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Heat Treat Ovens Some Basics

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by SteelSlaver, Dec 28, 2012.

  1. SteelSlaver


    Feb 17, 2007
    Having spent hours on the phone with several people making Heat Treat Ovens or Modifying Kilns I thought I would post some basic things you must understand to be successful. Please don't think any of this is real criticism of those who have made the leap. Just trying to help those on the verge of starting.

    The larger the volume inside the oven the more wattage you will need. Don't try to decrease the volume by adding stuff to the inside of the oven. This will only increase the amount of stuff the oven needs to heat up. (Thermal mass). Soft fire bricks are great insulators, but adding a good layer of Kaowool between the bricks and the oven case will make the oven lose less heat and become more efficient. It also gives the bricks some expansion room.

    Leaky ovens are poor ovens. Make the door close up as tight as possible. Plug anything like extra thermocouple holes with Kaowool. Where your element ends exit use Kaowool to fill any gaps not only does it insulate heat it will not conduct electricity.

    SSRs. I recommend 2 for 220v 0vens one on each of the hots and have a switch on the door that breaks the signal line from the PID to the SSRs. A simple push button swithc correctly mounted works. The voltage and amperage from the PID is low so it does't have to be an expensive complicated switch. GET THE NEXT SIZE BIGGER SSR THAN YOU THINK YOU NEED. Not much more and money well spend.

    There is an excellent drawing of how to correctly wire a 220 oven posted here.
    The heat led is unnecessary as the PID has one already. The rest of it is perfect. USE IT

    You are far better off in my opinion having your element connections outside of the ovens heat chamber. The heat and pressure of the connection is hard on something that gets around 2400f



    P (watts) = voltage X amperage.

    Amperage is voltage divided by resistance (measured in ohms)

    Having a 220 element doesn't mean much unless you understand what kind of 220 element. If you have a 220 element with 100 ohms it would use 2.2 amps of 220v and put out 484 watts and wouldn't make toast. If you have a 220 element that has 5 ohms it would use 44 amps of 220 and make 9,680 watts. Both are 220 elements. Which one you buy or have makes a lot of difference.
    5 ohms- 44 amps -9680 watts
    8 ohms -27.5 amps-6050 watts
    10 ohms-22 amps -4800 watts
    11ohms-20 amps-4400watts
    15 ohms-14.666amps- 3226 watts

    Series and parallel are also important fundamentals you must understand when working with more than 2 elements.

    SERIES means in line or one end of each element is connected to the other and power goes from one power line through one element then to the other element, through it, then to the other power line.

    PARALLEL means the elements are connected together at both ends. Power goes into both from the same power line and leaves both at the the same place at the other power line.

    example 2 8 ohm elements connected in series would measure 16 ohms (8 through the 1st then 8 through the second)(twice the length) use less than 14 amps and make around 3000watts.

    2 8 ohm elements connected in parallel would measure 4 ohms (goes through 2 paths and is twice as easy) use 55 amps and make 12,000 watts.

    Same 2 elements way different results

    YOU NEED ELEMENTS THAT MATCH THE WATTAGE YOU WANT TO ACHIEVE. There is actually no such thing as a 220 element in my opinion.



    I could put 100,000,000,000,000,000,000 volts across an element and not burn it up as long as I keep the amperage below what that gauge of element can handle.

    What you are buying when you buy a 110 element is an element with enough resistance to keep its amperage down to a level that it will not burn up. Same thing with a 220 element.

    The gauge of the wire is related to its diameter. The larger the diameter (and thus the smaller the gauge number) the more amps it will handle before burning up and incidentally the longer it will last at a certain amperage. 14 gauge Kanthol A1 will survive 20 amps much longer than 16 gauge.

    The larger the diameter (smaller gauge number) the less RESISTANCE THE WIRE HAS PER FOOT. (OHMS) Just like water and a pipe the bigger the wire the easier electricity flows through it.

    Where it takes 50 ft 14 gauge of Kanthol A1 to make 10 ohms it only takes about 30 ft of 16 gauge (.0510 diameter). The will both take the same amperage and produce the same wattage. The 14 gauge will last longer.

    Now lets say you went on Ebay and bought a 25' long piece of 16 gauge Kanthal A1 that is advertised as a 110 element with 8 ohms of resistance. You are stuck at 110 because at 220 it will use 27.5 amps and burn up pretty fast. If you place 2 of these in series you have 16 ohms and at 220v less than 14 amps and about 3000 watts. Incidentally 2 of these in series using 220 would mean that 110v drops across each element. You can't put them in parallel because then each would use 27.5 amp for a total of 55 amps and if the elements don't burn up before your shop you have some heavy duty wiring. Adding a 3rd wire in series would make things worse. Then you would have 24 ohms less than 10 amps and 2200 watts, more isn't always better! 3 in parallel would make the ohms drop to 2.6667 and amperage jump to over 80 amps.

    IF YOU WANT an oversized oven or a quick oven you need to think about your elements and get the ones you need.

