Troubleshooting This Converted Kiln O' Mine: Taller Chimney?

[Kristin_Gail]

I fired my kiln, Big Anthony, for the second time this weekend.  The first firing taught me an awful lot - and you folks, more, here.  (Well, many other threads, but that was the latest.)  This time was pretty well a failure.  I messed up the reduction (again), I screwed up the soda (again - worse than last time).  But I did succeed in not crash-cooling!  Woo for me!

 

I'm certain all the problems I'm having are 1. My fault and 2. Obvious issues.  The sort that, when identified, will make you throw your hands in the air and say, "MY GAWD!  How stupid can she be?  Yes, you have to close the lid!" or something equally as neophyte.  If I could have someone knowledgeable on-site, for maybe an hour or two of the firing, I'm certain all questions would be answered.  However, not having that option, I'm here to bug the heck out of you guys instead.

 

The issues:

• With my former little kiln, I could put it into reduction by moving the damper.  With just an 1/8" move, I could make it smoke, or shoot out a 4" flame, or nothing.  I labelled these options "bad, reduction, and neutral."
  It is not so easy with this bigger kiln.  Now I either get 1. Gobs of black smoke (damper pretty well closed); 2. Nothing (damper open 1"); or 3. Rapidly decreasing temps (damper open more than 1").  I can't find that "short flame shooting out" place.  This all assumes I'm firing with gas only.
   
• The damper has to be closed down to only 1" open in order to increase temps.  Open more than that, it won't go up.
   
• It stalls out at 1900°F.  I use wood after this, and when I do, I don't do it right.  Wood creates massive fires coming out of peeps when opened, accompanied by gobs of black smoke.  Each time I add wood (about every 10 minutes at 1900°, closer together at the end) I open the damper up to 3", to try to avoid this effect, while watching the temp drop rapidly.  Over a 3-4 minute period, I slowly close the damper back down to 1" as the smoke lessens.  Once it gets back to 1", the temp rises again.
   
• Many pieces are cracked - either where I attached handles, or where there was a paper-thin crevice (I have a nick on a rib I use on the outside of pots - it leaves the tiniest groove.  This groove grows in the kiln and in some cases cracks.), or in the foot.
   
• I don't think I'm using the burners properly.  They just don't sound right.  They sound loud and dirty and orange.  I've messed an awful lot with the primary air thingies, and it doesn't do much.  (I took a video; I'll see if I can find it.)
  It's a low-pressure system.  The dials go up to 10 PSI, but when both are burning, they max out at 5 PSI.  They're MR100s, running together on two 100-lb tanks.  The wind was strong this weekend, and when it really gusted, it made brief back-burning noises.
   
• It fires very uneven - at least two cones difference from top shelf to bottom shelf, probably much more.  There are only two shelves, and I put a third, empty, shelf atop all of it, in hopes of keeping the fiber roof from falling on the pots.

Okay.  So here are the facts about the kiln and the firing:

 

• I followed two of your recommendations this time.  1. I put a half brick in the flue and 2. I covered the hole above the damper with two pieces of kiln shelf:
  
  
  
  
  
  
   
• I'm still single-firing, but to save me from a tiring 24-hour firing, and upon the recommendation of Marc Ward, I fired slowly (9 hours) through 1100°F the first day, shut it up tight and slept.  Then, on Day 2, I fired quickly (8 hours) to Cone 6 (it was 200°F when I began on Day 2).
   
• When peering into the kiln, I can see the the majority of the flames are going right under the bottom shelf, directly out the flue.  The bag wall is 5" high, but the bottom shelf is a bit higher.  I'd say about 1/4 or less of the flames go above the first shelf, and none of them reach the second shelf.  Hopefully you can see what I mean in this photo about the height of bag wall vs. shelf.  I figure I need to make that higher than the bottom of the first shelf?
  
  

[split into two posts to accommodate all the photos.]

[Kristin_Gail]

 

• We added two more holes to the kiln - to add more soda and tiny additions of wood.  I now have gobs of holes all around this thing (well, technically speaking there are 11, not counting the burner ports), none of them being air-tight.  And the lid shows light, as well - although I do shove fiber in it as I approach 1600°.
  
