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Thread: Backyard Furnace - Optimum Chamber Size

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    Question Backyard Furnace - Optimum Chamber Size

    Hi Everyone,

    I have been using a charcoal based backyard furnace for melting aluminum and want to try making a propane based furnace. I have watched several videos on making them and I have a question. Is there an optimum space between the outside of the crucible and the furnace chamber inner wall? It seems that some space is needed for the heat to efficiently travel around the chamber before exiting the top, but none of the videos I have seen address this. Most talk about the thickness of the chamber walls and how to optimize the propane flame. So anyone have any recommendations?

    Thanks

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    Rather than a set amount of space between crucible and furnace wall, you will want to consider several factors:
    • the space needed for your lifting tongs to go in comfortably around your crucible.
    • the size of crucible you will probably be using, say, in a year or two. Building a good furnace and then finding that you need a crucible larger than your furnace for the work you are doing is a disaster.
    • It goes without saying if you happen to be using a steel crucible, do not go to the work and expense of building a good furnace to fit a steel crucible.


    Richard
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    Thanks Richard.

    I got the refractory cement today so here's crossing my fingers.

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    Read the instructions on your refractory cement. It NOT like regular concrete, it's a lot dryer. Good luck!
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    Thank you. I will keep that in mind. I saw a video where it looked very liquidy, but that was different than what I read. Seems it is supposed to hold its shape, more like a gritty and thick spackle. Is this what I should expect?
    Last edited by Mrasche; 03-09-2017 at 08:24 PM.

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    Senior Member machinemaker's Avatar
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    You also want to consider how much fuel you are putting into your furnace and if it will have enough time and space to completely burn inside the furnace. The time required to burn is also about its velocity in the furnace. As the fuel burns, it forms hot gases that are greater in volume than just the fuel and air. the smaller the space the faster it has to move to exit. Typically you want to surround your crucible with flame or hot gas, rather than pointing a small flame to a small area of your crucible. Just some thoughts.
    kent
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  7. #7
    Quote Originally Posted by Mrasche View Post
    Thank you. I will keep that in mind. I saw a video where it looked very liquidy, but that was different than what I read. Seems it is supposed to hold its shaped, more like a gritty and thick spackle. Is this what I should expect?

    I documented my first experiences with castable refractory starting here at post #68 & #69
    http://www.alloyavenue.com/vb/showthread.php?12107-Kelly-s-Furnace-Build-Log/page7

    And then the most complex shape here at post #76.

    http://www.alloyavenue.com/vb/showthread.php?12107-Kelly-s-Furnace-Build-Log/page8

    I learned a lot through the process and was able to successfully cast some pretty complex shapes. As a result I’d offer the following bits of advice.

    Buy a plaster of Paris (POP) mixing paddle to mix the refractory and build a vibration wand or better yet a vibration table that vibrates the entire mold.

    I mixed all but my last batches of refractory by hand and that was a ridiculous amount of effort compared to the mixing paddle. The mixers are inexpensive at the big box stores and I found the simple version with just bent rod that looked like an egg beater worked great mixing therefractory in a 5-gallon bucket. You will need a powerful drill with someg runt. Initially upon mixing the refractory will become very dry and stiff but will mix well in just a few minutes with the POP paddle giving you plenty of working time.

    Many here make vibration wands that are passed through the refractory after it is placed to make it settle and flow but I found full mold vibration was easier and worked even better. In my posts at the links above you can see my vibration table is just suspended on 4-springs I comment about using it and how the refractory behaved. Making a device to vibrate the hole rig can be as simple as an off-centerweight on a shaft mounted in a couple plain bearings (rod through wooden holes may even be good enough), and powered by a drill. It only needs to run for the time it takes to fill you mold. With the right vibration you’ll be amazed how well the refractory flows. By comparison, packing by hand or ramming the stuff is futile IMO.

    Also, I’d suggest you try a test run on a smaller object, like perhaps a furnace plinth before you commit your entire amount of refractory.You’ll also need something that allows you to accurately weight the refractory and water which may be a bigger deal than you think when you see the precision range of water. If you don’t have a scale, you’d be better off measuring the water volume and calculating weight.

    Best,
    Kelly

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    Thanks, Kent. I think your comment goes to the heart of what I am asking. I saw a video where there was less than a quarter inch from the graphite crucible to the chamber inner wall. When the flame was added, there were blue flames coming out from under the lid and out the access hole. The crucible heated up to a nice red hot, but I was wondering if the gas wasn't burning fully or if it would have gone faster if more space had been allowed between the crucible and the wall. I also wondered about how you get the crucible out. He used steel tongs from the Dollar store and I was thinking he would damage the crucible. It wasn't mentioned, so I really don't know, though.

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    Last edited by Mrasche; 03-07-2017 at 04:44 PM.

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    I cheat: I look at what these guys sell and take that into consideration: mifco.com Their engineering specs. and photos are a starting place.
    kent
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    There is beauty, power and excitement in simple old technology!

  10. #10
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    Don' know if there's a real optimum with so many variables but I'm with Rasper. Just enough space to get your tongs around the biggest crucible you'll use. Thick steel crucibles are OK for melting Al if you pay attention to their condition. How big a furnace are you making? Look at the many propane furnace fabs here including mine and you'll see what works. On the castable, Mizzou in my case, mix it exactly to the directions in smaller batches. Not all at once and get the air bubbles out between batches by tamping or vibrating. It won't look so wet when you start by as you debubble the aggregate settles and a dark slip rises to the top with excess water. Don't expect it to be like spakle or anything else. It's it's own thing when mixed right. You can practice on a couple plinths to get a feel for the stuff. Optimizing your flame will be about controlling your fuel and air. You'll want some sort of blower for sure. machinemaker's advice on mifco is good too to see what sort of dimensions work. Some design from the crucible size out. I started with an empty propane tank and designed in which left me with a #4 crucible.
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