Heat treat oven build help

[elijahjue]

Hay all I am trying to build a Kiln (Heat treat oven) for my knife making so that I can temper my blades but I am no electrician and I need help figuring out how much resistance I need in the coil that will be heating the oven I'm going to be running it on 220v the oven inner chamber dimensions are 31,1/2 inches deep 8,1/2 inches wide and 6,1/2 inches tall the coil slot is 11 feet long the inner diameter of the coil I made is 9.5mm I wrapped the wire around a 3/8in metal rod and the kanthal wire is

• 16 AWG, Average wire diameter: 0.0508 in. (1.29032 mm), 0.324 Ohms/ft at 68°F
• ALLOY: Kanthal A-1 (22% Cr, 5.8% Al, Fe balance). Rated for temperatures up to 1400°C (2550°F).

I am going to be heating this oven up to 1500- 2000 degrees and I need it to hold temp as evenly as posable for as long as posable Thank you

[Buzzkill]

I'm not really sure what you're asking of us.  You should have calculated the amount of resistance based on how much current (amps) you are willing/able to draw.  Once you know the target resistance you can calculate the length of wire needed to provide that resistance.    How did you determine how much of which diameter wire to coil before you knew how much resistance you needed for your system?  Are you going to run the elements in series or in parallel (assuming you have more than one coiled element)? Did you do any calculations for how many watts per square foot of surface area or cubic foot of volume you are trying to achieve?

Your PID controller will handle keeping the oven at a constant temperature once it's calibrated and programmed.  As long as your oven is insulated well enough to reach your target temperature, the controller will be able to adjust itself to keep a fairly narrow temperature range.

[timgunn1962]

There are many variables and it's a really good idea to sit down and work out exactly what you need to do and what you have to do it with BEFORE you start building. 

Some of the stuff I can infer from the information provided: You have a 220V supply available. What current (amps) can it provide? I am in the UK, where the domestic outlets are 230V nominal and are rated for 13A, limiting me to 3 Kw in round figures (If I need to, I can go to 16A or 32A, but it means the user needs to have non-standard, industrial, sockets to feed the oven). 

I'm assuming the 1500-2000 degrees is Fahrenheit? The length certainly seems to suggest Carbon steel temperatures. That is much easier than needing higher temperatures for Stainless steels.

Realistically, you don't really want to be getting too hung up about the stuff you cannot reasonably change, like the Voltage, the length of work you need to HT, etc. 

Watts per cubic foot or watts per square foot are not particularly useful unless you have all the information to hand and can deal with the math involved in calculating heat transfer rates. Understanding just enough to know when you are doing something different to established practice, and not deviating far from something you know works very well, is the path that most homebuilders of HT ovens take. Generally, it works pretty ok. 

You have obviously done quite a lot of the build already. My general impression is that you've almost certainly not loused it up beyond recovery. I've not used coils with diameters as large as yours, so don't have a feel for what will work: You may be completely golden.

If you've not already done so, Google "Kanthal Handbook .pdf" and try to get your head around as much of it as you can. 

My most recent builds have been 27" long, 7" wide and 6" high internally, using 3 kW of elements, and have managed 1300 degC, 2372 degF, during testing, so I don't think you'll have any problem reaching Carbon Steel temperatures on your dimensions, unless the IFBs you have used are really lousy insulators (by IFB standards), or you have significantly less power available than I do.

If you can let us know what power supply is available, we can work out the element power and resistance needed. Then we can calculate the surface loading. If you have any details on your IFBs that would be helpful too.

I used 16AWG Kanthal A1 on my first few builds (I've built 8 or 9 to date) and some of the guys who use them professionally experienced element burnouts. I went to 1.6mm Kanthal A1 for the 3 most recent builds and, as far as I know, they have held up well for nearly 4 years. I have always tried to maximize the available groove length, in part because I'm middle-aged and grumpy and find it easier to get the staples in to retain the coils in the grooves if I stretch the elements to the longer end of the recommended range. I run 4 lengths of the sidewalls as a rule. My elements are about 3/8" OD and the grooves are cut with a 10mm bit in a router. 

The 7" width of my build has been largely dictated by the need to bridge the roof with a single brick. I build without mortar because I just don't have the knack of using the accursed stuff. I did build one quick-and-dirty 42" oven, 9" wide, that used a 3 kW element each side. That used Ceramic Fiber Board for the roof and solved the issue of bridging the 9" gap. 

The last 2 ovens I built were 6 brick-widths long (27") and the 1" blanket I use as a door gasket compresses down to 1/2" in use, adding another inch for 28" total internal length (because some bloody irritating bladesmith had suggested, in passing, that it might be a good idea to build two ovens that could be joined together to make one for long stuff, they got built with removable backs with blanket gaskets too. I anticipated some other bladesmith telling me that a mere 56" of combined length is too short, so built the doors and roof from Ceramic Fiber Board with the facility to add layers to the door and middle for more length)