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Kristin_Gail

Real-Time Kiln Advice (Kiln Curently Firing)

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JBaymore    1,432

While all those evenly spaced out holes are probably a good idea once you get location and sizes right... they are liekly also overkill and more complex than you need in a kiln this small.  (I just designed the checker-wall at the back end -before the suetema - of a new anagama I'm designing we'll build this summer.  It's a big wall full of small holes for a large cross draft kiln that resembles what you are trying to do there with your bag wall.)  If it is easy to do, then do it.  But otherwise........ a simpler combination of full and halfs can likey work almost as well.

 

In the bottom couple of layers, do not have a hole line up directly with the axis of the burner flame coming out of the burner.  Other than that, with crossdraft circulation I think you need SOME opening down there as well ... not much ... but some.

 

For me, I want the flame to "break" across the angled face of the target bricks ... not splatter by hitting them flat on.  (Look up the term "flame quenching".)  After the target splits the flame, it then will hit the bag wall "flat on".  When I say "angled" I do not mean tipping the bricks upward..... I mean setting them vertically so a corner edge is right on the axis of the burner flame.  To have this a very even "split", a soap is best. 

 

Keep the target brick as far as possible from the burner nozzle in that small kiln.  I'd set the back edge of the target bricks right agaions the bag wall in this angled case.

 

The exact spacing of openings and non-openings gets "tuned" over multiple firings based upon results.  Once you have the basic kiln working OK....... change things very slowly to measure how the change affedcts results.  Whenever possible change only one thing at a time per firing.

 

best,

 

.................john

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Kristin_Gail    12

Bracing myself for another catch of something ridiculous discovered in the photos, here's the new bag wall and target bricks.

 

new_bag_wall.jpg

 

I didn't have any more regular bricks to use for the target bricks, so we cut up a large flat one we had leftover from the chimney.  They're about 2.5" across and 8" high.

 

target_bricks_01.jpg

 

target_bricks_02.jpg

 

The bag wall is now only about one inch away from the shelves - somehow I moved it back a bit when building.  If there's some sort of formula to decide how far to keep it from the burners, I'd love to use it.  Would be happy to move it forward if necessary.

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TJR    359

I think you can afford to move the bag wall an inch toward the burners. You don't have much space to lift your kiln shelves in and out of there.

Also, your primary air on the burners is still pretty far open. I would crank it down some more.

TJR.

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Kristin_Gail    12

Thanks, TJR.  I'll probably go move it an inch or so.  I've already built it three times today, why not go for four.

 

At the end of the last firing, I couldn't get it to stop reducing.  I played with the damper/primary air endlessly.  Apparently at the very end I had them wide open. 

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JBaymore    1,432

Was the chimney brick you selected at least HIGH DUTY?  If you used low duty bricks there maybe at the top.... in the firebox like that..... it'll melt a bit. Not a puddle.... but totally bloat and deform.

 

Yes you can move the bag wall away from the shelves.... that would actually be good circulation-wise and also thermally.

 

Do I remember from prior conversations that you are running on LOW PRESSURE gas to that burner manifold setup?   Like 11 inches Water Column or something?  If so... expect the P Air shutters to stay very much open... and still not entrain all that much primary air. Can't remember back that far and no time to look back in the various threads....sorry.

 

best,

 

................john

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Kristin_Gail    12

As I recall, these chimney bricks were super-duty.  I'm trying to find my receipt now.  I was told these bag wall bricks are super-duty, but have no way to verify.

 

And, yes, it's a low-pressure system.

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Mark C.    1,807

I agree on moving that bag way a bit towards the burners and away from shelves as thats real tight to shelves and leaves a lot of extra cumbustion space not needed (5-6 inches for a kiln this size is plenty)

 

Those target bricks look fine-those are close to a true soap brick which is a brick cut long wise in half (9" long 2 1/2 wide)

Good luck with the fire

Mark

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Mark C.    1,807

It would be a good thing to read a book on kilns like Olsens kiln building book-in there are all the brick shapes and dimensions

Its what all basic potters should know-it most likley can be had online soomewhere these days.

Knowing what soaps and splits are as well as all the arch and flat brick sizes are will help in the long run. we learned this in school 40years ago and I know them almost by heart

I'm amazed you build your stack without knowing any of this?

Knowledge is power they say.

