50 replies to this topic

[Sweany]

Quote

The T-Rex has a machined internal venturi. Without a venturi, it won't matter as the flame front velocity will outrun he gas velocity. Look at the pic above and you see the constriction on the burner tube? There is your venturi

*All I know is I can build these things from straight pipe and they work. I'm not a machinist.

The gas pressure has to induce suction in the tube for it to work and it can not do that, when the gas doesn't go down the tube. Usually the best bet is having the tip in the mouth of the burner tube.
My .02 on non-machined tube burners. This pic shows a forged end burner. It has a 1/4" pipe plug, drilled out and piece of 1/8" copper wire swedged in, the orifice hole is drilled in the copper wire. The tube is 3/4" x 11" long, and was one of my favorite burners.
http://www.farwestfo...&cat=&by=Sweany

Slotted hole type burner
My experience with most of these burners is the slots are over large to start with. If they were not overlarge. Why the choke?

Here is a pic of my T-rex clone. I'm not a machinist so I drilled holes instead of milling slots. It does not seem to matter to the air at all. This forge reaches welding temps quite handily.

http://www.farwestfo...&cat=&by=Sweany


My .02 build the side arm burner. Much easier to control the air and center the tip. Putting a plug in the end of the T and drilling center is way easier than drilling center on a Cast pipe cap.

http://www.farwestfo...&cat=&by=Sweany

#2 I don't think bigger slots will help.

#3 My 1/8" pipe nipples were 3" long leaving plenty of room for tuning. I couldn't find any Schedule 80, so I swadged down the sch 40 and reamed and tapped the nipple to accept the tip. Make sure the tip to nipple joint is air tight.

The Porter book has recomendations for pipe dia. length and Mig tip size. These seem to work pretty good.

*All the opinons here are strictly based on my experince, your experience may vary

[Fe-Wood]

So rather than say (write) anymore- Pictures. These are staight smooth pipes. Note the Air intake on both designs. They may be ugly but they work really well. I can run them down to about 1 PSI and up to ?
Hope this helps

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[Robert Simmons]

*shrug* I was just trying to help. I spent three weeks playing with this stuff 6 to 10 hours a day. Running a burner in a forge isnt the same as melting something with it. At this point I will just say "good luck" and move on.

[kcrucible]

Fe-Wood, on 13 August 2010 - 09:21 PM, said:

So rather than say (write) anymore- Pictures. These are staight smooth pipes. Note the Air intake on both designs. They may be ugly but they work really well. I can run them down to about 1 PSI and up to ?Hope this helps

I think the operative condition here is the ratio of slot area to pipe diameter. I'm becoming more convinced that I'm creating a partial vaccum due to not being able to suck enough air into the tube at any given time, which is slowing down the propane jet.

I'm glad to see your burner tip pulled considerably back away from the opening as I was getting much better performance that way (barring the whole, burning inside the tube thing.) That could also be a factor in terms of air... with too-small-intakes, blocking the route with the injector only makes things worse, even if, in theory, it would generate greater suction.


I tried another experiment last night...

Experiment #2
Replaced the 2" jet nipple with a 3" jet nipple based on the Porter suggestion on acceleration of gas, and also shoved the tip past the air opening.

Result
No real change that I could see. My initial suspicion seems born out. No great loss.

Unrelated, but important (I think) observation during this experiment:
On firing COLD and with decent pressure, I was having trouble getting the flame light/stabilize. After it did stabilize and pop back up the tube, warming the tube in the process, every subsequent lighting went into the tube immediately.

Theory
When the pipe gets warm, the air inside the pipe provides higher pressure for the fuel/air to push against, slowing it down. This slowing causes the flame to burn back into the tube. When cold, the additional air-backpressure doesn't exist and the burner operates ok (barring a need for better flame-retention, etc.) Vaccume created by insufficient air intakes could be contributing to the jet not being able to overcome this nominal pressure increase.

What do you all think?

I'm in the process of squaring off the ends of the intakes (according to porter, rounded edges do bad things to the airflow... and I was seeing some ugly behavior in my first firing) and increasing their widths a bit. It may be a couple of days before I can fire it up again.

