The Home Machinist!

Is anyone familiar with general guidelines on the wall thickness of the casting concrete? I am building my second propane fired furnace as the first one was a little too small inside. It was about 10 "OD and had a 3" wall of 2800 deg. concrete. My new one is 12"OD and will be 3100 deg. concrete. Should I stay with the 3" wall or is it more a matter of how hot the burner gets. And how large should the top exhaust be. Mostly I will be melting brass/bronze but might try cast iron. Any help would be appreciated. Doug

One thing for you to consider is the use of a lower temperature refractory material that has a greater insulating value. Because it isn't in direct contact with the flame, it won't see excessive temperatures so it will hold up just fine. The cost for the lower temperature refractory should be lower, as well as lighter in weight, and there's no real benefit in exceeding what you'd need.
If you haven't already made the purchase of the 3100° material, you may wish to revisit the idea.

You should be able to cast sections that are only two inches thick with no problems. If your can permits, I'd suggest a two inch layer of insulating material for the first pour, then two inches of the higher temperature refractory. I used that process when I built this furnace, although I elected to go with the 3,100° refractory to achieve higher resistance to destruction from flux. I did the melting of my product (precious metal wastes) directly in the furnace----no crucible.



Harold

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Wise people talk because they have something to say. Fools talk because they have to say something.

nicely built kiln. i am envious.

Harold, I have already purchased the 3100 castable, 100lbs for about $50 locally here in Cleveland. I have found somewhere a general rule for the top exhaust hole to be 1/3 to 1/2 the inner bore. When I built my first furnace I was unaware that the castable was not considered an adequate insulator but I now see much talk about hotfaces and insulating liners, such as you have mentioned. So other than the burning/melting temperature of the insulator, what determines the optimum thickness of the hardface? Or is that the only consideration? i have not been able to find any reference as to the insulating qualities of the castables. I have read a thread concerning the melting of cast iron in a crucible furnace as opposed to the cupola , where one of detractors of the crucible furnace for this application, was the difficulty of attaining the proper melting temps. It seems that with the key to this problem is purely a matter of total insulation wall thickness and burner size/ efficiency. I am not talking about all of the other dis-advantages met with melting iron in this type of furnace as I have been educated by yourself and others in other postings. A micro cupola will be my next furnace but for now its crucible. Since I have 3" wall total thickness to work with to get the bore size I want, I have been thinking of just sticking with the 3" castable wall and adding an insulating jacket around the outside of the furnace. Most of my pouring will be bronze but I want to try out iron. My first furnace did get hot enough to melt it. I have read a lot of your posts and want to thank you for all the info and taking the time to do so. Doug

It's been many years since I addressed the heat transfer capabilities of the refractory materials, so I don't recall much in the way of details. If memory serves, however, the maker generally provides technical data that includes the rate of transfer.

The only furnace I ever built with two layers was the one I showed, above. All others were single layer, and about three inches, just as you propose. That works fine, they just get hot on the outside. Applying an insulting layer there would be very acceptable, assuming it won't get abraded.

A couple things to think about. If you operate the furnace on top of a concrete pad, make sure you have adequate clearance between the bottom of the furnace and the floor. If you run long enough, you risk cracking the concrete from local expansion.

While some furnaces have been built that will melt cast iron, the end results are generally not very desirable. The prolonged heating cycle tends to burn off carbon, which destroys the desired qualities of the metal, but it can be done. The ability to achieve the desired temperature quickly is a definite advantage.

To be perfectly honest, I had no idea of a ratio of the discharge port versus furnace bore. When I built my first furnace, I had examined a McEngleven, and made some Kentucky windage adjustments, considering I had scaled it down for a #1 crucible (way too small for my eventual needs, so the next furnace was for a #8). All of my previous furnaces were fired by natural gas, and performed to my satisfaction. I could melt pure gold with no problems, which melts at a temperature very near that of copper. The temperature difference to melt cast iron, even one that is quite high in carbon, would be a substantial jump higher. I'm not convinced I could have enjoyed success. I make mention for reason. While I was more than satisfied with the performance of all of my creations, I may not have been had I hoped to melt iron. I believe, just as you do, that the design and heat transfer play a serious role in the degree of success one will achieve.

Like you, I had addressed the issue by starting to build a VERY small cupola. Only 4", which is reputed to be difficult to operate with success. I then encountered the opportunity to acquire a 50 kw induction furnace, so the cupola project was abandoned. The body was sold for scrap when I moved from Utah.

Thanks for your kind words. I try to share that which I have learned through the school of hard knocks. Some find it offensive.

Harold

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Wise people talk because they have something to say. Fools talk because they have to say something.

