Small Kiln Auxillary Melting Furnace.

Discussion in 'Furnaces and their construction' started by Melterskelter, Jan 30, 2021.

  1. Melterskelter

    Melterskelter Gold Banner Member

    A friend of mine had a small Skutt kiln available that I thought might make a decent furnace for aluminum melting. It is about 12" diameter and 14" tall with two banks of two coils (two coils up and two at a lower level) with a total resistance of 14A. It is wired for 220 VAC service. It used to have a kiln sitter controller but, all that stuff has been removed so that all the coils are terminated together with all four "positive" and all four "negatrive" legs joined together and it is plugged in for "on" and unplugged for "off." (You can not beat that circuitry for simple/bullet-proof.)

    So, I think that means it is approximately 3500 watts power and 15 amps draw. Correct me if I am wrong. This electric furnace/ resistive heating doesn't seem to come in gallons/hr units! The nameplate says 2500W which seems at variance to my measurements. Maybe the coils are not original?

    So, it looks like this unit should allow me to set up a programmable controller for not a lot of money.

    https://www.amazon.com/Inkbird-Temp...jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

    Is this unit junk? Is the SSR junk. I think I have read here that the thermocouple is something to toss. No big deal as there is a thermocouple already in the kiln---a type K.

    I think I will house the electrics in a steel box made of expanded metal or well-ventillated sheet metal as I understand the SSR generates a fair bit of heat.

    My plan is to set this furnace up like my diesel with the same hoist trolley system.

    I presently have a Bartlett programmable control on a front loader I use for heat treating small castings. I like its ease of use and programmability. Unfortunately that unit is not well setup for melting.

    Any guidance is appreciated.

    Dia.JPG Height.JPG Nameplate.JPG Overview.JPG Sitter.JPG

    Denis
     
    Last edited: Jan 30, 2021
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  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Looks like a pretty nice little kiln. That's pretty close to the size of my smaller furnace......10" bore by 14" tall but have two 4kw coils for 8kw total. From cold start it will melt 10lbs of aluminum in an A10 in 25-30 minutes. I'm guessing you would be the better part of an hour for same due to the lower power and higher mass, but as long as you have productive things to do during the heat that doesn't seem like much of a problem to me.

    When my resistive electric furnace had IFB, the metal temp would lag the furnace temp by 200-300F. Even though I had the PiD set at 1800F, it would never get there because the melt would reach pour temp before the furnace reached 1800F. With the much more massive dense castable version the temp difference/lag was only about 100-150F but subsequent melts were fast. With my low mass furnace it rapidly reaches 1800F in about 10 minutes, the PiD cycles and controls to that temp, then about 10 minutes later the metal slumps and melts, and then 5-10 minutes after that, it's at pour temp.

    The pottery folks don't like to open their kilns when at temp. They call it "crashing the kiln". As a melting furnace we have no choice. Avoid tool contact with the wall. The IFB is fragile. It also won't hold up to splashed molten metal contact. A thin sacrificial layer of loose Al2O3 blasting media on the floor will protect it. So will Satanite. If you accidentally splash aluminum on the heating element, it will alloy and fail in short order.....don't do that!

    Maybe different coils were installed or maybe it originally had the coils wired in series versus parallel. Lower resistance will mean higher power. If you find it to be underpowered or the melt times objectionable I wouldn't hesitate to design and install coils to put 8kw into that box.

    That thermocouple isn't very useful for this duty. The heavier gage TCs like this one are much more durable.

    NEW High Temperature Type K Thermocouple Core Ceramic Kiln Probe 2372°F | eBay

    The only thing that makes me wince a little about the one in the link above is those spade connectors. Unless they are the same materials as the respective TC wires, they will cause error. I use an Inconel sheathed k-type because they are only 1/8-3/16" in diameter and I can drill a hole and insert them anywhere. I also use them inside a carbon gouging rod for molten metal contact temp measurement.

    The big thing about both the TC and controller is make sure they cover your intended range. Many of the PiDs will not display or control to 2300F...some much lower. I can't speak to the one you linked but I didn't see its display/operating range.

