Forced ai propane burner

[kelvin2164]

I have a small furnace with a self blowing propane burner. It keeps blowing back and burning inside the tube. Works "ok" for aluminium, but seems when the temp gets up to bronze melting it's a constant battle to keep it working.
Anyway, I read a few posts on forced air burners and they seem a lot more controlable. I just need an accurate design. I was thinking 1 1/2" tube (cos that's the hole size in my furnace), 1mm mig tip for gas what about tube lenghts etc.
All help appreciated

 

[aonemarine]

a few pictures would help better diagnose the problem. I have a naturally asperated burner that works flawlessly. I also have put forced air to it to get to iron temps. Nothing fancy I assure you, I just duct taped a hair dryer to it and run it full tilt.
Need to know more about your whole set up, furnace bore, vent, tyre size, then detals on you burner and regulator....

 

[kelvin2164]

The furnace is made from a 20 litre fuel drum, lined with about 3" concrete, leaving a 8" core hole. The cover has a 4" dia vent hole.
The burner is 1 1/2" pipe with a 1mm mig tip in the end and a series of air intakes around the end of the mig tip. Typical off the internet stuff. The tip is ajustable in and out but seems to make no difference to operation whether before or after the air holes. The pipe is around 14" long. It was, but I've been cutting it shorter to see what happens and now it wont run properly at all. Muffling the air intake holes reduces the flame size, so it's not running lean, looks like more forced air would help.
The regulator is a modified oxy reg with the thread recut to fit the lpg bottle and seems to run well on higher pressures, up to full lpg bottle pressure (around 30psi I think, its in kpa, must learn to convert one day).
I do like the forced air idea. Are the basic dimensions the same, I would think it could be shorter as no venturi affect is needed. Which would help to prevent burning inside the pipe?

 

[aonemarine]

I would say your burner tube is too large of diameter and you are not getting up enough velocity thru the tube to get the air fuel to ignite in the furnace (combustion area) Try sliding another pipe inside your current pipe to reduce the inner boredown to 1" or even 3/4". this will move the combuston area out of the burner and into the furnace where it belongs. The burner tube should be no longer than 8" before entering the furnace, or flair if your trying to burn in free air. Adding a blower may help, but you should get your burner working properly without it first so you will be able to have better controll over the furnace temps.
The reil burner has been the best burner to build IMHO....

 

[Omnimill]

Lots of good data here that may help:

http://amalcarb.co.uk/downloadfiles/amal/amal_gas_injectors.pdf

 

[kelvin2164]

I'll try narrowing the bore next opportunity I get. I like the reasoning. Although the internet burners all talk about 1 1/2" and 2" pipes. I assume the narrowing starts at the end of the pipe and runs to the end of the mig tip.
Thanks for the Amal link. It's too late to read (and too tired) for that now, but will definitly digest it tomorrow.

 

[kelvin2164]

Burner update
I sleeved the pipe down from 1 1/2 to 1 1/4, but seemed to make little difference. I added another sleeve bringing it down to 1". Each sleeve being shorter, producing a primative venturi. Seemed to work very well. It still spluttered and blew back into the tube sometimes, but I managed to fix this by pulling the mig tip further out of the venturi? Also I had a 1.2mm tip in. Replaced this with a 0.6 tip and it definitly seemed to produce more heat. Although needed it closer to the venturi entry.
Thermal insulation? I have 2" of concrete as my liner. On turning off it changed from cherry red to no colour in about 3 minutes. Is this to be expected?

 

[aonemarine]

I hope you mean refractory cement and not concrete. I cant say I every watched my furnace cool down, but I dont think it glowed for very long at all...

 

[Ripcrow]

Sat a 44 gallon drum on Steel beams about 4 inches above concrete and was using it as a fire bucket for heat at a party until the concrete exploded,lifting the drum about 4 ft in the air and blowing hot coals into people's hair. Blew a hole in the concrete about 1/2 inch deep and a perfect circle the diameter of the drum. Be careful with concrete

 

[tomo]

I use a kids small bouncy castle compressor which is fairly low pressure but has a high volume of air delivery. I achieved good results melting bronze with LPG using this basic setup.