New Diesel/WO Furnace Sizing

Discussion in 'Furnaces and their construction' started by Jimmymmm, Apr 7, 2020.

  1. Jimmymmm

    Jimmymmm Copper

    After spending too much time trying to tweak my current furnace to iron melting temps, I have finally acknowledged failure and ready to move on to the next build.

    My primary focus is to ensure enough volume to generate the desired temps, making sure to leave enough space between the crucible and the furnace wall. I would also like to melt larger aluminum items without cutting them up, but this is secondary.

    I'm planning to use 2" of kaowool and 1/2" Mizzou lining. I currently have two containers to choose from for the new build: a 55 gallon drum (22" diameter) and an 18" diameter air compressor tank. If I leave 2-3" between crucible and wall, the drum leaves me with a maximum crucible diameter of around 12", and only 9" with the air tank.

    The steel drum seems awfully large, and I suspect it may take a great deal of time and fuel to heat it to iron temps. On the other hand, the air tank seems awfully small, leaving me with a small melt and potential issues due to being undersized.

    What experience can you offer to help save me from making stupid mistakes? Is there a calculator to help with furnace sizing, or is there an inside diameter and height that works well for you?

    Adding a thicker layer of Mizzou seems like it would just create a bigger heatsink. But would an extra layer of kaowool noticeably improve efficiency? I make my own crucibles so specific sizing isn't an issue.

    Any guidance will be much appreciated.
    Thanks
     
    Last edited: Apr 7, 2020
  2. Everybody seems to have something different that just happens to work for them. It's kind of hard to give definite build info unless you build a close copy of someone's existing build. I'd encourage some more modifications to your existing test bed until you have something that works well for you and then make a final "production model" based on the knowledge you've gained on the patched, modified, jury rigged test bed. I could describe what I have but it runs counter to existing oil burning experience (that works) in that it has a relatively small bore with minimal crucible clearance, with a relatively higher burn rate. That said it appear to be faster to melt metals, including iron.
     
  3. Petee716

    Petee716 Gold Banner Member

    So are you planning to use your ribbon/oil hybrid burner or something else? You seemed to be having some promising results according to the linked thread but I'm not sure what your final decision has been. I also didn't see what materials your current furnace is constructed out of. Based on your o.p. the furnace's volume seems to be of concern. The one factor that seems to be key in getting over the top with iron seems to be insulation value of the furnace. Disclaimer: I have yet to melt iron. Your planned wool/mizzou design in itself will be adequate. Even better might be wool/kastolite30 li. It has the same temp value as mizzou but is considered insulating. Mizzou might still be a very good option because it doesn't conduct heat very well although not considered insulating. Regardless, your furnace will be heavy.
    I'm not certain of what amount of iron you are looking to melt but the 13" bore that you're proposing would fit a crucible that will take 35kg of copper or more.
    https://www.lmine.com/index.php?main_page=product_info&cPath=1_2_142_374&products_id=5732
    At that weight you are looking at a 2-man pour or mechanical help which may or may not be in your plans, but it is still a lot of metal. We're also finding out that less furnace volume is sometimes the most effective. The 55 gallon drum is a helluva commitment in space, weight, and material cost as well as the fuel to feed it, quite possibly with a lower return in effectiveness. Out typical furnace designs just don't seem to size up well.

    Pete
     
  4. Jimmymmm

    Jimmymmm Copper

    Going back to my current furnace, I have spent upwards of 35 hours of burn time trying to tune it, with no real progress. I should fill in more details in that thread, but basically there just isn't enough volume to get the heat I'm looking for. With an empty crucible I can reach ~1250c, but that takes 90+ minutes... and also requires periods of adding propane to get over several plateaus. Once I find the right mixture, I can see a one degree increase in temperature every 10-15 seconds, but any adjustment in either fuel or air will send the temperature plummeting. I once reached 1450c without a crucible, but that required over 90 minutes as well, and the only way to gain any increase over ~1300c was to add propane to the diesel burn and have flames shooting 2-3 feet out the vent. I determined that there wasn't enough volume for the burn, and I also suspected that the airflow velocity was too high... possibly due to the ribbon burner. I built a 2" moya burner to feed from a 1.5" air line in order to increase pressure and reduce velocity, but only reached 1100c after 70 minutes.

    After that experience I determined that a new furnace was required, but wanted to ensure I didn't make the same mistake (again) of too little volume. I certainly don't need 35kg of melt, but became (hyper)focused on not repeating past problems.

    I'm open to any suggestions that could improve my current furnace. I'm encouraged that you can reach iron temps with minimal space between the crucible and wall, but I'm not convinced it's possible with my current setup.
     
  5. Jason

    Jason Gold

    Jimmy, you may or may not have seen this. Congratulations, you have found the top of the bell curve with your current setup. You have to increase the amount of fuel and the amount of blown air at it. Hopefully you will do it more or less equally. (but not likely) I bet your rig becomes too unstable to maintain steady combustion.

