Muses About A Low Mass Electric Furnace

Discussion in 'Furnaces and their construction' started by Al2O3, Jan 22, 2019.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I had been focusing on coating the plug and building it out. That's sort of the build it from the outside-in approach of hand laying the same pieces of cut and rigidized wool. Then paint the hot face surface. I'll give that some more thought. One thing I can tell you based on my recent doinking around with wool and colloidal silica soaked doesn't do what you want or stay where you put it without coercion. The stated thickness is just for reference. When I cut a piece of 1" thick wool off the role it becomes more like 1.25-1.5" thick. When I apply liquid rigidizer, it compresses or sags.

    Expanding upon the hot face discussion in your thread, as it relates to my electric furnace basin, body, and lid:
    • I need something to further densify the wool for the furnace body to support the coils. That moldable ceramic fiber is the front runner at the moment.
    • For the furnace basin, that same material is definitely the hot face front runner here because it is most commonly used to line molten aluminum transfer troughs....can't beat that for the basin. It needs an IFB pedestal in the center with a thin layer of dense castable to make it durable and insure it can support a full A20 but the rest can be rigidized wool.
    • The lid can just be a rigidized jelly role of doesn't need a hot face. There's no high velocity air and it just sits there.
    As you lose mass materials lose durability. I think this may be the best compromise of the two for me.....but still working it.

    ......and I am (still) having fun!

    oldironfarmer likes this.
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I need to start another project like I need another hole in my head, but with the better part of 2ft of snow on the ground and sub-zero temps crimping my casting style, what the heck. I’ve settled on construction and materials.

    I sent an email to my friend where I buy all me refractory materials. Low and behold they stock the moldable and pumpable ceramic fiber. I also was able to purchase 5 gallons of commercial lost foam coating from one of their sister companies……I’ll report on that later.

    I’m planning to use the pumpable version of the moldable ceramic fiber to build up a ¼” to ½” hot face. The pumpable version is a tackier and more spreadable mixture whereas moldable is more putty like for hand placement. They both have similar properties at about 26lb/ft3 density and are very good insulators…..about like ceramic fiber board. As previously mentioned, this material is also rated for continuous metal (aluminum) contact and non-wetting so should be easy to clean up any spills and drips in the basin. The rest of the refractory liner will be rigidized 8lb/ft3 ceramic wool.

    1 Ins-Tuff Pumpable.jpg

    So per my previous post, here’s where I’ve arrived on the low-mass electric furnace. This will be a dedicated aluminum duty refractory liner for my existing lift off electric furnace. The entire furnace mass is estimated at about 22lb for ¼” hot face. A ½” hot face would add about 3.6lbs. 4 ½ lbs or 20% of this total mass is added by the denser IFB pedestal and dense refractory cap in the basin. I figure I need that for durability and strength to support a full A20.

    2 Low Mass Furnace Sketch.JPG

    I kept the forms from my previous dense refractory liner build so all I needed to do was reassemble the plug, make some more of the Polystyrene heating element groove profile, and glue it on the plug. The plug is mummified in packing tape and the whole thing coated with carnuba wax.

    3 Coil Groove.JPG

    I'm gonna be busy with the day job for a while but I’ll keep pluggin’ away at it.

    oldironfarmer likes this.
  3. oldironfarmer

    oldironfarmer Silver Banner Member

    Looking good! Exciting build!!
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks Andy. Hope it turns out and performs well. We'll see.

  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    You know you're in the right place to buy refractory supplies when this marks the entrance to their parking lot....

    Ladle Driver Marker.JPG

    ...and these are the flower pots at their entrance door.

    Ladle Flower Pots.JPG

    I picked up the moldable and pumpable ceramic fiber. Will experiment with it a bit before I start on the furnace refractory liners.

    Moldable Pumpable.JPG

    Tobho Mott and oldironfarmer like this.
  6. Melterskelter

    Melterskelter Silver Banner Member

    And in what city was that?! As a former Iowan (40 yrs ago) I am surprised that such a place exists in an area that I did not know had a lot of foundry work. I wish there was a similar place near here in the PNW.

