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Kiln Conversion Updraft Downdraft Chimney?

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It was a nice day yesterday so I decided to fire the kiln. First change was to take down the chimney again to the height of the kiln. The tube was precarious and I don't think it was changing much with the forced air burner.

 

I need some kind of safety and pilot on this burner, it's too fast below 400c and keeps going out with a bang. After 400c it is very good but low end is bad. Not sure how to do that yet or have the money to. Need to stand by for the first 2 hours and hope the pots can brave 200c an hour.

 

The flame out the kiln was a brilliant blue with green tips. Hard to get a good photograph of it. The reduction across the kiln was good on the burner side and bad to oxidised on the flue side. I think this may be partly down to shelves too close to the wall on that side.

 

Is it also something to do with my flame colour? Do I really need to be getting some sooty yellows? I need an oxygen probe instead of talking flames.

 

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I also need a better thermocouple, this one is way off and hits cone10 at 1140. It was only half a cone difference top to bottom.

 

Another interesting note, in my body reduction which I hope was 900c~ the brick I used to close the chimney built up a yellow tinge also smelt very eggy. Thinking some of my glaze materials might have a lot of sulphur. Just a thought. Maybe the clay.

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JBaymore    1,432

............... it's too fast below 400c and keeps going out with a bang. After 400c it is very good but low end is bad. Not sure how to do that yet or have the money to. Need to stand by for the first 2 hours and hope the pots can brave 200c an hour.

 

This aspect of burner design is called "Turndown Ratio".  It is the relationship between the burner running at full BTU output and the lowest STABLE BTU output setting.    The first step in improving this is the use of a good flame retention nozzle.

 

best,

 

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

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............... it's too fast below 400c and keeps going out with a bang. After 400c it is very good but low end is bad. Not sure how to do that yet or have the money to. Need to stand by for the first 2 hours and hope the pots can brave 200c an hour.

 

This aspect of burner design is called "Turndown Ratio".  It is the relationship between the burner running at full BTU output and the lowest STABLE BTU output setting.    The first step in improving this is the use of a good flame retention nozzle.

 

best,

 

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

 

 

I do have the homemade flame retention nozzle. I think it does a good job but the fan I am using doesn't have much control at the lower end so it blows the flame right off, once I get somewhere I can use more than a smidge of gas it does well.

 

Do you know anything about flame colour John? Do I need the sooty yellows or is my blueish flame ok?

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Joel,
  
One way to better control of the fan is to add a tee in the air line between the blower (fan) and the burner to split the air into two streams.  Install a butterfly valve on the side of tee and use it to divert air away from the burner. Run the blower without throttling the air intake to the blower.  The blower will stay cool, and you will have control over air to the burner. 
 
Re the 'yellows' along with the 'eggy' smells could be due to odorizer added to the propane to alert you of leaks.  Some regions mandate that gaseous fuels have the odorizer to alert everyone for leaking gas.  In the US it is required in natural gas, not sure about bottled propane.  The favorite odorizers here are sulfur based.  Check your fuel supplier.
 
LT

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It's a little complicated, I have no studio but some work left over that's bisque and a gas kiln in a garden. No space to really make more work or a kiln to bisque in yet. 

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Joseph F    865

It's a little complicated, I have no studio but some work left over that's bisque and a gas kiln in a garden. No space to really make more work or a kiln to bisque in yet. 

 

Once you get your early temp ramps under control you could single fire! ^_^ Doesn't really solve your no space to make stuff though. Can't wait to see your stuff again once you do get it all sorted out. Life improves. It does. Just keep going.

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It's a little complicated, I have no studio but some work left over that's bisque and a gas kiln in a garden. No space to really make more work or a kiln to bisque in yet. 

 

Once you get your early temp ramps under control you could single fire! ^_^ Doesn't really solve your no space to make stuff though. Can't wait to see your stuff again once you do get it all sorted out. Life improves. It does. Just keep going.

 

 

I have a small kiln that runs off domestic power, just trying to decide if I want to poison myself and fire a bisque in my bedroom. It's been a weird year for me even outside of the pottery but things are slowly improving :D baby steps.

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JBaymore    1,432
I do have the homemade flame retention nozzle. I think it does a good job but the fan I am using doesn't have much control at the lower end so it blows the flame right off, once I get somewhere I can use more than a smidge of gas it does well.

 

 

Do you know anything about flame colour John? Do I need the sooty yellows or is my blueish flame ok?