  
   

• The chimney height is built per Marc Ward's specs, following that formula I now can't find but is standard.  It's 8'6" from the bottom of the flue to the top.  Hole of chimney is 9" x 9".  Exit flue is 9" x 4.5".  Internal dimensions are 27.5" wide, 41.5" long, 30" high.  I put a Venture-effect layer about 3' or so up from the exit flue.
  
  However.  Marcia has suggested a couple times it sounds as though the chimney isn't tall enough.  And, now that I stop to think, I realize I do have this thing situated in a rather tight urban space.   It's in the half-built shed, behind the white garage:
  
  
  
  
  
  While the top of the chimney is taller than both the kiln shed roof and my (white) garage roof, it is probably 4' lower than the roof of the tan-coloured garage of the next-door neighbour, which is only 20' or less away.  Then there's the pottery shed - seen in the first photo - and its roof is higher, as well.  And another neighbour's shed, about 30-40' behind me, is also higher.  My husband and I are fairly convinced this is my major issue.  The thing is just shoved back there in a hollow, so we're having a difficult time getting a draw.  
   

I have an awful lot of questions regarding the logistics of increasing the height of the chimney. (Should I also reduce the diameter of the hole, by shoving a 12' or longer 6" culvert pipe down the entire thing?  Or go 9" so it can sit on that Venturi-effect layer?  If so, do I need to somehow mortar around that place?  Or should I just extend from the top, up?  If so, what sort of pipe should I use?  How would I seal from the brick to the pipe? And oh so many more.)  But for now I just wonder if y'all agree, that might be what's causing me fits.  If the chimney were higher, would I be able to open the damper more?  And stop having problems reducing?  Have a happier-sounding flame coming from these burners?  Magically start introducing soda properly instead of clogging multiple garden sprayers and ruining kilns full of pots?  (Okay, maybe it will not be the magic panacea I want it to be...)

 

​Phew.  I tried to include as many details as I have, but I'm certain I've left out something important.  I am forever - forever! - thankful for any advice.  I kept thinking, while firing, that if I could just Facetime with one of you, for just ten minutes, you could probably tell me exactly what's going on.  I need a mentor that's closer than a thousand miles away.

 

 

 

 

 

[neilestrick]

This was a lot to take in, so I'll do my best here. It would all be much easier in person!

 

- If the burners are running really orange, then they are probably running with too much gas or too little air. Try for a blue flame and adjust from there.

 

-If it's really uneven, I would raise the bag wall.

 

- That's a really small kiln to try and fire with wood. If it's really flaming out and smoking badly, then you're probably putting way too much wood in at one time, or the damper is too closed up. That and the fact that it's not really set up properly to fire with wood. You're trying to use wood in a kiln that's designed for gas. Wood kilns have some requirements for air flow and such that you're just not getting with that setup. Plus you're not really going to get the true benefits of wood and ash unless you fire up to cone 9 or higher, which may not happen with that kiln with its limited insulation. At cone 6 you'll get some flashing, but any ash that accumulates on the pots may not melt well unless you salt or soda it well, too. Plus you'll get the flashing with the salt/soda anyway.

 

-If the damper is that touchy, then the exit flue may be too closed up now, however with your burners running too orange it's hard to know for sure. You need to get the burner situation figured out first.

 

When firing, set the burners so the kiln is climbing at the rate you want, and with the degree of oxidation/reduction you want, then adjust the damper so you get just a touch of back pressure out the top spy hole (downdraft kiln, right?). You may have to work your way through these 3 settings several times to get everything in balance. Many people who fire with venturi burners never touch the air adjustment on the burners. The only adjust the gas and damper. So really work on getting them set properly.

[Chantay]

You might try and post some of these questions on clay art also.  A number of wood burners post there.

[Kristin_Gail]

Neil, I'm amazed you made it through that ramble. I so appreciate your perseverance.

