Furnace Fail & Reconstruction

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

  1. _Jason

    _Jason Silver

    That's why when I eventually build my shop, I'm wiring it with it's own service
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I pay $.087/KW-hr so at 44.5kw-hr it cost me $4 to fire the refractory. Good news is I figure it costs me less than 10c to melt 10lbs of aluminum. First melt is 30 minutes so 8kw at 30% duty cycle for 1/2hr = 10 cents. My shop is also separate service/meter and our house utility bill is typically 10x to 20x my shop so I get no grief....I pay'em both.

    Best,
    Kelly
     
    _Jason likes this.
  3. Negativ3

    Negativ3 Silver

  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Wrong thread -3 :) But thank you.

    There are a couple things mentioned in that link. One was the affect of inductance from the contactor coil and the other a snubber or resistive/capacitive load to bleed of leakage current. Would you please go have a look at the schematic on my SSR thread and let me know what you think?

    Best,
    Kelly
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Got the resistive coils made and installed tonight. Each is 63 feet of 14 ga Kanthal A1. Resistance of 13.4 Ohms. Each coil produces about 4.3kw at 240vac.

    Here’s a video of them being wound up on the winding fixture I made. Takes longer to measure the wire length than it does to wind them.



    Here are the close wound coils.

    37 Close Wound Coils.JPG

    ...and a coil with approximately a 3:1 stretch.


    38 Stretched Coils.JPG

    Here are both coils installed. Each coil makes three revolutions around the bore. The coil leads are doubled up to reduce resistance and temperature at the junction.

    39 Coils Installed.JPG

    40 Doubled Up Leads.JPG

    Barring the unforeseen, I should be back in action this weekend.

    Best,
    Kelly
     
    Melterskelter and Negativ3 like this.
  6. PatJ

    PatJ Silver

    That is a really slick little element winder.
    Works like a charm.

    I need to learn how to wind valve springs for hit-and-miss engines.
    Anybody got a video for that?
     
  7. OCD

    OCD Silver

    Never mind, seen it wasn’t thread after the second look.
     
    Last edited: Jan 10, 2018
  8. Jason

    Jason Gold

  9. Negativ3

    Negativ3 Silver

    Simple yet elegant coil winder.

    Could you show a little more detail of the end of the rod where you have the wire captive by what looks like a brass bushing plus the locking collar?
     
    Last edited: Jan 10, 2018
  10. OCD

    OCD Silver

    If ya bought them then you wouldn’t have that cool winder gizmo.
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    You have a sharp eye -3. Here are some close ups from when I wound the original coils. It is a bushing and a stop collar from a drill bit. I had a bushing that happened to be the right size. I cut and shaped it with a die grinder. The clamp could be anything that keeps the wire from rotating on the tube. You might even cut a slot in the end of the tube and bend the wire through it as long as you can extract the coil from the mandrel, (otherwise you could just drill a hole through the tube, but you couldn't extract the coil). The coil relaxes when you stop winding and does not grip the mandrel....in fact it unwinds a bit.

    2 Coil Winding Fixture.JPG 3 Test Coil.jpg 4 Winding Test Coil.JPG

    Originally I used a 3/8-16 threaded rod as the winding mandrel because I thought the thread lead of 16/in and the wire gauge of 14 (~1/16" diameter) was great coincidence and the wire would laydown on the root of the threaded rod as it was wound and the thread would advance the rod as it was being wound. It turned out that a smooth rod was better because the wire diameter and the thread lead were so close, after about every inch or two it would bind and either climb or skip a thread. With the smooth rod it naturally advances. The wire guide slot I cut with a thin cut off wheel so the wire points at the OD of the mandrel. You might be able to drill a hole to do same but you will need to extract the mandrel to remove the coil if it still has an unwound leg. In the video you might notice more hardware on the winder jig. I experimented with trying to spring load the wire guide to control the winding tension. It didn't work well and controlling the tension with hand grip was simpler and worked much better.

    My close wound coils are about 3ft long. The mandrel needs to accommodate the close wound length. If you cant reach the drill because the mandrel is too long, you'll need help. You also need to come up with a spool to feed the wire as used because it will coil and tangle ahead of the winder if you don't. Heck, you might even get by without the wire guide for a quick job and just guide and tension by hand. I will say that getting a nice uniform close wound coil is important for uniform coil spacing when you stretch it. I strongly recommend following the guidelines for coil design here.

    http://www.hi-tempproducts.com/pdf/the-kanthal-furnace-mini-handbook.pdf

    My advice to anyone who wants to wind their own coils is go for it. Just experiment with a couple short lengths of wire first to de-bug your winder. It's pretty simple to wind your own coils to your exact specs and much less expensive then buying them. Kanthal A1 is about $40/lb which is 100ft of 14 ga....lighter gauges much more feet.

