Furnace Fail & Reconstruction

Discussion in 'Furnaces and their construction' started by Al2O3, Dec 27, 2017.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Well bad news/good news.
    • Bad news: The resistive coils and refractory insert for my electric furnace failed.
    • Good news: Now I get to build another one.
    So here’s what happened. Yesterday I had a good long casting session poured thirteen cups, 20 castings, with five melts. During the last melt I was charging a half full crucible and about 6” above the crucible I lost my grip on a 2” diameter slug from the cup/sprue of one of my previous pours. It splashed. Careless of me but I didn’t think much about it at the time.


    This morning I opened the furnace and was going to clean the crucible to get ready for another casting session when I noticed there were two breaks in the resistive coils. My furnace has (had) two separate coils and as fate would have it, a blob of molten aluminum landed on each one……this resulted in broken resistive coils in both circuits. -No more heat!



    I have a spool of Kanthal on hand so I thought I’d just pull the refractory insert and replace the coils. The insert is K23 2300F IFB mortared together and shaped with cylindrical sheet metal casing like pictured below.

    1 Skins and Wool.jpg
    2 New IFB Modules.JPG

    So I removed the sheet metal skin, removed the wool, and removed the coils. When I went to pick up the IFB cylinder, it separated into two pieces.

    3 Two Pieces.JPG

    I got out the mortar and thought I was going to put humpty Dumpty back together but when I tried to move the bottom section it fell apart in my hands. Apparently fractured from many heat cycles. Ugh.:(

    4 Many Pieces.JPG

    It wasn’t just a cylinder, but also had two nested spiral grooves for each resistive coil.

    5 Coils Scrapped.JPG

    So I’m considering my options. I’ve been very happy with electric furnace as far temperature control and melting. It worked very well, especially for a resistive electric. I’d like to replicate it dimensionally along with the coil retaining grooves. This time it needs to be more durable so I can rebuild it next time. I gained some experience and confidence in casting complicated refractory shapes in my large furnace so I think I can cast it in refractory with the coil retaining grooves and all. I may have to come up with a burner to cure it because the refractory may be electrically conductive before it is fired. Hmmmm, instead of pouring castings and machining them it looks like I have a new project.



    Best,
    Kelly
     
    Last edited: Dec 27, 2017
  2. _Jason

    _Jason Silver

    I bought Gingery's book on making an electric furnace. He cast his and used a hose set around a cardboard cylinder to allow for the heating element. I wonder if cast refractory would hold up better than IFB in this application?
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I'm thinking dense refractory would be quite a bit more durable,....even insulating castable. I think I have that Gingery book too...somewhere. I was thinking I might glue a spiral of half round foam onto the inner furnace form and then just burn out the foam for the coil retaining groove....same idea though. I'm just concerned about the durability of the foam when filling the cylindrical mold with dense refractory. I think it would work with the insulating refractory I have used because that stuff really flows under vibration but the dense castable is much thicker more difficult stuff as far as flow. -Still thinking it over.

    Best,
    Kelly
     
  4. PatJ

    PatJ Silver

    That is a bummer, but I sure know the feeling.
    I had the same problem with the furnace I build using insulating fire bricks.
    I fired it once with the oil burner and it was toast. It was a tough lesson in firebricks.

    What about wrapping some of that hard plastic water line (red or blue) around a sonotube (as Jason suggested).
    Seems like that would work if you could find the right size, or perhaps use a little more flexible hose to help with removal.
     
  5. ESC

    ESC Silver Banner Member

    I used a castable refractory 20 yrs ago for my resistance furnace. After the cure there is no conductivity as I remember. Review Gingery's method, but I think some closed cell backer rod or similar on a rolled sheet metal inner form would be strong enough for packing dense refractory. I save my forms since all the furnace linings are consumable.
    Just a note for next time. Even though it creates a hot spot, when I lose a coil and am not ready for a rewind, gently overlapping the coils in the groove allows it to be used. They are brittle hence the gentle.
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I had already spliced one of the coiled elements. Only problem I had with my furnace was the coils wanted to grow out of the grooves even though I had them anchored every 90 degrees. This occurred more if I pushed the temp to 2000F+. I think they grow a bit and then contracted when they cooled and pull themselves out of the grooves as they shrank back to room temp. I previously broke the coil trying to stretch and coerce them back into the grooves. I may try little dabs of mortar to retain them in the groves on this go. Also think I need to make the grooves a little larger so they can slide in the groove. My grooves had some tight spots.

