After testing various burner options in a couple different furnaces, I wanted to try a pressure nozzle and eliminate the need for an air compressor, which runs too much feeding the siphon air. My current configuration is using straight used automotive oil, pressurized in an air tank to get it to the siphon nozzle efficiently. My thought was that it might be quick and easy to simply replace the siphon nozzle with a pressure nozzle, then crank up the air pressure behind the oil to get it to atomize. I understand that pressure nozzles typically require 100-150psi to atomize, so the plan made sense on paper. However, at 110 psi I get nothing but a solid stream of oil. I suspect the psi rating is designed around heating oil, and probably warmed heating oil at that, rather than the cool waste oil I'm using. I'm working on getting a bigger, better air compressor (for different reasons), but I don't expect it to reach more than 150psi, which probably won't solve my problem anyway. I can cut the oil with diesel fuel, but would prefer not to if I can find another solution. Are there different pressure nozzles I should be looking at for burning waste oil? Do you have any idea of the psi I may need to atomize waste oil directly? I suspect I may be missing something else, any pointers will be greatly appreciated.
I'm using pressure fed nozzles but I have access to light fuel oil so it atomizes easily. Some other forum members just preheat on propane then have the oil stream hit some hot surface in the furnace to vaporize the liquid, like the refractory lining of the furnace. Forum member Ironsides just has a 1.6mm hole with forced air blowing around it and I think it's not even pumped, just gravity feed with the air blowing it into the furnace chamber 1:12 minute mark.
This makes sense.. I see I have a couple of new things to try. I inadvertently experimented with a solid stream hitting the hotface when my siphon nozzle clogged awhile back, but I wasn't able to effectively optimize the air/fuel mixture at temperatures over about 1100c. It's certainly worth revisiting as I didn't spend much time once I realized the problem. I'm certainly going to try the method Ironsides is using, this would make a very simple solution if I can get it to work as expected. I'm currently using a 2" inlet pipe, but sometimes feel the diameter may be too large as I'm getting plenty of air but not the expected velocity for the flame to make several rotations. I have seen varying sizes of tuyeres in the various threads here, is there a common calculation or typical size to use with waste oil?
Well I started with 4" bore pipe and have necked it down to about 2.5" with multiple nozzles taking up more space in the nozzle, a lot of people here seem to opt for 2" and it works fine for them. I think Ironsides has 2" bore air pipe for his furnace, maybe he can weigh in you can send him a personal message on the forum. I know he has at least one video discussing dimensions of his furnace. I think getting one of those cheap S type thermocouples and making a porcelain heat shield for the length of it would aid in getting fuel air mixtures set right. Also trawling through Melterskelter's posts will get some dimensions on his reliable furnace that is used almost exclusively for iron melting.
I now recall reading about your multiple nozzles because I thought it was such an interesting idea, but assumed it wouldn't fit into a 2" tuyere. What kind of nozzle are you using, and how is this working out for you. The more I reread many of the posts, the more I realize how much I didn't yet understand until I put things into practice. I followed many of the examples when designing my latest furnace, which is finally tuned quite well, but I haven't spent much time on the burner. The porcelain heat shield for the s type thermocouple is a great idea for tuning the higher temps. I'm going back to reread and relearn the stuff I've forgotten. Thanks for taking the time to respond. I seem to get stuck in my own thoughts and need a refresher course on occasion.
I'm using pressure fed garden mist nozzles, I haven't tried any waste oil through them yet. Small oil droplet size will allow faster burning and is dependent on small nozzle opening size: the smaller the nozzle the finer the droplet size and the faster it vaporizes and burns. I'm doing something wrong with my furnace, possibly the 11 inch bore diameter with an A25 crucible leaves only a 1" air gap all round, to test this I need to buy a smaller A16 crucible at some point to see if the larger gap slows the burning gases enough to transfer heat better. I'm using a car engine oil pump driven by a motor but a suitable fuel pump would be a belt driven power steering pump connected to an AC motor with a fuel pressure regulator to set the pressure. https://www.aliexpress.com/item/4000641081499.html?spm=a2g0o.productlist.0.0.3bf63d3e63kkjF&algo_pvid=141c6703-52df-476a-a42d-a69e9f525bea&aem_p4p_detail=202110311511101213021340748560030660721&algo_exp_id=141c6703-52df-476a-a42d-a69e9f525bea-4&pdp_ext_f={"sku_id":"10000004792331761"}
Waste oil will not atomize through nozzles it is too thick, there are two ways to atomize waste oil, mixing diesel with waste oil or heating up waste oil. I use 80 % waste oil and 20 % diesel. I have tried heating up waste oil so when I got to 80-90C waste oil is like water and will atomize through a nozzle.
I initially bumped into this same problem. I could reach higher temps quite quickly with a couple of stacked firebricks in the furnace, but struggled when the crucible left only a 3/4" gap. I dropped down a size in crucible and it helped solve about 70% of the problem. You confirmed exactly what I'm seeing. I was hoping to avoid mixing diesel with the waste oil, but it seems much less problematic than heating the oil. Based on your video with the 1.6mm nozzle, I want to try injecting a fine stream of oil directly into the air stream before the tuyere. I turned a new nozzle and started with a 0.8 hole, and will test it tomorrow. My idea is to inject straight diesel with the nozzle for pre-heating, then switch over to oil for the higher temperatures. Like most other things in my life, it seems I have to learn the hard way to gain any real knowledge, but please jump in if I'm simply repeating mistakes others have already learned. I should be able to test some of this tomorrow, and hopefully begin narrowing down each variable toward finding a very repeatable process. Like I said, I hope to avoid mixing diesel, but I'll head that direction if my current testing fails miserably.
I checked last night, and it looks like waste oil has a flash point between 450 and 480*F. Most pressurized drip burners seem to only run at 5-10 psi with the end of the nozzle at/near the end of the burner tube. I'd start there, just like Ironsides demonstrated above, and only stray if necessary.
One thing I forgot to mention with a pressure pump and regulator, is the fuel gets heated by the churning through the pump to about 50-60 degrees C: too hot to touch. My pump is made from stuff lying around, the motor is too large and runs too fast so the speed is reduced through a pulley drive to about 1000 RPM so the engine oil pump doesn't pump too much. Even then most of the oil is recirculating through the regulator back into the pump inlet.