Quote

*shrug* I was just trying to help. I spent three weeks playing with this stuff 6 to 10 hours a day. Running a burner in a forge isnt the same as melting something with it. At this point I will just say "good luck" and move on.

Thanks for your insights Robert I appreciate your participation. I may very well end up creating a step-down venturi tube (1" -> 3/4" for most of the length -> 1"/flare) if I can't get the pressures and velocities right in the straight pipe. In the end it would effectively be the 3/4" t-rex, just that the rex machined pipes would be replaced by castable and sheathed in black iron.

I haven't quite given up on the 1" straight-tube yet though.

[Nakedanvil - Grant Sarver]

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Many hundreds of hours on this one.

[kcrucible]

So, a giant rosebud tip in the floor of the forge? Very cool... how do you keep them from melting?

Regarding my burner...

Experiment #3
Widened and lengthened air intakes. Probably about 1/3 area increase. Squared off the ends of the slots. Moved the tip back a ways so that there's plenty of room for airflow.

Result
No change.

Observation
Dialing up and down the pressure on the regulator doesn't seem to change anything at all... no faster hiss of propane, no increase in flames, etc. It acts like it's on or off.

I guess I should hook up a guage and make sure it's actually working properly. Perhaps the real problem is the fuel supply. Are there any other good tests for the regulator?

I don't suppose anyone here lives in eastern MA and willing to let me hook into your supply for a test?


Past that, all I can really think of to do now is the smaller jet orriface. Supposedly the 045 tip was acceptible for a 1" pipe, but maybe not.

[Nakedanvil - Grant Sarver]

Pretty hard for it to get hot with all that cold air and gas going through all them holes. Air cools the burner, burner heats the air! It's a solid chunk, but it just discolors, not scaled at all. And the forge has been run very hot. Nearly impossible for the flame to pass through the holes to the other side.

[kcrucible]

Grant, that is actually a Flame Retention Device? I saw one reference to such a device to keep the flame from crawling back up the tube. Just a plate with some holes drilled in it? Any specific features required to make it work? Angles, etc?

Pretty much confirms what I'm seeing... included here for reference.

http://www.potters.o...bject81078.htm/

Quote

The phenomenon you are rexperiencing is called back-burning, and it is hard
to understand why Peter at Summit Kilns would tell you this is okay. What
kind of burners are on that conversion? Most of the updraft toploader gas
kilns and conversions use the GACO MR-750, the same burner that is popular
for most raku kilns. It is a good little burner, with excellent
flame-retention properties, and it should not be backburning. Any
well-designed, properly adjusted, properly operated cast-iron or fabricated
steel tube or venturi burner should never experience back-burning. That
said, we do have some old Norman gas updraft kilns at the Craft Center, and
they are equipped with refractory clay venturi burner tubes, and at low
settings they do tend to backburn.

Generally, backburning is caused by too little primary air entrained into
the gas-air stream, and/or too little gas pressure at the orifice. If there
is not enough primary entering the burner tube, the speed of combustion
exceeds the speed of travel of the air-gas mixture, and the flame moves back
up the burner tube, often right back to the orifice, creating a smoky
yellow-orange flame. I am assuming that your burners have adjustable air
shutters at the back opening, and you must make sure that those shutters are
open wide enough to admit adequate primary air. You may have to do some
experimentation.

In a tube burner or venturi burner, primary air is drawn in by a slight
vacuum created by the velocity of gas escaping from the orifice. If there
is not enough gas velocity, then inadequate primary air will be entrained,
and the gas-air mixture will move very slowly down the burner tube.

You didn't say whether your kiln is natural gas or propane. If by chance it
is running on propane but with natural gas orifices in the burners, that
would explain the problme. Household natural gas pressure is normally 7 WCI
(water column inches) whereas set-regulated propane is usually 11 WCI, and
thus the propane orifice must be smaller. That's enough to make a big
difference. And if a burner setup is designed to run on an adjustable
propane regulator, then the pressure at the orifice can be as high fifteen
or twenty PSI (pounds per square inch) pressure, in which case the installed
orifice is often smaller still.

