MIFCO furnace questions, answered

Discussion in 'Furnaces and their construction' started by garyhlucas, Aug 12, 2019.

  1. garyhlucas

    garyhlucas Silver

    So I should thank you all for asking lots of questions, as I realized I am doing some things wrong. First was the mention of a plinth block under the crucible and I see that MIFCO sells them and there wasn't one with this used furnace. I see there is a bunch of slag in the bottom of the furnace that I should remove and the firebrick in the exhaust port needs to have the mortar repaired. I can see that a plinth block under the crucible will bring it up almost into the lid. So it may heat better and keep furnace gases out of the metal. This furnace has a double cover and the ID of the hole in the main lid is 7". So it will be much easier to charge the furnace with the crucible raised.

    I reshaped my grab & pour tongs a bit with a grinder and now it is easy to use in the furnace.

    I did as asked and took a bunch of pictures. In the process I also discovered the furnace has a stop where you push the burner into. I had it too far out. I found the manual for the furnace at MIFCOs web site and it has lots of useful information. They also sell repair and relining kits. I attached two pages from the manual that are quite important to understanding the burner.

    In the photos I took you can see that the pipe in the side of the furnace jacket is a 2-3/8" OD and the burner pipe is 1-1/2" Sch 40 pipe. There is a bulged area, apparently made from 2" Sch 40 pipe with the ends rolled in to fit against the 1-1/2" pipe on the furnace end and reduced to a 1" pipe on the blower end. That bulged area is really important for a natural gas fired furnace as you can see from the drawing! It is a venturi that accelerates the combustion air and creates a Vacuum in that chamber to actually suck the gas in. Natural gas is supplied at really low pressures so the blower could very well blow out the flame by cutting off the gas flow. So this venturi keeps the gas flowing. I also attached the BOM so you can see what parts were used.

    I think a big part of the cost of this furnace was in the controls. There is an electric gas valve, motor starter for the blower, a pressure switch for the air and a UV sensor and fire eye controller. This came out of a school and was used indoors so safety was very important.

    Attached Files:

    Tobho Mott and Melterskelter like this.
  2. garyhlucas

    garyhlucas Silver

    Here are the BOMs for a C10 and a C20 size furnace. I also added a picture of the tongs with an elbow and nipple on the end to add some counterweight for balance and it is above the crucible so it reduces pouring torque and makes a good handle for controlling the pour.

    One other thing, I have been dipping foam in the drywall compound and realized it comes out much better if you go in real slow and watch to not trap bubbles, and the coating is more uniform if you come out very slowly too. I've got some stuff ready to pour. I really need to get some cast aluminum scrap or more silicon for the 100 lbs or so of 6061 that I already have.

    Attached Files:

    Tobho Mott, Jason and joe yard like this.
  3. Jason

    Jason Silver Banner Member

    WOW.... look how tiny that exhaust hole is. I'm looking forward to pouring over this info. Thanks Gary! nice job buddy. What the heck is the hole in photo 3 for?

    Any chance we can see the business end of that burner?

    Found this on the site...

    STEEL WELDED CONSTRUCTION of heavy gauge furnace shell and reinforcing members conform to industrial specifications. Hot rolled bar is used for support rings and lid bands. The two piece furnace cover is recessed in a cast chrome iron top protection plate. The lid derrick is front operating, with removable handle. Lid swings to rear of furnace when open. A drain hole is located in the bottom of the furnace for drainage of overflow of metal. The exhaust port is at the rear of the furnace.

    Am I to believe this thing has 2 exhaust holes? One little one out the top and a side exhaust? If this is true, this would be the golden ticket that would explain the efficiency of this expensive rig. Think about it, hot air rises so don't let it. Make it work to go out the side, slowing down the loss of heat right out the top. Boy are we a dumb bunch?! In picture 4, it looks like the exhaust on the outside of the furnace, hard to tell in the shadow of the sun.
    Last edited: Aug 12, 2019
  4. PatJ

    PatJ Silver Banner Member

    I have seen a couple of furnaces in the UK with side exhaust that goes up a stack on the outside of the shed (the furnace being inside the shed).
    I am not sure I would want to operate a furnace inside with carbon monoxide and low oxygen, but I think they operate their furnaces with the door open, and the walls of the shed seem to be cast concrete, so obviously it can be done safely (still not sure I would attempt it, and my walls are flammable).

