Building a small electric furnace / kiln

I've been working through the design of a small electric furnace. It's looking reasonable for a chamber with an 8" square base that is circa 10" tall. The cheap kanthal wire I have found looks a bit dodgy at 1300C but should be ok a bit lower. Two questions hoping some one else has done something like this.

I'm going to wind my own elements and intend to do it on a lathe and wonder about tensioning as it seems to be pretty springy stuff. I intend to screw cut a mandrel to the pitch I need or use all thread and then stretch as needed.

I have 2 options on the element carrying sides. Cut up insulating bricks or make them from castable refractory. A lot of the heat up time in these is caused by the bricks so I wonder just how thin they could be if the bricks were cut down or made from the castable. I intend to use ceramic blanket for most of the insulation.

Another option for these might be ceramic board, RFC. It's semi fluffy stuff but is seems that it can be routed or cut with a knife. I believe it can be hardened using a silicone colloid but can't really find any info on what that involves in practice. Sounds better than other options as it's thermal mass is lower. Pricey though.

Yeh I know propane is easier and quicker for smelting aluminium. And also that the furnace would be much easier to build.

John

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Hi John,

first of all I would experiment on different diameter bars with a short piece of wire to see what diameter the wire springs to when tension is released using your lathe gearbox to give the desired pitch of coil. You can stretch the wire out to give different pitch as required. When you arrive at the right diameter drill a hole, a close fit to the wire diameter through a piece of bar of the right diameter and length across the diameter near one end. Thread the wire throughout the bar and in the tool post sandwich the wire between two pieces of softish wood. Clamp the wood lightly enough to give good tension to the wire, select the pitch required and stet the lathe in as low a speed as possible. Once the desired length of coil has been wound. Stop the lathe, and release all the tension carefully before cutting the wire, hopefully you should end uo with the right coil.

Regards,

Bob

Get hold of a copy of Tubal Cain's book 'Hardening tempering and heat treatment'. He goes into making electric muffles quite a bit.

hi i am also in the process of building a furnace, the main one is to be out of a stainless steel beer keg, the refractory i will be ordering is from artisan foundry supplies. artisan foundry .co.uk, its tri-mor monolithics up to1700c, can melt iron if using a waste oil burner, similar to collin pecks design, , the price for the refractory is£41.99 for 25kg. i looked at the electric design however not so good with electrics.

I've heard of a few of these "waste oil" heaters and wondering what kind of oil is burnt in them? Hopefully not used engine oil which is truly nasty stuff. On the other hand, used cooking oil would probably not be so healthy for the neighbourhood either, as it would give everyone the munchies each time you fired it up. What is normally used?

Myfordboy on youtube has some pretty comprehensive video's on making furnaces including oil burning. He also shows a method for vibrating the casting which the materials used need to get rid of air pockets etc and assure it's all well compacted.

Things may have changed in respect to small electric kilns/furnaces. There seems to be some about now that heat up very quickly. There are calculations about but they assume a certain rate of heat loss against temperature. This sets the power needed. I'd guess that the calcs are based round a single layer of ordinary insulating brick and very little else so if I add more that also has less thermal mass the elements may burn out due to overheating.

One person seems have got round the overheating problem by using a cheap triac controller ahead of the main temperature controller so just turns a knob to limit the max power the controller can provide. It sounds like it should work but I'm inclined to try and make it easy to fit more elements in series to reduce the max power or start with too many and remove as needed.

From the electric "cigarette" brigade it seems that kanthal can be annealed by heating to orange heat. There is some info about on winding furnace elements that use a turn or so round a reel to provide the tension but I suspect that will harden the stuff again. That method is a bit like wrapping a turn of electronic hook up or copper wire round a terminal screwdriver and pulling it to straighten out kinks and bends. I have wondered about winding it around a mandrel, keeping it tight and then heating the lot up with a propane torch. Tension via blocks of wood might be a decent idea.

As to calcs there isn't much about and zero on constructing small commercial ones that heat quickly. Potters can't be keen on saving the planet. One kiln is being sold on ebay at the moment - outside got so hot it started a fire. This is all I have managed to find on the sums for normal types. I suspect it would be a good idea to check the actual resistance of dubious kanthal wire.

Interesting thought at the end of the Russian offering.

There is plenty of talk about not using the cheap elements from China off ebay. It might be possible by buying ones that state 2 or 3 times the power needed and using them in series. The surface loadings are far to high used as they are listed.

John

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Edited By John W1 on 12/12/2015 18:05:09

I think a kiln to fire pots in requires gradual ramping of temperatures, the whole process takes a very long time.

Properly designed controllers and thermocouple sensing are used to give more consistent results than manual operation.

Rapid heating would likely have disasterous results, there are exceptions where a rapid firing cycle is used, but I think that is with gas fired kilns and particular types of clay.

Reading about modern kilns indicates that they have much better insulation than older units.

Please excuse me if any of the above is out of date, I have not been involved with ceramics for quite a few years now.

Edited to remove teaching granny how to suck eggs syndrome.

Edited By V8Eng on 12/12/2015 19:51:33

Just a thought - Pottery and messing about with clay appears to be the new "Bake-off". People are probably buying kilns that they will only use a couple of times. Wait a year or so and there might be quite a number on the second hand market.

Somewhere I read that one does heat treatment and melting slowly but silver soldering quickly. If so a pottery kiln seems to make sense to me except for the highest temperatures.

JA

JA.

That's an interesting thought, have to admit that the programme did nothing for me, but one mans meat etc!

My firing was done by a professional potter which made life much easier at the time, but if (good) S/H kilns became plentiful I might give it all a go again.

