Oil Furnace swirl experiments

Discussion in 'Furnaces and their construction' started by Mark's castings, Sep 18, 2019.

  1. Hi Pat, I've had the issue of incomplete combustion/pyrolysis with my furnace which has relatively high rates of fuel and airflow. At the outset, the end goal was to sacrifice efficency for speed as human time/labour is worth more than the cost of waste oil. Given the resources I'd love to instrument a transparent quartz tuyere and furnace chamber to see what's really happening. At present with the swirl modifications, the furnace now gets to a yellow heat with the same higher levels of fuel and airflow that would give orange heat or result in red heat with ramjet like oscillations . It now has an internal precombustion/swirl chamber formed by a refractory disc and the plinth that allows high rates of fuel and air to burn at a yellow heat instead of the orange heat (photos in post 1 of this thread) so it has more complete combustion than previously. Almost all small fuel furnaces will show large amounts of unburnt fuel if you place a woven platinum mesh in the exhaust stream to see a secondary flame from the catalytic combustion of the unburnt fuel.

    It also melts 9 Kg of iron in 32 minutes from hot, which is fast enough for me. Post 12 of this thread goes into some depth about the physical changes made to (theoretically) increase flame propagation speeds by boosting flame turbulence, preheating the fuel spray with a recirculating flame and increased flame path.

    iron furnace 1.jpg
     
  2. PatJ

    PatJ Silver Banner Member

    Wow, that is a big blower.
    I initially used the output of a shop vacuum for combustion air.
    Then I decided that bigger was better, and went with a dust collector blower that was perhaps 1 hp.

    The more fuel and air I tried to force into my furnace, the cooler it seemed to run.
    At one point I was trying to burn 10 gal/hr.

    I finally made a valve tree, where i could vary the flow of fuel easily from 1-10 gal/hr in 1 gal increments.
    And I used a lid that had an 8" opening in it, so I could clearly see what was happening inside the furnace.

    And I ran tests at night with all the outdoor lights turned off, so I could accurately judge the color temperature of the interior of the furnace.
    Much to my surprise, right around 3 gal/hr was what produced the hottest furnace interior.

    And I had to discard the large dust collector fan, since it produced far too much combustion air.
    I ended up using a Toro variable speed leaf blower set to the lowest speed.

    My buddy Clarke uses an Ursutz burner, which is a precombustion chamber, and he likes it, but an Ursutz burner has to be made from high temperature refractory if it will last.

    I hear what you are saying about your precombustion/swirl chamber, but you could also try melting iron without that, and dial the fuel flow and associated combustion air back to 3 gal/hr.
    I think you may be surprised at the results, and how well 3 gal/hr works with melting iron.
    It would be easy enough to try, and you could hook up a dump valve (you may already have one in that piping somewhere) to get rid of excess combustion air.

    .
     
  3. OMM

    OMM Silver Banner Member

    Like Pat already mentioned… I think you have too much air, and then you need to compensate with fuel.

    The quickest easiest is choke the intake with a piece of plywood (and tape if it doesn’t suck on) then adjust fuel.

    You could also add a blast gate in your 4 inch duck work, Then adjust fuel.

    It might take you 10 minutes longer for the same melt at half the fuel and half the electrical power for the blower.
    1E039FCB-82E3-479B-9777-900466FEE8AD.jpeg
     
  4. PatJ

    PatJ Silver Banner Member

    It seemed counter-intuitive to me, but I found out that 3 gal/hr melted faster and got the melt much hotter than fuel rates above or below that.
    It is not a matter of sacrificing melt speed when you turn down the fuel and combustion air, you actually speed up the melt by turning down the fuel and air flow.
    At least that is how it worked out for me.
    Worth a try.

    .
     
  5. OMM

    OMM Silver Banner Member

    The hottest point of your flame is slightly above the furnace. This is where more outside air is mixing with unburnt fuel making a beautiful bud. If you pull that flame down in just a tiny bit, and get everything into a perfect balance… Your efficiency will go through the roof. At the end of the blue cone, you will be able to melt steel and probably five minutes.
    9D30C916-4928-4E11-8DC9-7AF737E2014B.jpeg
     
  6. OMM

    OMM Silver Banner Member

    Giving air and fuel to mix properly and to be burnt efficiently creates a perfect cone at it’s hot it’s point. I did up this drawing about two weeks ago. (It is a pretty bad sketch). But this is where I want my bud, (or slightly below).

    D4935BFD-709E-406F-B2DC-09D363EC6C5C.jpeg

    The further the redline (hourglass shape)is from the IFB, I am figuring should take the worst beating, in radiating. Entry very lean and exit very hot.

    Most guys that I’ve seen, find their lid falling apart the quickest.... and it should be based on my scale drawing.
     
    Last edited: Oct 7, 2019
  7. I'll just add some further detail about how the furnace is constructed: there is a butterfly valve on the output of the blower just after the first 90 degree PVC bend: the brass lever handle shows it's about 30 degrees open in that photo (in post #21). The power consumption has been confirmed to be proportional to the outlet opening and draws much less current than the rated 2HP. It's a high pressure but relatively low volume unit which you can see by the large diameter and narrow chord, it'll be replaced soon with a commercial 1HP unit of similar design.

    The flame is run a bit deliberately rich in the photo as I'm breaking in a new crucible and wary of cooking the graphite out of it with too lean a mixture, my cheapo crucible lost about 1/4" depth of graphite from the outer surface with all the experimenting. The fuel nozzle assembly is attached to the 1/2" straight copper tube you can see in line with the tuyere pipe on the left of the photo (see post #21): by inserting and withdrawing the copper tube the flame can be made to go inside the furnace by changing the length of the flame path.