    The biggest PITA about the cheap PiDs is having to change temp with the up/down arrows. Takes forever. Would be so nice to type the number and hit enter. Equally, with the ramp and soak, you have to change the temp and time in the first step of the program .....but they are inexpensive.

    Both of mine just have a heat sink mounted with fins on the outside of the box and SSR on the inside. The SSR is mounted to the heat sink with thermally conductive grease. I bought my first heat sink and cast my own on the second. You could do the same with the heat sink in your link. Just cut a hole in the box and stick the fins outside the box.

    http://forums.thehomefoundry.org/index.php?threads/electric-furnace-controller.458/

    Best,
    Kelly
     
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  3. Melterskelter

    Melterskelter Gold Banner Member

    Kelly,

    Thank you for the very informative post. The information is very helpful. I hear you on the up/down arrows instead of a numerical keyboard. I am guessing I’ll not be changing the set temp very often. If the becomes a royal PITA, I’ll get a numerical keypad unit and replace this one. But, I see the numerical touchpad units sell for in the neighborhood of 200 dollars or more.

    1800F is the optimal furnace temp set point for aluminum? What about bronze?

    The melt time will be no issue. I’ll be in the barn with the kiln and I can mold or mull sand while the kiln is doing its job. I have started making the frame for the furnace.

    Denis
     
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    1800F is my aluminum set point. It's arbitrary but fast enough for me and easy on equipment and materials, and as I said, in previous furnace versions it didn't matter what the set point was, at least for the first melt. Sometimes I back it off to holding temp but try to minimize the time the melt is in the molten state.

    For bronze, it depends on the heating element material. The max operating temp for Kanthal A1 (FeCrAl) and NiChrome (80/20 NiCr) are 2640F vs 2282F respectively. But the local temperature of the coil in a 3-sided groove will always be several hundred degrees higher than the furnace temperature. For Kanthal elements and bronze I would start 2300F set point but I suspect it will be a gradual climb and you wont spend a high percentage of time there. Life of the element is dramatically shortened at 2600F but the charts still show hundreds of hours of life which would last a long time for me. NiCr is probably marginal for Bronze duty but depends upon the bronze. -I need to try some day.

    Best,
    Kelly
     
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  5. Melterskelter

    Melterskelter Gold Banner Member

    One more question that occurs to me is what is the best base to place the kiln on. As it is, the base is just bound together IFB. That is great for insulation, but would be very easy to breakout if something drops onto it. So, I was thinking that backing it up with something that would conform to it well and would remain in good contact through heating and cooling would be ideal The idea of using Kastolite in maybe a 2 inch layer and thet sitting on an inch or two of sand or pea gravel might be a way to go. The prior owner just had it on 3 IFB bricks leaving the center completely unsupported. I don't think I want to do that. Thoughts anyone?

    I did make a base of angle iron and plywood with outriggers so I can use my hoist and trolly to unload the furnace. I practiced with the hoist and had no trouble carefully lifting out the crucible without any contact with the side walls. PID controller is coming Tuesday.

    Denis

    Don't tell Tobho but I may be casting an aluminum sword for my grandson.
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Is this to be support for the exterior/underside of the kiln floor?

    Best,
    K
     
  7. Melterskelter

    Melterskelter Gold Banner Member

    Yup, I want to rest it on something that will reduce risk of cracking bricks. A shelf inside may also be in order?

    Denis
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    On the exterior, I would think a an inch or two of sand contained by a shallow ring or tub would be sufficient for anything you could throw at it.

    Best,
    Kelly
     
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  9. Tobho Mott

    Tobho Mott Gold Banner Member

    I have cast little aluminum pegasi/unicorns flying over rainbows for my niece... No judgement here; you totally get a pass for that if it's a gift for a beloved child.

    One thing though: If the little dude has a sister, she will be needing some aluminum armour ASAP once you had over that sword. :D

    I also still have not yet added programmable controls to my little kiln... So I will be following along closely here.

    Jeff
     
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  10. rocco

    rocco Silver

    I'd give consideration to spending the extra bucks necessary to get a ramp/soak type pid controller. It gives you detailed, automated control over not only the temperature but also the rate and duration of your heating and cooling cycles, that's definitely not required for simply melting metal but would add versatility to your set-up should you ever want to do more with it.
     