Mark

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Kristin_Gail    12

I'm very bad at losing information quickly. I read not all of The Kiln Book, but certainly much of it, and all of The Art of Firing. And this was only a number of months ago. I didn't immediately use what I had learned, so I lost it.

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JBaymore    1,432

Thanks, Mark. I had no idea what John meant by a soap brick. It tickles me that I accidentally used (close to) one.

 

Kristin,

 

FYII........ I just assume that when I throw out a term that you don't happen to understand at the time... that you'll look it up. Either in a book or via a google search.

 

best,

 

.................john

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Kristin_Gail    12

Yes, and I do. I have to look up an awful lot of terms you use. And read your responses aloud to my husband, numerous times, before they sink in. And am forever thankful that you take the time to explain the whys behind the answers I seek. I was being flippant and silly about the soap brick. I remember clearly the illustration of brick cuts in the book, and am able to refer to it.

 

You folks have been very patient, overly so, with me and my questions. Unfortunately, my degree is in Communications, not Ceramics. My education has been obtained via community courses, workshops, conferences, videos, books, and in asking questions of those who want to help. Which means many things, but in particular there are an awful lot of holes, which I'm attempting to fill.

 

I would be far too embarrassed to ask the questions I ask here of any of you in real life. I know many of them are ridiculous, and must be frustrating to read. But I'm thankful someone takes the time to answer.

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bciskepottery    925

You folks have been very patient, overly so, with me and my questions. . . I would be far too embarrassed to ask the questions I ask here of any of you in real life. I know many of them are ridiculous, and must be frustrating to read. But I'm thankful someone takes the time to answer.

Actually, Kristin, Thank You . . . this thread has been one of the most interesting discussion on the site -- mainly because you have asked the questions and posted the pictures. I've learned a ton from both your questions/pictures and everyone else's responses and suggestions.

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Kristin_Gail    12

Well, then.  How about a video?

 

 


 
This happens immediately when I light the burners, and it happens whether they're facing into the kiln or just out into the air.  

 

After a long discussion with Marc Ward ... I'm still at a loss.  This is backburning, is it not?  Messy blue flame within the throat of the burner.  Not just in the front (head?) section.  When you turn the gauge down lower, the flame within the throat becomes long, lean and clean, and goes all the way back to the orifice.  (This only happens for a split second, as I turn it down to off.)  There's no carbon in the burner.  When I scrub around in there with a toothbrush, the bristles get a little red from rust, bus certainly not black.  And there isn't much at all of the rust.

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JBaymore    1,432

This is a totally CLASSIC case of back-burning. Yup..... you called it 100%.

 

Do NOT continue to fire with that situation happening. The burners are not made to get as hot as that is going to make them. Plus that heat energy will slowly work its way back through the conductive metal of the plumbing pipes until it hits some components that can take that kind of heat even less than the burner casts can......... and then start to melt/screw up. Things such as the guages, BASO valve seals, and maybe even back to the flexible tubing you are using.  Luckily the P air flow will provide SOME cooling action there.

 

The burners themselves will just rust out faster than they should, maybe crack, maybe warp. Which will screw up things like the retentiaon nozzles more! And the already minimal venturi action that they have to get primary air.

 

Unfortunately this is a problem with those cheapie MR burners. As I said LONG ago in one of these threads, "You get what you pay for". I know a lot of potters use those things....... it is becasue they ARE cheap. Potters do a lot on the cheap... and end up paying later. ("Pay me now... pay me later.")

 

Those burners have terrible turn down ratios (look that term up relative to venturi and power burners). They have poor retention nozzles (look up how those nozzles actually work..... and you'll understand why I say this about those burners). They pull in very little primary air (look up venturi function and see how important the quality of the shape and surface of the interior casting at the throat is.. and then look closely at the MRs construction).

 

Your low pressure setup is not helping you either with this.

 

Unfortunately, it is also POSSIBLE that early on, you might have damaged the retention nozzle part of those burners, making them even more touchy then they normally are.

 

By having those insulating firebricks as a bag wall, you were potentially creating significantly excessive firebox temperatures. (I remember looking at some of your earlier images/videos and sayiong to myself "I don't understand why that looks so hot, but she's not climbing".) 

 

The amount of radiant energy coming off the glowing refractories is linerally proportional to the temperature of them. So the hotter it is, the more heat energy is radiating back out of the burner ports toward the nozzles.

