HT oven build!

[Taz575]

Decided to build up a heat treat oven during my vacation! Chamber is 27" long, 7" wide and 6.5" tall and I used coil over quartz tubes across the top of the kiln like a few others have done. Auber Ramp/Soak/Wifi PID, dual SSR and heat sinks, 240v and a 3840 watt, 16amp element in 13 AWG Kanthal A1. I used RedBeard's wiring style/diagram for the electronics.



I suck at welding, but I welded up an angle iron frame for it. I used a self closing door hinge and rigged a spring mechanism to hold it tighter. I used a different limit switch as well than what others do.








The rest was the generic K23 IFB build:


Gotta play with sealing the door better and 4 of the tubes broke when I was taking it up to 1900. The tubes were a very tight fit in the coils, and when I spaced out the coils, my jig was off, so there was a lot of tension between the elements. New element and tubes are ordered already.

 

[Sean Jones]

Interesting design. Most of the ovens I've seen have the elements along the side or the sides and the top. Any particular reason for having your elements just along the top?

 

[Taz575]

I saw Jarod Todd's build on BFC.com; he used the coil over tube method and loved it. It keeps the walls full thickness for better insulation and he found it made it more even temperature wise. A few other guys have done them this way and liked them over the coils in the walls. Coil in the wall means some of the heat is blocked in and that can lead to longevity issues with the coils. I was able to use 13 AWG element coils in mine, so it should last a while once I get the right sized tubes!

 

[Sean Jones]

I saw Jarod Todd's build on BFC.com; he used the coil over tube method and loved it. It keeps the walls full thickness for better insulation and he found it made it more even temperature wise. A few other guys have done them this way and liked them over the coils in the walls. Coil in the wall means some of the heat is blocked in and that can lead to longevity issues with the coils. I was able to use 13 AWG element coils in mine, so it should last a while once I get the right sized tubes!

Thanks for the explanation. That makes sense

 

[Taz575]

Got the new element and tubes installed and sealed the door better with a rope gasket; I had a 1/8"+ gap between the door and the body firebricks. Took 2.5 hrs to get to 1900 on 240v (same as the first run with the gap in the door!), so something isn't wired correctly somewhere! Going to explore that tonight; I found one of the switches for the elements may be wired backwards; power in to it is on the outbound power leg and the light doesn't light up when the switch is on or off, so that is where I am starting to look.

 

[Taz575]

Got the DPST re wired, but the light still doesn't turn on. I will put it on the positive leg of the SSR next time and see if that makes a difference instead of being on the negative leg, or run the contacts on the negative side of the switch?

I did another run last night to 1530 in around 80 minutes. I checked and the voltage into and out of the SSRs to the elements is 233 volts, so they are getting pretty much full power. It flies up to around 1000-1200, then really slows down. After I shut it down for the night, I started removing the Kaowool looking for gaps/glowing. Side bricks look good, but at the top, where the roof brick sits into the wall bricks is a lot of heat loss. The Kaowool was MUCH hotter there! With the elements in grooves at the top of the brick, that's to be expected. The grooves for the tubes aren't fully covered by the roof bricks (walls aren't perfectly square) and I used a 1x1" square brick piece to cover those, but they get moved out of the way easily. I cut down some firebricks to make a larger block to square up the top of the oven and seal those corner areas better. I am going to stuff Kaowool into those grooves to plug them up better and try again this weekend. I may end up redoing the bricks, too to give better coverage there, or getting a large piece of insulwool board to use to span the top more completely and then put the bricks on top of that.

I am going to mock up a 3" wide blade in a fake foil packet and see how the height is compared to the coils with my blade holding fixture. When I rebrick, I may make the chamber a little lower. It's around 6.5" high currently, may bring it down slightly to have better overlap with the side walls. I want to make sure I won't hit the coils with the packet though, even though I have the door kill switch working and a manual kill switch, too.