  11. Melterskelter

    Melterskelter Gold Banner Member

    That is a good suggestion. I do have that functionality in a front-loader controlled by a Bartlett 10-key-padded card. I can do various ramped up/down/hold programs on it and it works well. I use it for conditioning new crucibles, for instance. But, as a front loader it is less well suited for melting. My backu plan is to buy a Bartlett card or similar if I feel to limitted by this unit.

    Do you have a particular programmable unit you favor?

    Denis
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    My small (10" bore) furnace is just PiD and my large (14" bore) furnace is ramp and soak. I use the small one almost exclusively for melting and the larger mostly for heat treating curing etc. Both are Auber PiDs. They were discussed here recently in another thread and I think several members use and recommend them.

    https://www.auberins.com/index.php?main_page=index&cPath=1

    The big reason for Auber is they have clear and understandable manuals and they support their products but I would not be surprised if they are just privately labeled versions of the others you can buy on eBay, Alibaba, and the various other import sources. You can download and view the manuals at the Auber site. They are probably $1.5-2x what you can by the other imports.

    I have 3 others PiDs that I got for $15 each with SSR and heat sink. Though there are some subtle differences, most of the ramp and soaks program essentially or exactly the same as the Auber. I can't say what other quality matters could exist. As I previously mentioned, some don't have much/adequate temp range, and I believe some have codes where they can be changed but they don't typically come with that information. I remember Mark here on the forum explaining some of the reset codes.

    Best,
    Kelly
     
  13. Melterskelter

    Melterskelter Gold Banner Member

    Well, I put 7 pounds aluminum in a n A20 and plugged it in with just direct unswitched power. Plugged in = On. Unplugged = Off No user manual ;-)

    It took 2.5 hours to melt the batch and get it to pouring temp of 1400F
    The air temp only registered 1250. So, I guess most of the heating of Super Salamander was due to direct radiation from the coils and not conducted air heating.

    The length of time needed to heat the aluminum was longer than expected. On the other hand, I felt comfortable doing some molding while the heat went on where I would ususally not want any significant distraction while burning diesel. In my diesel furnace I would have been done melting in less than a half hour based on prior experience. This process was all inside the barn on this very rainly day. No way would I have been comfortable doing an outdoor pour.

    I wonder how much different the melt time would have been with 17 pounds of aluminum vs the small melt today. Might not have been hugely longer as the mass of the crucible is pretty significant compared to the aluminum itself.

    I might think about adding a third set of coils. That would mnean disassembling the kiln, cutting the groooves, making the coils etc. For now I will just use it as is.

    Denis
     
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  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Wow, I was expecting better than 2.5 hr. So the (big?) differences between your kiln and my furnace are:

    3.5kw vs 8kw

    Reading back, did you measure the current or calculate it based upon the observed resistance? If you tried to measure the resistance with a multimeter the oxidation of the coils may have increased the measured resistance. In other words is it 3.5kw or the name plate 2.5kw.

    Unknown/Unfavorable difference in thermal mass and insulation.

    Any idea what the kiln weighs? Even though I used that moldable ceramic fiber it's closer to the mass of ceramic Fiber than IFB. Wool is 8lbs/ft3. IFB is ~70lb/ft3. That mass is probably the biggest difference in warm up time. I'm guessing the IFB is 2-3" thick. What's the wall?

    12" ID vs 10" ID

    Too early in the morning for physics, emissivity, and Stephan/Boltzman constants for me, but IIRC, radiative heat transfer is a 4th power function of the temp difference and 2nd power for the distance from the source. So twice the distance is 4 times the reduction but it would depend on how much of the total heat transfer is occurring through radiation (which common sense-wise, might be quite high, especially at big temp differences). It's also traveling through air and not a vacuum......You may want to search and check me on this bit.

    It all doesn't bode well for melting bronze in the kiln though. One thing is for certain, you'll convert the electricity to heat at near 100%, so Scotty, I need more power! Aye-Aye Captain?