 

Add to that, stuffing the burners too close to the port (I think I remember this discussion about them being too close earlier on.... but maybe not)... and that puts them closer to that high heat energy source... and then the inverse square electro-magnetic radiation law factor comes into play on the radiant energy.  You get way more increase in the energy absorbed than what it would seem by halving the distance.

 

Plus too close to the kiln, and the flow of secondary air around the burner nozzle does not help to cool the nozzles (which further affects the turn-down ratio and retention quality). Then to have the kiln WAY dampered down for overall flow, and that can cut the flow of cooling secondary also.

 

So they may be more touchy than they should be..... or may even be damaged beyond repair (I'm NOT saying that they ARE damaged.... just it is one more variable to look into).

 

Sounds like Marc was headed in that direction a bit.... asking you to clean the insides.

 

Try cutting down the primary air.  Close the shutters a lot.  This may fix the backburning issue, which is a tactic many potters use to fix this.  BUT..... and that is a BIG but........ this screws up the firing controls you might have had with a better burner.  You then become much more dependent on the flow of secondary air into the kiln system.  Plus very dependent on the mixing qualities (look that one up) of the flow characteristics in the interior of the kiln structure to get the atmosphere inside the kiln running anywhere near evenly (pockets of heavy reduction and pure oxidation).

 

Too much stuff going on the do "brain surgery" from afar here.  Sorry.

 

best,

 

.........................john

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Mark C.    1,807

Seems your gas pressure /regulator burner issue is still front and center.

I almost hate to say this but a revamp of this system would save you these headaches

How about some nice ransome burners?

I think another call to Marc Ward would help with this as your burner system is not yet right.

Are you sure your orfice size is coirrect for propane?

Mark

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Kristin_Gail    12

Oh, I'm definitely not using them.

 

I mentioned to Marc the possibility that I had damaged them somehow, with the high heat coming from a firebox made with a soft brick bag wall. He said he was a production potter for twenty years, and his kilns always used soft bricks as a bag wall - and there's no way having a soft brick bag wall could have damaged the burners. (I'll let you two experts settle that one amongst yourselves!)

 

I wish I had videos from the very first time I used them. In fact, I might. I'll check. But in my recollection they seem to have always done this same thing. (Also, remember how the regulator was turned way down when I received the system? They didn't burn right until I figured out that the regulator was adjustable.) But then, why is there no carbon inside? Marc said that it would immediately build up in there if they were backburning - and they may have been backburning for 36 hours or more.

 

It sure does sound as though I've damaged them - just by using them this way (they were never closer than 1.25" from the side of the kiln when firing).

 

Every time I fire the kiln and get frustrated, my husband mentions that we've been trying to move to the country for years. How about we just buy a lot and, before building the house, build a wood-burning kiln? A fast-fire sort, that still gets its colour from soda? (Because that would be so much eaier to figure out ... hardy har.)

 

Marc came to the conclusion, from my description, that it was not backburning. But I later sent him the videos. I'll give him another call this morning.

 

And will see what happens when dialing down the primary air.

 

And will look up all the new-to-me terms you used, John.

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neilestrick    1,381

Your other option is to build a single power burner that comes in parallel to the bag wall. I've fired many small kilns with that design and they all worked great. It wouldn't be affected nearly so much by the chimney height, you could get plenty of power out of it, and would be easy to control. If you've already got the safety valves, then you could build the burner for $200 or less, depending on where you source the burner tip and blower.

 

 

post-6933-0-20657600-1400851508_thumb.jpg

 

post-6933-0-20657600-1400851508_thumb.jpg

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Lockley    2

I think your problems with this kiln would indeed be solved with a power burner.  Naturally aspirated systems are a little tricky to operate in my experience.  In my checkered career I've built a large number of naturally aspirated systems.  I've found low pressure systems to be the most difficult to make work.  Naturally aspirated burners are incredibly sensitive to down stream and up stream resistance . Too much resistance in front of the burner and it will back burn.  To much resistance behind the burner it burn orange and smoky.  There is another problem, which is referred to  as resistance to mass transfer, meaning that hot gases are harder to move in volume because the individual molecules are moving so fast in a chaotic pattern.

