Another electric furnace

Discussion in 'Furnaces and their construction' started by Old Jim, Jan 23, 2023.

  1. Old Jim

    Old Jim Copper

    I decided to build a 6000 watt electric lift off furnace. 10 inch inside diameter, 14 inch outside diameter by 15 inches tall not including the base and lid. The body will consist of 10 annular rings of 1-1/2 inch CFB of about 22 lbs/cubic foot density. There will be (2) 3000 watt elements each consisting of (3) coils. Number 15 Kanthal seems to hit the sweet spot in terms of surface loading, furnace wall loading, resistance and expanded length. My intention is to layer the coils alternately from one element to the other spaced 1-1/2 inches apart by virtue of the thickness of the rings.
    Right now the CFB is 6 weeks away so I’ll be working on the other bits I’ll need. Here is a pic of the circuit I plan to use to control the furnace. I would appreciate any advice and comments on this before I put it together.
    Thanks
    Jim BC232D55-63A1-455D-B24E-BE1539F06098.jpeg
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Hi Jim,

    The SPST switches are fine to use for selecting the active heating elements but don't rely on them for denergizing the heating elements for safety when you open the furnace because one side will still be energized 120vac above ground (you are using US 240 VAC 1ph, correct?) if you touch it with a metalic tool. Use your DPST switch to denergize the heating elements and move the control circuit for the PID and SSR above that switch so you don't shut down your control circuit when you open the DPST.

    I like the lamps. I haved them too. My routine for opening the furnace is to turn off the individual switches (your spst), check pilot lamps are extinguished, and open my equivelant of your DPST which is a solenoid actuated contactor controlled by latching push button switch which is also illuminated. I also have a power meter that continuously displays current and voltage. Opening my furnace means no lights on the panel.

    ........obviously I never want to be shocked.......unless it's to restart my heart. :)

    Best,
    Kelly
     
  3. Old Jim

    Old Jim Copper

    I can do that. Thanks
    Jim
     
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    One other minor point, I use two SSRs and my SPST switches just open the control circuit to them. Your circuit eliminates and the cost of an SSR for a 15 spst switch. The reason I opted for two SSRs, if one fails, I still have half power. It slows me down but doesnt end my day and the chep Chinese SSRs have a reputation for failing. 6 years on, mine have not. You'll also want a heat sink on your SSR(s).

    Best,
    Kelly
     
  5. Old Jim

    Old Jim Copper

    I have an Inkbird ITC-106VH PID. I also have a second Inkbird SSR-40 DA. Originally I considered driving both of them in parallel with the PID, but could not find any power consumption specs for the SSR’s. I have e-mailed their tech support. The PID output is rated for 30 mA at 12 VDC. I will heat sink whatever I end up using.
    Thanks again
    Jim
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    My SSRs say they are 3-32vDC and DC3-25mA, so I'd assume the higher current goes with the low voltage and vice versa putting them in the 50-60mW range. If so, at 12vDC that would suggest ~5mA per SSR, plus 2mA of leakage. I think they're all pretty similar so driving two should be no problem but probably good to confirm.

    Best,
    Kelly
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Is that CFB 2300F or 2600F rated variety? I think either will work but the higher temp stuff is also slightly more dense and durable. How are you planning to cut the annular layers and coil shelves? The other thing I think you will notice is the interior of the board is softer and more "cottony" or friable than the stabilized exterior.

    Colloidial silica is used as ceramic fiber blanket stabilizer (aka rigidizer). Painting the cut surfaces with that might be helpful or wiping on moldable cermaic fiber might do same. I mention it because I've found some variability in how the stuff responds to cutting from one source to the other, so probably a good idea to do a little experimentation with your preferred fabrication method before you cut into a bunch of stock.

    Sounds like a good build. Very similar to mine in size, about 75% of the power, but depending upon how big a crucible and charge, I think you'll like it.

    Best,
    Kelly
     
    Last edited: Jan 24, 2023
  8. Old Jim

    Old Jim Copper

    Haven’t heard from Inkbird yet. I’ll give them a few more days. My SSR’s also accept 3 to 32 volts dc input but don’t have a current specification listed. My CFB supplier is Armil CFS in South Holland, Illinois. I’m using their Duraboard HD 2300. Its density is spec’ed at 26 pcf Their Duraboard 2600 is spec’ed at 14 pcf. They don’t have a 2600 HD. Based on your previous comment I ordered the higher density product so it would be less fragile. Thermally it is only slightly more conductive than the 14 pcf stuff. The rigidizer sounds like a good idea. I don’t have any firm ideas about how I’m going to cut and carve the CFB. I don’t want to blow a bunch of fiber dust into the air even though I will be wearing a respirator and probably a Tyvec suit. I’ve got some 3/4 inch CFB I can play with. I’ll let you know how it goes.
    Thanks
    Jim
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That’s interesting. Pretty sure the higher temp stuff I last bought was denser than the lower temp. You should be fine. It cuts easily with a utility knife which may be ok for coil shelves but might be hard to manage in thicker stock and rounded cuts. They make smooth knife blades for saber/jig saws. Fine Coping saw or hack saw blade might do the trick.
     
