Thinking of a homemade furnace - confused on controls

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Mattybock
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Thinking of a homemade furnace - confused on controls

Post by Mattybock »

I simply don't have the massive wad of cash to buy a used oven ($600), even if they could be found in my little nook of the plains, so I have to build one, which is really where I figured things were going to turn to all along. The bits I work on are long and thin.
Perhaps a meter and a half by 20 centimeters square is the internal size I need, and that's hard to come by. I understand how the elements work, being thermal resistors, and how the bricks are cut and assembled into a casing and all that.

The big problem is the electronics involved.

I'm going to be running it off a 120v wall plug, on a 20 amp breaker. The oven need get to only 850 degrees (about 1,550 degrees F). But the issue is what things I need and how to put them together. I'd like to avoid a digital controller device if possible, as I'd probably get the wrong thing and mess it all up. Is it possible to replace the digital box with a knobbed potentiometer? Do I need a trasnformer or a rheostat or what?

Any help?
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Dave_C
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Re: Thinking of a homemade furnace - confused on controls

Post by Dave_C »

First things first:

Can you get an electric element that will produce that temp and not ruin the element? (on 120 volt power)

If you can then the rest is sort of simple. You will need a bulb type thermostat that covers the range you intend to work in. (this may be the hardest part to find) Then determine if the thermostat contacts can handle the 20 amp load. If they can just wire it up and plug it in. If not, then use the thermostat to control a contactor and let the contactor carry the heavy current of the heating element.

You could always add some safety limits (If you can find them in the range you need) and maybe some sort of switch to turn the power off rather than just unplugging it.

That should do it,

Dave C.
I learn something new every day! Problem is I forget two.
Mattybock
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Re: Thinking of a homemade furnace - confused on controls

Post by Mattybock »

Now that straightened out the whole mess! Thank you ever much.
There is a 120 volt commercial furnace that can be had for a bit over a thousand dollars, so the kanthal wire must exist, but I see no reason in spending money I don't have to get something via a purchase, when it's much more satisfying to make it myself.

Once more, thank you.
HDB
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Re: Thinking of a homemade furnace - confused on controls

Post by HDB »

If you want to use/make an electric furnace, use a PID. You can get PID + K typ thermocouple + SSR for like 50 bucks. Check Ebay etc.

You can calculate the amount of kanthal you'd need (U = R * I or voltage = resistance * current).
and
P = U * I (power= voltage * current)

Similar question:
source:http://electronics.stackexchange.com/qu ... tt-consume
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The input voltage is 220v in single phase.
How many watt consumed for Kanthal A1 (heat wire) AWG18 for 30m at 1400 degree Celcius ?
How many If I use NiCr 60 or 80 with AWG18 ?

Does anybody know the formula ? Does your formula apply for NiChrome wire ?

I read the following below and confused :
Answer
AWG 18 has a nominal diameter of 1.016mm, and a cross section of 0.8107mm^2. The resistivity of Kanthal A1 is 1,45Ω-mm^2/m at 20 celcius. So the total resistance of 30m will be 53.65Ω at 20 degr.

The temperature factor of resistivity Ct at 1,400 degr. is given by the manufactures as 1.05 so the resistance at this temeprature will be 56.34Ω. So the consumed power will be P=220^2/56.34 or about 860W.

1400 degrees is the maximum contius temperature that you can use A1. As for V80 this temperature is the melting point, so you can not use for the specific application. You can check now by yourself the use of Nichr 60 alloy.
So...

U = R * I => R = U / I = 120V/20A = 6Ohm
P = U * I = 120V * 20A = 2400Watt

So you can use 2400Watts max for a 20A fuse @ 120V with a 6Ohm kanthal wire.

So how much wire is that? That depends on the dimensions of the wire, but the calculations are also similar to the ones I quoted.
Be aware of the fact that the resistance of the wire will decrease if the temperature increases.

It's really not that hard to make such a kiln/oven. Also you want to check youtube for diy electric kiln.
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steamin10
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Re: Thinking of a homemade furnace - confused on controls

Post by steamin10 »

I came to the party late here, and was going to suggest pottery kiln furnaces, that are aluminum capable, or parts to build in that style for controls and elements.

Going higher in temp will require a fuel gas, tho and some light fire brick. A venturi burner is easy to make, and a shop vac can provide blower air for big burners. Actually a furnace with a 10 lb LP bottle is easy to rig, and is what I use.