An interesting point here is BTU input. I can melt aluminum, no problem with the furnace top open, and indeed, my first was built from a 55 gallon drum cut to the 2nd band. Anyway it is a question of input and burn time, Getting a swirl aroudn the vessel so that the atmosphere in the furnace rotates about 3 passes around before exhaust, is the best you an hope for. some furnaces vent out the back with a solid lid, and others right through the center. On my furnace, I have a 3 inch burner (!) and a 3 inch exhaust through the lid. As the furnace walls heat, so does the burn time shorten for the fuel. This is good. The problem is how much gas can you feed this thing (BTUs) in any time frame. I have found a raw open line for 3/4 inch, is not enough BTU to get iron melted, there just is not enough delivery value, on house gas. So generally I use 100# bottled propane, to get higher heat faster.

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Big Dave

Be Alert! The world needs more Lerts.

I checked both the Noxcrete castables paperwork and have found nothing about their insulating qualities. At 3" the 2800 variety definitely had enough to keep the outer metal shell from glowing red as the small amount of cast iron inside was melting. Maybe I will try that again and measure with the 1500 deg pyrometer I have. But I see now that better insulators and less castable should be used as it does take a while for the walls to heat up to yellow. At this point I only have a 20 lb propane tank and understand the need for a 100 pounder. My desire to melt cast iron, in very small amounts like 5 to 10 lbs., is job related as well as a curiosity. One of my first jobs was in an alloy steel mill with electric furnaces. Wish I was paying attention. In addition to being 3rd helper on the ingot pours I got to quench very large quantities of steel in huge water and oil baths. Quite impressive. From what I seen so far I wouldn't be surprised if the little furnace could reach 3000 deg if designed properly. On a side note, I recently drilled open a 2000 lb cast chrome iron cannonball safe from around the 1870's. Once I figure out how to post pictures where should I post it? Doug

If you are going to make a small furnace, like a lab furnace, as the size goes down, you just lenghten the intake tube for the burner, as there is not enough time for the atmosphere to react in the furnace proper. Probably 12 to 16 inch blow pipe into the side of the furnace space offset so you do not target the vessel. It should come off the wall and roll around.

A little experimentaion will get you close to run with this. For low temps like aluminum, a blower is not necessary, and a vented "Riel" type burner will due with the right venturi, moderating the gas pressure.

Kiss rule: only make it as complex as needed to do the job.

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Big Dave

Be Alert! The world needs more Lerts.

I cant speak much on castable and such materials, as the furnace that I built used furnace clay and sand, to fill in between the gaps of large kiln brick. They brick were used from a housbrick kiln, being religned, and were set on a floor of the same brick, radialy on end, to fill the space for a 55 gallon drum. The resulting pit, will hold a #16 crucible easily, and has a homemade burner for the aluminum run there. The Commercial McEnglevans, with blowers, are used for bronzes. I dont do iron, as it is too hot for my oilsand, and really eats crucibles for breakfast, with the high heat and long run times.

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Big Dave

Be Alert! The world needs more Lerts.

Posting pictures is quite easy. Below the box in which you compose your post, there's a button to click (it says Browse) which permits you to access the pictures you have stored on your computer. Select the picture you'd like to post, then follow instructions. You can attach more than one, plus place them in line if you choose. All pictures that are hosted on the forum are posted as attachments.

If you choose to provide links to pictures hosted off the Chaski board, insure that the pictures are not sized larger than 800 pixels in width. Those that are must either be resized, or they must not execute without being clicked. Pictures wider than 800 pixels spread text for many of the older monitors still in use. That makes reading text difficult, and is often the reason a post goes ignored by some readers.

You can post your picture(s) and text on this subject in The Photo Album.

Harold

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Wise people talk because they have something to say. Fools talk because they have to say something.

Yep! It's really troublesome, in particular when using either graphite/clay, or silicon carbide crucibles. That's due, in part, to iron's affinity for both carbon and silicon. Crucibles are dissolved by the iron. Yet another reason to not use a crucible furnace for melting iron.

Harold

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Wise people talk because they have something to say. Fools talk because they have to say something.

So I guess it would be help to know the insulating properties of whatever is used as the "hotface", to ensure that whatever insulator is used behind it, is not damaged by the temp that does reach it. For instance, if a particular insulator material is damaged by temps over 2000 and the hotface sees a reduction of temp at 500 per inch(purely for argument sake as I don't really have a clue what it is), running a furnace, having a 1" hotface, well up over 2500 is pretty much guaranteed to damage the insulator layer. My comments here are pretty much just thinking out loud on how to make the furnace itself into the most efficient it can be and not so much because I will attempt a few iron pours. I get that a tilting furnace or cupola would be much better suited for that. Oh, the agony of it all! I just want to melt some %(#@ metal. Doug