    I presume the hole in the lid is for viewing the melt and perhaps feeding in some scrap, and not for venting exhaust.
  5. Jason

    Jason Silver Banner Member

    Hey Pat... Million dollar question...
    Are they hotter and more efficient?

    I would like to see a tape measure on that exhaust hole inside the furnace and maybe on the outside portion too. Wonder if this thing could run a fuel burner? Those of us running waste burners have larger exhaust holes in the top. I'm curious what the outcome would be with the small side exit. My estimate is inside the furnace, that hole is 1.25" x 3.25" so it's got around 4sq inches of hole... Compared to say my 4inch round exhaust at the top, I've got 12.5sq inches of exit.
    Last edited: Aug 12, 2019
  6. garyhlucas

    garyhlucas Silver

    Yes the exhaust goes out the side through a rectangular opening. It is about 5” x 2” on the inside of the furnace. With the crucible up on a block the hot gases swirl around it not over it and go right out. I would guess the intent is to not have the gases contact the molten metal. I’ll grab some pictures of it heating up on the next pour.
    Jason likes this.
  7. Melterskelter

    Melterskelter Silver Banner Member

    Great work, Gary. Thank you!

    Look how far outside the furnace air and gas are mixed---at least 12 inches. I am guessing the furnace has not been fired since it was painted as the paint is uncharred the full length of the tube. It will be very ineresting to see how far downstream the burn tube gets hot enough to turn the paint black. Please post a pic post firing. Sorry to beg so much but this is an important learning opportunity. The results will not be directly transferable to our oil furnaces. But, clearly, Mifco chose its mix point very deliberately after testing various designs. And the general principles are aplicable to our oil burner designs. I have a feeling I will be moving my nozzle up the burn tube and away from the furnace a ways.

    I don't think I'd worry about the slag I see on the floor.

  8. Jason

    Jason Silver Banner Member

    Yes and no Denis. Without seeing in the burner tube it's hard to see where the flame front will form. With a blower/gas pressurized system like this, you HAVE to mix the fuel air back aways. The lower the fuel pressure, the further back up the burner pipe you must go. Look at this photo, this was the shortest I could make that 2" pipe to be able to burn lazy household natural gas. The flame appears on the outside of the refractory block.

    Yes that's a crap hose from lowes running natural gas to the base of that long pipe.
    Microwave blower with a lighting rheo on it for speed control. I can cobble some serious crap!
  9. garyhlucas

    garyhlucas Silver

    Nope the furnace has been fired twice since painting! The ignition electrode sparks against the inside of the 1-1/2 x 6” nipple right at the opening inside the furnace. So all the heat is inside the furnace.
  10. Jason

    Jason Silver Banner Member

    I rest my case.;)
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Gary, did you happen to time your melts? I'd be interested in what time you observe to melt 10lbs (or whatever alloy or weight of charge) of aluminum to a given pour temp, both from a cold start and second heat after the furnace has come to temperature. If you're melting with propane, bonus points for weight of LP used for each melt.

  12. PatJ

    PatJ Silver Banner Member

    I just don't see the laws of physics being re-written just because it happens to be a MIFCO furnace.
    I think a MIFCO is going to have about the same melt times as any other well insulated furnace.

    That being said, I also stated that a Ursutz burner would never work correctly with a furnace to melt iron, and Clarke has proven me so very wrong on that one.
    So if the MIFCO does have a magic design, that would be great to learn about, but I am skeptical.
    If there is a better/faster way to melt metal though, I am all for it.

    It looks like a well designed furnace, and I like the hot face segments.
    I priced some hot face pre-fabricated segments at the local supply house, and they were priced like gold.

    I would also be interested in melt times.

    Generally speaking, the interior of the furnace looks pretty good.
    The slag on the floor is nothing to be concerned about.
    No need to do anything with that unless you just want a cleaner look.
    You can patch up the cracks/joints with patching compound.
    The hot face segments look solid, so that is a very good sign.

    If your furnace starts to look like this, then perhaps think about cleaning up some slag (photo by scavenger).


    Last edited: Aug 13, 2019
    joe yard likes this.
  13. garyhlucas

    garyhlucas Silver

    Too excited the first time around to observe time, except that second melt was way faster. I will time the next one. I have a scale out there because I wanted to see how much propane is burned, and to measure the charge weight for adding silicon. Trying to get some cast scrap but so far not successful in finding a source. I am thinking traffic light pole bases, from when a car whacks them.