    Any tricks for coil retention? I have several Kiln construction books and the best scheme I've seen in kilns captures the coil but it does so between layers of bricks with hairpin coils. The most common seems to be downward slanted grooves that form a bit of a pocketed shelf. I'm think I may try to cast such a profile.

    Best,
    Kelly
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Got a late start today but still managed to get the forms built.

    6 Casting Form.JPG

    It took me longer to mummify the forms in packing tape than it did to build the forms. If I can get the grooves figured out I should be ready to cast tomorrow.

    7 Mummified in Tape.JPG

    Best,
    Kelly
     
  8. _Jason

    _Jason Silver

    Probably can't buy one this time of year, but maybe a foam pool noodle?

    Or some automotive hose perhaps?
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    All good thoughts. I decided to just make a custom profile. I'll post up todays progress in a bit.

    Best,
    Kelly
     
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Another late start today but had to run some errands including picking up some refractory. I spent the better part of the afternoon working on the provisions for the heating element grooves.

    I decided I wanted to try to make an element groove profile that would allow the coiled elements to expand and contract but stay retained in the groove. I needed about 16 feet of it. So the profile was hot wired and routered in polystyrene foam as shown below. The opening is very close to the 1/2" coil diameter and the cavity about 1/16” larger than the coil, the idea being the coil will lightly interfere in the opening but be able to float and change length within the groove….but not grow out of the groove….That’s the plan.


    8 Groove Profile.jpg

    I found a sonna-tube suitable for the OD (12 1/8") and then just cut a section out of it to reduce its diameter for the ID (9 3/4") and taped the seam with filament tape. Both ID and OD are mummified in packing tape to provide water resistance and a free releasing surface. There are two separate grooves fashioned in a double helix, or double lead thread with a 3” lead for you machinists. Each groove making three trips around the plug for about 8 feet of groove length each. This is the same scheme I previously had on my IFB refractory insert. I laid out the double lead on the bore plug with a felt pen, stretched/formed the profile strips into hoops by hand, and then glued them to the plug with hot melt glue.

    9 Plug With Grooves.JPG

    9.1 Groove Close Up.jpg

    Here's the bore plug in the mold. I just need to tie the center stud off to the outer forms and it’s ready to be poured.

    10 Plug in Mold.JPG

    Here’s a look down at the gap between the outer wall and the groove profile. The gap is about 5/8". I’m going to rely on heavy vibration to flow the refractory into and fill the mold. The plan is to burn out the groove profile and maybe the inner cardboard sonna-tube as well. About the only thing I have found that adheres well to packing tape is hot melt glue. I don’t want to break the groove while the refractory is still green so probably safer to burn it all out.

    11 Groove to Wall  Gap.JPG

    Hoping to cast refractory tomorrow.

    Best,
    Kelly
     
    Last edited: Dec 29, 2017
    OCD likes this.
  11. Jason

    Jason Gold

    While I think your foam snake is a work of art... Typical kiln elements are trapped behind a small section and keeps them in place. Usually without the aid of clips. With your skills in foam work, you can totally make this. That chunk is about 3/8" of an inch in thickness.
    Like this. Profile picture on the right of this image.

    lgBrick_C1.jpg
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Got at it early today. Finished the mold up and mounted a couple turbine vibrators on opposing sides.

    12 Mold Complete.JPG

    I kept my vibe table from my previous builds. It’s just a platform that sits on four springs in the corners. I set the mold on the vibe table mixed up the refractory and shoveled it in and the vibration did the rest. It’s was a high flowing dense refractory with a placement density of about 150 lbs/ft3. It called for 5% water addition but said up to 6.5% as job site required and I figured it did, so I used the higher content. It flowed well and I’m pretty sure everything stayed put.

    13 Cast Refractory.jpg



    I mixed the refractory in three batches because that’s about all my ½” drill can handle with the drywall paddle and that thing is a beast, but the refractory is some thick dense stuff. It required 62lbs of refractory to fill the mold. Heck, the refractory body in my large 14”bore x 19”tall furnace only weighs 72lbs but it only has a 5/8” dense refractory wall and insulating refractory flanges. The wall on this one is 1 3/16” to accommodate the heating element grooves protruding 5/8” into the wall.

    The deed is done. The way-yay-a-ting is the hardest part.

    Best,
    Kelly
     

    Attached Files:

    Last edited: Dec 30, 2017
    Bentation Funkiloglio likes this.
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    There are some constructed that way but I wouldn't/couldn't give up that much wall thickness to that approach. Slanted shelf should suffice.

