240V 1 Phase burn out oven element burned out in 3 places?

Discussion in 'Furnaces and their construction' started by John Gaertner, Dec 26, 2021.

  1. John Gaertner

    John Gaertner Silver

    I have a self made 18" x 18" x 18" kiln/furnace that I have been using for doing lost wax burning out and sintering of the shell investment. The element was professional made to order (thank goodness I ordered 2). I was using the oven today and for the first 6 wax invested patterns, everything was fine. I have a PID temperature controller and both Voltage and Current meter/gauges to monitor the system. Running 240V AC single phase on brand new #10 wire and cord on dedicated circuit in my shop. Everything was fine with regards to the power.

    I removed the first batch and installed a second batch of patterns in a "hot oven" for burn out and everything seemed to be fine. Then I could hear the solid state relay going on and off a lot more than normal. Opened the lid, poof, flames as always from the wax, and I could see the element was damaged and glowing in 3 places. I have since let the oven cool and inspected the inside of the oven and the Kanthal element is burned through in 3 places.

    Any suggestions on how to avoid this in the future. The hottest I had the oven, today was 1300 F. I have had it a lot higher in the past. Could it be the soot from the burned wax, shorting out the element? Any positive suggestions will be appreciated. JG.
     
  2. John Gaertner

    John Gaertner Silver

    Hello Forum, So like any other person who is anxious to find a solution I Googled "kiln heating element failures". This upload, down, below, is to a very good article. I am still open to suggestions. But it looks like some careful modifications are required on my burn out oven. I think the burning wax may have killed this element, but I am not sure. Seems weird because I have used this oven many time before. But I have never put a second batch of wax patterns into a hot oven?

    What other means are there for doing burn outs of wax or PLA? Gas/propane heat? How do you control the temperature? Do others have a hole in the bottom of their burn out oven for the melted wax to fall through, out of the oven?
     
    Last edited: Dec 26, 2021
  3. Hi John, that doesn't appear to be a valid URL to that website article, Chrome appears to be munging it up for no good reason.
     
  4. John Gaertner

    John Gaertner Silver

    Hello Mark, I am sorry, I should have checked the URL. The title of the PDF file is 5 ways to extend the Life of your Kiln's element. Here is the file as an attachment. I hope this works. John Gaertner
     

    Attached Files:

    Mark's castings likes this.
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    John,

    If it's a single heating element, I find it quite remarkable that it has failed in three locations because that means all three failures need to have occurred instantaneously since the first fail would cause open circuit. Or do you have parallel elements?

    Per you attachment, you do want to avoid foreign material contacting the hot coil. The heating elements are very reactive at those temps. Any molten metal contact will quickly alloy with the coil, reduce it's melting temp, and quickly fail. Wax residues may ignite and locally raise the temp or chemically attack the hot elements.

    For evacuating fumes, the common method is use a fan to create negative pressure inside the furnace. This can be done with the fan downstream but you must also introduce a dilution stream to reduce and monitor the combined venting air temperature to acceptable limits for fan operation. This is a common method used for ventilation hoods. There are also schemes that keep the fan upstream of the furnace and use an ejector schemes to draw the fumes from the furnace. There are pros and cons to each and they are not hard to make. Do an internet and YouTube search.

    Where are you measuring the operating temperature? The local temperature of a coiled element in a shelf will always be hotter than the furnace environment, usually several hundred degrees F. If you place a cold object in a kiln/oven, and it fills up a significant volume of the oven, and portions of the cold contents are close to the furnace wall and location where you are measuring temperature, it can create locally much higher coil temps than are being measured.

    Say for example you had a 10" bore x 16" tall furnace and you were measuring temperature in the middle of the lid. Then you place an 8" diameter x 14" tall flask full of room temperature investment in the furnace. If full power was applied to the heating elements in short order the temperature in the 1" gap can greatly exceed the sensed temp at the top center of the furnace. The solution to this (at least as far as the furnace and heating elements are concerned) is a slow(er) heating schedule.