    Best,
    Kelly
     
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  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I'm on the move today so cant do the calcs, but if you wanted a pretty good estimate of the relative contributions of the first two subjects above:

    Increases/decrease in warm up time will be approximately proportional to power input. -The easy one. So I'd expect 2x power =~ 1/2 the time to temp.

    My furnace base, body, and lid was 39lbs all in.

    http://forums.thehomefoundry.org/in...w-mass-electric-furnace.607/page-4#post-14126

    Weigh or estimate your IFB mass. Assume both are essentially Aluminosilicates, so you could look up the specific heat of such or just approximate with Al2O3, and calculate the energy thus time required for the temperature rise at your power level. You probably have some additional heat loss through the wall but I'm guessing the biggest chunk of the difference is in the power and mass.

    Then there is the mass of the crucible and metal, but by comparison that is the same for my furnace.

    Best,
    Kelly
     
  16. dennis

    dennis Silver

    From a less-rough draft:

    "As we passed an obvious spacecraft, I heard a voice stolen from ancient Scotland muttering in Gaelic from the old times, something about “This is all she'll do, Captain. If I take it any higher we'll be eating the core for a snack. Worragh, Worragh, Worragh.”"
     
  17. Melterskelter

    Melterskelter Gold Banner Member

    I have not weighed my kiln but I am sure it is less than 50 pounds given the ease of lifting it compared to a 5 gallon bucket of sand which weighs 50 pounds.

    I have not measured power input, just the Ohms of the coils and from that inferred power.

    I probably will add a bank of coils and might change the wire in the 2 double banks that are there to a lower resistance wire to increase power. I wuppose that the more coils there are producing heat for a set amount of power, the more efficient heat production will be. Or is that true?

    I would be sure that the coils themselves would be brought to a lower temp and thereofre should be more durable if 3 pairs of 2 coils produce the same amperage as two pairs of 2.

    I think I will buy a clamp type amp metere to determine the actual amperage of the furnace.

    Slow heating by itself is not a show stopper as I can mull and mold while the furnace is heating. But, yes, I would like more power...just becuz.

    I am using an A20. Are you also, Kelly?

    Denis

    PS My PID came yesterday. Kinda quirky documentation online. But, I think I get it. It seemed to measure accurately in a water/ice bath and in boiling water. I've yet to incorporate it into the furnace. No need at present. Plus I've had little time to devote to it as I have been super busy machining castings and shipping them. Hopefully that is temporary as I enjoy casting a lot more than machining and crating. Most of all I am glad to have a purposeful intriguing and productive activity to focus on. I see my neighbor rocking rocking rocking in a rocking chair. That would be torture.

    Denis
     
  18. dennis

    dennis Silver

    Amen. (To the torture part.)

    Am following this thread with interest, as I need a (much) smaller electric kiln for heat-treating tools. (Especially wood-working tools, to make patterns.)
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    My 30 minute figure was with an A10. A20 is longer, + another ~10-15min after the initial charge to melt but if I want a full A20 I have to add to the melt several times cuz I don’t have a practical way to preheat charge and can’t pack the crucible full enough cold.

    So let’s say 45 minutes for most of an A20. That’s at 8kw which is 2.3 x 3.5kw. So if I reduced the power in my furnace to 3.5kw I’d expect 104 minutes.

    In retrospect, I don’t think the mode of heat transfer inside the furnace matters much as long as it well insulated because the electric conversion to heat is near 100% and it is essentially a closed system except heat loss through the wall. I can comfortably touch the external surfaces of my furnace with my basre hand. I suspect your kiln outer surface is hotter and that’s loss.

    On furnace mass it doesn’t sound like a huge difference but the wool is such a good insulator you don’t end up raising the temp of a lot of it, and IFB is not quite as good and if the outer wall may be hotter, your heating more pounds of refractory.

    In the end it doesn’t make sense to change the structure, but double the power and half the melt time. Triple it........I think you could put 8-9kw into it and make a big difference.

    Best,
    K
     
  20. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    MS, if you get a chance can you measure the OD of the coil and the wire diameter? Also the l x w dimension of your coil grooves?

    Best,
    Kelly
     

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