 

The burn problem you show in your video tells me that the total velocity  of your air fuel mixture is to slow for the diameter of the burner.   How to solve it? Either reduce the resistance in front of it or increase the gas velocity entraining air by increasing pressure.  Given your previous discussion I vote for moving your splash bricks toward the center and moving your whole bag wall back an inch or so.

 

Lockley

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JBaymore    1,432

I feell like I am running an online class here.........

 

If the flame front is seated ANYWHERE except in the plane of the front face of the retention nozzle narrowing flange, on the exterior (kiln) side of that plane,.... it is not burning where it should be burning by design.  Period!  End of story.  No metal burner is designed to have the flame burning inside what is called the mixing tube.  Basic combustion engineering.  Look it up in INDUSTRIAL references.

 

The video you shot clearly shows backburning unless there is some amazing distortion in the image.  The flame is burning inside the mixing tube.

 

The reason it is backburning is the relationship between the flame speed of the gas air mixture, and the rate at which that mixture is flowing out of the burner......and the inability of the retention nozzle to keep the flame seated where it should. 

 

Flame speed is the rate at which a flame front burns through a mixture of fuel and air.  Flame speed is determined by a number of factors. One is the particular fuel gas used. Another is the ratio of primary air mixed in with the gas. So flame speed is not a total constant.  It does vary a bit.

 

So............

 

Think of a canoe on a slow moving river.  If the canoeist is paddling upstream at the rate the river is flowing, relative to a point on the shore, the boat sems to just stand still.  This is when the flow out of the burner matches the flame speed exactly. (This only happens at one set ting for a give na gas/air mixture.)   

 

If the canoeist is paddling faster than the stream is flowing, he moves upstream.  This describes backburning.

 

If he is paddling slower than the river flow, he moves downstream relative to a point on the shore.  This represents the condition often known as "fluffing off".  The flame sort of "blows out".  Just before this happens... ther can be a gap of many inches between the burner nozzle tip.... and the burning gases.

 

All this above is a good analogy to bring to bear when thinking of how a burner is working.

 

Now you have to look at the kinetic energy flow that is mechanically induced by the flow of gas molecules coming out of the burner's orifice.  As gas molecules come out they "bump into" the air molecules occupying the burner's mixing tube.  This pushes them down the mixing tube.  This creates "space" and more air molecules come in the back of the burner to replace the ones moved away.... because nature abhors a vacume.   

 

Now when the potter tries to have the burner running at a low setting (to keep the kiln from jumping in temp and blowing up pots) the kinetic energy of the gas coming out of the orifice is minimal.  If that orfice also is relatively large to get he BTU volume of gas needed for later in the firing, and the pressure it is being ejected out is quite low, then the energy to 'bump into' air molecules is even more affected at the low settings.

 

When the burner is in this condition, the canoeist is paddling faster than that river of gas and air is flowing. 

 

THIS is where the retention nozzle is supposed to do one aspect of its complex job.  The main way it is supposed to do this is to cause flame quenching to happen right at the very large thermal mass of the tip of the burner. As the flame front is burning outside the burner nozzle and back toward the orifice in the mixing tube, when it reaches the relativelty cold mass of metal of the nozzle, that absorbs heat energy and stops the combustion reaction.  The flame can't burn back into the tube........ no energy present to sustain combustion. (There is another action that the nozzles do to help with this... which involves how the flow of material exists the slight narrowing flange at the nozzle ... more on this in a minute.)

 

So now we get into the aspect called "turn down ratio".  Every burner has one.  This deals with the range over which the burner and nozzle system can deal with the canoeist's actions.  It is the ratio between the maximum stable output of the burner, and the LOWEST setting that the burner can maintain a stable flame.  If you try to turn the burner's BTU output below the low end of that turn down ratio, the canoesit paddles faster than the river... .and the burner backburns.

 

On the other side of things, when the river is raging and the canoeist can't keep up with the flow, the nozzle's job again is to keep the flame seates at the tip.  There are a number of types of designs to do this, but what they all do is to try to slow down a small portion of that raging river, mainly by creating some eddy currents in the laminar flow through the center of the mixing tube, and keeping a small "ring of fire" seated at the nozzle tip.  This acts like a continous pilot burner to keep igniting the fast moving flow around the edges as it comes out of the burner. 