    When I was an electrician I fixed a lot of parking lot poles after cars took them down. I'd Tig weld the bases back together with it sitting on a propane plumbers pot to get it up to heat. I'd cut off the dented part of the pole and weld it back on the base. No one can tell 30 ft pole is now 28 ft. Almost always the poles were obsolete and you had to match the bolts in the concrete. Plus the poles were about 2K each plus expensive shipping so fixing them was always the way to go.
  14. Jason

    Jason Silver Banner Member

    We had a saying in the used car business... Liars don't figure and figures don't lie.
    It will be interesting to see how this thing functions over time. I can't help but think there is something to this side exhaust. How about a phony inquisitive customer call
    to a rep at a certain company to pick a brain?:D Anyone able to keep a straight face and make the call? Once upon a time I was really good at that, but not so sure now.:oops:
    Last edited: Aug 13, 2019
  15. Melterskelter

    Melterskelter Silver Banner Member

    Very interesting that the igniter is at the opening to the furnace. So they are blowing air down the tube fast enough to exceed flame propagation speed....hmmm. What recommendations do they make for determining optimum air/fuel mix, flame color, flame appearance etc. The igniter is a hot coil like a car cigarette lighter, a hot glow plug, or is a sparking device?

    How close to the igniter does the flame start to emit light?

  16. Jason

    Jason Silver Banner Member

    Poor gary.. this is going to be a game of 100questions. Everyone has to take one for the team sometime.:D

    I gotta look again at the mifco site, they should spec minimum pressures and flow rates I bet. Some of those optic eyeball things only see certain colors. The flame sensor on the f16s was like that for the afterburner.
  17. garyhlucas

    garyhlucas Silver

    The ignition is a 15,000 volt transformer, good hot spark just like an oil furnace. The UV sensor looks straight down the pipe into the furnace. The furnace does a purge cycle at start up. First the blower starts and runs for a few seconds then the ignition comes on and the gas valve opens. If the flame fails the gas valve closes and the blower continues to run then the controller trips and you have to push the reset button before it will start again.

    The manual talks about being able to throttle the heat up and down so you can heat up slowly to avoid cracking the lining and doing the same when shutting down for a pour. I see that with continuous ignition the flame can go out while adjusting and you have about 10 seconds to get it going again before it trips. When I had one propane tank and regulator it wasn’t as powerful but kept running as the gas pressure fell.

    An interesting experiment I’d like to try. HDPE as fuel. If you heat HDPE to 900c in the absence of air it turns into a high btu fuel gas about equivalent to natural gas. It just so happens that we produce a LOT of scrap HDPE at work cutting round parts from flat sheets of material from 3/4” to 4” thick, and lots of short pieces of HDPE pipe too. I’m thinking of trying a closed stainless pot in the 7” opening of the furnace with a pipe connection. If the gas coming out burns then maybe tee it into the burner just to see if it works.
    Melterskelter likes this.
  18. Jason

    Jason Silver Banner Member

    Dumb guy in the room here.:oops: What is HDPE fuel? I thought that was the stuff plastic bottles are made out of?
    I'm the kind of guy that after running that furnace once, I'd rip all that safety crap off it and stick it in a box. I have auto ignitors on the airplane and I only let them do there job on start. After that, I prefer to run the show. Sound logic seems to elude a lot of these enginerds.:rolleyes:

    Full newer manual here. http://mifco.com/wp-content/uploads/2015/01/B-C-Series-Operating-Manual-revised-8-2018.pdf good reading actually.
    Last edited: Aug 13, 2019
  19. OMM

    OMM Silver

    HDPE has a flash point at 900°F. This is taking a waste furnace to a whole new level:).

    HDPE production has fuel byproducts. If you work in a plant that produces plastic products there is HDPE fuel byproduct. It is just thick juice/oil remains that are usually discarded/ recycled.
    Last edited: Aug 13, 2019
  20. Jason

    Jason Silver Banner Member

    Anything poisonous come off that stuff when burning? Most plastics are pretty foul when set on fire. :eek: I torched an old visa card in the garage once and wish I didn't. Seriously NASTY!:eek:

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