 

I never intended to make this a wood-burning kiln. I was just going to throw in a little stick or two when I added soda, because someone, somewhere, said it would make the flame longer and take the soda deeper into the kiln. When I wasn't able to get to temp, and the wood was just sitting there, I ended up throwing it in to get me over the hump. I'd rather fire the kiln as it was intended, for sure. But can't figure out how to do it.

 

I have made acquaintance with a very kind man who teaches pottery at a craft college a couple hours away. My next course of action is to do whatever sweet-talking I can to get him over to help (cookies help, right?). Because my constant messing with the burners, the pressure gauge, the damper, isn't getting me anywhere. Same flame, same stalling. Perhaps he'll have the magic touch.

 

And I'm definitely raising that bag wall!

 

Will playing with the burners and damper, with the kiln empty, teach me anything? Or do I need to do it with a full kiln, as I have been?

 

I'm still perplexed about all the cracked pots. Am I just firing too fast through a certain temp range? I loved the idea of blaming it on the accidental crash cool last time, but that obviously wasn't the case.

 

Forever thankful!

[neilestrick]

What kind of clay are you using? Is it mostly S cracks in the bottom (inside or outside), or vertical cracks down from the lip, or spiraling cracks around the body?

[Kristin_Gail]

Oh, some of the pieces broke almost right in two. Not a crack, really, just a big ol' fracture. One mug broke open from rim to halfway down the side, straight down, two cracks directly across from each other.

 

Most of the other cracks radiate out, semi-horizontally, to one side or the other from where the handles attach.

 

I put in a thin platter I had bisqued ten years ago, made with a red clay from Laguna? It was on the top shelf, which was I assume very cold. It just plain ol' broke. The bit that hangs off past the foot looks to have sagged down a bit and sort of torn off in a section.

 

I used two different clays for everything else: Mid-White and Mid-Smooth Stone from Tucker's in Ontario. The whole kiln load is sitting on the shelves still. I didn't get to take a good look at everything yet. Tomorrow I'll separate each pot by clay and slip, to see if there is a correlation. And to take a better look at the cracks. And take photos, of course.

[Guest JBaymore]

What burners are you using?  Those MR thingies?  That is part of the issue with level of "control".  They do not entrain all that much primary air becasue they are cheaply manufctured castings with poor venturi fuunction.  Even the BEST quality venturi burners can entrail no more than about 70% primary air......... so you are always dependent on secondary air with that sttyle of burner. 

 

So that puts you quite dependent on kiln entrained secondary air.  Which brings you to the function of the combination of burner ports sizing, exit flue sizing, size of the chimney interior cross section, the height of the chimney, and the temperature of the exit gases at the top of the chimney.  I think you have an issue with this for a combination of reasons.

 

The comments about 'orange and noisy' also tends to lead me to think the burners might be doing what is called "back burning"........ the flame is not seated at the tip of the flame retention nozzle (those MRs have VERY primitive function in that regard)....... but is combusting inside the burner mixing tube.  NOT what you want, if that is happening.  Screws up firing fuunctions.... and will deteriorate the units fast.  Getting the tip of that burner too hot (too close to the kiln) helps to cause the nozzle to not retain the flame properly.  You need a cooling flow of secondary air flowing around that tip to keep the metal there cool (relatively).  (Can you post a Youtube video of the burners during a firing?  How about a still of the burners on the kiln?) .  How far away from the outer face of the kiln wall at the level of the burner ports is the plane of the face of the tip of the buirner (hope that is clear)? 

 

Looking at that kiln design from that one interior image,..... of COURSE the flame is going right thru the bottom.  More cowbell  ...... uh.....bagwall .  Higher but perforated.  NO hole directly lined up with the axis of the burner ports.  Tune these openings over multiple firings to spread the heat energy out appropriately.  (Spread a lot of cone packs in the firings until you know that kiln.)

 

You say you have an exit flue from the chamber into the chimney that is 4 1/2" by 9" and that you have blocked off HALF of that?  So that means that you have a 4 1/2" x 4 1/2" opening, correct?   That is only 20.25 square inches amd with the surface area of the exit opoint divided into two sections half that.  Too small with too much entry friction losses. 