 

Some types of retention nozzles not only have a slight restriction plate (flange) in the nozzle to cause the eddy currents I mentioned (and also assist in flame quenching), but also have a set of small holes drilled in this plate.  This creates a small ring of individual stable little pilot flames to help do this job.  Looks a bit like the ring burners on a home gas stove.... and works on the same principle.

 

As the materials exit the burner tube, due to friction with the surrounding air (and kiln stuff) that starts to slow things down.... and the flame front eventually catches up with the velocity of the gases... and everything is now burning nicely.  This is why you often see that cone of nothingness in the center of the column of flame coming out of burners.  It is burning only around the edges.... but in the center it is still just gas and air mixture. 

 

If the kiln system is inducing draft, this complicates this further.  And resistance to gases flow (as mentioned in post above) near the outlet of the burner also can affect the flow out of the burner.

 

The better the burner,.... the better the turn down ratio.  The beter the burner, the better the retention nozzle properties.

 

The higher the pressure of the gas supply the more kinetic energy the gas molecuels exit with, the more flexibility you have for control.  And more ability to entrain % primary air.  The better the quality of the engineering and execution of the throat casting of the venturi on the mixing tube, the higher the percentage of primary air the burner can entrain....and again more conmtrol options.

 

As I said before.... the low pressure system is NOT helping you.

 

 

Burners are made of metal alloys.  The retention nozzle end of burners are made of specially formulated alloys that are intended to get relatively hot. They are also made in thick profiles, to help them not only cause flame quenching( to help with retention and specifically prevent back-burning), but tio also allow for some normal detrerioration and 'rusting away'.   

 

But like all metals, with heat and corrosive atmosphere, they eventually and slowly do deteriorate.  They are not "forever items".  The burner designers EXPECT certain situations to exist when they engineer the design of the burners for use.  It is expected that the nozzle area will be exposed to certain conditions.  It is ALSO expected that over time,....... becasue of the exposure to the heat energy.... those nozzles will deteriorate and eventually need replacement.  The amount of time this will take is based on the environmental expectations (planned life of the nozzle).  The hotter they get, the faster this deterioration will occur.  As they deteriorate, their retention qualities deteriorate.

 

Higher quality burners have better retention nozzles.  They do the 'retention job' better, and last longer.  You pay for this.

 

This is not "conjecture".  Or "opinion".  Ask a burner MANUFACTURER or design engineer.

 

With the PARTICULAR design of that kiln, the PARTICULAR circumstances you have described, and the IFB bag wall.... as I said it is possible that excessive temps may have damaged the burner nozzles.  I'll stand buy that statement also.  It is a good point that I was not remembering correctly that you had the burners stuffed up close to the kiln... so that fact certainly decreases the possibility. 

 

It does NOT impact what insulating firebircks can do to refractory temps when used in bagwall situations. 

 

I've seen lots of what by industrial standards would be called "poor practices" done by all sorts of 'experienced studio potters'.  It does not make them "correct".  I've seen lots of firebox area refractories un-necessarily deteriorated because of practices like using IFB for bag walls.  (For target bricks...... not so much of an issue.... full bag walls..... much more so.)

 

I stand by my statement that excessive heat can damage a burner retention nozzle.  I have pictures.  (Also have one nozzle out of four on my gas kiln that, at about 30 years, is showing its age ;) .)

 

I've taught this crap at the undergrad and grad level for about 35 years at MassArt, BU, and NHIA, and literally have built hundreds of kilns.

 

best,

 

.................john

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Kristin_Gail    12

John, I think you are running an online class.  Although you teach this at the undergraduate level, I feel like it's a masters-level class, and I haven't had the proper four years' schooling to prepare.  I'm currently researching your terms.  

 

And I'm certainly not arguing with you at all on any points - not in the least.  Just reporting on my conversation.
 
Thank you for your analogy of the canoeist.  It makes complete sense.  I have a B-2 Ransome burner that I used on my old kiln, and I have seen with that one both the fluffing off and backburning, with orange flame coming out through the primary air valve.  (Incidentally, our initial plan was to run this new kiln with that B-2, plus another identical B-2 - but at the last minute he decided to go with this low-pressure system instead, as it would fit the size of my kiln better and was about the same price.)  In any event, your description helps me to understand what I was seeing with the burner/kiln system.
 