EDIT: Thats 100 lb, thank you.
Last edited by steamin10 on Wed Feb 25, 2015 12:11 am, edited 1 time in total.
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Harold_V
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Re: Thinking of a homemade furnace - confused on controls

Post by Harold_V »

HDB wrote:Be aware of the fact that the resistance of the wire will decrease if the temperature increases.
Are you sure about that? Seems to me temperature increases resistance. I welcome your thoughts.

Harold
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Dave_C
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Re: Thinking of a homemade furnace - confused on controls

Post by Dave_C »

I have to agree with Harold as the temperature increases so should the resistance. A lose connection in a fuse box will make a hot connection as it will resist flow of electrons. A heating element will not heat unless there is resistance to flow of electrons. A perfect conductor (gold being one of the best) causes very little heat as it does not resist flow of electrons as does NiCrome wire. Certain heating elements are made of material that purposely resist the flow of electrons and thus they make heat. The resistance will increase until the element is a short circuit and then burn out. [too much voltage, too much heat = burn out]

It looks like the OP actually ask us about controls and we ended up focusing on the heating elements? That seems to be helpful as well...

Dave C.
I learn something new every day! Problem is I forget two.
HDB
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Re: Thinking of a homemade furnace - confused on controls

Post by HDB »

Harold_V wrote:
HDB wrote:Be aware of the fact that the resistance of the wire will decrease if the temperature increases.
Are you sure about that? Seems to me temperature increases resistance. I welcome your thoughts.

Harold
Oops, you are very right. My bad. Resistance inceases if temperature increases.

So the 6Ohm @20°C would be 6Ohm * 1.04 (Temperature factor of resistivity) = 6.24Ohm @ 1000°C -1300°C

So 120V²/6.24 = 2307Watt @ 1000°C -1300°C

Partial source: http://kanthal.com/en/products/material ... nthal-a-1/

So still use 6ohm kanthal wire (at 20°C).
Hopefuldave
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Re: Thinking of a homemade furnace - confused on controls

Post by Hopefuldave »

Hi Matty, interesting project, and something I'm working towards! What's the desired use, heat-treatment / annealing, melting for casting, investment-casting burnout, enamelling or ?

Running from 120v/20A will limit the heat input to your oven (and hence loss through the oven walls becomes significant), consider having an external frame or panels around your (insulating) firebricks and adding more insulation, 2 inches of mineral or glass-fibre wool should be good for the temperatures you want.

150x20x20cm is actually quite large, so you want to keep as much heat as possible inside, it may be worth using a few high-thermal-capacity bricks inside to add thermal mass (here in the UK people throw out "night storage heaters" which were intended as thermal stores charged during off-peak periods when electricity was cheap, then released the heat thoughout the day/evening - the firebricks used are ideal for this) - put the door on the end (smallest) face!
The "storage" firebricks will slow the heat-up to the required temperature and initially use more energy, but reduce temperature variations in the oven when you have to open the door - maybe a compromise, you'll have to decide which way you'd prefer to go?

Kanthal wire is available from China on Ebay, I got enough for a 3KW (at 240V x 13A) heat-treatment furnace I'm planning for less than a tenner shipped, then you need ceramic terminal blocks (they're cheap from China too), high-temperature-insulated wire (or glass-fibre + silicone sheathing over what you've got) and a controller - as has been suggested, a PID controller would be ideal, as every time you open the door you'll dump a lot of heat to the outside world: they're not expensive nor are K-type thermocouples to measure the temperature (if you don't mind Chinese "quality" and "reliability").

The PID controllers have the advantage that they "learn" what's happening in the oven and predict the heat input needed, so keep the temperature more even - useful if you're using the oven for heat treatment, not as vital if it's just melting aluminium or Mazak etc..
Setting them up is really quite trivial as long as you can guess what the Chinese manual means to say - if you can cope with a chinese machine-tool "instruction" book, you'll be fine!

Just my ha'pennorth...
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rrnut-2
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Re: Thinking of a homemade furnace - confused on controls

Post by rrnut-2 »

Mattybock,

If you pm me with your address, I will send you a controller for your furnace. I have a few Eurotherm instruments sitting on the shelf.

At the moment, I can't do deliveries. :D

Jim B
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