 

The kiln is 19.8 cubic feet.  The MR 100s are rated at 153,500 BTU each at 5 pounds pressure using propane.  Call that 300,000 in round numbers.  For a 20 cubic foot insulating firebrick kiln you need a MAXIMUM capacity (hardly ever used) of about 200,000 BTU / hr.  You have some SERIOUS over capacity firing capability there.  Even though you have that calculated 200,000 BTU/hr. design capacity....... in most cases .... you'll never need it. 

 

So your issue is likely not total BTUs available, if the MRs are only entraining about 65% primart air.  Which I HOPE they will do.  That will give you the 200,000 BTUs.... (but might also put 100,000 BTUs up the chimney as basically unburned fuel).  BTW.... you should not have trouble getting reduction in that kiln due to the lack of aeration of the fuel with that type of burner.  A sime\ple adjustment of the damper should be all that is needed.  What might be harder though is getting good MIXING in the chamber so that the reduction is even.

 

The very common novice mistake is to turn UP the burners when the kiln is not climbing........ often you need to turn them DOWN.  What is happening in some cases is that you cannot burn the fuel inside the kiln (so not getting the heat energy there).... and the extra heat energy is going out the chimney either as unburned fuel... or is heating up the chimney excessively.  With the low bag wall you have... and the poor primary aeeration those MR buerner produce (compared to a quality venturi burner) I suspect this is the case.  300,000 BTU inside that kiln should shoot it up like a rocket.

 

The "venturi" in the chimney is also a question here.  Because of the math that is involved in true venturi operation, the shape and texture of the narrowing of a ve nturi is a critical factor in the fuinction.  Just narrowing the chimney down is not necessarily doing what you think it is.  Now it can have a positive effect if the chimney iteslf is already oversized for the kiln unit.......... it is like closing the damper a bit.  Couple that narrowing with your tiny exit flue from the chamber ... and thire is another area to look at.

 

Sorry... it is hard playing "doctor" via a forum situation from afar.

 

best,

 

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

[Kristin_Gail]

John, I explained to my husband earlier today that, if I were to go back and read all 2,000+ of your posts, I would have a complete education in ceramics. And here you've done it again. What you've shared here is absolutely logical and comprehensible, even to, well, me.

 

The bag wall is one of those "shut the lid, duh!" instances. Of course the flame will go straight out - I could tell you that from the photos. But I'm too deep in the forest sometimes.

 

Everything you've said about the burners is something I've felt to be true, but didn't understand why or how. They *do* seem to be back-burning. And I *know* that's bad. But I can't figure out how to fix it. I believe they did burn cleaner on the first go 'round, when the brick was not in the flue.

 

And I've felt that, too, I shouldn't be turning these burners to max PSI so early, or ever - but couldn't understand what I was doing wrong to make me do such a thing.

 

Okay. Much time now to digest all of this.

 

And I do have a video of the burners going. But I'm embarrassed to post it. Of course they're back-burning! Of course they're too damn close to the kiln! I'm sheepish. But I will find it now.

[Kristin_Gail]

I'm sorry this is all choppy and pixelated; it's my first time uploading a video from this iPad thing.

 

 

The burner support is not bolted down - throughout the firing I moved it closer and farther from the port. In this particular video, they're pretty close, maybe 1" or so? But I couldn't get the flame to change by backing it off, playing with the primary air, etc. (But I was not coupling this with damper changes.)

 

When I initially light the pilot, it does not draw into the kiln. I have to first build a fire in the bottom of the chimney, to heat it up. Then it draws, but not without a fight. Is this normal?

[Guest JBaymore]

OK.... it is hard to tell from that video... but the throaty, gargling, clearing throat type sound I THINK I hear there is a classic hallmark of "backburning". Once it starts... it is hard to stop it except by shutting off the burner and re-lighting it.

 

OK.... on to some of the kiln design and operation course content that I teach (and give in workshops also .... plug ....plug ):

 

So what tends to cause this backburning problem?