    Best,
    Kelly
     
  6. John Gaertner

    John Gaertner Silver

    Hello Everyone, Thanks for your help! Its great having other people to ask help from and bounce ideas off.

    After lots of thought I think three things occurred. One, I put room temperature wax invested parts in a 1000 F oven on my second batch of invested parts. I should have let the oven cool down more. I normally put the first batch in a 750 F oven. Two, I think two of my invested parts may have been learning on the wall of the oven and this could have lead to a hot spot and the shorts in the oven element. The element did short out in 3 places. Three, I need to get the melted wax out of the the bottom of the oven. Right now it pools in the bottom of the oven around the bottom layer of fire brinks and I think it just burns forever. I will try cutting a hole in the bottom to let the wax pour out quickly. If anyone has a suggestion about this, please post.

    Kelly, answers to your questions. I have a ceramic covered thermocouple located at the center of the chamber, through the wall. Its reading is displayed on the front control panel as is the Voltage and Current, and set temperature. I have a single 0.060" thick Kenametal element that I ordered from an commercial oven making company. Anyone know what kind of fan you would use to pull the wax fumes out so the fan is not destroyed by the heat? doesn't the fan just cool your oven down? I do get smoke escaping from under the lid. I just replaced the fire cord on this oven but the glue I used failed on this heating.
    I may try a mechanical attachment on the next seal I ordered.

    Thanks, Jgaertner
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If your elements are just coiled Kanthal A1 wire, you should look at making them yourself. Much less expensive and timely. There are how to threads here if you are interested.

    John, this is what I was trying to describe in my previous response. The fan isn't anything special because the air stream is cool.

    Kiln Evacuation System.jpg

    Yes, but if tuned properly, it is minimal and the effluent can be just 10F above ambient. It doesn't take much air flow to vent the furnace and a fraction of an inch of negative pressure is all it takes with sufficient duct size. You may need to cut an additional hole in your kiln. The commercial kiln manufacturers sell evacuation systems just like this and are listed on their websites. My kiln is a purpose built melting furnace so it has a 4" hole in the middle of the lid. That's where my venting system is installed. The drain hole in the bottom is sufficient leakage. It's hard to achieve zero odor but when tuned properly they can work quite well. The flex hose doesn't even have to be metal, but the aluminized stuff is recommended in the event of malfunction or condensing wax vapor. Occasional inspection for the latter.

    Best,
    Kelly
     
  8. John Gaertner

    John Gaertner Silver

    Kelly, Thanks for your help! I really appreciate it. I was looking on line last night for this kind of system. Not found any, yet. Will keep looking. Jgaertner Edit. Found this link for a Kiln supplier who is kind enough to share their vent manuals. Now it makes more sense to me. https://www.theceramicshop.com/store/category/33/115/kiln-vents/
     
    Last edited: Dec 27, 2021
  9. Zapins

    Zapins Gold

    I made a propane burn out kiln. The temperature controller is handled by a pid with solenoid and a cleaver bit of tube work to pass some of the gas around the soldiered to act as a pilot light. Sv seeker came up with the design for a self regulating propane burner on youtube in his video when he cast the propeller or anchor winch system a couple years back.

    Here is the build thread. Lots of links to relevant burner setups. It works well. Fairly simple to make too.
    http://forums.thehomefoundry.org/in...c-shell-burnout-kiln-for-lost-wax-zapins.586/

    20190126_183135.jpg
     
    John Gaertner likes this.
  10. mytwhyt

    mytwhyt Silver

  11. John Gaertner

    John Gaertner Silver

    Update on my 220V AC burn out oven repairs. I received a box of 9 insulated fire bricks I ordered a couple days ago on Amazon. I posted their description/picture on another thread for folks who did not read this one. They are great and were packed perfectly. Not a single one broken or damaged. I have welded up a stainless steel straight sided funnel that fits in the bottom of the oven. It has a 3/4" square spout that goes outside of the oven, through the bottom. I plan on making a fan powered vent like others have already described and build a stainless steel damper that I can open and close from the outside of the oven to control the air flow. I will post some pictures when its all done. One step forward and 2 steps backwards, I always say! Jgaertner
     
