One Burner 10 Cubic Ft Gas Kiln?

[Bill Kielb]

42 minutes ago, tomhumf said:

Just under 0.5 bar it looks like, kiln is at 850C.

I'm pretty sure it will make bisc temp, just wondering how it will cope with a cone 7 reduction firing. Seems really strange it fires best with chimney only and inch open. Not sure how easy it will be to get it to reduce. 

Too big or too tall makes your chimney overly sensitive to adjustment. 9x6 = 54 sq inches so depending on height 300-500 Kbtu  is an approximation for 54 sq inches.  So you seem to be going ok, I would turn up the gas a bit and observe performance including how it will go into reduction and what that will expand your damper position to. Take lots of data, maybe decrease your chimney height A bit and observe  at various pressures. Lots and lots of data helpful here. Rate of climb, reduction level, temperatures, chimney draft would all be great at this point. 

[tomhumf]

Ok thanks, cone 06 just dropped final temp was 1015C. That was about 8 hour firing with 1 hour under 100C and 1 hour holding around 850C. Pressure was very slightly over 0.5 bar in final stages. 

For test purposes I very briefly pumped up the gas around 900C and nearly closed chimney and seemed to be getting reduction flame from peephole. 

I'm a bit confused how chimney heigh relates to it's cross section. If I reduce the height would I be able the damper more? Seems crazy to be firing it will only 1" x 6" of  the chimney in use. 

How would I measure chimney draught? 

 

[Bill Kielb]

2 hours ago, tomhumf said:

Ok thanks, cone 06 just dropped final temp was 1015C. That was about 8 hour firing with 1 hour under 100C and 1 hour holding around 850C. Pressure was very slightly over 0.5 bar in final stages. 

For test purposes I very briefly pumped up the gas around 900C and nearly closed chimney and seemed to be getting reduction flame from peephole. 

I'm a bit confused how chimney heigh relates to it's cross section. If I reduce the height would I be able the damper more? Seems crazy to be firing it will only 1" x 6" of  the chimney in use. 

How would I measure chimney draught? 

 

If you increase the height of the chimney you will increase the draw. If you increase the size in sq. Inches, you will increase the draw by reducing the friction losses through the orifice / opening. You are working at kiln temperatures but I have attached the basic formula for calculating and a chart commonly used for sizing. Your flows will be greater than the chart because your temperatures will be higher for a direct connect chimney for a kiln. Even though the chart is not specific to your use it does provide an idea of the effect of height on a chimney.

In the chart notice for a natural draft 8” diameter chimney we go from 370kbtu to 450kbtu just by raising the chimney 4  feet in height. So in general, higher means more draw.

Draft would be measured with an appropriate item such as a magnehelic or probably easy to make an inclined water column gauge for temporary use.

Final thought is your damper will almost always will be very sensitive to adjustment as moving it changes length and width so area changes. The velocity of the chimney, pressure drop through the orifice are exponential relationships so tiny positional changes have exponential results. Most commercial kilns in reduction are susceptible to very minor movements of their rectangular damper. This is hard to see by eye, but very apparent when using an oxygen probe during reduction. It’s not uncommon to see significant changes in reduction levels moving your damper in and out as little as 1/32nd of and inch.

 

[tomhumf]

Ok, I should have probably worked all this out before I built it. 

I'd assumed I could make the chimney oversized and use the damper to reduce it to the required level. It seems that assumption was false. 

My previous mini kiln has a 3" x 6" chimney, around 4' tall. It seems to work very efficiently with this burner. 

The bisque firing made temperature at 0.5 bar - that's around 120kbtu according to my burner suppliers data. 

I think I should be able to fire to cone 6 / 7 at this BTU. From the ward burner chart posted early in the thread I get :

13k BTU /hr ( fast fire ) for 9" HTI x 8 cubit ft = 104 kbtu.

From the Selkirk gas vent chart the closest size would be 141 kbtu, which is a 6ft tall 5" diameter. That's 20 sq inch , so the nearest internal square brick size would be 6" x 3". 

So I'm thinking of rebuilding the chimney either at that size or 6" X 6" and making it shorter. The problem with a shorter chimney is I have wooden roof beams near the chimney hole. I would need some kind of metal extractor tube above the chimney opening I suppose. 