    You also need to think about placement. They need to make the oven head evenly. Hard to put elements in the door, so get them as close as safely possible and then keep any run across the back wall straight (no coils) that way the back wall isn't making a bunch of heat while the front wall (door) makes none. The smaller the oven chamber the more likely to keep oven heat even. Also the longer you take to get to final heat the more likely to be even. That doesn't mean you need to make a slow oven. Mine is fast, but I take it to near my desired temp then hold it for a bit until everything has a chance to get that temp then climb the last 50-100 degrees to my desired temp. This keeps everything even, and gets rid of any overshoot problems.

    Think about where you want the thermocouple. I like near the center and the tip down a bit past center height wise. That is where my blades are going to be. If I was going to make a long oven for something like swords, I think stick my pids thermocouple 1/3 the way back from the door and add another thermocouple and PID just to read the temp about 1/4 of the way from the rear. You could also set it up with 2 elements and 2 pids and thermocouples and have a pid run the element in the front and another run the element in the rear. With the correct wiring you could use 2 15 amp elements at 220v for a 30 amp oven with 6600 watts and have a very uniform heat the whole length of the oven. But, even on a 40 inch long oven if you kept the height and width to say 5x5 and used a single 20 amp element (50' of 14 gauge) for 4400watts it would be fairly fast and even if you laided out the coils evenly.

    I will try to help those who seek it but, please learn the basics of electricity before you jump into such a project. It really isn't that complicated. But, it can be dangerous, frustrating and rewarding.
    Last edited: Dec 29, 2012
  2. SteelSlaver


    Feb 17, 2007
    a 16 gauge element can handle 220 volts but, it would have about 11 ohms of resistance to keep it down to 20 amps and it would put out 4400 watts. I believe it would have a pretty short life. Better to get it up to about 16 ohms and run 14 amps for 3000 watts. This would take 47 ft of wire.

    Anyone can make elements. You need a drill that will turn very slowly, a long piece of all thread that has the inside diameter of the coils size you want and a vice. Take a piece of approx. 1/4 plate and drill a hole in it slightly larger than the all thread. Put a double nut an inch or so up the one end of the all thread then stick it through the plate, then add another double nut on the end. Fix the plate in the vise and then stick the drill on the other end. Now tape the very end of the kanthol to the drill chuck and have a friend SLOWLY turn the drill while you CAREFULLY feed the wire onto the threads. If you run out of all thread before you run run out of wire. Remove the coils you have made and start on the other end of the wire. Just be careful, go slow and have a buddy that will stop if you get tangled up. I have made several 50' elements. The only difference is I used my lathe instead of a drill.
  3. SteelSlaver


    Feb 17, 2007
    Info on Kanthal A1 Not listed but, I got 14 gauge from them.


    Also on the door. Mine closes up tight with a bit of Kaowool for a gasket. I have very little decarb with unprotected carbon steel because very little oxygen gets in the oven. What is initially in the oven about it because of the tight case and door. No reason oxygen is going to try to flow into a oven heating and expanding the gasses already in it. I believe I could create a oxygen free environment in my oven with a very low flow of argon through the small stainless valve and tube I have in the top.

    Here is a place I just found that has 50' of 14 gauge for just $20 or if you wanted to built a larger oven the have 13 gauge and 50' of that would make a little over 8 ohms and use 26.7 amps and make 5800 + watts. You would need a 30 amp circuit and a set of 40 amp SSRs to run it. Hmmmm I see a killer big oven in my future. LOL

    Last edited: Dec 29, 2012
  4. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    Thank you for posting this. I have had the same email torrents and tried to explain series/parallel/watts/amps/volts. Hopefully your concise explanation will help people understand Ohm's law. It is only multiplication and division, so I never could figure out what was so hard about it.

    In designing a HT oven, start with the desired output ( watts) and then calculate back wards to get all the required information needed for the coils.

    The only thing I will add is that in building a device like a HT oven, the heavier the build the better it will work. Larger capacity SSRs, thicker gauge Kanthal coils, heavier gauge wiring. I feel about 90% of the problems encountered come from one of these three things.

    I'll put this in the stickies.
  5. SteelSlaver


    Feb 17, 2007
    As I said, I will be glad to help those who wish to make their own ovens. I have learned making and using mine. Trying to help others has given me some insight into the pitfalls that some encounter. I am not trying in anyway to belittle those who I have helped. I am glad I could help them. This post is meant to try to keep others from getting things they don't want, ending up with ovens that work poorly and give those who are preparing some better basics so they don't end up having the same problems as others.

    What Stacy said is very true a bit of over building goes a long way. Under building and not doing it right the first time will cost you time, money and be frustrating. My oven isn't pretty, it is heavy duty and very durable and works well.

    I will try to take some pictures and post, but have new computer and haven't got the photo editing and handling part down very well yet
  6. llecardu


    May 31, 2008
    great info, gota work on those pics, how would you calculate watts needed for a given size oven?

    edit: never mind-- answer look up wattage of a production oven
    Last edited: Jan 17, 2013
  7. Brian Ayres

    Brian Ayres

    Feb 4, 2011

    This is awesome. My friend has an Evenheat that went through a fire and melted the controller. This will help him get it going again on his limited budget.

    Thanks for all your time posting this.

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