choke/air intake design considerations

[Greenbuggy]

Hi there I recently discovered this site and it looks to be a real wealth of information, and inspiration for some projects I'd like to make as well.

I have recently constructed a freon keg forge, with space for 2 burners although for now I have only one installed. The forge is lined with Kaowool, soon to be covered with plistix, and the burner is very similar in nature to the ron reil EZ-burner.

I recently picked up a copy of Porters "gas burners for forges, furnaces and kilns" and have been reading thru it. I see a lot of parallels between his forge burner designs and the mongo burners on Reil's site. Also some definite similarities to the hybrid burners with regards to the sliding chokes. Naturally I'm wondering about how to improve what I've already got built and to that end I did some cleanup and polishing on the inlet and outlet of the black pipe fittings in my ez-burner with some slight improvement in the temperature of the forge.

I know that the refractory coating I'm planning on using will improve the heat retention inside the forge and make parts get to heat faster, but I've got something I've been wondering about burners as well. Most of the higher performance ejector burners I've seen like those in the Porter book and the hybrid burners, use a sliding choke over milled slots. I have to wonder if some improvement couldn't be gotten by milling a series of holes in the tube off-center, or using a rotating choke that is flanged in such a manner that it causes a spiraling column of air as it moves through the pipe, intermixing with the gas a bit better before entering the combustion phase. My inspiration comes from seeing burn barrels with triangle-shaped openings bent in on the bottom to cause the air to spiral around the burning trash to ensure a cleaner, hotter burn.

I have an old bridgeport mill and some time to blow so I wouldn't be opposed to killing a few 3/4" black steel pipes to test my theory, but would like to know if anyone else has contemplated doing something like this, or seen something like this in action before I start making chips. My understanding of fluid dynamics is a little rusty, you know?

[David Ab]

Hi Greenbuggy,
I'm a beginner in terms of making forges and burners, and this is really just me thinking out loud, but here's what I remember reading about fluid dynamics, the ASHRAE books, experiments done at MIT, and a few other sources, and some stuff I've been thinking about:

1. A smoothly-rounded entryway means more fluid can get in more easily, as opposed to an entry with sharp edges. Even smooth sharp edges will allow much less fluid to get in, than a smooth, rounded entry. A half-toroid shaped air or gas inlet will allow the most fluid in with the least friction.

2. Wind tunnel photos show that a surface with a "golf ball" texture creates lots of tiny little vortexes and the main flow of gas or air can then "slide" over those vortexes with significantly less friction than you would get with a smooth or even a polished tube. Which is why they put all those dimples in golf balls. Whale flippers have bumps on them that increase the flippers' ability to move water, so I guess that means that "innies" decrease resistance to flow, and "outies" increase the ability to more easily move larger quantities of fluid? (I haven't fully wrapped my head around this one yet...)

3. A longer tube gives a flow of air or gas more time to "settle in" and become smooth. A short tube actually takes more energy to move the same amount of fluid. There's a formula that involves the diameter and length of the tube, the amount of fluid moving through it, the viscosity of the fluid, and the speed of the flow.

4. If you have to have a 90 degree elbow in a pipe used for fluid transfer, the wider, and smoother the bend is, the more fluid that will get around the bend easier; the turbulence caused by a sharp 90 degree elbow, on the other hand, can decrease the volume of flow through the tube/pipe by 40%. So, sharp bends in the tube would mix the air with the propane.

5. I think- not sure, but think- that if you have gas and air going through a pipe and you want them better mixed, you can run the pipe into a significantly larger-diameter pipe, which will cause more turbulence and space for the mixing of the two. One factor to consider, though, is that propane is heavier than air.

6. A toroid-shaped air outlet allows more fluid out of the pipe, given the same input pressure and volume. I think that a toroid on the end of the burner tube would make for a shorter, fatter flame, but not sure. I know the shape of the outlet on a fan or blower has a lot to do with how the air moves out of it, so it would have to affect how a flame moves out of it, too.

7. I've been wondering about shaping the inside of the burner tube like a car's muffler. Mufflers use interior baffles to break up the flow of fluid, and I'm thinking that baffles would also therefore mix up the gases in a burner tube.

