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"venturi" burners


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I've been messing around with naturally aspirated burners lately, which I haven't done in quite a while. In the process I've been looking at all the common designs -- the Michael Porter/Rex Price "T-Rex" type, Frosty's t-burner, Ron Reil's stuff, the Zoeller sidearm, etc. I've also been looking at commercial naturally aspirated burners for kilns, glass furnaces, etc., like Ward and Ransome burners, and even the venturi burners on things like commercial turkey fryers. One thing I have noticed is that the successful blacksmith burner designs involve a larger area intake necked down to more or less a straight pipe, which then runs straight to the flare. In contrast, the better commercial burners seem to use a more traditional venturi shape -- large intake area followed by a short, necked-down section to reduce pressure and increase speed of the air/gas mix, then a long, gentle flare -- after which they finally straighten out again at a diameter close to that of the intake. In fact I've seen it said it's wrong to call most blacksmith burners "venturi" burners, because they really aren't venturis. I'm not sure that's exactly right, because they do operate on the basic principle of a venturi, but it's at least true that most of them don't use the full, traditional venturi shape.

I'm wondering if the full venturi shape offers added efficiency over the straight pipe burners. Anyone have any firm ideas about this?

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I think my $10 in plumbing parts work very well, which is one of Frosty's T-burners, especially after seeing those prices.

I wonder what a hybrid burner T-Rex looks like inside.

There has to be a reason for commercial burners being built the way they are, and I am sure it is fluid dynamics. It has been 11+ years since I used any fluid dynamics, so I am no help here.

Phil

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Oh, I know some of these designs work really well. I'm just wondering if they max out on efficiency.

I'm asking this because I've kind of got the idea to try to build a semi-naturally aspirated cross between a Pine Ridge style ribbon burner, and a Giberson ceramic burner head. I suspect a venturi has to be very efficient to work with those sorts of burners. (Pine Ridge says their ribbon burner needs forced air, but Joppa Glassworks typically pictures the Gibersons in use with Ransome venturis.)

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When I say "semi-naturally aspirated," I'm thinking of using a small, high pressure compressed air jet shot down a venturi -- an eductor/injector/aspirator, or choose your favorite term -- in place of a blower, and introducing the propane somewhere downstream, as on a typical blown burner.

BTW, although I've never studied fluid dynamics, I did wonder if a straight, relatively narrow pipe doesn't create extra drag. There's also the fact that the pressure in front of the restriction on a more traditiional venturi -- where the flare is -- actually goes up, compared to the pressure in the necked-down area. Not sure if that's important.

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Nozzles and diffusers are opposites. In subsonic flow situations a nozzle is a reduction in cross section and a diffuser is an increase in cross section. A nozzle increases flow speed and a diffuser reduces flow speed. (in supersonic flow the geometry reverses to the same effect)

Phil

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Right, but having a L-O-N-G diffuser means less speed but higher pressure when you get to the end of the burner. Having a certain length also helps with proper mixing, but all the models are based on smooth flow, which is the LEAST efficient, turbulent flow is faster! (for a given cross section and pressure difference) This may be among the reasons why a blown burner seems to perform better.

Having a long straight tube seems to serve the same purpose as an expanding tube.

On air fuel mixing, I had a practical on that lighting the brush pile yesterday. The air was eerily still, and the fire pit is...well, a pit about 10 feet across and over a foot deep. A bit of gasoline(my real problem, I should use kerosene or diesel), say a pint, run up onto a board at the edge, brought the torch to it to light and BOOM!!! The vapors pooled under the brush and blew some of the brush out of the pit! Other than loosing the hair on my arm I came out OK.

The principle behind this is (besides don't use gasoline to light the brushfire) if you have good mixing before the fuel/air mixture is ignited you get a faster and therefore hotter burn. If your air and fuel is injected into a box unmixed, it will mix at whatever rate it wants and burn when it has enough oxygen, which may be after it has exited the forge body.

Phil

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I had a wild thought, when I was in school senior projects were hard to come by. Maybe we can see about getting some engineering students to do the math on several burner designs, then do practical testing and find out what designs perform best AND explain why, so better "home made" burners can be built.

Downside is it might need a budget.

Phil

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Sounds like a pretty good idea. Wish I knew some engineering students! If we're talking home built burners, the budget probably wouldn't have to be very big.

Of course it looks like this book should answer just about any burner question one might have. Lots of tantalizing stuff in the preview. Too bad I can't afford it. It'd take months to digest. http://books.google.com/books?id=cCJ_YyAEqnQC&lpg=PP1&dq=industrial%20burners%20handbook&pg=PP1#v=onepage&q&f=false

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I happen to have a T-Rex, so just now I went down and pulled the injector assembly and started examining the inside with a flashlight. I couldn't tell if it had a necked-down throat (Rex makes it hard or impossible to remove the back end of the burner), but my flashlight was almost the exact diameter of the burner tube at the business end, so I turned the burner upright and dropped the flashlight down there. It slid all the way through to the intake ports. No restriction there worth mentioning. It's basically a straight pipe.)

By the way, I know turbulence is good for mixing, but is it good for the induction ratio? I know Mike Porter mentions several times in his burner book that he's trying to promote laminar flow. And I'm pretty sure the T-Rex is just a commercial version of MIke Porter's burners.