Smaller chimney makes sense as my current 9" x 9" fires best with damper almost shut. Do you think I should make it smaller? 

[Bill Kielb]

1 hour ago, tomhumf said:

Ok, I should have probably worked all this out before I built it. 

I'd assumed I could make the chimney oversized and use the damper to reduce it to the required level. It seems that assumption was false. 

My previous mini kiln has a 3" x 6" chimney, around 4' tall. It seems to work very efficiently with this burner. 

The bisque firing made temperature at 0.5 bar - that's around 120kbtu according to my burner suppliers data. 

I think I should be able to fire to cone 6 / 7 at this BTU. From the ward burner chart posted early in the thread I get :

13k BTU /hr ( fast fire ) for 9" HTI x 8 cubit ft = 104 kbtu.

From the Selkirk gas vent chart the closest size would be 141 kbtu, which is a 6ft tall 5" diameter. That's 20 sq inch , so the nearest internal square brick size would be 6" x 3". 

So I'm thinking of rebuilding the chimney either at that size or 6" X 6" and making it shorter. The problem with a shorter chimney is I have wooden roof beams near the chimney hole. I would need some kind of metal extractor tube above the chimney opening I suppose. 

Smaller chimney makes sense as my current 9" x 9" fires best with damper almost shut. Do you think I should make it smaller? 

The Selkirk is an example only and applies to natural draft situations for appliances which are lower than your operating temps and your configuration is a direct connect as well. This chart is for draft diverter stuff or natural draft hood which has dilution air. I was only using it to illustrate that higher means more draft as well as bigger size.  

It seems that your real life test run proved the size could be smaller So I probably would look to restrict the flow temporarily in the kiln if possible by say 10% - 20% of the area just to see, maybe that’s perfectly acceptable and gives you more range with your present damper setup and height. I would also  add a manufactured  top to it to spark arrest, divert the wind and keep the rain out. Once the top Is added it will change operation slightly as well.

Properly sized chimneys provide the proper draft pressure (negative) while maintaining enough velocity to entrain as much moisture from the sidewalls of the chimney. So smaller over all area means higher velocity, and greater height maintains flow at the higher velocity. This is combustion theory and no one I know involved with kilns spends that much time to learn it. In other words a bit overkill for most, but understanding the basics may change your thinking towards tuning.

0-1 bar seems very acceptable as far as operability and in reduction, you might simply raise it towards 2 or 3 bar without changing much to go in and out of reduction as desired, so this may be a very workable orifice / power arrangement / kiln build  ....... as is.

My feeling, you seem to be in a fairly workable situation so If I  could  test tune or block the opening in the kiln bit by bit with your  existing chimney height  where you need it to be for safety and that nets let’s say a two inch damper opening at midfire temps I think  I would be happy.  Sneaking up on it this way is probably most practical. I would definitely add a top to it as they are available  square to round pre made, they spark arrest and reduce the variable influence of wind on your draw significantly and adding it will probably require opening your damper a little bit more as well.

A couple  final thoughts , calculating  this stuff is great but it’s all an approximation in the end and no guarantee it will be spot on. Testing and tuning to your comfort and firing style can make the difference between a good kiln and a great kiln I always like to fire.

Second, just to mention because it may influence your operation and thinking, gas pressure and Btu are not proportional or linear. That is to say, increasing gas pressure 10% actually increases thermal output more like 20%. I mention it because when guessing at the current output of a burner using a linear guess often results in significant error. Additionally folks have difficulty when firing as early in the firing they tend to turn their gas up more than necessary, yet later as losses increase at a higher rate, and draft increases significantly, they may need to turn the gas up a bit more than  early in the firing. Sort of counter intuitive unless one has a solid sense of where the energy is going and why.

And finally in reduction the efficiency of the flame really goes down. If I needed 100 kBtu to fire to cone in oxidation, it would not surprise me if I needed 200 kbtu in medium reduction. It’s that thermally inefficient.
 

hope that helps more than it confuses.

[tomhumf]

6 hours ago, Bill Kielb said:

let’s say a two inch damper opening at midfire temps I think  I would be happy.