8. I think that Corin's burner draws in the primary air through the back end of the burner tube, and put in the gas from the side. This would make sense because the ideal mixture for propane and air is over 90% air to under 10% propane, so you want to make it as easy as possible for lots of air to get into the tube. It would be easy to put a toroid shape on such an air inlet, and also easy to angle the gas inlets so they shoot the propane into the pipe to induce a spiral mixing flow as you were talking about. But I don't know how to choke off the primary air flow to adjust the flame...

But like I said, I haven't even built a forge yet. I have most of the materials, and am going to start on it in a few days.

I'd like to hear how yours works.
David

[kcrucible]

I recently picked up a copy of Porters "gas burners for forges, furnaces and kilns" and have been reading thru it. I see a lot of parallels between his forge burner designs and the mongo burners on Reil's site. Also some definite similarities to the hybrid burners with regards to the sliding chokes.

That's not a coincidence. Porter, Rex, and Ron basically all collaborated on the fundamentals of the design according to Ron's site. They each had different takes on it as they developed it further.

Most of the higher performance ejector burners I've seen like those in the Porter book and the hybrid burners, use a sliding choke over milled slots. I have to wonder if some improvement couldn't be gotten by milling a series of holes in the tube off-center, or using a rotating choke that is flanged in such a manner that it causes a spiraling column of air as it moves through the pipe, intermixing with the gas a bit better before entering the combustion phase. My inspiration comes from seeing burn barrels with triangle-shaped openings bent in on the bottom to cause the air to spiral around the burning trash to ensure a cleaner, hotter burn.

You could try it, but I think you'll kill the top-end performance. The venturi's work by using air velocity from the tip to create a vaccume and suck in air. Anything which makes it harder to pull that air into the mixing chamber is going to screw up the fuel ratio. If you want better mixing you'd probably be better off with something to swirl the gasses at the exit, rather than the input. Porter goes to great lengths to assure the smoothest, fastest, airflow at the intakes as possible (a little overboard in my opinion... a scooped choke?)

[Mikey98118]

Hi there I recently discovered this site and it looks to be a real wealth of information, and inspiration for some projects I'd like to make as well.

I have recently constructed a freon keg forge, with space for 2 burners although for now I have only one installed. The forge is lined with Kaowool, soon to be covered with plistix, and the burner is very similar in nature to the ron reil EZ-burner.

I recently picked up a copy of Porters "gas burners for forges, furnaces and kilns" and have been reading thru it. I see a lot of parallels between his forge burner designs and the mongo burners on Reil's site. Also some definite similarities to the hybrid burners with regards to the sliding chokes. Naturally I'm wondering about how to improve what I've already got built and to that end I did some cleanup and polishing on the inlet and outlet of the black pipe fittings in my ez-burner with some slight improvement in the temperature of the forge.

I know that the refractory coating I'm planning on using will improve the heat retention inside the forge and make parts get to heat faster, but I've got something I've been wondering about burners as well. Most of the higher performance ejector burners I've seen like those in the Porter book and the hybrid burners, use a sliding choke over milled slots. I have to wonder if some improvement couldn't be gotten by milling a series of holes in the tube off-center, or using a rotating choke that is flanged in such a manner that it causes a spiraling column of air as it moves through the pipe, intermixing with the gas a bit better before entering the combustion phase. My inspiration comes from seeing burn barrels with triangle-shaped openings bent in on the bottom to cause the air to spiral around the burning trash to ensure a cleaner, hotter burn.

I have an old bridgeport mill and some time to blow so I wouldn't be opposed to killing a few 3/4" black steel pipes to test my theory, but would like to know if anyone else has contemplated doing something like this, or seen something like this in action before I start making chips. My understanding of fluid dynamics is a little rusty, you know?