    With the swirl chamber and narrower air nozzle, I'd estimate the path of the flames is now 2-3 times longer than it used to be: previously the 4.5" bore pipe meant the airflow was slow enough that the flame would travel 6-8 inches into the chamber then perform a 90 degree turn and travel vertically up the chamber and out the lid with no swirl at all. Now it forms a kind of spinning flame cyclone (very even distribution) which must have a longer path before exiting the lid.

    So the nozzle unit can be slid up and down the tuyere to tune combustion and minimise flames out the lid.
    air nozzle assembly 2A.jpg

    Air nozzle cone (first used in the last session) to boost swirling in the furnace and overcome lack of swirl.
    air nozzle assembly 1.jpg


    Each 3/16" thread nozzle flows 3 litres per hour @ 60 PSI and allows flow to be adjusted in 3 litre/hour increments. Fine tuning is accomplished with a needle valve on the fuel line.
    nozzle assembly.jpg

    nozzle assembly 2.jpg
     
    Last edited: Oct 8, 2019
    OMM likes this.
  8. PatJ

    PatJ Silver Banner Member

    I experimented with a lot of different nozzle/burner/blower configurations for six years.
    Single nozzle, multi-nozzle, multi-burner, large blowers, small blowers, a wide range of flow rates, etc.
    In the end, only one rule seemed to hold no matter what, and that was "less is more".
    I think there is merit to the dual 180 degree burner setup, but probably not worth the trouble of all the extra plumbing.

    Edit:
    One thing that someone mentioned here is to angle the burner tube down slightly to counter the tendency of the air/fuel flow to climb the back wall of the furnace.

    Another thing is to slow down the combustion and compressed air velocity as much as possible, again to help prevent the back wall climbing.

    A white paper says you can improve combustion by 30% by pulsing the fuel pressure, but nobody here has tried that. The white paper does not describe the exact mechanism used to pulse the fuel pressure, but I assume it is a motorized valve, and perhaps a rotary valve.

    As with all flames, there is a hot part of the flame and a cool part (such as the candle experiment, where the center of the flame is cool).
    So the idea is to create as much of the hot part of the flame as possible.

    .
     
    Last edited: Oct 8, 2019
  9. You could probably have two pumps and nozzles to implement the 30% pulsation although rapid pulsation would be difficult. I'm implementing anything that can boost the flame propagation speeds, turbulent flow being the most effective boost, followed by temperature increases. By looping the flames back onto the fuel spray, the turbulence and temperature/pyrolysis will be boosted and should drastically boost flame propagation speed... in theory. To be objective I should run it next with just the air cone nozzle and no refractory disc and see if it still gets hot enough to do iron.
     
    Last edited: Oct 8, 2019
  10. PatJ

    PatJ Silver Banner Member

    I told my buddy Clarke that he would never get an Ursutz burner to work correctly with a foundry, and he proved me wrong and uses an Ursutz every day.
    That being said, I don't think the Ursutz does anything for him other than eliminates the need for an air compressor.

    The one simplification that I tried that I could not make work was going from a spray nozzle to a drip nozzle.
    There are a number of folks here that use drip nozzles with great success, but I found them to be much more unstable than a spray nozzle, with lots of flame fluctuation.
    The Ursutz is the same way, it fluctuates a lot when running.

    The spray nozzle burner is rock solid, does not fluctuate when running, and with about 10 psi pressure on the fuel tank, it never requires adjustment during the melt.
    It burns crystal clear with no smoke at all.

    If somebody shows me a burner that performs better than a spray nozzle type without a lot of complication, then I am all for it.
    Until then, I will use my relatively simple and trusty siphon nozzle.

    I do believe there are gains that can yet be made with foundry oil burners, but it is not easy to get positive results with experimentation, as I have found.
    A well designed Ursutz made from a high temperature refractory will outperform a spray nozzle burner, but the downside is that it can work too well and damage the plinth and crucible, as Clarke has discovered.
    The Ursutz is a large and unwieldy burner compared to a spray nozzle burner.

    I do feel that experimentation is a good thing since you will have a much better feel for oil burners and how they operate after all the testing is done.

    Edit:
    If you transport your furnace and foundry equipment to various iron pours as I do, having a light weight furnace is important (my furnace is about 140 lbs total with lid and frame). A Toro leaf blower is also relatively light and compact.
    In order to free myself from air compressors, I am going with a gear pump design, which uses a slightly different spray nozzle. That experiment begins next year when I get caught up with work.

    .
     
    Last edited: Oct 8, 2019
  11. Petee716

    Petee716 Silver Banner Member

    I guess I never saw your nozzle before. Geez that's a doozy. I was wondering how you were delivering so much fuel. I hadn't realized how large your tuyere was either. I was trying to visualize the problem you were having with the flame going straight up but now I understand. It would seem that the pressure increase you get with the downsized air pipe has remedied a lot of the trouble.
    Incidentally, a couple of my budge clay graphite crucibles look like the one you showed. I agree, too lean.
    A peristaltic pump might produce the pulsation you guys are talking about. I think Donkey over at AA runs one.

    Pete
     

  12. The main reason for the 4.5" tuyere bore was so I could add 3/4 to 1" thick refractory lining and have 2"+ bore remaining. Then I could start combustion was back in the tuyere like a similar oil fired furnace I've heard about. A quick area calculation shows it has 4 times the surface area of a 2" pipe, so 1/4 the airspeed in the pipe: it's the small details that can trip you up. Those small nozzles are cheap, available at the hardware store/Ebay and are of all-metal construction.
     

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