  7. oldironfarmer

    oldironfarmer Silver Banner Member

    I'm guessing Davenport.
  8. DavidF

    DavidF Administrator Staff Member Banner Member

    Friggin white shit!! :mad::mad::mad:
  9. PatJ

    PatJ Silver Banner Member

    That spiral foam is just downright creative stuff.

  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes, just North of town....and on my way to work no less. I'm blessed.

    I hear ya. Record snow fall for January. I extend my middle finger to the Polar Vortex!

    Thanks Pat. Lets hope it works!

  11. Melterskelter

    Melterskelter Silver Banner Member

    "That spiral foam is just downright creative stuff."

    It's called the Kelly Vortex---patented, ya know. ;-)

  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I do get some mileage out of that Pink stuff.

    I had to play around with the new refractory materials. The first pieces I troweled on the moldable version with a putty knife. After I got the material applied between 1/8 and ¼” thick, I just dipped the putty knife in water and smoothed it out on the rigidized wool samples I previously prepared.

    1 Moldabale on Rigid Wool.JPG

    This worked well, dried firm (24hr @ room temp), and gave good surface rigidity, like a very dense Styrofoam or very similar to refractory fiber board if you have ever worked with that material. I’d have no hesitation about troweling it onto a surfaces of the rigidized wool furnace refractory. This gives me some options for building the lid and basin because I can make and rigidize the wool structure first and coat it with the moldable ceramic fiber hot face afterward if I choose.

    For the furnace body, because of the coil features, I decided it was better to apply the hot face to the plug and build out. I think I could apply the moldable version to the plug but for some reason the pumpable stuff was about half the unit cost. It’s much wetter and brushable…..sort of.

    So I made a couple of sample pieces to experiment with the pumpable ceramic fiber. They are just foam (what else right) but I wiped them down with carnauba (paste) wax for release agent. I brushed on some of the pumpable stuff and wearing a latex glove, dipped my finger in water and roughly formed it around the samples at about 1/8-1/4” thickness and let them dry at room temperature for 24hrs.

    2 Pumpable Dried.JPG

    It is carvable and sandable when dry. That’s good to know because I think that means it can be easily patched and repaired if ever needed.

    3 Carvable.JPG

    When I peeled the samples off the foam, it was clearly not yet dry in the thicker sections. There were some slight shrinkage cracks at the thickest areas. I’ve concluded the pumpable stuff will need to be applied thin, in multiple layers, and dried with some modest heat and air movement.

    4 Not Quite Dry.JPG

    However, I do think this is going to work. Here they are demolded. Other than the uncured areas, they look fine. I hollowed out the center of the coils shelves with a razor knife and stripped them out.

    5 DeMolded.JPG
    6 DeMolded.JPG

    Since it cant be fired, I don't know if applying the wet rigidized foam on top the dried hot face will wet and reactivate the hot face. -Probably to some degree. I suspect it will take time and temperature. When the time comes to demold from the furnace body plug, I’ll probably heat the inside of the plug with a light bulb to around 150F. That should reactivate the hot melt glue I used to stick the foam profile on the plug and allow me to strip out the cardboard SonaTube and leave the foam coil form in place. I’m going to be very tempted to just burn out the coil profile as it will minimize the risk of damage cause by mechanically stripping them.

  13. It has a very similar appearance to the wollastonite-phosphoric acid cement. A mix of the two sets up as a chemical reaction releasing heat and the resulting cement melts at 3040 degrees F or 1670 degrees C. I've used it to repair a small electric furnace lining. The result is a lightweight but fragile cement used to repair pottery kilns. It's main advantage is the cure time measured in minutes.
    Last edited: Feb 1, 2019
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That is an interesting material. Not quite sure how melting temp may correlate to sustainable working temp. It was a bit far into the read but the density at ~2gm/cm3 (~125lb/ft3) is pretty close to a dense castable but I suppose the cement itself could be mixed with low density materials.

