Fitted a K type thermocouple to my furnace to help fuel air mix tuning

Discussion in 'Furnaces and their construction' started by Mark's castings, Jul 20, 2022.

  1. Fitting a large K type thermocouple on the inside of my furnace bore has really made a difference to tuning the furnace fuel/air ratio for the highest temperatures. I bought a 10 inch long K type probe with the ceramic beads over it from Ali Express a while back and last week I drilled through the refractory from the outside and angled the thermocouple into the bore about halfway up the side of the crucible. The thermocouple junction is naked but sits inside a small ceramic cup which I packed with kaolin clay to give some protection from the combustion gases. So while the furnace is running you can set a given fuel flow and vary the airflow and see what the temperature does in realtime: this really does take all the guesswork out of running an oil fired furnace in a big way. I learnt that I'd been running too lean all this time and maximum heat occurs with large yellow rich looking flames spitting 12" out the furnace lid. Also it became apparent the furnace needs constant adjustment as it heats up and the fuel system also warms up and changes characteristics. I can highly recommend a thermocouple as a tuning aid for your furnace, this unit has fairly thick wires so it's reasonably rugged and still works after reaching 1305 degrees C/ 2381 deg F.

    Gettin up there!!

    K type furnace probe.jpg

    Running the furnace at the hottest setting:
    furnace mixture setting.jpg

    Probe tip in the bore, hole sealed with kaolin clay.
    thermocouple install 2.jpg

    Some brass screw terminals to thermocouple extender wire.
    thermocouple install 1.jpg
    Last edited: Jul 21, 2022
    Tobho Mott and Tops like this.
  2. Petee716

    Petee716 Gold Banner Member

    It sure would be helpful to be able to monitor temp that way. I ruined my last iron melt by trying to blindly maximize my furnace temp and ended up too cold. I believe you’re well into the upper limits of a k-type. I wonder if going over it’s limit 15-20% will ruin it or just affect its accuracy at those temps?

  3. They say a K type's life is reduced from oxidation at the upper end of it's range (1300 deg C according to Wikipedia) , this one has fairly heavy gauge wires so it may have a decent life. K types eventually die from metal diffusion at the junction reducing output, the solution back in the day was the Australian development of the N type thermocouple but they are kind of rare. It was surprising how small a change would cause the temps to plummet and the ideal settings changed as the furnace got hot and no doubt as the fuel pump began to warm the fuel up recirculating through the pressure regulator. I said in the past that tuning it was as bad as tuning an Italian sportscar without realizing how true that was.

    K-type Thermocouple Sensor Ceramic Furnace Wire Thermal Detector Probe 2372°f 1300°c .png

    I've just been watching the video footage of the initial startup of the furnace where I shut off the fuel tap completely and let it run off the excess fuel sprayed onto the cold refractory walls and I noticed I had some blue flames as the mix went lean. I had initially thought it was due to the copper scrap I was melting but then that would cause green flames and not blue. While it was running like this there was an excess of air with the butterfly valve more open and the thermocouple temp was plummeting in this mode.

    lean burn 2.jpg
    Last edited: Jul 21, 2022
  4. Petee716

    Petee716 Gold Banner Member

    I think you're on to something very useful here!
  5. That's why I thought I'd post it here, all of a sudden I'm getting reliable feedback on temperatures as the furnace runs. I'll need to find another 100 degrees C to get temps to 1400 deg C if I'm going to melt grey iron (1260 deg C) reliably.

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