Thanks that's all very helpful. I will try restricting the chimney opening to kiln then to start with and test.

From your quote above I take it the optimum damper setting for neutral temperature climb changes over the course of the firing? You should start with it closed more and open as the gas levels increase? 

[Bill Kielb]

36 minutes ago, tomhumf said:

Thanks that's all very helpful. I will try restricting the chimney opening to kiln then to start with and test.

From your quote above I take it the optimum damper setting for neutral temperature climb changes over the course of the firing? You should start with it closed more and open as the gas levels increase? 

Yes, it does!

The rough optimal gas to air ratio for clean combustion is about 10:1. Atmospheric burners get roughly half of this air from their primary air source (shutter behind burner) which is drawn in proportionally by the Venturi action and the velocity of the gas injected through the orifice.    

The remaining air for combustion, known as secondary air basically comes in around your burner. As you fire, you turn up the gas gradually which means more Primary air and gas and more secondary air.  More mass in the kiln so often as one fires they need to  open the damper slowly as the kiln climbs to keep it climbing and limit the reduction.

When you force the kiln into reduction by closing the damper, you are limiting the secondary air and forcing incomplete combustion. Push the dampers in far enough and you start forcing flame and soot out the secondary air port. Way too rich actually.

Its good early on after You are reasonably finalized With the kiln to test run into reduction and very slowly tune the primary air shutter for best rate of climb While in reduction. Often this is the onetime when moving that shutter as little as 1/4  To 1/2 turn will have a noticeable effect.
All other times you will swear it has Virtually no effect. Of course an O2 probe and rate of rise temperature logger is immensely helpful to do this If ya got em. The rate usually is dramatic so a pyrometer and counting off one, two seconds per degree is usually a suitable indicator. Just like flames out a sight port enable operators to visually observe these tiny pressures effectively, tuning your primary air is doable this way and will help maximize your power at top temperatures.

Picture of a nice positively pressurized reduction indicator at sight port below. Picture is a down draft Geil car kiln.

[tomhumf]

Bill you're awesome, thanks for your very generous explanations. 

With the risk of becoming really annoying with this kiln, can I ask advice on the bag wall? 

The bag wall is currently 9" high and solid all the way across, kiln is about 35" tall to the middle of arch. I put in some angled bricks at the back and some other deflector bits half way down. 

The shelves are a bit big, I've order some 12" x 12" ones which will give an extra inch of space around. 

The cones pictured are left to right : bottom front, bottom back, middle shelf, top shelf. The kiln was only loaded two thirds of the way up for my first firing. 

I'm thinking I maybe need to lower the bag wall, or leave some spaces for the heat to get to the bottom shelves? 

[Bill Kielb]

1 hour ago, tomhumf said:

Bill you're awesome, thanks for your very generous explanations. 

With the risk of becoming really annoying with this kiln, can I ask advice on the bag wall? 

The bag wall is currently 9" high and solid all the way across, kiln is about 35" tall to the middle of arch. I put in some angled bricks at the back and some other deflector bits half way down. 

The shelves are a bit big, I've order some 12" x 12" ones which will give an extra inch of space around. 

The cones pictured are left to right : bottom front, bottom back, middle shelf, top shelf. The kiln was only loaded two thirds of the way up for my first firing. 

I'm thinking I maybe need to lower the bag wall, or leave some spaces for the heat to get to the bottom shelves? 

Mmmm, even more complicated! I am not a fan of thick bag walls when they take up so much room in the kiln. Many kilns run just fine without them actually. So after red heat most of the heating done in the kiln is by radiation. Just not enough energy in any airstream to heat by convection. So the bag wall becomes a radiant surface and also isolates the flame from impinging on the pots.

Bag walls and target bricks should be placed for a reason and by experience. Often they are sort of  added just because. Target bricks are interesting too in that the Coanda effect often makes the flame wrap around the target brick. So armed with this knowledge I would say a thinner  bag wall  would  be more desirable. Your flame will naturally rise in the rear so not sure there is a reason to deflect it upward with target bricks

Many kilns configured this way run without either, still others are successful with half of a target brick at the midpoint of the flame trench effectively splitting the flame in half while reflecting some back towards the burner entrance. The flame tips will be lowest near the burner because of the velocity anyway.