This does sound like a problem either with the orifice size, the gas supply
pressure, or the primary shutter settings. Open up the primary air shutters
and see if that has any effect. Make sure that your burners have the right
orifices for the kind and pressure gas you are using. If you are using
propane, make sure the set-regulator or adjustable regulator has sufficient
capacity for your burners. If you are using natural gas, make sure that the
supply line is of adequate size for the burner system.

[Nakedanvil - Grant Sarver]

Good question! It's not apparent that it's not a plate. It's a solid 2" diameter slug about 2" long.

[Dragons lair]

Just a thought on this, Just becuse ya got a burner that burns perfect clamped inna vise don't mean it's gonna
work in your forge or visa versa. Run your test in the forge your gonna use. Tune the burner to the forge.
Ken.

[kcrucible]

So, correct me if I'm wrong, but the principle is basically to take the fuel/gas from the mix tube and inject it into multiple smaller jets, which each have higher velocity than the mix tube velocity. The velocity of the individual outlets are now fast enough to prevent the flame burn velocity from exceeding the flow rate? Basically 1 side of a venturi tube at the exhaust end?

The only real question is whether the reduced area would cause detrimental back-pressure for me.

Question for the peanut gallery... what size tips have you used on 3/4" and 1" tubes? Currently I'm at 045 for 1". Maybe dropping to 035 will increase the velocity enough to run properly?

Quote

Just a thought on this, Just becuse ya got a burner that burns perfect clamped inna vise don't mean it's gonna work in your forge or visa versa. Run your test in the forge your gonna use. Tune the burner to the forge.

Of course, my inability to tune it in a vise is causing me to be really pessimistic about being able to tune it in a forge. There's a fundamental problem here that goes beyond the normal scope of tuning (fuel/air mix, etc.)

[kcrucible]

It just occurred to me... maybe the tank itself?? A little googling and hmmm...

http://www.grillman....ankregs.htm#q12

Quote

Q: Roofers and other commercial customers are trying to use OPD equipped tanks on torch applications and they are not getting an adequate flow through the valve. What can we do about this?
A: Some of the early OPD valves had an insufficient flow capacity for certain types of service such as high pressure torch applications. Market competition has resulted in OPD's becoming available that have significantly greater rates of flow.

The OPD getting in the way... That would explain why my regulator doesn't seem to be adjustable, right? Still, my initial firing was working more or less properly...


A little more and I had this:

http://www.gashosesa...latorfacts.html

Quote

A second cause might be the automatic activation of a surge protection device within the propane regulator. If, for example, you turn on the tank valve before you fully turn off each of the burner knobs on the grill the surge protector will likely sense a leak and activate. The fuel flow will be very low. The remedy is to turn everything off, disconnect the tank, and reconnect everything before starting over.

I'd turned off the tank directly rather than dialing-down the regulator at several points. So I went outside, disconnected everything, hooked it back up, and cranked up the pressure... whooosh. The gas was now so fast that I couldn't light it. I dialed down the pressure lower and lower until the flame stabilized. Unfortunately, at that low a pressure the flame went back into the pipe!



But at least now I have a problem that is knowable and common... the flame is blowing off the end of the burner despite the flare (might not be long enough to have hit the sweet spot.) My previous problem was... *drumroll* User Error.


So now I'm considering the flame retainer without serious reservations. It should continue to work at higher velocities once lit right? (like my oxy-acetylene torch.. need to light it at low pressure, after that you can crank it up.)

The other alternative is to just widen up the flare, I think. If the whole point it to find the magic spot where velocities match then the narrower positions aren't worthwhile anyway.

[kcrucible]

Well, I think we're on the right track now... I did some research on flame holders and ultimately came up with "there's no right or wrong way to do it." Documented here.

Here's some pretty pictures courtesy of the government on this page, and finished off with a cool video of a "jet engine" using a fan grill for a flame holder. I happen to have a LOT of fan grills lying around, so figured I'd give it a shot. The profile seems very much like what Robert made with the concentric rings in his "My New Gas Forge" post and I'd seen other references to a concentric ring arrangement, so figured it was worth a shot.