 

Every gas burner has something called a "turn-down ratio". It is the relationship between the lowest setting that the flame will be stable on the burner and the highest output of that same burner where the flame is also stable. The MRs have a totally crappy turn down ratio.... becasue they are cheap casts with poor retantion nozzle properties. (Sorry to say........ you get what you pay for.) This means that at LOW BTU output settings... the flame does not tend to stay seated on the retention nozzle, but instead burns inward toward the gas orifice source inside the mixing tube.

 

Why this happens relates to a property called "flame speed", and that is what "retention nozzles" are designed to deal with.

 

Every gas to air mixture ratio (and type of fuel gas) has a rate at which a flame front propogates through it. Think of the flame front like a canoe on a river. If the river (gas air mixture) is flowing at the rate that the canoist is paddling (flame front burning).... the canoe sits still relative to the bank (nozzle tip). If the canoeist is paddling slower than the river is flowing, the canoe goes backward down the river bank (called "fluffing off" on a burner). If the canoeist is paddling faster than the river is flowing, the canoe moves upstream relative to the bank (backburning).

 

At very low settings, the flame is burning toward the gas orifice inside the mixing unit faster than the gas and air mixture is coming out of the burner. And the cheap retention nozzle on the MR likely cannot deal with keeping the flame seated outside the burner on the tip. So now when you are first running the burner........ as you asre slowly heating up the kiln........ the flame is burning inside the burner. This is so low, that you likely don't even notice it.

 

While this is happening, the flame is quietly heating up the burner metal. The hotter the metal gets, the worse the flame quenching action of the retention nozzle gets. So even if you can get the flame re-seated......... the turndown ratio got further screwed up because of the hot retention nozzle. So it comes "unstuck" again easily.

 

As you continue turning up the burner, the flame will not tend to reseat itself on the retention nozzle even when the flow of material increases a lot. IN fact eventually.... the flame can "fluff out" as the speed fo the flow exceeds the propagation of the flame front. SOmetimes you can see this where there is a space between the tip of the burner..... and then you can see flame starting a little buit away from the front of the nozzle.

 

Having installed gas burners with too much BTU output exacerbates this whole issue. This is again exactly becasue of the turndown ratio. At the lowest stable flame setting for those burners, the BTU output is so high, that the kiln unit jumps with too fast a climb. So the potter trys to throttle them back. This gets the burners below the turndown ratio BTU putput..... and bingo..... unstable flame problems. It becomes a choice between screwing up pots and screwing up the flame for the firing. A lose/lose situiation for the potter.

 

You might have been better off with slightly lower BTU burners. While it actually screws up the air entraining characteristics of a given unit a bit when you go outside a certain range of orfice sizes, ........ which are poor on those burners to start with......... changing out the orfices to smaller ones might be in order to help this issue a tad.

 

IF there is decent secondary air flow into the burner ports (see my comments about exit flue size and "chimney venturi" above), the spacing of the face of the burner nozzle to the plane of the burner port face should be about 1 inch +/-. 2 inches away is almost always way too far, and 1/2 inch or less is always too close. Heat energy radiating from the open burner port is also heating up the burner nozzle. You have to have a balance of flow to keep it cool enough to work properly to its design specs.

 

Hope this helps to understand this.

 

You shouldn't have to "fight " to get early draft.  A short while on the Ransome pilots there should soon get things going OK.  A fire in the chimney will start it instantly....... but it shouldn;t take TOO much to get it going. 

 

Those Ransome pilots seem to have a VERY yellow and short undirected flame.  They should burn sharp and mainly blue (like your kitchen gas stove ring burners).  Particularly with the high gas pressures you are running.  This leads me to think there it SOMETHING ELSE WRONG HERE.  Looks like the gas pressure is maybe NOT what you think it is at the burner manifold in this image.  Do they look that yellow and fluffy and weak at the BEGINNING of the firing?  Or do they slowly get this way as the firing progresses?

 

Wish I could see this kiln in person..... it is hard to tell WHAT is going on from this stuff.  Wrong angles of images, etc.

 

best,

 

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

[Mark C.]

I have been away in SF a few days-spent part of today at Petter Pugger manufacturing plant today as well on way home getting parts so I missed this post.

