Heat treat oven build

Kevin McGovern said:
I ran the numbers through the calculator Dan Comeau has on his blog. I played with it to get to the wattage i wanted (4000) and to fill the existing grooves in my oven. My oven is 220v. I wound up using 2 elements each with 22' of 16AWG wire. you can drop the length of the wire somewhat to get more watts, but the result is early burnout of the coils, and if they aren't long enough to run the length of your oven 2x on both sides, you may have uneven heating. Basically, I used as much stretched element as i could fit, and adjusted the gauge to reach the watts I wanted. Make sense?
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You mean run them in parallel?
 
Well, here is what i have figured. Forgive me, the only thing i could find was in metric so i had to switch everything to that.
My surface area inside my oven is 2348cm.
My power at my outlet is 122v with a 20a breaker.
The way i understand it is, the higher the resistance the less watts or power in your oven? With 50' 20g wire my resistance is is just over 49 ohms. Which equals about 300w.
For my oven, i figure i need between 1850w to 2100w.
35' of 14g a1 gives 7.43 ohms resistance, which will give right about 2000w power.
Coil at about 5mm id.

Does this look right? Or am i way off?
I was sick af when i was doing this so i may be way off.
 
JustinWoody said:
It let me download it but doesnt do anything. After reading that, i am way more confused then i was before!
My oven is 4"x5"x18". So about .2 cubic feet? Seems 120v would be plenty for this.
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You need to figure out the watts you want first. Then you change that number in the top chart.it will then calculate how much wire yo need based on the gauge and list them by gauges in the boxes farther down.
 
If total wattage of all heating elements does not exceed the capacity of a single SSR, you dont need multiple SSRs.

You need multiple SSRs for each element if perhaps you wish to create a multizone heating scheme.
Or your voltage supply does not use a grounded conductor.

But said that about grounded conductors...
Its always good practice anyway to switch both conductors of any appliance what may have exposed energized parts. Also switch both conductors of any plug connected appliance.
 
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If you've read the post on DC knives that was mentioned earlier, you would've come across a term called surface load. It's the amount of load that your element can take. 20 gauge wire in my opinion is way too thin and would have a surface load of somewhere around 5-6. Whereas the kanthal a1 handbook mentions that a surface load of about 2.5 is ideal and shouldn't cross 3 for a coiled element if you want to have a long lasting element (for ribbons and strips it could be as high as 7). If you're still planning to stick to the 20 gauge wire then I'd wish you luck, but I really think it'll burn out a lot quicker than you'd expect.
I'm making a similar sized oven 18x5.5x5.5 and I'm using 14 gauge kanthal A1 wire wrapped with 12mm od coil. It's about 17 ohms on a 240V mains which is roughly around 26-28 ft (I don't quite remember which). I could post the images of the calculations and research I did to get to this point but that would have to wait because I left those at work.
Also for an oven your size 2kw is slightly on the underpowered side. The watts per surface area should be about 0.8-1.1 per cm2, as in many commercially available kilns. It would take very long to get to temp if you're ever planning to do stainless.
With my setup on a 240v line with the elements mentioned above, you end up at 3.3kw. Which draws 14.1 amps and should reach up to temp fairly quickly. And since it's a thicker wire it's surface load is around 2.3 (if I remember correctly) . Which should make it last longer.
Also you need to keep in mind that as the elements heat up, their resistance changes slightly which reduces the current through it which drops the output power. So even with these 2kw elements, you'd be getting atleast 4% lower power not considering other losses.
I hope this helps. I'll share the images for all the calculations that I've done for this setup on monday.
Goodluck
 
Tehemton said:
If you've read the post on DC knives that was mentioned earlier, you would've come across a term called surface load. It's the amount of load that your element can take. 20 gauge wire in my opinion is way too thin and would have a surface load of somewhere around 5-6. Whereas the kanthal a1 handbook mentions that a surface load of about 2.5 is ideal and shouldn't cross 3 for a coiled element if you want to have a long lasting element (for ribbons and strips it could be as high as 7). If you're still planning to stick to the 20 gauge wire then I'd wish you luck, but I really think it'll burn out a lot quicker than you'd expect.
I'm making a similar sized oven 18x5.5x5.5 and I'm using 14 gauge kanthal A1 wire wrapped with 12mm od coil. It's about 17 ohms on a 240V mains which is roughly around 26-28 ft (I don't quite remember which). I could post the images of the calculations and research I did to get to this point but that would have to wait because I left those at work.
Also for an oven your size 2kw is slightly on the underpowered side. The watts per surface area should be about 0.8-1.1 per cm2, as in many commercially available kilns. It would take very long to get to temp if you're ever planning to do stainless.
With my setup on a 240v line with the elements mentioned above, you end up at 3.3kw. Which draws 14.1 amps and should reach up to temp fairly quickly. And since it's a thicker wire it's surface load is around 2.3 (if I remember correctly) . Which should make it last longer.
Also you need to keep in mind that as the elements heat up, their resistance changes slightly which reduces the current through it which drops the output power. So even with these 2kw elements, you'd be getting atleast 4% lower power not considering other losses.
I hope this helps. I'll share the images for all the calculations that I've done for this setup on monday.
Goodluck
Click to expand...
This helps a ton. Your kiln will actually be bigger then mine. Mine is 18x5x4. I wasnt planning to stick with the 20g wire. Just seemed more plausible to run 2 wires in parallel as according to the calculator on the previously mentioned website shows i will have about 72" of stretched wire, after it is coiled. This is with a .287" od though.
 
The two SSR setup is because in the USA, 220 is fed from both ends, not to ground. This means if you use one SSR, the coil will stop to be energized, but will still float above ground at 120volts. If you touch the coil and are grounded, it will severely shock you. And I mean severely as in potentially fatal.

I realize that few people and even commercial makers use this setup, but two SSRs are far safer than one.
 
Stacy E. Apelt - Bladesmith said:
The two SSR setup is because in the USA, 220 is fed from both ends, not to ground. This means if you use one SSR, the coil will stop to be energized, but will still float above ground at 120volts. If you touch the coil and are grounded, it will severely shock you. And I mean severely as in potentially fatal.

I realize that few people and even commercial makers use this setup, but two SSRs are far safer than one.
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Well that is very good information! Thank you!!
 
that few people and even commercial makers use this setup, but two SSRs are far safer than one.
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Safer on 120V ovens too.
If plugged to a receptacle whats wired backwards (whats very common btw, especially in residential.) heating remains at 120V above ground what is very dangerous.

Further, simply contacting a grounded conductor can give a shock. Intensity of shock is generally low and will vary depending on numerous conditions. None the less, can be dangerous.
 
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In 120vac you dont need 2 SSRs if you can rig the hot wire going from wall to SSR and the neutral wire going from heating element to wall on a double pole switch (hot on one pole, neutral on other pole, or just use two switches for each wire) The neutral wire is not guaranteed to be exactly 0V relative to ground, and so this should probably be done regardless of if your house is wired properly or not.
 
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Thanks for all the help. I will be deleting this account due to baby kids on here bitching about old posts. As of now, my heater works pretty good. I have some fine tuning to do and have an electrician wiring me some stand alone plugs to my workspace.
You all have been a great help, i will see if i can close this post.
 
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