Wow... the nature of the flame couldn't have changed more. What had been a mighty torrent of blue gas erupting from the end of the pipe was now whisper-quiet. It lit immediately, even at 30 psi. No burn-back at all! The blue flame was now concentrated into a concentric "cross" shape of a much more intense blue color, but surrounded by an aura of yellow.

High Pressure


Low Pressure



I think it's clear that while I'm holding the flame like a champ now, there's too much obstruction (both metal and gas backpressure) happening and it's totally choked off the ability of the jet to pull in sufficient oxygen.


So, I'm thinking about clipping out a few of the spokes here to let things flow a little more freely. Sound like a good plan? Any suggestions on which to remove, or doesn't it matter so long as I get rid of the "tight" spaces?

Probably the best solution would be to leave two wires heading into the center and install a cone welded between the two of them. This should yield the "funnel-shape" flame found here (more or less)





Still, this fan-grill flame holder seems like it might be an excellent solution to someone using forced air.

[Sweany]

Robert, I hope you didn't think I was dissing what you said. I was trying to clarify what I said.
After all it's not a discussion if everybody has the same opinion is it?

[Sweany]

Now if you really wanna expand your mind.

http://www.process-h...000f932a8c0____

This is the start of a series of articles on process burners.

[tlreif]

A quick note on the t-rex burners. I make my own knock off version of that burner. The reason for the larger air intake (By the way it is 1 1/4 pipe.)is that you have an increase in velocity. The club I belong to makes a similar burner to yours and side by side at same pressures the t-rex burner has a flame that is much longer. I like to have my tip even with the bottom of the intake slot. Also the burner flare is highly important. it must be a 1:12 ratio. Without this you may be causing this problem. It definatly will effect the stableness of the flame. I am no super expert but this is what I have found

[Robert Simmons]

put it in a forge and see what it does there. It will be COMPLETELY different from running it in open air.

As for flame holders, If I had it to do over again I would use a cone mandrel and expand a 1/2" inside diameter pipe to 3/4" inside diameter and weld that one so the cone sit higher than the rest of the tube exit. It should cause the flame to glue to the end.

And you have flame shape you do because of wake turbulence. That is why my flame holder is a long cylinder 2 inches long.

But anyway, put it in the forge first with the forge buttoned up for work -- because you will be shocked at the diff.

[kcrucible]

Robert Simmons, on 18 August 2010 - 03:46 AM, said:

put it in a forge and see what it does there. It will be COMPLETELY different from running it in open air.

As for flame holders, If I had it to do over again I would use a cone mandrel and expand a 1/2" inside diameter pipe to 3/4" inside diameter and weld that one so the cone sit higher than the rest of the tube exit. It should cause the flame to glue to the end.

And you have flame shape you do because of wake turbulence. That is why my flame holder is a long cylinder 2 inches long.

But anyway, put it in the forge first with the forge buttoned up for work -- because you will be shocked at the diff.

Last night I removed the flame holder grill entirely and ground out my flare to a wider cone. This has the effect of slowing down the exit gasses by itself AND introducing exit turbulence by virtue of the castable aggregate mix... some nuggets now sit higher than the rest (the hard stuff), some areas that had nuggets got pulled out forming depressions, etc.


I went to fire it up and the flame is now stable and pretty good looking (in a chaotic flame-licking sort of way rather than a nice flared cone), though it needs just a touch more air and the low-end is a bit touchy, not dialing down quite as much as I'd like in case I ever want to use it as a kiln, annealer, etc.

I just moved the jet tip closer to the solid tube portion, which should help out the low end. Not sure if it will help (better vaccum) or hurt (more constricted opening) the fuel ratio. After I fire it up again I should be able to tell what needs to happen. But I'm pretty close now. At this point I could throw it in the forge and see how it works I think.

Now I just need to finish the forge... I have been focussing on the burner, since without a burner I've got nothing but a big paperweight.