I will agree on points made above-I use the same burners in my salt kiln-secondary air flaps are always wide open I never close them up. (have 4 with natural gas) salt eats them up and they are cheap so thats a fine use for them. Not the best as John says but cheap and they get eaten fast . Kilning my ransomed with salt would break my heart.

Your bag wall need to be way taller with some holes as mentioned-think more about the flame path getting to the damper slot and how to force the flame UP thru the load then DOWN

Your damper slot may be to restricted.

Back to the burners in our 24 cubic salt we have 4 burners and never have them full on.

If you have to start a fire to get draw consider making the chimney a bit taller-use metal if you have to-it does not have to be brick all the way-thick metal will last a awhile but will go away over time. You have so many points to cover you should be able to get a draw in mere minutes-you can test by kiting a newspaper and watching the smoke go out the chimney .

Our salt kiln has 7 holes (1 for draw rings-6 for salt intro) but the plugs fit tight. make sure your kiln is as tight as you can get it. You can use a small hand torch to introduce heat to bottom of stack (chimney ) to get that draw going in mere minutes-then light a crumpled newspaper at front burner port-smoke should draw thru kiln up stack this test is fast and cheap you can do it without a load in kiln as well to test draw.

I must go to bed today has been a long one.

Mark

[Kristin_Gail]

I'm headed out to make a video of the pilot - I think it's always soft and orange, even from the beginning.

 

I'll also test the draw. Heat up the chimney, light a newspaper.

 

Marc Ward described this as a low-pressure system. It has the sort of regulator I can't adjust - is this how all MR100s run? I had no idea they were cheap! Meaning both cheaply built and inexpensive. They were expensive on my budget.

 

John, you're absolutely right that these things shoot the temp up way too fast, initially. The pilot can only get the kiln to just past 100F (with my current configuration). Then I turn on one of the bigger burners, as low as it will go without squealing, and the temp tries to jump up hundreds of degrees per second. I don't know what I'd do if I didn't have a lid to prop open 2.5" for the next few hundred degrees.

 

I have another Venturi burner setup, a B2, which I used with my little kiln. Could I use this maybe to heat it slower in the beginning?

 

Mark C, what does "your damper slot may be too restricted" mean? It's wider than the 9x9 hole, so I don't think I understand this part.

 

But I must be learning something, folks, because I now can understand almost all of what y'all are telling me - which was not the case when we started this party, back last summer.

[Marcia Selsor]

I missed a lot of your info especially about the venturi gas burners. I fired with oversized big berthas selected by the Facilities engineer at my college who did not bother to discuss things with me.. Very touchy, too big, but good burners. I did lots of adjusting with the primary air disc and the damper.I turned up the gas, opened the discs more to make the kiln temp climb. Maybe one turn another but never had it open more than a 1/2 inch.In balance with the damper, they fired even.My problem was holding them back.

After 20 years, several times per week during the school year, I could fire those kilns blindfolded.

I agree with Neil, Mark and John. cut back on the gas and use the primary air. I'd reconfigure the bag wall and unblock the flu.Orange flame is too much gas.

You still have never mentioned you altitude which could play a big factor. The higher your location the more height needed in your stack.At Banff we had an Olsen wood kiln stalling at 1900. It was the stack. Les Manning called Fred Olsen who said that at 6000 ft altitude, we needed much more height to his original design.

Marcia

[neilestrick]

Looks way too gassy to me. Throttle it back. You may need to get an adjustable regulator.In addition to raising the bag wall, also put in a target brick to disperse the flame. This can just be a brick set into the fire box about 2/3 of the way in from the burner, with the 2 1/2 inch side facing the burner.

[Marcia Selsor]

You need a hard flame to bounce off a target brick. A gassy licking flame won't bounce.

Just cut a brick diagonally and place against the bag wall in front of the burner. Not having a target brick causes inefficient turbulence at the base of the bag wall.

Marcia

[Guest JBaymore]

Woah......... LOW PRESSURE!   THAT explains some things here.  Totally changes some of my comments.

 

I read your 0 to 10 POUNDS comment and the 5 pounds comment at the very top posting .... and went off of that.