Getting back to elements , the OP could ask UK based kiln manufacturers about supplying  Kanthal etc, maybe standard size spare elements could be utilised?

Edited By V8Eng on 12/12/2015 20:28:38

Kanthal is a brand name. Most of the wire that is around seems to be described as kanthal like now. The same sort of alloy. People do phone suppliers and ask for elements to be made and they will do that but it can result in more grief when in real terms there is no easy way of determining what wattage will be needed. If I did that I would go for a lower wattage than I think I need that could be shortened to increase it if needed without upsetting the surface loading figures to an unacceptable level. Those set how long the element is likely to last.

Anyway i have bought the wire which cost me £11 and am thinking in terms of 2kw. The sums suggest that is around 30% more than needed. I could reduce that by extending the length of the element using the same wire. The wired came in the form of 5 10m spools and each spool should be ok for 1kw with 120v across it so 2 should be ok for 240v and plenty left to add more if needed. It was a buy 4 and get one free deal. I have seen the real thing at £1.79 per m but don't want to spend much until I know it will work.

The lower temperature range low density bricks seem to have better thermal insulation properties but kanthal needs one grade up from those really. These are usually listed as 1400 to 1500C, or grade 26

It pays to look around for suppliers of the basic materials. Prices vary significantly. Vitcas is one of the main suppliers in the UK. They do have an on line shop. They have lots of things not shown in the shop. I managed to get some high temperature ceramic blanket off ebay, left overs from some one who made a pizza oven using materials that were a lot better than needed really.

For propane furnaces the forge people may have a simpler route than casting refractory. They just line a barrel with ceramic blanket and replace it now and again. Maybe just add a cast base for smelting aluminium. There is info about on the web. The higher temperature range blanket might be needed. The sub 1300C stuff might not stand the heat. It can be made more rigid on the surface if needed but so far I have no idea what the liquid used to do that can stand. The same sort of thing can be done to board but there is even less info about.

John

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John, I'll be following with interest - I'm collecting info (for an electric build) at the moment. There are a couple of pdf documents around, you may already have them.

The Kanthal handbook

Dudley's element paper

Heat_Treatment_Furnace

I'm sure you will find them instantly with google, but give me a shout if not. Also, please share if you have found other useful titles.

Roger

Thanks Roger. I hadn't found any of them so far. The Kanthal handbook answers a few questions I had. Dudley show more or less what I intend to try. On the heat treatment furnace there is a series of video's showing the construction of a similar unit. They are long but of interest especially the testing which starts at the end of the one before this one.

There is more testing in the one after that. I did talk briefly with Myfordboy - he said forget it and go for propane. He does converse with some one who is trying electric who keeps burning elements out. Trouble is I like a challenge. On the propane front there is an interesting video from a forge person on making a torch. Long and slow with complete instructions rather than "gee look how big a flame I have". His torch looks to be very efficient in terms of heat output and fully burning the gas at least. Interesting little forge too. Myfordboy shows a cheap way of making a needle valve to control a torch and one or two other things as well.

I obtained my wire from here.

**LINK**

Before I do anything though I need to decide on how to make the sides. The bottom will be brick but I'm wondering if I can avoid using it for the sides. Hence in part the earlier question about minimum thickness that will take the heat. Bricks can be cut in half, which I feel would be ok and castables can be cast at any thickness. I'm keen to keep the weight down as well so if at all possible ceramic board would be the best option - if that is possible.

John

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John, I know you have visited Traction Talk, have a look at this thread as the guy has a couple of electric furnaces.

They want MONEY off me Jason for that link. If it's for a for sale add I've put in too much time now to do that. I've looked around at all sorts and the shape and size is generally a problem

One other link I meant to add was these people on ebay. They have been selling this one and another 1400C version for a long time now. Might be of interest for propane or oil.

**LINK**

Not sure how price compares with others.

John

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Have you actually tried to register John, no money needed

but if you can  then he has a 800w furnace using the guts out of a storage heater that will melt 1.7kg of ali No 4 crucible in about 1hr for 10p which is a lot less than propane, also has a larger 3kw one that takes a No 8 cruicible

Its not a for sale item, just a thread about his home casting setup

This link may be easier

Edited By JasonB on 13/12/2015 18:15:00

I registered a month or so ago Jason, it shows me as logged in at the bottom of the page (ajohn the name I usually use) but any time I try to look at anything I get a message about sections (3 of them) that can't be viewed without donating. I DON'T WANT TO LOOK AT THEM (shouting)

Same thing happened following the link using a browser that is already logged in.

It's a bit like hitting a nag screen. Hope it doesn't catch on.

Edit - thinking further I am not sure if I ever received an activation email.

John

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Edited By John W1 on 13/12/2015 20:50:41

Would have thought you can ignor that and still get access to the main site, there is some good stuff in the supporters but mostly of interest to traction engine buffs

No same thing happens on all links including donate.

John

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The Mikesworkshop site has information on building furnaces for melting ali - he makes his own refractory, details on the site, from materials available at supermarkets and garden centres. Though he uses propane firing the same mixture should be usable with electrical heating I'd have thought?

JW1, thanks for those links. I would actually like to use propane, because I only have a single-phase supply, and the main (supply company) fuse is 60A. By the time the air-con, pool pumps, cooking oven, lighting, machine(s), etc are running, there's not much left for a furnace. However, I see accurate temperature control with gas as being a lot more trouble, and probably more expensive, than with electricity.

I have also run into the same problems with Jason's link to the traction-talk forum. I have been through the activation email process (and I can now log on and see the forum) but it also said that I have yet to receive notification of acceptance by the moderators. Maybe after that it will be visible...

JH, a useful link, thanks.

Roger

Edited By Roger Head on 14/12/2015 00:07:01