduce it to 1.250/1.50, or do you start with 1.250/1.50 fittings and reduce smaller?

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I find that for most uses 2 inch is too much, unless you're going for a 55 gallon drum forge. 1.5" is plenty. I tried to build a 3/4" burner, but I had trouble getting enough air through it. I think that it could work, but it needs some experimentation. The 1.5" reduced to 1.250 works the best, I think.

 

Geoff

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Thanks for your work in putting this together. My question is could you use a "T" at the end somewhere in order to have two nozzles? I'm putting together a hard firebrick forge with a 4.5" x 3.75 x 18" chamber (304 cubic inches-correct?) , would a forge that size need two nozzles or does this design produce enough heat to fill that size chamber? I ask because I have absolutely no idea how to decide if I need one or two and this will be my first blown burner build; I've looked at some propane forges online and it seems like a fine line for size in relation to 'x' number of burners, (all Venturi type).

Thanks for the guidance,

D Gill

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I have built a 2 burner version, (if by that you mean 2 outputs and a single fan) and it works pretty well. I was using it in a HT forge, and so was trying to get it to burn at very low heats. It's not as stable as I would like, but at hotter burn settings it works just fine.

 

Your chamber is small diameter, but very long, normally I'd say that one burner would heat that. I heat closer to 1000 Cu with just one burner, but you might go with two for that design.

 

I'd like to ask, why such a long forge?

 

g

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That's exactly what I was meaning, one fan and one gas line with two nozzles.

 

WOW! One nozzle heats a 1000 Cu - I had no idea - that's amazing!

 

It might be silly why it's so long - it's the length of two of the brick, end to end length-wise, because I don't have a way to cut the hard brick. I wish I could post a picture of what I have laid out on the garage floor, but I can't figure out how to do that. I thought the longer size might come in handy sometime for doing longer blades or blacksmithing stuff.

 

I don't have a real reason I guess, just trying to think ahead and cover all the bases without having any experience.

 

I would appreciate any advice.

 

D Gill

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Yep. Venturi burners have to be closely matched to the forge size to work at all, but blown burners are not nearly as picky. The same nlown burner can work with a 150 cubic inch forge and a 1000 cubic inch forge just by adjusting the gas and air.

 

What they (or any gas burner except perhaps for a long narrow ribbon burner) don't do well is long tiny chambers. You will need more volume to get an even heat over that length, and remember hard firebrick is not an insulator so you will want or need some fiber as well.

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For general smithing projects, hook and scrolls and stuff, I use my welding forge turned down from the full blast condition it needs for welding. It's 8 x 7 x 18. For knives and other edged tools I use a vertical forge with about a 5 inch hot zone. This is about all the steel I can forge by hand in a single heat. I just forged a 34 inch backsword in that. For heat treating, you need to be able to heat a long length all at the same time, for me, that is a third forge.

 

Therein lies the issue. Forges do one thing well (depending on the design). My vertical forge is efficient and well suited to the kind of forging I do the most. I don't try to weld in it, and I don't heat treat in it (except for little stuff) because it doesn't do that kind of stuff very well. OTOH, forges are cheap to build, and if you are feeling cheap, one burner could be moved from forge to forge.

 

The brick floor in your design will cost you fuel to heat and keep at heat, and it will be slow to come up to heat. If you are not welding, then you probably don't need it, and unless you are forging every day, the hard floor won't extend the life of your forge al that much. I've been running my all fiber vertical forge for 4+ years, and it just now needs a rebuild.

 

Geoff

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Thanks again for all the great advice, you guys are awesome!

 

I'm a bit discouraged though. I'm cheap, on a budget and I like to do things myself with what I have because there's not much money for anything. And what I have is about 40 hard brick with access to more. I'll make the chamber bigger after your advice. My plan was to assembly the brick and coat the chamber with a refractory mortar from a local (an hour away) big box store. I had thought about using Satanite coating, depending on how much it costs to ship.

 

I suppose, on a 'good, better, best' scale this isn't the best; but is this reasonable? Is this usable? Can it work well enough to do basic blade forging and such? Maybe some pattern welded blades in the future?

 

I'm sure this is getting away from blown burners and into forge design and building so thanks for the patience. And again thank you both for your willingness to share your time and wealth of knowledge and experience.

 

Dave

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I just re-lined my forge today. It had a good 10-year run, I think, or close to it. The lining was uneven, the floor was saturated with old flux, the burner was a simple 1 inch pipe sticking in there... it worked, it welded, it did everything I asked, but it was time to start over.

 

The worst damage to the shell was at the front left, and back top openings, where years of heat had eroded some of my metal, but not enough for me to care... maybe a half inch.

 

Tore out all the old insulation, cut off the old pipe inlet, and cast up ribbon burners for both me and my main student right now (he's building a new forge, figured I'd experiment with him) using about 5# of castable and a ugly welded manifold of cheap home depot c-channel, and 1" pipe. For my forge, I patched the old inlet hole (low mount) and opened up a new one (high mount), welded in the burner where I wanted it and sealed it up good. Laid in 2 inches of Kaowool, by far the most expensive thing in the whole project, and then mixed up about 7 or so pounds of Mizzou to line the interior with. I use latex gloves and smear it by hand, and find I prefer the Mizzou to Satanite because it goes on just a little thicker, and I think it's a bit stronger and so more resistant to me being not perfectly careful about how I poke the forge with sharp things I'm making. I also laid it in a bit thick and runny on the floor, so I have a pretty nice flat spot to rest things in. My experience also shows Mizzou holds up to flux better than most of the alternatives.

 

The cool part is, I got most of it on video with some explanation of what I was doing, so once I cobble that together, I'll put it on Youtube and link it here for everyone's benefit.

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As promised, the video of my forge re-build. I hope it is educational. Certainly not the only way to do it, but it's running very well and I'm happy with it.

 

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that was an awesome video! thanks for sharing it, it really helped me to understand some things.

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Thanks for the awesome video, Chris. I feel a lot more confident going into my build now! I had the pleasure of speaking with Larry Harley today, and he talked about how you want to get the fire to swirl like that, so seeing yours in action really drove home his point.

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I got mine put together today- I can't seem to find a good balance on the flame. It's either pouring out of the front or sputtering a little. Also the fan is getting fairly warm. Would it be bad to extend the pipe farther away from the forge? Here's some pictures and a video of it running- thanks'

 

 

 

https://youtu.be/n-FzaOsoVqI

image.jpeg

image.jpeg

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Two things.

 

1) What are the thumb screws for?

 

2) How big (CFM) is your fan

 

I like the look of your build, very tidy. Run the fan full speed, but close off the intake about 50%. This should make it run rich (lots of soft billowy flame out the door). Then adjust the needle valve to lean it out. You want a bit of flame out the door. You may want to shorten the pipe below the reducer (it may not make any difference).

 

I run a 100 CFM fan with the intake closed about 80% most of the time, with the gas pressure above the needle valve at 1-2 psi.

 

The fan case should be galvanized steel, so you can build a shutter out of the magnetic sign material. Or make a cardboard box to put over the fan intake with a moveable shutter.

 

The brickpile works, but is not very efficient, since you have to heat the majority of the brick mass before you can get much work done. Something like ITC100 painted over the inner brick faces will make your forge a lot more efficient.

 

Let me know, I'll keep an eye out for your reply.

 

Geoff

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