 

You have MR100's running on probably 5-6" Water Column is my guess now.  And Ransome pilots on same 5-6" pressure.

 

Inches Water Column is a different measurement from pounds per square inch.  One pound per square inch is about 27.68 Inches Water Column. 

 

5" water column pressure squirting out of a small gas orifices has a LOT less kenetic energy than 5 pounds of pressure. 

 

It is that pressure that is the driving force for the gas molecules bumping into the air molecules in the mixing tube and sending them moving along toward the burner nozzle.  Becasue "nature abhors a vacume"....... more air molecules rush into the area through the primary air opening from the outside air to replace the ones in the burner that got moved away.  The "venturi effect" serves to make this air entrainment bigger than it would be if that effect was not there..... but the amount of the increase is directly dependent on the exact shape and surface of the throat (narrowest) area.  (The MRs are not great at capitalizing on this principle.)

 

The Ransome pilots are also affected by this low pressure issue.  But they should be stronger even on low pressure than they look.  Are they running at full line pressure in that video?  Or have you throttled back the pressure on them with the needle valve?

 

MR100s running on low pressure gas will not entrain much primary air at all.  You are therefore VERY dependent on secondary air for combustion.  VERY.  GASCO only rates those burners (with stock orifices) at 100,000 BTU/hr. at 9.5" Water Column (WC).  They are designed to run on a minimum of 3.5" WC pressure.......at 51,000 BTU per hour.  So if the orfices are "stock" sizes (for propane) then those burners are just about right BTU rating for a 20 cubic foot IFB kiln.

 

But without combustion air....... you can't REALIZE those BTUs in the kiln.  And the MRs aren't going to give you the air you need. The rest of the system has to do that.  The driving item for that system in this case is the chimney.

 

Personally I'd lose that "venturi" business in the chimney.  My guess is that it is doing nothing more than acting as a partially closed damper.  And I'd also increase the chimney height by at least four feet.  And open up that exit flue into the chimney to the original dimensions.

 

As to what to make the top of the chimney out of...... there is some more science involved here in the decision making process.  Natural draft induction in a chimney is directly a function of the temperature of the gases at the exit point (top opening) of the chimney.  The higher this exit temperature, the more the capacity of any given diameter chimney.  The reason for this is that the effluent gas molecules at the top of the chimney have to "get out of the way" for the molecules below them to come up and exit.  If they don't move out of the way, they "stop up" the flow.  The greater the differential of temperature between them and the surrounding air at that point, the more "boyant" they are in the outrside air....... and hence they rise faster.

 

So maintaining the temperature of the stack gases is important in natural draft systems (such as this).  Metal piping lets heat energy pass through it peretty easily...... and will affect that exit point temperature.  When the overall system is "marginal" in function, like this one, I'd go with brick.  But it can be low duty bricks... and even red brick.

 

So.,.. more to chew on.

 

best,

 

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

[Kristin_Gail]

It's frigid outside, and blowin' to beat the band.  Have postponed burner-lighting video for a bit.

 

[Deleted photos of crappy pots, because it's a bit like giving them to your mother - they'll come back to haunt you.  I've decided the main culprit of pottery cracking is my method of applying slip to bone-dry instead of leather-hard pieces.]

[Mark C.]

Mark C, what does "your damper slot may be too restricted" mean? It's wider than the 9x9 hole, so I don't think I understand this part.

I should have said you put a brick in your EXIT flue slot and it may be to restricted-take it out

We seem to all agree about raising the stack height -I like Johns idea on use whatever brick you can find-low fire brick found at lunber stores for fireplace work would work fine and is cheap.

Low presure natural draft kilns is what all three of what I use.

Mark

[Kristin_Gail]

Ah, okay.  I get it now, Mark.

 

My husband says the slab under the kiln/chimney will not support another 4' of bricks, and is trying to get me to put an 8" diameter single-wall stovepipe down to that dreaded Venturi-effect layer, which would hold it up.  Shove fiber all around the corners.  Add height as needed.  I want him to show me the numbers on the slab - will it really fail under four more feet of brick?  Film at 11.

