Furnace Design Question

I'm building an aluminum melt propane furnace in which I may also want to someday melt brass - and maybe even occasionally some cast iron. I've
researched many home foundry websites and am following the advices found there but a few design details are missing. I am using a scrap 3/16" SS tank I found at a salvage yard. The tank had been cut in half by the scrap yard for crushing, but I have welded it back to its original shape. The tank is 18" in dia and I am currently planning 5" walls of 3000 deg cast refractory giving me an 8" center. I'm getting ready to cut the height of the tank to 16" not including its rounded bottom. This will leave adequate tank wall material for a 4" - 5" thick lid. I also found a nice piece of 4" schedule 40 SS pipe from which I plan to build a crucible. This will leave about 2" of space between the crucible and the refractory. Its going to be a heavy beast and so I plan to build mechanicals to help handle the lid.
My questions primarily concern the tuyere:
1.) I understand that it should enter tangentially to the inner wall of the refractory but how far up from the bottom? Some drawings I see show it at the top of the plinth (barely touching the crucible), and only one other reference that I was able to find that mentioned its location said that it should enter at the bottom 1/3 of the crucible. Is this critical?
2.) Is it best to extend the tuyere into the refractory or to use a temporary mold for it and terminate the actual metal pipe at the wall of the furnace? I'll likely be using scrap SS pipe here also. I've read about it being done both ways as others have admitted to being as unsure as I am. What's optimum?
3.) Does anyone see anything wrong with the dimensions that I am planning? I am wondering if the space between the crucible wall and refractory is important and if I'm currently planning too much? If I build a larger crucible someday is 1" clearance ok?
4.) The tank has a rounded bottom and a 2" hole. I suspect this is too large for a drain hole? Should I close it up to something much smaller?
5) With the diameter tank I am working with I have seen at least one reference that suggested replacing the outer one inch of refractory with one inch of Kaowool. What will this buy me? Is there any performance advantage worth the cost and hassle of working with this stuff?
I haven't settled on a burner design yet but so far I am leaning to one of the blowerless designs. I'm still reading here and there is alot of good info available on the web. I thought I would get serious about the burner while the refractory is curing.
Any other comments are greatly welcomed before I do any irreversible fabbing. Thanks in advance.
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Take a look at "Gas Burners for Forges, Furnaces, and Kilns" by Michael Porter (I got mine from Amazon). Looks like a more thorough treatment than Ron Reil's website and incorporates several designs from 1/2" to 1.25" burners.
I haven't built one yet - probably next summer.
Pete
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snip------
In my opinion (I don't like the idea of melting in metal vessels), it would be a huge mistake to design around a steel (or stainless) melting vessel. I would strongly suggest you decide on a volume of metal that will fit your needs, and design around that size commercial crucible, especially if you have aspirations of melting iron. Be certain that you allow room for the lifting device of your choice to get in the furnace to pull out the crucible.
The tuyere should enter the furnace at the crucible rest so the flame doesn't impinge directly on the crucible, but instead swirls around the inner wall of the furnace.
I envy you your container. I've always used steel vessels and in time they look like hell due to oxidation. Your stainless one should hold up very well.
Harold
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Don't use one at all,... http://webpages.charter.net/dawill/tmoranwms/Casting_Rvb.html
Joel. phx
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Thanks for the tip. I've decided to design for a #12 standard crucible and will aim the yuyere as you suggested.

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5"
Ouch!!! Use 3, or 4" walls tops. More room, less heatup time. The insulating refractory I've used in my last few furnaces barely gets hot on the outside after half an hour of melting. Your crucible might be ready to pour in 15 minutes from cold. (By its nature, my furnace melts slower. Fine with me.) Even if your refactory is so bad that it's conducted three inches already, you've got two inches to go before you actually see the worth of so much refractory. Until then, it's just soaking up more heat, particularly if it's the heavy rock stuff. Extra thickness only serves when you are keeping the thing hot for long periods.

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4" pipe is nice stuff but you can easily handle 20 or 30 pounds in such a tank!

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Not really. But I'd do it at the top of the plinth, just a little above the bottom in other words. And put in a drain hole. That way spills will go out the bottom instead of into your burner. ;-)

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My latest reverb. furnace has a plain hole molded into the refractory, I place the burner and start melting. No tuyere tube, no flare (the close fit to the refractory serves as a flare).

Nothing at all, but you can get a lot more usefulness from such a shape as I have mentioned.

