Possible explanation for my oil fired furnace tuning issues.

I think I’ve lost track of the nozzle you are using. Does it not atomize the fuel?

Denis

 

It's the array of brass mist sprayers pressurized to 50 PSI to give a fine spray.

 

Right, right. Now I remember. You have done a lot of work. It would be nice to see you rewarded with some reliably melting iron.

Denis

 

Today I tested the 1/8" drilled plate in the furnace with mixed results but ultimately successful. I spent maybe 50 minutes to an hour playing around with various settings and seeing how hot the refractory glowed as a guide with yellow to white glowing refractory as the goal. In the end I shut the furnace down, lifted the crucible out and then up ended the refractory disc and removed it with tongs. The disc normally sits on the plinth to form a doughnut shaped pre-combustion chamber under the crucible. Once I had the crucible sitting on the plinth I then ran the furnace again and to my surprise I still had the same flame swirl pattern around the crucible to give fairly even heating. The drilled plate makes a slight noise difference and eliminates the Boeing 727 exhaust crackle sound but it's main effect seems to be making the fuel-air mixture tuning much more consistent: all I can think is that the 4" bore air plumbing allowed the high velocity air to take different paths past the fuel nozzle and make it hard to set the mixture consistently. I wasted a lot of fuel in the first hour or so but once the refractory disc was gone, I could throttle fuel consumption back a bit

Setting the right mix is very sensitive but for the first time I had the refractory glowing white in the furnace chamber (by eye) but camera photos showed yellow after the auto shutter speed set the exposure to 1/1250th of a second which I don't think it's ever done before.

I poured a 4" diameter slug with the iron but again had trouble setting the fuel air mix right after skimming the slag and heating it back up again.

The photo is a 1/1250th of a second shutter speed shot (edit: actually 1/640th shutter speed), the white crescent shape shows the edge of the 5" lid hole, the white hot chamber refractory going down past the crucible and a white flame over the crucible and out the lid from the lower edge of the hole. There's about 5" of air gap between the crucible and the inner lid surface.

 

Good progress Mark.

I associate yellow exhaust with too rich a mixture. Do you agree?

Cheers Charlie

 

In my opinion it does, it also seems counter intuitive to run the furnace richer than I usually do but it does get hotter with white heat on iron denoting about 1200 deg C. Yellow flames indicate carbon still burning, just like that lazy bit of flame from a vapour leak on the middle right of the photo in post 24. I found a transparent orange flame with as little air as possible gave a good result with preheated iron scrap slumping into the crucible in around ten minutes. My camera doesn't give a good rendition of the flame colour due to the IR filter and auto shutter speed unfortunately.


So for my particular setup, the flame below is too rich as you can't see the rough texture of the refractory through the solid yellow flames (or even the lid hole refractory).




The shot below is leaner (but still richer than I have been running) than the photo above and you can see the rough refractory glowing yellow through the flames as well as the top edge of the crucible and exhaust hole. At this setting the refractory will begin to glow white after a few minutes. Both photos are 1/800th shutter speed which is like looking at the actual flame through welding goggles. Both photos have the same fuel setting with the lower one having the air flow slightly increased.

 

A nice report! I enjoy your lucid explanations and references.

I hope to build a new furnace soon, running on waste-oil and diesolene, so the experience of others is often helpful,

Cheers Charlie

 

My particular experiments are from getting a non optimal design to actually work through sheer stubbornness. I think larger designs based on a 44/55 gallon drum are more tolerant to tuning as I suspect the larger chamber gives a longer dwell time in the furnace. I'd have to try some smaller crucibles to increase chamber volume.

 

Mark, one more way you might consider (I have not done this) for objectively checking furnace temperature would be to use a steel rod of some diameter ?1/4”? that you would place through the lid into a selected spot in the furnace. Leave it for ?30? Seconds and withdraw it and photograph it using a manual setting on your phone. Compare photos. Or use smaller diameter rod and measure melt/burn length loss. Or use rod bent so it has a 4” horizontal leg. Insert, hold, withdraw and check for droop.

Any of those methods could be used with relatively short intervals between the time an adjustment is made and resulting flame temp is measured. I kinda like the droop test which could combine the burn-away, color, and droop in a single measurement.

Denis

 

That's a method I hadn't even considered and I have the steel bars already. I'd been thinking along the lines of an optical pyrometer or one of those cheap pink S type thermocouples on an extension rod. I saw an old optical pyrometer come up for $50 recently but it only went to 1200 deg C.

 

Well, it is just a daydream that might or might not be useful. Seems like it could yield useful info, though.

Denis

 

Another variable worth exploring is the exhaust hole. I think mine is about 4.5". I get to where my fuel and air seem balanced, then cover to hole as much as I can till it starts to have an effect on the burn or gets too noisy. Of course everything changes as my hi-mass furnace heats up, but I would bet it makes a difference.

 

That's another factor to consider, right now I have a vapour leak possibly on the bottom that will need fixing to make things pressure tight or tighter than it currently is before I can start resricting the exhaust hole.
At least I can flip it over easily and access the bottom hatch where the plinth is poorly grouted in. This last run I was using the 1/4 brake rotors over the lid opening to pre-heat and covering the opening a bit.