    Best,
    Kelly
     
  12. crazybillybob

    crazybillybob Silver Banner Member

    So I might be over thinking this but when you say the leads are doubled does that mean there's 2 coils in parallel or you have added another short piece of wire twisted together (through the insulation and junction areas to cut down on heating there). I'm assuming the second but for clarity have to ask.

    CBB
     
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The auction you listed has more info about the coil than most and based upon that info the coils also appear to better than most as far as gauge and it even lists the material which is between NiChrome 70 and 80. Have you used them yet?

    Best,
    Kelly
     
  14. Negativ3

    Negativ3 Silver

    Thanks Kelly.

    Pat, Machinery Handbook goes into spring manufacture, mandrel size etc required.
    A good video for DIY...

     
  15. Jason

    Jason Gold

    Kelly... I haven't used them yet. I still have 3 out of my 4 coils working in my kiln. I figure when I lose one more, I'll install the complete new set and roll from there. At 10bucks a piece, I had to give them a go. Paragon wanted a couple hundred bucks for a set of coils. I only gave 25 bucks for the thing and put about 20bucks worth of new wire in it. The three coils get it to 1700 perfectly and the ceramic shells are ready for bronze after about 45mins. Works great for me as a holding point for the shells while I do everything else.
     
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Got everything reassembled tonight. Have some small things left to do…..

    41 Reassembled.JPG

    .......But back in business boys. Was only down for two weeks.

    42 Coils at Temp.JPG

    Interesting thing with the new coils, no dim LED light when the SSRs aren’t energized. It’s just set to 500F but doesn’t appear to be enough leakage current to partially illuminate the LEDs. I didn’t change anything except the coils and new junction wiring…..Hmmmm.

    43 Control Panel.jpg

    I’ll post up the details after completion this weekend.

    Best,
    Kelly
     
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I usually don't have much good to say about the cheap eBay coils but that auction you listed offers more info than most and they appear to been better than most as far as gauge. The 5kw coil only has .3" OD and is somewhere between 15ga and 16 ga. The 2.5kw coil described in the auction is about 19 ga. The material is a bit of a question mark. They get bad mouthed on the pottery kiln forums.

    Just curious, do you know how the resistance of the eBay coils compares to the ones in the Kiln and the existing drive circuit? From the auction I'd say it's in the vicinity of 22-23amps @ 220vac and 9.6 ohms. Does your kiln have four 5kw coils and one is out to lunch?

    There's lots of ways to make a 10 ohm coil. You need more length/metal when you use heavier gauge wire; they're durable but cost more. I could make you a 10 ohm coil out a couple bucks worth of smaller gauge resistance wire but it may not last long. You offer up your testicles much more freely than I do, but feel free to send your left one out David so he can use it as a BP ornament and I'll send that cheap coil along to you.:D

    Best,
    Kelly
     
  18. Jason

    Jason Gold

    I measured my wire.. It's 15ga, sorry no ohm measurement, it's all taped up in an envelope. I've seen those "kiln experts" on the forums too pissing about cheap ebay coils...
    I tend to believe they are a bit BUTTHURT from getting the hard screw from specialty companies. Resistance wire is resistance wire. It's all markup at the end of the day.
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The doubling comment referred to twisting a short piece of wire to halve the resistance and reduce temperature. Tripling or quadrupling even better. Also, there does happen to be two coils/elements in parallel as well but even though they are electrically wired in parallel when the circuit is energized, there are separate conductors to each coil from the controller so they can be independently (or simultaneously) operated.

    Best,
    Kelly
     
  20. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    One thing I should have mentioned, and Jason's post reminded me, there are many combinations of wire diameter and length that will provide a target resistance but the wire needs to be able to handle the current without runaway temperature and resistance. The design guide specifies an allowable limit to the current density. This is a function of the total surface area of the coil since most of the current is carried near the surface. Sounds fancy but it's just Current/(Pi x Diam of wire x length of wire).

    If you're just going to melt aluminum or do burn outs, something like Nichrome 80 might be a better choice than Kanthal A1. It has better hot strength but slightly lower temperature limits. I used Kanthal because cost was essentially the same and I thought I might explore the limits up to 2400-2500F for bronzes or whatnot but I never see much need for more than 1800-1900F to melt aluminum...and most resistive alloys have very good life within those limits.

    Best,
    Kelly
     

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