My best observation is these things work well by rigorous testing and a need to solve a problem. So, is it too hot on top or bottom or front to back. Are there Reduction dead spots in the kiln. Is the centerline of the burner too low or high? Fortunately kilns for ceramics go at a relatively slow speed which means they fire pretty evenly. I would start adding these things if I discovered a need, not necessarily before.

I am not a fan of taking up so much useable firing space with a heavy bag wall, but I understand the convenience of using the brick. Before I would speculate on possible fixes I would want to know the nature of the problem which will take some firing and data gathering.

also firing speed has a pronounced effect  so at the end of your firing in the last 200f you will want to maintain about 100f/ hr to get your cone to drop at temp anyway. Often moderate  speeds even things out significantly.

My thought would be to monitor top and bottom temps throughout the firing and take data at various firing speeds. Same could be said for front to back but that becomes pretty tedious for most. It’s hard to tell in real time with cones  especially what speed might even this out.

Nice looking kiln!

[tomhumf]

Ok thanks, I would rather not have a bag wall if possible and get a couple more inches stacking space. 

In my old little kiln I had an area near where the burner hit the back wall which got super hot and was unuseable space. 

I added the bagwall to my new kiln to try avoid any spots like this in my stacking space. I also wanted the deflectors to protect the walls from getting battered. I would rather replace the deflectors than have to rebuild a wall. 

I think I'll try taking the heavies out totally, move the shelves right an inch or so and leave the deflectors where they are for now. Hopefully this will even the temp a bit and I can go from there.

[tomhumf]

I just did a quick test restricting the chimney, all temps are in C. They are both higher % than you suggested - 25% and 50% but i just wanted try it without having to cut bricks.

Seems like a big difference over 5 mins but not sure how good of a test it is. I suppose I should check again with the damper closed more. This much restriction probably wouldn't work as well at higher temperatures too.

Damper full open

Starting temp 100 - 5 mins @ 0.5 bar

 

Original chimney opening

End temp 343

 

Block chimney opening 25% - 3" X 4.5" (24" long)

End temp 366

 

Block chimney opening 50% - 6" x 4.5" (24" long)

End temp 386

 

[Bill Kielb]

26 minutes ago, tomhumf said:

I just did a quick test restricting the chimney, all temps are in C. They are both higher % than you suggested - 25% and 50% but i just wanted try it without having to cut bricks.

Seems like a big difference over 5 mins but not sure how good of a test it is. I suppose I should check again with the damper closed more. This much restriction probably wouldn't work as well at higher temperatures too.

Damper full open

Starting temp 100 - 5 mins @ 0.5 bar

 

Original chimney opening

End temp 343

 

Block chimney opening 25% - 3" X 4.5" (24" long)

End temp 366

 

Block chimney opening 50% - 6" x 4.5" (24" long)

End temp 386

 

Nice work!

With some patience you will tune this thing to work nicely. Hot spot in back, I understand so maybe a target placed mid kiln sticking 1/2 way out in the flame trench. Bad drawing attached. So knce you do the heavy lifting, let us know what worked best. Might be helpful to a future builder.

[Mark C.]

Just put a split brick (1/2 brick) in the flue space inside kiln-to reduce the space-that will fix and too large chimney issues.

10 cubic feet is a lot of space to heat with one burner as all along. The kiln is super insulated which helps-that one burner better be a BIG one.

I think your bag wall is to short (more height) and that shelve on sude wall by the flue could be tighter aganist wall. You really want the flames/heat going up thru load and back down under bottom shelve for the downdraft to work well.

This is same advice I gave another on a trash can (electric gas conversion) a few days ago . Many issues are from the flames/heat just flowing thru the bottom of kiln out the damper without geeting the heat/flame thru the load of pots especially up top.

you could add a piece of pipe to extend the chimney stack just by putting in place during the fire to see if this helps without much work. 

My thought with 1 burner it all has to be pretty dialed in to make the heat rise you want-you do not have extra power with 1 burner.