  12. John Gaertner

    John Gaertner Silver

    Update on my 220 V AC burn out oven repairs. I installed the insulated fire bricks in the bottom of my oven and have the stainless steel funnel finished. Are there any suggestions for a material I could buy to fill the voids that the fire bricks do not fill? My funnel is not so accurate that I could cut and file more bricks. Was wondering about that white fire "wool" that some people use in their propane furnaces. I have one and that stuff seems great. I also have some colloidal silica that I could soak the "wool" with after its been mushed to the shape of the underside of my funnel. Any suggestions?

    Jgaertner

    Repairs to burn out oven (1).JPG Repairs to burn out oven (2).JPG
     
  13. Zapins

    Zapins Gold

    I had an issue when I made a funnel for my first dewax furnace. The metal gets so hot that the liquid wax bursts into flames instantly and keeps burning. You might have to take it off and keep a 2 to 3 foot drop distance to the collection pan. Otherwise its going to cause a lot of fire.
     
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If service temp is < 2300F, the moldable ceramic fiber is easy to apply, will tolerate minor movement. You can buy it by the gallon, 5gal, or calking tubes.

    Best,
    Kelly
     
  15. John Gaertner

    John Gaertner Silver

    Thanks for the warning. As built the funnel carries the melted wax out through the bottom of the oven and I plan, into a bucket of water. My oven sits on legs that I can raise or lower the height. My hope is, if I keep the lid closed and sealed, there should be very little opportunity for a fire to start. But you are correct. as soon as you open the oven, right now it wooshes into flames because of all the melted wax that was accumulating.

    I guess the first burn out will tell?
     
  16. John Gaertner

    John Gaertner Silver

  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If you don't have a local source, You can order it from your arm chair here. It's all the same stuff regardless of brand. It's easy to apply from a calking tube but it's great stuff to have on hand so I'd get a gallon and apply it with a putty knife and your finger.

    https://www.ebay.com/itm/254519449811?hash=item3b428a98d3:g:np4AAOSwlHJeUDSY

    https://www.ebay.com/itm/254519450959?hash=item3b428a9d4f:g:np4AAOSwlHJeUDSY

    Best,
    Kelly
     
  18. John Gaertner

    John Gaertner Silver

    Thanks for those links. I tried to get more information at the Distributor but they want you to open a commercial account with them? No thanks. I will see about ordering some from this vendor on eBay. Jgaertner
     
  19. Zapins

    Zapins Gold

    I did something similar. Tried to direct and funnel the wax away using a similar design to yours and a funnel/spout to pour it into a pan off to the side.

    Also I have found that if you catch the wax in a pan of water then try and remelt it later and reuse it it becomes unusable as it absorbs water and sputters/sparks. I have not been able to figure out a good way to extract the water from the reclaimed wax. Doesn't seem to boil out with heat and it doesn't separate out either.

    My initial wax adventures
     
  20. John Gaertner

    John Gaertner Silver

    Just reporting back... I bought some "Rutland brand 2000 F Furnace Cement" to glue the sealant rope around the top of my burn out oven lid and to repair some areas and it worked really well. You can buy it on Amazon for less than $7 per 10 oz. caulk tube. You can smooth it with a putty knife and I pushed my furnace seal rope into a bead of it and the seal really stuck well. Its flexible on the top 2/3'rds, where the lid comes down but its held in place with this caulk. I just ordered 2 more tubes to have on hand. Waiting on the 2400F insulation material to fill the void around my SS funnel.

    Jgaertner
     

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