  10. Old Jim

    Old Jim Copper

    Just heard from Inkbird and they said the ITC-106 VH controller would drive 2 SSR’s in parallel. Time to redraw my schematic, again. Thanks Kelly for the carving advice.
    Jim
     
  11. Old Jim

    Old Jim Copper

    Since I am waiting for receipt of the CFB I decided to build some of the bits I will need when it gets here. My primary interest is lost foam so I built a foam cutter using an 18 volt 3 amp power supply from a defunct lap top, a $7 Chinese motor controller, a Radio Shack switch and some stuff I had laying around. It works great! I will probably make the fence a little higher.
     

    Attached Files:

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  12. Old Jim

    Old Jim Copper

    I also built the controller I plan to use for the furnace. Attached is the circuit I finally used along with some pics of the completed control. Tomorrow I will try to power it up using some light bulbs and a bucket to simulate the furnace. The wiring is not as chaotic as it looks. Really.
     

    Attached Files:

  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That looks well done Jim. That milled arc for adjusting wire angle is an interesting feature. Don't think I've seen it done that way before. Hot wires are easily built low-tech machines but I bet you find it to be one of the highest return on investments in your lost foam casting equipment arsenal, and with a little ingenuity it's amazing how far you can get cutting and assembling pieces, not to mention keeping the mess to a minimum.

    Best,
    Kelly
     
  14. Old Jim

    Old Jim Copper

    Thanks for noticing the arc. That was fun to make. I have a Bridgeport and a 10” rotary table but that arc has a 12” radius, so I clamped a couple of pieces of stock to the table with an appropriate overhang and then clamped the aluminum angle to that. Fortunately the cutting forces were low so it worked. Today I tested the PID. I used a bucket with a couple of light bulbs mounted underneath and with the k-type thermocouple stuck thru a hole in the top. I wired it for 120 volts for test purposes and all went well. The weather is supposed to warm up some this week, so I think I’ll go to work on a vibration table.
     

    Attached Files:

  15. Old Jim

    Old Jim Copper

    I built a vibration table for compacting sand around lost foam patterns. Here are some pictures:
    The base is 1” x 2” rectangular tubing. It holds 3 double locking casters and a bracket for the air manifold. Next is the underside of the platform showing the 3 GT-8 turbine vibrators and 3 rubber isolation bushings from an air conditioning compressor mount. The top of the platform has a ring to clamp the flask to the platform. Plumbing up the turbines. They have British Standard Parallel Pipe ports! Wonder who thought that was a good idea. Here it is plumbed and another with the proposed flask sitting on it. I ran it and it seems to work. I placed the turbines to emphasize horizontal motion in the X and Z axes based on what I’ve read here on this forum. I need to paint it when the weather improves and then I’ll loom the tubing and assemble it with blue Loctite and Nyloc nuts as appropriate. I’ll also run some compaction tests.
     

    Attached Files:

    Tobho Mott likes this.
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Looks well done.

    This is what counts. How are you going to run the test?

    The simple method:
    1. Fill the flask with sand by just shoveling it in and strike off at the top.
    2. Vibrate, level the sand, and measure settling/compaction.
    3. Pour that sand on the floor and refill the flask either while continously vibrating if you have the compressed air capacity, or in 5-6 stages, level the sand and measure settling/compaction.
    Comparing the three will tell you how much compaction you are achieving and whether you have a enough power to vibrate a full flask.

    As an aside, three wheels is good since all three will always be in contact with the ground, but mine was a bit tipsy. Just be aware when you move it because if you roll over some sand or small pebble........over it goes. Ask me how I know! I have a flask extender that makes the flask twice as tall (and twice as tipsy). That's why mine now has four wheels and wider base. Still must be carefull. I still have my original.

    Best,
    Kelly
     
  17. Old Jim

    Old Jim Copper

    When I round up some sand I’ll test it per your recommendation. As I fooled around with it some more I concluded that my air supply was too restrictive so I’m going to change the inlet manifold to give more volume. Right on cue the weather was beautiful today in NE Ohio so I took it apart and shot some paint on it. Once I put it back together with the new manifold(s) and run the tests, I’ll post a picture. As always, thanks for the help.
    Jim
     
  18. Old Jim

    Old Jim Copper

    I spun up new less restrictive manifolds and painted the flask vibrating rig. I got this sand from Maynard’s. It was the only dried sand they carried. I am somewhat disappointed with it. It has a lot of very fine sand as well as the occasional piece of gravel, but I can sift that out. Per Kelly’s recommendation I filled the 13-1/2 flask (bucket) in one shot, vibrated it for about 5 minutes and it settled 1-1/2 inches. About 11%. I topped it off and dumped it out and refilled it while running the vibration continuously. I got less than 1/8” additional compaction. I’m pleased with that. In the meantime I going to try to find some more uniformly graded dry sand. Lowe’s and Home Depot don’t seem to carry the Quick Crete brand so I’ll keep looking.
     

    Attached Files:

    Tops likes this.
  19. Tops

    Tops Silver Banner Member

    Jim, the stuff I finally got from Menards for lost foam is this:

    Handy Sand - 50 lb
    Model Number: 1891344 Menards ® SKU: 1891344

    Dry, no rocks, about 50 or 60 mesh. It was kept in the Garden Center so dry but at ambient temperature. Tech Mix was the brand name.
     
  20. Old Jim

    Old Jim Copper

    Thank you for the tip. I’ll give them a visit tomorrow.
    Thanks again.
    Jim
     

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