    I remember your furnace now. You had some kind of ribbon burner. I thought we sorted this out. Dude, dump the ribbon burner! (And I love the ribbon burner concept, but for NG or propane and not waste oil) We just aren't there yet in my opinion. Build a siphon burner and you'll see a huge difference. The video I saw of your furnace was burning too lazy. It should sound like a jet engine when running.



    15830316184373162158480379183817.jpg

    Listen to what it should sound like around the 12:30 mark. If it's not roaring, your setup is buttkiss. No offense buddy, just trying to help you to get it up and running correctly.;)

     
    Jimmymmm likes this.
  6. Jimmy, for further experimentation, is it possible to remove the ribbon burner patch over the hole and fit a two inch pipe/tuyere down low and tangential to the bore of the furnace?. That seems to be the common point that everyone has on their furnaces before going their own way with various burner designs.
     
    Last edited: Apr 8, 2020
  7. Jason

    Jason Gold

    That's a good idea Mark. This would give him a starting point. Go with what works to get the hang of a properly setup rig and experiment later.
     
  8. It's common to every working setup I've ever seen whether propane, diesel or reconstituted Yak fat. If you change every parameter trying to get it to work then the odds are slim of hitting the right combo and there are too many factors to go into. Start with a known good foundation and experiment from there: it's taken me a over year of mucking around to get something I can use reliably.
     
  9. Jimmymmm

    Jimmymmm Copper

    I recently removed the ribbon burner and piped in a 2" tuyere for testing. I remember typing this into my first post above, but there's nothing there about my moya burner. Maybe I'm losing it. Or maybe I'm getting old. Or both. Either way the text still isn't there. Apologies.

    Anyway, I read every thread and learned more about burners than I really wanted to know. I determined that I needed more volume or more oxygen, and possibly less velocity. I replaced the ribbon burner with a 2" pipe, stepping up from the 1.5" upstream airflow in order to increase pressure while reducing velocity. I also milled/drilled a couple of drip nozzles with different sized holes ( .8mm, 1mm, 1.2 and 1.5), and tested varying levels of pressure in the oil line. I didn't see any improvement whatsoever. In fact, my last burn over the weekend with a 1mm nozzle netted only ~1100c, which is right about when I threw my arms up in the air and started looking around for a new furnace shell.

    I'm prepared to continue testing as I really want to make this furnace work. But all signs point to a lack of volume to effectively burn diesel. I ordered a siphon burner which will be here early next week to help validate, or hopefully invalidate, my assumptions.
     
  10. How tall is your plinth the crucible sits on?. I was able to get a worthwhile improvement by casting a refractory disc to sit on the plinth that came within a half inch of the walls. It formed a chamber that guided the flames back onto the oil spray and boosted burning speed. My furnace is noticeably hotter with it than without. The link below has some photos in the first post showing the difference in heat:
    https://forums.thehomefoundry.org/index.php?threads/oil-furnace-swirl-experiments.855/
     
  11. Jimmymmm

    Jimmymmm Copper

    My furnace ID is 9" and 14" in height. I have a built-in plinth that is an inch or so tall, and spans the width of the furnace. I also place a 6.5x2" plinth under the crucible. My primary crucible has an OD of 6.75" at the top.

    I closely studied your flame swirling thread earlier, which prompted me to initially build this crucible slightly wider to shrink the distance. If I recall correctly you left about 15mm free space. I think I'll build a similar wide refractory disc for testing, as well as a wider crucible to help narrow down the issue. If I'm lucky, I'll find that my assumptions about air velocity are wrong and I'll be able to get this baby hot and bothered.

    As you mentioned earlier, I need to complete these tests before installing the siphon burner next week to keep each test to a minimum number of changes.

    Btw, where do you find yak fat?
     
  12. It should be possible to graft a furnace body extension on top to allow for say 4 inches of plinth and an inch or two of refractory disc on top of the plinth. Are you using a steel crucible?, you could shorten it to get space for the plinth/disc inside. I ended up with 1.5 beer kegs in an abortive attempt to increase furnace volume below the crucible without getting hotter combustion and this allowed me to sit a disc on a taller plinth to form the doughnut chamber. So for me the extra volume and height did not work until the flames could be made to take a longer path by swirling.

    I had a go at lost foam casting this week and it shows my furnace in operation burning diesel fuel at about 10 litres over 20 minutes duration to get the AT-30 crucible 3/4 full of molten aluminium in under 18 minutes. It starts making a chuffing noise at the 16 minute mark which is a wave of molten aluminium swirling around the crucible, this is a bad thing as the dross gets stirred back in the melt. There's over an inch crucible clearance all round: 8.5" crucible in an 11" furnace bore.



    As for Yak fat, they are running around all over the place here. Attached in a photo of a tame Yak I keep as a pet:

    cat-yoga.jpg
     
    Last edited: Apr 9, 2020
    Jason likes this.
  13. Jason

    Jason Gold

    I got one of those!