You definitely want intimate mixing before the mix enters the body and ignites. From what I can tell, a "short," fast, efficient burn lets you pump more heat into the furnace before you get to the point where any extra fuel comes out as dragon's breath. Forcing the fuel/air mix into a bunch of tiny streams rather than one big one seems to be a way of promoting that kind of burn. I think that's why the Gibersons and ribbon burners work well.

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ILL the book...several times if needed. $300 is not a little price tag, and their disclaimer about internal use photocopies is not funny either.

Some of the faculty are still around, maybe I can call someone see what it takes. Might need Glenn onboard as well. (Boss, you listening?)

Phil

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It also may be good if we could provide several burners prebuilt and tested. I have a T-burner I can send with a Zoeller flare on it...don't want to but I am not using it. I am not sure how well they are about returning parts like this, so I would have to consider it gone even though the cost is mostly labor. One of Zoeller's creations, a sidearm and a Reil burner would be one the list too.

I would expect they spend one semester doing mathematical analysis and then fire them the second semester and see if they are right.

Comparing to commercial burners would also be an expectation, so somehow having those provided might be necessary (that's where the budget comes in)

I graduates winter 2000 from Valparaiso University with a BSME, for what it is worth.

Lastly what metrics and expectations would we give the students?

Phil

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In IFI gallery a "smithy" posted by Joshua M. is some pics of my homemade firebrick forge with a 2" blower flanged on a 2" stainless pipe. A 3/8" fuel supply pipe is inserted into the air stream 3" from the burner end the diffuser just barely visible is rings of s.s. pipe welded across the end of the pipe burner this makes the flame front start right at the end of the burner and rotate around inside the barrel shaped forge also the burner is fixed to point UP inside the fire box. Fuel is EITHER or BOTH propane at 5psi and nat gas at house pressure.3/4 " pipe supply, reduced to 3/8" With nat gas alone the blower controll plate must close 1/2 of the blower, untill the forge gets white hot. about 30 min with nat alone, then it can be opened 3/4 of the full open , with propane alone the blower can be full open . This rig never gets close to welding heat so MANY scroll bars can cook , making a high yellow -orange heat good for powerhammer or scroll bending. From cold bar in a HOT forge, to Little Giant.. 10 min is enough.

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Well, based on the original burners that we could provide it would be what is happening, and are they the top of the line for their complexity, or can they be significantly improved with little added complexity?

Fuel pressure range of 0-30 psi, possibly up to 60 psi.

BTU output for each given design, plotted against fuel pressure.

Can they design a "plumbing parts" burner that will perform better and be built without a machine shop?

What design has best wind tolerance for out door use?

Limitation of dragon breath, CO, scale formation and understanding that some of these are opposites of others.

What else?

Phil

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Well, based on the original burners that we could provide it would be what is happening, and are they the top of the line for their complexity, or can they be significantly improved with little added complexity? Fuel pressure range of 0-30 psi, possibly up to 60 psi. BTU output for each given design, plotted against fuel pressure. Can they design a "plumbing parts" burner that will perform better and be built without a machine shop? What design has best wind tolerance for out door use? Limitation of dragon breath, CO, scale formation and understanding that some of these are opposites of others. What else? Phil


Seems like ability to handle back pressure ought to be a consideration. Burners that look great in free air don't necessarily perform well in a forge or furnace. Also, for want of a better term, tunability in a forge environment? You said 0-30 or 60 psi, so I guess that covers turndown ratio, which was going to be my other suggestion. I think that's related to the "tunability" issue.

And how could we quantify the amount of heat you can pump into a forge of a fixed size from a burner, until you start losing unburned gas out the front? I'm sure the combustion gurus have a term for this. I don't know what it is.
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Naturally aspirated/venturi forges can get up to welding heat with the right forge design. I don't know the all the physics behind behind the venturi effect but I know from my own experience with my venturi forges that you can easily alter the fuel/air mixture to obtain the best 'neutral' flame - with a simple sliding plate held in place by a magnet. I just move the plate to reduce the amount of air drawn in by the propane. For initial heating or forge weldingI use up to 12psi (propane) and full open venturi. For reheats or normal forging maybe half that.

Here's a couple fo photos:-
Dual burner venturi (2" diameter exhaust pipe venturi, 3/8" gas line, .030 MIG nozzle flowing into 3/4" burner pipe - nozzle position variable via threaded rod)
FinishedForge5.jpg

The proof is in the forge:
NewForgeBurnerDesign3.jpg

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Naturally aspirated/venturi forges can get up to welding heat with the right forge design. I don't know the all the physics behind behind the venturi effect but I know from my own experience with my venturi forges that you can easily alter the fuel/air mixture to obtain the best 'neutral' flame - with a simple sliding plate held in place by a magnet. I just move the plate to reduce the amount of air drawn in by the propane. For initial heating or forge weldingI use up to 12psi (propane) and full open venturi. For reheats or normal forging maybe half that.


I know they can get there; I'm just wondering if the more traditiona venturi shape of many commercial burners makes them more efficient, and better able to handle back pressure.
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So reefera4m, do I know you?


Maybe - I used to frequent this forum before my old 'puter died. I may have met you at the Mt Vernon Hammer-in but I was just passing through and couldn't stay.

Personally I just like to make my own tools whenever possible - forges (propane and coal/coke), air-over-hydraulic mini-presses, hardies, etc.

Besides the 'puter problems I've been rebuilding by shop - two bits of advice - good insurance and lots of digital photos.

Here's a photo of another 'tool' I made:
Press-Front.jpg
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