    For heat transfer it's more effective for the crucible to have a good line of site view of the coils for radiant heat transfer. It's just a practical matter to have them housed in slots at all. The Kanthal design handbook de-rates service temperature for coiled elements contained on three sides since it can make the local temperature at the coil hotter than the kiln/oven temperature. I would think that configuration would exacerbate the problem.

    I've seen coiled elements actually cast and encased in refractory slabs or annular cylinders.

    For the highest performance kilns the elements often just sit on cantilevered cupped shelves so only half or less of the coil is shrouded. If you use NiChrome 60 instead of Kanthal A1, you can get by hanging the element in external insulators because NiChrome 60 has more hot strength whereas Kanthal A1 does not and sags...but has higher temperature tolerance by a couple hundred degrees F.

    Best,
    Kelly
     
    Last edited: Dec 30, 2017
  14. Jason

    Jason Gold

    My kiln brick is soft and pins can be used to help secure the elements. But with the grooves, it's easy to put them back in place. I heat the area with a torch to a dull red and then shape them as needed with needle nose pliers. My old turd will easily melt aluminum. Sorry bud, but I'm not buying that placing them behind a block changes the temp much. If it cant do the job, then more elements are needed.

    I thought you were going to dump this electrical junk anyway and go to NG?

    This I believe is the better solution for element holders, but a little tough to execute in a round can. I couldn't find this photo earlier for you, but it doesn't matter. Done is done.
    e18-3-sidebrick-w-el-holder.jpg
     
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Facts are stubborn things.

    Would never do that. The electric is great. Very versatile, great control for melting low temp metals, programmable for heat treating, and operating cost is almost nothing. The NG is for my larger furnace but even it has an 8kw insert for heat treating and aging. -Not a one trick pony.

    Yah, saw those a couple years ago. Wasn't impressed by them and like you said, not practical for round furnace.

    Best,
    Kelly
     
  16. Negativ3

    Negativ3 Silver

    Looks really clean Kelly. Would you say the rattler as shown is a good investment? I have tried motors with offset weights with limited success.
     
  17. Al Puddle

    Al Puddle Silver

    What is the curing plan? How long are we going to wait?
     
  18. PatJ

    PatJ Silver

    Looks good.
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    For me I would say very much so. I use them to pack lost foam and refracrtory molds. However, they are a bit of an air pig so if you don’t have the air compressor to pull them, then maybe not. CF is ft3/min and the other liters/min. I bought the GT-25 from a main land China source. GT is “Golden Turbine” which is the original brand and these are copies but at much lower price points. They may be a shorter/quicker ride for you. Search “GT Turbine Vibrator” on eBay, order the search lowest price first, and there will be sellers that offer many sizes. Here’s an air consumption chart.

    Vibe CFM.jpg
    Might start another thread about your motor vibrator.
    Thanks Pat. Are you getting pretty close to completion with your build? The weather has been a bugger!
    The refractory instructions say 50F-80F is the ideal casting temp so we had that covered at about 60F (although that may slow the ambient cure a bit) then, 24 hrs ambient cure before any heat. Most of the recommended heat schedules contemplate thick structures whereas this is very thin by comparison, but it was a little richer in water. The hole in the top of the bore plug form is there to install a light bulb. I have a 200w bulb that will go in there today and I expect that will elevate the temp to 150F-200F. After it sees 24hrs at that temp I think it should have enough green strength to demold everything and hopefully the internal plug temp will have softened if not reactivated the low temp hot melt glue I used to glue the foam in place enough to remove the inner sonna-tube too…..tomorrow.
    I fished the old heating coils out of the trash and am making a meg-shift heating element out of the remnants…should be about 4kw or maybe a little more. I’m going to use that with my furnace controller to fire it. Here’s the schedule. I think I can be a little more aggressive with the early ramp but will observe the hold periods at the upper end. With the foam in the grooves, it will have to move outside for burn-out and man it’s could out there (0F with wind)…so will play that by ear.

    Castmax-Heat Schedule_Page_1.jpg
    Best,
    Kelly
     
    Al Puddle likes this.
  20. PatJ

    PatJ Silver

    I am close to "tea time" with my furnace.
    I put a light bulb in it last night and wrapped it in a tarp, just in case there was some small amount of moisture lurking somewhere in the shell or lid (better safe than sorry).
    We are looking at single digit temperatures here I think today or tomorrow, which is extremely rare for these parts, and is generally a boon for the plumbers since pipes always burst all over the city when we go into the single digits.

    My burner works fine in the cold, but molds are another story, and I have found that they need to be near room temperature in order to fill completely (depending on the exact mold).

    I may try to finish my insulation today and then it should be "iron-time".
     

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