On a side note- you used hard brick on burner ports which is best -except the soft brick on top  in burner port -which will fail over time. If you can stick a =hard brick in that spot. Thats a hot spot the roof of any burner port.

I like that tall arch .

[tomhumf]

Thanks Mark, I swapped out the brick above the burner port. 

I'm going to experiment with the bagwall and chimney height, I've got a bit of room to add or take away in the angle iron. 

Is there anything you can do to stop HTI bricks making as much dust when you're sliding the door together? I'm trying to be careful with them...

[Mark C.]

Those look like k26 and tney are gritty by nature. -soft brick are not great long term door stack material. They shed the grit as you are noticing .They are fragile and other than vacuuming them I have  no other ideas. The best solion is to put those bricks in a swinging front door so you do not stack them every time. That little kiln would work very well with a small swinging door. You biuld the frame and hinge and make the door.It could be 1/2 the thickness with some say fiber backing.

Are these bricks china made or something else?

[tomhumf]

Right, they are gd26 rated bricks in the UK, I think the same type you mention. I'm not sure where they were made. 

I'll just have to be more careful for now, maybe a swinging door would be a good option for the future. 

[tomhumf]

Full list of my tests in case they help anyone in future. I have removed all but two hard bricks on the bagwall at the middle and back of flame trench and left the deflectors in place. All tests are with original chimney height unless noted otherwise.

I'm going to do another bisque soon with the setup I've retested a few times which gets to highest temp.  

 

Damper full open, 2 brick bagwall

Starting temp 100 - 5 mins @ 0.5 bar

 

Original

End temp 343

 

Block chimney opening by 3" X 4.5" (24" long)

End temp 366

 

Block chimney opening by 6" x 4.5" (24" long)

End temp 386

 

Block chimney opening by 6" x 4.5" (24" long)

Decrease chimney height by 2 courses

End temp 358

 

Block chimney opening by 6" x 4.5" (24" long). One brick bagwall

End temp 363

 

Retest - Block chimney opening by 6" x 4.5" (24" long)

End temp 394

 

Block chimney opening by 6" x 4.5" (24" long) increase chimney height by 2 courses

End temp 348

Retest end temp 383

 

Retest - Block chimney opening by 6" x 4.5" (24" long)

End temp 396

[Bill Kielb]

1 hour ago, tomhumf said:

Full list of my tests in case they help anyone in future. I have removed all but two hard bricks on the bagwall at the middle and back of flame trench and left the deflectors in place. All tests are with original chimney height unless noted otherwise.

I'm going to do another bisque soon with the setup I've retested a few times which gets to highest temp.  

Nice work!

This probably becomes much easier if you measure the draft in the firebox.

[jrgpots]

Could you create a crossdraft on the top of the chimney by placing a blower across the top creating a venturi effect....kind of like an airbrush.  The rate of the blower could change the drawing of the chimney.   Has anyone done this?

[Bill Kielb]

3 hours ago, jrgpots said:

Could you create a crossdraft on the top of the chimney by placing a blower across the top creating a venturi effect....kind of like an airbrush.  The rate of the blower could change the drawing of the chimney.   Has anyone done this?

Generally at the tops of chimneys we want to minimize the Influence of the wind because it creates unpredictable variances in the firebox. The more consistent we can make the draft the more efficiently and consistently we can tune the results. In the past, 30 feet and taller chimney or oil burner work for sure we installed a barometric relief  which varied the amount of air up the chimney but maintained a consistent draw at the boiler / incinerator / appliance.

In the current day, the only way to squeeze the last bit of efficiency out of a boiler/ water heater/ furnace,....... 90%:range is to have a draft inducer motor that runs really high temp but meters the amount of air through the firebox to extract a predictable amount of energy from the fuel. Placing a fan above the chimney to induce draft would likely create more variability in the system, which for Kilns in reduction or ordinary oxidation for that matter would be difficult with respect to operation.

The time tested use of natural draft hoods solves this and lowers the operating temperature of the stack at the cost of additional dilution air required. Additional dilution air could potentially require conditioning if the kiln is in a conditioned space. (Heat / cool - air conditioned space) We solve that issue by providing proper sized combustion air supplied to the space where the kiln is firing.