    I gave some serious consideration to just casting the furnace body as a monolith with the coil permanently molded in place like many of the commercial tube furnaces. My thought was to wrap the heating element with a thin film to keep the material out of the interior of the element, then wrap it around the plug and fill it up. It's possible it may have been able to be cured with the heating elements cast in place.....that part was attractive.

    Commercial Furnace.jpg

    I decided against it for two reasons. I'd like to be able to replace the heating elements. I could use the same foam profile on plug method described earlier and just cast a monolith, but curing it might be a bit tricky, and at 26lb/ft3, the material is still >3x as dense as rigidized wool. So I figure if my purpose here is lowest mass, I'll stick to that purpose,.....with hopefully a few small compromises for durability and practicality.

    Hoping to do a little bit of work today.

    joe yard likes this.
  15. I think you're doing the right thing in not embedding the wire in the refractory, in my limited experience I embedded heating wires into that wollastonite furnace cement to mimic the original furnace construction, it lasted about half an hour before failing probably because of the thin wire I used at the time. It must reduce energy radiation from the wires and make them run hotter, so you make the wires less exposed and safer at the expense of lifetime.
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I managed to get the first coat of moldable (actually the pumpable version) ceramic fiber on the plug today. I just troweled it on at about ¼” thick.

    4 First Coat.JPG

    I needed to come up with some way to accelerate the drying, so I made this duct.

    5 Duct.JPG

    ….and then added the gear motor and blower.

    6 Drier.JPG

    Here it is in action.

    It will need another coat or two of the moldable fiber and probably days of drying and then I’ll start building up the rigidized wool.

    Funkster, _Jason and oldironfarmer like this.
  17. Melterskelter

    Melterskelter Silver Banner Member

    I think a little BBQ sauce is called for

    Very. Cool!

  18. PatJ

    PatJ Silver Banner Member

    I really like seeing creative solutions to somewhat unique problems.
    That is what it really takes in my opinion to operate a home foundry.

  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Funny, I almost borrowed an infrared heat lamp to use instead of the forced air and was thinking the same thing. I chickened out on that because I didn't have a good way to measure/control temperature and didn't want to take the chance of melting the foam or the hot melt glue holding it in place (yet!).

    Thanks Pat. Seems like with enough time and modest heat I should be able to get the hot face dried out......but it's going to take a while. I figure I need to allow first coat as much drying time as my patience will permit. The plug side is a barrier wrapped in packing tape so I can only drive water out of the exterior surface. Multiple thin coats are much better than one thick one in this regard. Brushing didn't work very well because it was hard to get the material to cover, especially the taped surfaces, so I resorted to troweling. I think the next layer will going on much better. I would have been better off use the moldable material instead of the pumpable for this reason and because it has less water content.....but they only had 1-gal units of moldable and 5 gal of pumpable. With the latter being half the unit price, we'll make it work.

    The sample parts were probably coated a little thinner and they were not fully dried after 24hrs. I have a provision to place a light bulb in the interior of the plug. Figured I could run it on a PiD and thermocouple to protect the foam and glue. When comes time to demold, I intend to do this to melt the glue holding the foam. The cardboard tube is cut and taped lengthwise, so I was planning to remove the MDF supports, cut the tape and peal back the card board......that's the plan anyway. It air dries to reasonable strength but still needs to be fired which will occur naturally when the coils are installed.

    I have a different scheme in mind to apply and dry out the rigidized wool layers. The project won't take a great deal of personal time, but may take some calendar time.

  20. oldironfarmer

    oldironfarmer Silver Banner Member

    Wise move. I stuck a small pattern in my toaster oven and set it on 100F thinking the foam should easily handle 140F attic temperatures. The first heating cycle shrank it up into a ball. Your situation is quite different but I was surprised how quickly the toaster oven did it in.

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