Yes, there are a lot of parallels, since I contributed burner designs to his site from 2000 to 2002. You will also find that Rex Price states openly on his site that his burners were designed with my help. Read the book more carefully and you'll abandon hole drilling as I did early on. The rotating choke was also abandoned, because it only works properly in the full open or full closed positions, interfering with smooth air intake in all other positions; this mainly happened because it turned the rounded ends on the Hybrid burners into points (very bad thing to do). So, the next question would be, why not try it on my high speed tube burners, which have squared and beveled ends?

Answer: The length to width ratio of air intakes strongly affects a burner's performance. If you make their width variable through use of a rotating choke, the variance in burner performance will become a real aggravation (in high speed burners) to the operator. On the other hand, varying their length from the rear with a sliding choke keeps performance predictable. Rotating chokes can be found on some Bunsen burners, but this kind of burner isn't anywhere near as dynamic as the weakest burner design employed in gas forges today. Air from side intakes already spirals as it enters the mixing tube area, as does the air coming in through bell reducers on the ends of linear burners (Reil type).

A burn barrel is something midway between a burner and the equipment the burner is supposed to heat. Ideas will not transfer from it to a burner that is a single heat source within forge or casting furnace equipment. Also, turbulance is good in certain areas of the burner, and very bad in others. You want to keep your turbulance confined to the mixing tube as much as possible. In both the air intake area and the flame nozzle, turbulance is a problem--not a blessing.

I think what you really want to do is to experiment with the burner design to make it your own; bravo! Most of the people who read my book do. So, if I were advising a guy with a mill what to do in such case, I would say "try something in between my design and the Hybrid burners." Do not give up the rectangular bevelled air intakes, but instead, use the next size pipe and dimensions, with a little added length beyond the air intakes to insure a good fit, and bevell the inner face of the pipe you slide inside it for a mixing tube. Line up the bevells of the mixing tube with the forward bevelled ends of the air intakes on the larger tube before permanently affixing the two parts together. You should be able to use directions from the burner size you want to build to make the burner mixing tube and nozzle, along with directions from the next larger burner size to make the air intake portion, end cap, etc. I plan on trying this design myself and including it in my second burner book if it works out as well as expected. One other thing: I reduced the number of air intakes from six down to five larger intakes on my Generation Five 1" burner. The new slots are 3/8" wide by 2-1/4" long, with about 5/16" wide ribs between them. The new burners run just as well as the older models but have much stronger ribs. If you are building a 3/4" burner (mixing tube size in this case), try these dimensions.

Another change I'd recommend is to use MK brand MIG tips, which have 12-24 thread. This allows you to use smaller diameter tube for your gas pipe, creating a better gas jet assembly. Also, I don't like the quality trends with the Tweeco MIG tips these days. Gentle hint: OnlineMetals.com has exhaustive dimension listings on their parts and reasonable pricing on cut parts and shipping, so there is no need to panic when you start swimming down at the deep end of the pool. Further instructions, like on soldering brass to stainless upon request :-)
Porter

[DClaville]

Thanks you for that post Porter, it is very informative and confirms some points in burner design i have had to mind but not been 100% sure off

the last couple evenings I have been building a big 1" main tube burner and went with only 3 air intake slots 1" wide 3" long in the mixing chamber (1 1/4" pipe) but am very unsure if it would have given a better mixing and burning with 6 slots of a smaller size?

I will dig out the digi-cam and take photos and vids of the burner and make a new thread tomorrow

Cheers
DC

[reekanmantell]

Answer: The length to width ratio of air intakes strongly affects a burner's performance. If you make their width variable through use of a rotating choke, the variance in burner performance will become a real aggravation (in high speed burners) to the operator. On the other hand, varying their length from the rear with a sliding choke keeps performance predictable.

I have recently purchased a copy of "Gas Burners for Forges, Furnaces, & Kilns" and have to say it is a very informative read! I am confused with the content of the book versus the above quote, in the book the choke is always mounted to close off from the end closest to the burner flame while the above quote seems to indicate the choke performs best when closing off from the end opposite the flame. In addition the book suggests flaring the choke on the side opposite the flame to increase performance. Is there a noticeable performance gain from installing a sliding choke that closes from the side opposite the burner (as is the case with hybridburners), or does the burner perform best with a flared choke installed as in the book?