Here's the retention nozzle on this burner, plus the pilot.  
 
burner_tip.jpg
 
When I spoke with Marc, he mentioned that, with this particular burner, the flame can begin within that last section.  In fact, he isn't even that worried about the blue flame being in the thinner, throat section.  He would be worried, however, if it were orange.
 
However, that's not what you're saying at all.  And I'm not comfortable with it either.
 
I dialed down the primary air as far I could while still seeing inside the burner from the back, which is about 1/4 closed, and I can still see blue within the throat.  The flame does appear to shrink (get closer to the tip), but it's difficult to tell, really.  
 
I closed the primary air completely, and - viewing from the front - the flame front is still within that front head section of the burner - and possibly further, within the narrow throat section, but I'm unable to see just how far it is.  
 
These two descriptions are not dependent upon the amount of pressure.  No matter how high or low the number on the gauge, the flame still begins within that section.
 
Both burners behave identically, and I'm performing these tests with the burners moved away from, and not facing, the kiln.  He has suggested that, if they were placed properly next to the kiln, and the kiln system was set up properly, the amount of draw from that system would change this backburning issue.
 
He has also offered to replace the burners, but also doesn't think there's anything wrong with what I have.  This would leave me with the same system I currently have, just new and possibly not damaged, if I have done so.

 

Neil, if it comes down to me building a burner - I'm ready for that.  Really.  But holy hell, imagine the amount of help I'd need.  Oh, my. 

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Benzine    610

I feell like I am running an online class here.........

 

 

I've taught this crap at the undergrad and grad level for about 35 years at MassArt, BU, and NHIA, and literally have built hundreds of kilns.

 

best,

 

.................john

Hey, I'm getting a credit for this, so just sign off on the paperwork, when you get it, and don't ask questions!!!

 

Honestly, I've learned more, applicable information, just reading this topic, than I have in one year, of school required professional development, where I actually do get a credit.

 

Also John, I don't want to toot my own horn, but I have built a grand total of one, really simple kiln, soooo.... *Brushes shoulder*

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JBaymore    1,432

When I spoke with Marc, he mentioned that, with this particular burner, the flame can begin within that last section.  In fact, he isn't even that worried about the blue flame being in the thinner, throat section.  

 

Yeah it can begin in that last section.  But it shouldn't.  This is becasue those MRs are pretty minimal burners.  They will not hold a stable low level output flame.  They have terrrible turn down ratios.  There is very little function to those "retention" nozzles.  GACO doesn't even call them "flame retention nozzles"  on their own product brochures... they call them "flame retention features".   They are nothing more than a slight restriction to the diameter of the mixing tube.  At higher settings they will cause that "eddy current" that I mentioned, and keep the flame from fluffing off.  But they don't do much to prevent backburning.

 

Technically those burners should not be turned lower than the level at which the flame does NOT burn back inside the mixing tube.  THAT is the lowest level they are really designed to be run at.  When you light it (not installed on the kiln), try turning the gas up a bit stronger than you have been before you hit it with ignition to see if the flame then "seats" outside the mixing tube.

 

BTW......... GACO only lists them for use with gas pressures between 3.5" W.C. and 9.5" W.C. .

 

Note the method that the Ransome pilot uses for its stable flame retention.  It is almost like a gas stove "ring burner".  Very stable flame (has to be as a pilot).  That approach is too much 'resistacne to flow' for a larger burner........ but that "lots of small holes" shows the concept of flame quenching and the slowing of the flow of gases below the flame speed. 

 

While it was not as much "on axis" to the burner port as the MR on your kiln (hence not quite as much radiant heat hitting it) ....... note the condition of the Rasnsome pilot's retention head metal... and the condition of the tip of the MR in the photo.  All of that stuff was brand new when you built the kiln, wasn't it?

 

http://www.potters.org/subject19495.htm

 

What orifice # or size is in your existing Ransome B-2?  I have an "experiment" in mind for you.

 

best,

 

.........................john

 

(PS:  I know that you are not arguing.  ;) )

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Kristin_Gail    12

I don't see any numbers on the B2 orifice.  I can tell you that it's almost exactly 1mm in diameter.  I don't know if the size would have been listed in my original receipt or not.  It's possible I could find it.

 

Here's that one (with a six-year-old holding the penny).

 

B2_orifice.jpg

 

I used this burner on another electric-to-soda conversion, set 1-2" from the burner port.

 

10.jpg

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