Update: Moving the tip up (about 1/4" from the end of the slot) improved both the low end and the fuel mix. Now I get good mixing from low (est 1-3 psi?) to the middle-high range of the regulator (about 20 psi?) Going higher starts adding orange, indicating oxygen starvation, right? I'm widening the slots a bit and tapering them inward to increase air intake area and reduce friction/turbulence at the entrances to see if it helps. At this point I could also consider a cast-venturi tube at the jet to increase suction, but widening the slots seems the more obvious of things to try first.

[kcrucible]

tlreif, on 17 August 2010 - 03:19 PM, said:

A quick note on the t-rex burners. I make my own knock off version of that burner. The reason for the larger air intake (By the way it is 1 1/4 pipe.)is that you have an increase in velocity. The club I belong to makes a similar burner to yours and side by side at same pressures the t-rex burner has a flame that is much longer.

But is a longer flame any better? The point of the burner is to combust a given quantity of fuel fully. If that happens, the same number of BTUs should be added to the forge, regardless of the velocity. If anything, I'd think that a slower flame is better because the hot gas will linger in the melting/forge chamber longer, pushing the heat into the material you're working rather than shooting out of the exhaust/front... less dragon's breath.

Higher velocity has the advantage that it should make it easier to get enough oxygen to fully combust, but I don't think that the velocity of the air/fuel/flame itself is neccessarily an advantage? Maybe I'm wrong. Happy to be corrected by people with actual experiance of the difference instead of just theory!



http://www.scribd.co...eating-Handbook

Quote

Natural gas delivered for heating purposes typically has a heating value somewhere between 800 and 1150 BTUs per cubic foot. The exact heating value for a spe- cific gas can be obtained from the supply- ing utility. The heating value for propane is approximately 2550 BTUs per cubic foot of gas.
...
Example calculation: A heater with an in- put rating of 125,000 BTUs per hour is to be supplied with gas having a heat value of 990 BTUs per cubic foot. The heater will use approximately 127 cubic feet of natural gas per hour (125,000÷990 = 126.263).

The volume of the fuel is a product of psi and the orifice diamter. Velocity is just how fast it's moving and a byproduct of the first two + size of the container, right? At the same pressure and jet size, the same number of BTUs is being put into the forge as far as I can tell.

Quote

I like to have my tip even with the bottom of the intake slot.

Yes, after I moved the tip up things got better. I'd had it up at this point before, but because of other problems it wasn't obvious that it was an improvement until after I could get the flame stable.

Quote

Also the burner flare is highly important. it must be a 1:12 ratio. Without this you may be causing this problem. It definatly will effect the stableness of the flame. I am no super expert but this is what I have found

I'd thought I'd done the 1:12 ratio, but maybe I misunderstood what it meant. I'd found one website that described it as a doubling of size over the course of 12 of the sizes. ie. if starting from 1" diameter, it becomes a 2" diameter after 12". When manufacturerd to those specs I was not able to keep the flame lit at decent pressures. This might be because my flare was too short to hit the "magic width" where the velocity of the exit gasses would match the flame burn rate.


Last night I widened up the outer sections of the flare (effectively decreasing the ratio I guess) and introduced turbulance inside it to create a hybrid flare/flame holder. This is working very well, though with a shorter flame than I'm sure you're used to. But so long as the same number of BTUs is output, I can't complain.

It works ok up until I start hitting the higher end of my regulator, at which point I'm not pulling enough oxygen to fully combust the fuel. This is where flaring out the air intake or using a venturi tube might come in handy. Still investigating.

[Robert Simmons]

A longer flame is indicative of more burning fuel as the flame must originate outside the tube and then be burned. If the flame gets longer than it is simply taking longer for the gas to slow down, mix and ignite. Inside the forge, the back pressure will interfere with the velocity and 02 content and therefore the burner will burn yellow rich. Adding air in as I do, while not the only solution, helps add more O2 to the flame and finish the burn while not increasing the gas velocity. I think blown burners have to be really large to slow things down. My burner is externally blown. Another problem you will see is when you put it in the forge and put fire bricks on the door, the flame characteristics will change dramatically. Such is the pain of atmospheric forges.