1" on either side will be good for tight crucible tongs, should you use a ceramic crucible later (you'll want it for bronze, and obviously iron). You can cut it down to 1" total space (1/2" either side) for a metal crucible, but it cuts off airflow a bit.

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Yeah, if there's a lot of burn coming out you might want to close it up a bit. Simple matter to use a bit of dowel and just duct tape it in place. Duct tape makes great hole-covering refractory mold material.

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Absolutely nothing at all given your specified design. Reason being, the outer 2 or 3" of your refractory aren't even doing anything because heat is still being conducted out to it. It'll be hours before it reaches the kaowool and starts putting heat across it. Plus so much weight will crush it, on the bottom. However, 1 to 2" of wool plus a half inch or so of "hot face" castable on the inside will make a skookum furnace. Let's see, that's 14" i.d.? Like, #50.......
The only problem is crushing the stuff on the bottom... you'll have to use solid refractory for the base.

You won't be disappointed. Reil inspired designs can be scaled endlessly, I've made a 1/2" i.d. model (using a *cast* jet of .012" or so), although it had stability problems due to a bad burner tube design. I've heard of them up to 1 1/2 or 2" i.d., a 1/16" jet hole for the propane. No reason you couldn't go up to 5" burner tube (about 4 feet long) and a 3/8" orifice, except no one has a need nor want of two tons of molten aluminum. God, that burner would swallow a 500 gallon tank in minutes...
For your furnace, a 1 1/2" (tube (i.d.) or pipe) burner will work well. About 1 foot long from venturi to end; not counting flare. You can either cast in place a tuyere tube, or mold it in the refractory alone as I did. I place the burner in the furnace, turn on the gas and light it. DON'T light from the vent hole as the furnace will be filled with gas of the exploding variety... I do it all the time (eheheh...) but I'm not too curious how far your lid will fly if you try it on something like that. So throw in a lit ball of newspaper or something.

Just one more. Cut that curved bottom off and use it for the lid. Enlarge the hole a bit, 3 or 4" dia. should work well for something this size. 2 or 3" total height, depending on the curvature. If it curves more than 2", you're probably better off scrapping it and welding a plate in its place. For the bottom, do that anyway so it's flat. Convienient to not have your furnace rocking around while you melt. ;)
Meh...long enough post... volumes I could add but will wait for now...
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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On Thu, 12 Aug 2004 13:33:09 -0500, Tim Williams wrote:

This brings up a question I've had about Reil-type burners. Can you give an estimate as to the rate of fuel consumption for a 1 1/2" class burner?
Is there a way to calculate the fuel consumption rate of a given burner during the design phase of making one? How about BTU output for a given burner?
Thanks
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You could look at the following info for some idea.
http://www.joppaglass.com/burner/lowp_chrt.html http://www.joppaglass.com/burner/highp_chart.html
I have looked into this for estimating the gas usage for a glass blowing glory hole and all the info is available on the net. Just wish I had bookmarked more of it.
Some info that I noted.
for proper combustion 23.86ft3/ft3 propane http://energy.growmark.com/propane/Chapter1.html
2500btu /ft3 propane gas http://www.engineeringtoolbox.com/9_510.html
http://www.propane.ca/Resources/page1.asp http://www.altenergy.com/propaned.htm
From the joppa orifice size/heat output/pressure chart you can get an idea of the btu output. From that you can then work out the weight of propane required to fuel it for a given time.
Artemia Salina wrote:

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Thanks much for the comments. I plan to reduce the thickness of the refractory as you suggest and I just cut the rounded bottom off to make the top.

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Terry, I see you're in Austin. If you ever want to come by and look at my furnace setup, I'd be happy to play host for an evening. Seeing something in real life might provoke a lot of thought. I've even got some scrap laying around if you want to see it work.
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Seems my signature didn't show, but I'm in Austin too.
Gary Brady
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Hello,
The flame should enter at the base of the crucible, but the crucible should be a few inches off of the bottom, like sitting on over-turned bowl. This allows crud and refractory to fall and not impede the flame path, clean it out every once and a while.
To keep your plumbing from consuming itself: The furnaces that I've seen have a refractory block that acts as the flare with a steel plate on the outside acting as a flange to attach your plumbing. These have been oil burning furnaces, but even the T-Rex suggests using a cast iron flare for his, so maybe a cast refractory one wouldn't bad either.
Also, a flat cover allows you to set items on top to preheat them (scrap, ingots, skimmer, etc.).
Good luck,
Eide
I just saw this page, thought you might be interested: http://www.backyardmetalcasting.com/guest_willsburner.html

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