 

(p.s. How in the world would I lose that Venturi thingie in the chimney without taking the entire chimney down and re-building it from that point?)

[Mark C.]

Hopefully the slab was at least 4 inches thick and maybe had some rebar or steel mesh in it?? Poured on undisturbed ground?

What can happen sink into ground?

Mark

[Kristin_Gail]

4" thick.  On gravel. No rebar/mesh.  He's convinced it will crack and move, pulling the chimney away from the kiln.

[Kristin_Gail]

This is the pilot burning. The t-valve is all the way open, but if I turn it all the way closed, nothing changes.

 

 

 

This is what happens when you turn on the MR-100.

 

 

 

I remember my little B2 burner used to shoot out a clean, straight blue flame when you had it shooting into nothing like that.

 

This is the hose setup:

 

 

[Guest JBaymore]

Unfortunately, by taking the whole chimney down and re-building it. Easy for me to say.

 

Dead load on a concrete foundation is a valid concern. The 9x9 chimney has an 18" x 18" footprint. Hardbricks are at about 8 pounds apiece. It takes six per course of 2 1/2 " high. At 8 feet that is already about 39 courses, plus the solid floor courses (lets call that 8 per course for 3 courses). So the brick total so far is about 2035 pounds, so that is a dead load on the slab of about 904 pounds per square foot. Add fopur more feet of hardbricks and that goes up by another 960 pounds to about 2995 or 1198 pounds per square foot.

 

What is the design of the concrete slab? How thick? Is there rebar in it? How large is the slab? How close to one edge of the slab is the chimney?

 

Dropping single wall metal pipe in there is not really a good choice at all. It is going to detreteriorate very quickly inside the jacket of bricks due to the temperature and the corrosive action of the stuff coming from the wares. Plus by inserting a round into a square, you are decreasing the square inches of flue cross-section (ducting) from 9 x 9 square and 81 square inches to at most (for a perfect fit) 9" round for 63.6 square inches. On a kiln you are trying to increase the darft on.... not a good choice. Flow in a pipe is directly related to cross-section also. While a flue XC of 64 square inches might actually be a good size for that kiln if everything else were "good" (better burners, etc.) .... in this case it is likely not. Even 8 inches of round would liekly work... if it was the correct material and if the burners were better.

 

A kiln is a "system". All of the parts work WITH each other. What works on one situation if you look at the components as "stand alone" elements.... may just not work in a diffent situation.

 

Plus single wall metal pipe will lose heat to the environment quickly above the brickwork.... dropping the exit point temperature. Again, on a kiln you are trying to increase the darft on.... not a good choice. At the least use double wall (with airspace) gas appliance venting pipe for the top 4 foot extension. And go larger than 9" diameter to try to maintain the cross section better. Use low temoperature castable to connect the brick and the pipe. The pipe will eventually rust out within the covering castable at the join..... buit it'll take a while. Better yet use an actual insulated flue pipe like the common "Metalbestos".... which has not asbestos in it... but ceramic fiber. It is lined with stainless metal.... will last a LOT longer. And it will maintain flue exit temperature well.

 

Again.... hope this is helping.

 

best,

 

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

[Guest JBaymore]

Just saw the last posting that came in as I was doing mine.

 

Yup.... MRs on low gas pressure.  The character of that flame tells it all........ no primary air.

 

Your kiln part of the system will have to make up for the burners weaknesses.

 

Comparing a Ransome to a MR is like comparing a Piper Cub to a Learjet.  And comparing a Ransome to a Pyronics or North American burner is like comparing the Learjet to an F-15.  Ransome casts are true venturis.... far better casting and machining than the MRs that barely qualify to use the term 'venturi'.  The Ransome B-anything will burn blue-er for sure. It is pulling in more primary air at any given pressure setting....with that steadily decreasing (as a %) as the setting gets lower.

 

Where did the regulator on the tanks come from?  (BTW>.. are they yoked..... hooked together with a Y connection then to the regulator?)  Did the GAS COMPANY supply it?  Or is it one you got from Marc?

 

best,

 

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