For kilns, we rarely have too little draft as we can simply increase the height of the stack and close our damper more as needed. Since this is a direct connect flue,  it needs to be tuned nicely for ease of operation. It has to function over a wide range of temperatures  and he will seek to tune it so late in the firing there is a reasonable amount of variability available in his damper. The problem with linear slide dampers is we open and close them in a linear fashion but that motion changes the area by a square relationship as in square inches..

Some work has been done with a round (convex)  slide damper that fully embeds to add a form of fine tuning to this action. Generally folks just test and experiment a little to get reasonable results with a plain old rectangular damper.

[tomhumf]

I'm planning to fit a hood over the chimney top tomorrow. I thought I could use the corrugated roof piece I cut to make the chimney hole.

Is there a good distance between top of bricks and the hood, I'm thinking around 8" ? 

[Bill Kielb]

2 hours ago, tomhumf said:

I'm planning to fit a hood over the chimney top tomorrow. I thought I could use the corrugated roof piece I cut to make the chimney hole.

Is there a good distance between top of bricks and the hood, I'm thinking around 8" ? 

Generally chimney caps are set up as non restrictive so an 8X 8 flue is  generally spaced off by 8”just so as not to be restrictive.

[tomhumf]

Results of second bisque firing similar to first. Cones are left to right : bottom back, bottom front, middle, top. Why are the back and top. Ones black, is it reduction? Not seem them bubble like this before. 

Next time I'll try taking all the bag wall and deflectors out I think. 

[Bill Kielb]

1 hour ago, tomhumf said:

Results of second bisque firing similar to first. Cones are left to right : bottom back, bottom front, middle, top. Why are the back and top. Ones black, is it reduction? Not seem them bubble like this before. 

Next time I'll try taking all the bag wall and deflectors out I think. 

Likely yes.. they do darken and bubble a bit but those do look reduced  Cone 04, Iron bearing cones? Interesting question is how fast did this go? If it went 500 f degrees per hour, not sure this kiln will be able to be fired  at that speed and remain even. When we fire the old Alpine updrafts if we go fast early the difference between top and bottom can be 300 degrees or more. When we get into solid red heat later in the bottom becomes 50-100 f degrees warmer than the top. Firing the Geil downdraft I believe always is challenging to get the bottom cone to fall with the top. About half cone difference. A couple well placed pyrometers would give you an indication of the effect of speed, damper, gas pressure. Finally might be the picture but the cones initial setting  angle looks a a tiny bit different.

So if it were me I would expect the top of this kiln to run cooler because it is a high arch, more surface area for conduction losses. 
Again is there a speed that evens this out? Without a perception of firing speed it will be hard to figure.. I would also measure at least two points of temperature at a time to understand the when and why data. Without this data through the firing it’s gonna be hard to figure out with cones only.

what is the flame centerline in relation to level and height within the kiln?
With the orifice combination does the burner impinge on the back of the kiln, do we need to broaden this flame yet have the same energy Output?

So my final thought, take a step back and observe, it fires hotter on bottom and back for sure So a test fire observing temperature in various areas and the relationship to gas, damper, flame centerline, firing speed is likely how I would formulate what next to try.

[tomhumf]

This is my firing data first column is time,second column is degrees C

I did about 2 hours under 100C, around 7.5 hours total. I tried to keep it slow up to about 500, gas ran out at one point when I was away from kiln so I had to changed bottles.

These are cone 06 cones.

You say bottom firing cooler, but the cones on left are the bottom cones.

I miss labeled the first two, should read 

Cones are left to right : bottom front, bottom back, middle, top.

So the top has been hottest both firings, but yes on bottom shelf at the back looks hotter.

I'm wondering if 2 layers of brick on arch is too much. Also, I made the top layer of the floor hardbrick, maybe that was a bad move? I could possibly replace most of it with soft brick. 

4.30 kiln on

5 97

6.45 110

7.40 185

8.10 259

8.35 330

8.56 448

9.21 539

9.46 653

10.11 722

10.30 800

11 gas ran out 650 , changed bottle

11.30 860

12 967 kiln off, cone bit over melted, opened chimney after closing up