    20190627_185336.jpg

    Except he likes to hump the rug on the back patio.:rolleyes:
    My wife caught the little pervert one afternoon! This is the kind of guilt that washes down the drain.:eek:

    20191016_191525.jpg
     
  14. Now you know why it's called "Shag-Pile".
     
    Jason likes this.
  15. Jason

    Jason Gold

    Lets see the failure!
     
  16. Zapins

    Zapins Gold

    I grafted an extension onto my furnace to make it taller:
    [​IMG][​IMG]

    And here's the dog tempting fate.
    IMG_6842.jpg
     
    Last edited: Apr 9, 2020
  17. Jimmymmm

    Jimmymmm Copper

    It's been a busy week, with some successes and some failures. Although anytime I learn something I consider it a success.

    I made a disc with a diameter slightly smaller than my furnace, and included an angled vent to enhance swirling as the flame tries to make it's way up and over the obstruction. Mark, I'm not sure if this is what you meant, but it's how I envisioned it. Once it dried out for a couple of days, I christened it by laying it on top of the crucible full of aluminum. The good news is that I had a crucible full of molten aluminum in about 13 minutes. The bad news is that I could only reach about 950c. Something learned.

    https://imgur.com/a/n08IhlB
    [​IMG]

    My next test was putting the disc on the plinth but under the crucible. This was mostly a failure as it took a very long time to reach 1000c, probably somewhere around 45 minutes. It also dumped diesel onto the furnace floor as it couldn't burn it all, which appears to have seeped through cracks and possibly soaked into the kaowool lining.

    I made a larger crucible (wider and taller) to fill more of the space in the furnace. It needs a couple more days of dry time, but I'm anxious to get it fired and see how it impacts the burn.

    In the meantime, I'm ready to install the siphon burner that finally arrived. Is the best installation method to run it down the center of my 2" pipe and center align the siphon burner at the end of the tube? If you're feeding this through the air pipe, how are you sealing off the entrance hole to keep air from flowing out?
     
    Last edited: Apr 17, 2020
  18. Hi Jimmy, That's an encouraging speed increase, does your furnace have a lid on it?, they need to have a lid to improve efficiency and reduce heat loss. I had a 4mm thick steel disc of identical shape to your photo with a cut out in the rim but ended up using full circle as that distributed the flames the best around the crucible. The area of the flame gap between the furnace wall and the disc was 60% or so greater than the area of the 4" tuyere pipe if I recall correctly. Do you have room to raise the plinth height with a refractory puck sitting on top?. I also had diesel on the furnace floor until I moved to a mist spray nozzle arrangement and run things a bit low until I get the refractory hot enough to vapourize the fuel, after that it's not an issue as the hot walls vapourize the fuel. Also what sort of air blower are you using?.

    Edit: The furnace wall could be patched to be a bit more circular or alternatively the disc shaped to have a constant gap with the furnace wall to even out the flame distribution.
     
    Last edited: Apr 16, 2020
  19. OMM

    OMM Silver

    I actually spit/spray diesel onto the floor & walls. All my brick are loose laid, tightly fit and mechanically secured. I personally cannot work with pooling as it will saturate my wool. I need to get it hot and fast. After about 5-8 minutes on diesel, I can switch over to 50°C WVO. I am five burns in and two melts of aluminum and I am actually starting to like WVO better. The reason why I like it better is it is a little bit more sluggish through the needle valve at the same 60 psi as the diesel through the same needle valve. My WVO is free. Diesel cost me about $.30 to start up. If I ran all diesel, I think I could probably do $10 per hour.

    In the next few days I'm going for a copper melt.

    Now, one thing I did find with the last double melt of aluminum. My sidewall bricks became a little loose. When the furnace cooled they became tight again.

    For anybody who followed my build thread... I think the stainless steel straps expanded a wee bit and then contracted afterward (the brick is as tight as day one after cooling). Neither the floor or lid brick had the same reaction as the walls. They remained tight. It was less than an 45 minute burn. My laser thermometer maxes out at 1000°C. I couldn't take a temperature reading of the brick... but the molten aluminum was 800°C, when I poured it.

    I'm not going to change anything for the copper melt. But... I do think, I will have to make some minor changes to go to cast iron.
     
    Last edited: Apr 16, 2020
  20. Jimmymmm

    Jimmymmm Copper

    I built this furnace from an air compressor tank. I cut off the top, welded in a number of large fender washers, then followed the same kaowool and Mizzou formula from the rest of the furnace and mounted her back on top. I think the exhaust hole is about 4".

    Interesting side note, once I reached ~850°C the other day I placed the exhaust hat covering about 60% of the hole, and I watched the internal temperature increase about 5°C per second up to 1000°C. Obviously it couldn't sustain that, but I haven't seen the temp increase that fast before.

    I have 4-5 inches of height to play with. Additionally, I can build a shorter crucible if more height is needed for testing. I have played around with the idea of more patching after removing the ribbon burner to make it more circular, but I'm still not convinced I shouldn't build a bigger/better furnace, so I'm saving my supplies for now. I'm interested to see how well I can get this one to perform though.
     

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