Micro venturi burners?

[kraythe]

I have been thinking about building a ribbon burner with some spare castolyte that I have around here somewhere. However, the one problem I have with the setup is one of safety. If you lost the power to the blower you had better re-establish it quickly or be seriously on the ball with turning off the gas or the flame will back up into the plenum and burn back into a large amount of mixed air and propane with unfortunate results. So the question was, how could I do this safely without causing all of these problems but gaining the benefit of the ribbon burners. After all, if it was just me I might be more inclined to take risk but my family depends on me so I have to be more careful.

It seems that the prime benefit of the ribbon burner is the increase in surface area of the flames. The increased surface area allows the burner to more efficiently transfer heat to the forge. At least that makes sense in my mind. So I began to wonder how I could get the increase in surface area without the ribbon design and the idea struck me of using micro venturi burners. These burners would be constructed on a 1/2" size or even 1/4" size instead of the normal 3/4" size. The burners could be arrayed into the roof of the forge to provide distributed coverage and remove hot spots. With the proper gas flow they should draw air without needing electricity. (oh by the way I dont use black iron but rather welded Ts to reduce turbulence.

I was thinking to use 1/4" tube with 1/2" tube on the top of the T. The gas jet would be located exactly centered on the 1/4" tubes and would probably be pretty small. I would have to figure out a viable way to introduce gas into the system. My current method of MIG tip isnt going to work and it seems to me the gas jet will have to be much smaller than the .025 tip I am using in my 3/4" burner. I havent quite figured out that part. The flares would be as I have them now, essentially they are cast out of castolyte with a nipple for attachment embedded in the refractory and then coated with ITC-100 to provide heat transmission as well as smooth out the flares. The 1/4" tube would be about 2" long following the 8 to 1 ratio that i have seen works well. The cross tube on the T would only be 1" long or so. What I need after that is some kind of needle to introduce the gas.

So what do you think? Am I psycho with this? Can you think of a safe way to introduce the gas?

[pkrankow]

Just get a solenoid valve and run it in the same circuit as the blower.

Alternately, as long as you are with your forge, don't worry about it. If the blower quits it will become very non-energetic, with large lazy yellow flames, and you just turn it off.

Phil

[ThomasPowers]

Would the smaller burners make them more sensitive to back pressure?

I've had a number of power outages with my regular blown forge and never had any problems---the lazy propane flare mentioned above was all I saw.

I've not tried it with a ribbon burner---yet---but expect I will get that chance soon.

[MattBower]

The main advantage of the ribbon burner isn't just surface area, I don't think. It's also really good mixing of fuel and air. Not sure how that affects your analysis.

I think the main problem with tiny venturis would be drilling lots of tiny holes.

[Fe-Wood]

I agree about the lazy flame with a power outage and/or a solinoid. I saw a realy nicely designed bruner using 3/8" Ø x3" long bits of pipe stacked together to form the burner body. looked like a easy to build design.....

[maddog]

A 1/4" venturi burner would be a struggle. There will simply be too much back pressure from the pipe. The friction on a fluid flowing through a pipe rises very rapidly as the diameter decreases.

Head Loss
Friction Table.

But if you are going to try, I'd love to see the results. Commercial propane torches do go as small as 5/16" perhaps smaller but I dont know how much heat they put out.

Burning back into the plenum is undesirable but not really dangerous in my experience. The burner will make a lot of noise and combustion in the wrong place will do damage but it doesnt explode or go out of control. If the plenum containing combustible mix is kept small, it shouldnt be too bad.

I've been toying with the idea of separating the plenum from the burner block. A grid of 1/4" steel tubes would take the mixture from a mixing chamber into the back of the case holding the burner block and feed into the holes in the refractory block. That would give a lot of surface area for the heat from the burner to dissapate in the air. Making the tubes smaller dia. than the holes in the refractory block would also created a barrier against burning back into the plenum. But I think this would need a force aired system and a fair amount of pressure.

[kraythe]

Hmm I am encouraged by what you say about the lazy flame. I can deal with that. I also thought aobut cutting a hole in the feed tube and welding it over with a thin layer of steel to create a pre-calculated weak spot in case of an explosion. I looked into solenoids but it seems they would cost as much as my blower!!

Another thing that interested me is people talking about building the burner tubes out of steel tubes and then using refractory only for the ends of the tubes. Would you cast them like flares for venturis? I can imagine a cluster of 1/4" tubes in a bundle acting as a burner but I think they wouldnt stand up to the heat as well as castable tubes.

[maddog]

Hmm I am encouraged by what you say about the lazy flame. I can deal with that. I also thought aobut cutting a hole in the feed tube and welding it over with a thin layer of steel to create a pre-calculated weak spot in case of an explosion. I looked into solenoids but it seems they would cost as much as my blower!!

Another thing that interested me is people talking about building the burner tubes out of steel tubes and then using refractory only for the ends of the tubes. Would you cast them like flares for venturis? I can imagine a cluster of 1/4" tubes in a bundle acting as a burner but I think they wouldnt stand up to the heat as well as castable tubes.

If you haven't looked at the Pine Ridge Burner page yet, it's a must.

Pine Ridge Burners

He uses steel tubing to meter the mix into the block. Notice the depth of the refractory block to hold back the heat.

In this thread Tom from Pine Ridge discusses the design of his burners. Very illuminating.

Ribbon Burner Thread

I too have thought about drilling a hole in the plenum and sealing it over with foil or inserting a solid rubber grommet like they do with pressure cookers as a designed failure point. But I doubt the amount of gas/air mix in a small volume like that has enough energy to be dangerous even if it detonates rather than just igniting. There is an open path back to the airgate. Even if that is closed the extra volume in the connecting pipe should take the oomph out of it. In that thread or another, people have reported experience with their ribbon burners igniting in the plenum. All that happened was the plenum got very hot.