 

 

Unless you have cast those home-made nozzles from well-engineered designs... they will not work as well as commercial ones.  And even they have trouble maintaining the flame on the nozzle at low settings (on a high output burner).  This is one reason why larger burners usually have pilot burners.  On really low input... it is run on the pilots untikl the lowest stable setting of the main burner is OK to run.

 

As to flame color........ no, you do not need to have yellow.  In fact... SOOT is not a good situation... because that indicates that a portion of the fuel is going to carbon...... and a carbon particles are not as good a reducing agent as CO.  If you are trying for carbon trap... well... then you want that sooty flame (VERY early).  But for "normal" firings (whatever they are) you want just enough deficiency of oxygen to get the job done at the correct times.  No more.... no less.

 

That being said... it is also very much about mixing.  It is possible that in certain locations in the ware chamber that the character of the flame you are seeing at the exit flue and the peeps is NOT what the ware there is "seeing".  You might have a blast of hot air with almost literally no fuel in some places.  And in others you might have amazingly oxygen starved hot fuel and air ratios. 

 

So kiln and burner design and the matching of the two also gets into cause consistent mixing to happen before the combustion products start interacting with the wares.  Forced air burners can be GREAT for this. 

 

Unfortunately, for your specific kiln/burner combination.... you are going to have to do some experimentation to get it right. 

 

A lot of potters use an approach of "overkill" to get consistent results.  Excess reducing gases in the kiln beyond what is needed, and present for more time than what is needed.  A "shotgun" approach.  If you fire enough pellets at the target.... one is bound to hit something.

 

An example on this "what is needed when" is that I did some tests with a certain celadon glaze recipe.  To get a good celadon color I found that I needed to start reduction before cone 04 down.  And that I could do anything I wanted to the atmosphere after cone 4 down...... oxidation, reduction, neutral... didn't matter.  That is for one specific glaze.

 

best,

 

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

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Ok  :D good to know I don't need to be aiming for a yellow flame. I do remember reading about carbon but who knows where. I think my shelving could probably do with a little more space round the edges to let it flow a bit more.

 

That flame nozzle was engineered with a drill, blood, sweat and tears  :lol: you can see a close-up shot in the short video. 

 

I recorded some footage with the burner, sometimes it was blown out by the wind and others it went back to burning at the orifice I think. After watching it multiple times the wind seems to be the main culprit but also my fan control speed.

 

I did a few back of the envelope calculations and worked out I use 40,000BTU at 1PSI. If there is 2516BTU/ft3 in propane then I am using 16~ft3 of propane an hour? With a ratio of 30:1 I need 480ft3 of air with that gas so 8ft3/min? Does that maths look good?

 

Right now the blower I have does 54ft3/min so I think it is a bit too much for the size of kiln. Going to look into a more suitable replacement.

 

vid https://drive.google.com/file/d/0ByISKVubkZcXWF8tRjVFcTJvZGs

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I decided to buy a smaller fan, I am getting bored of the big transformer and hard to control blower. First mistake, going for nearly bang on the air flow that I need. Sitting here I am remembering old conversations about static pressure and other things long forgotten.

 

You can see the fan is tiny. It did work but not up to 1psi where I need it to be. Ordered a larger 12v fan that should (fingers crossed) do the job. 

 

The fan control is now a potentiometer and npn transistor. Who knew you could use them as amplifiers, well it makes sense now I know but anyway.

 

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The data sheet says continuous current of 0.8 amps for the transistor, the one there is 0.1 and I bought a new one that does 0.45 amps. 
 
I am not exactly sure what I am talking about. It looks like it should be happy with 0.8 amps. http://uk.rs-online.com/web/p/bipolar-transistors/6254966/
 

Depending on the fan that transistor might need a heatsink?

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Joe_L    37

Even if its ok for current capacity you still need to watch the power dissipation (voltage drop across the transistor at each speed setting x current at that setting), needs to be less than Maximum Power Dissipation (0.625W in this case). Also watch the temperature, put your finger on it and see its not getting hot at various speed settings.  I'm not suggesting you have a problem, just something to watch out for if failure mid-firing would be a nuisance.

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Ok, seems like I might be 3x over at low speed settings  :wacko: 1.8w and it is getting hot, full speed it doesn't get hot and seems to be 0.4w. Confused me at first why low speed made more heat but the transistor is having to get rid of more right?

 

Even if its ok for current capacity you still need to watch the power dissipation (voltage drop across the transistor at each speed setting x current at that setting), needs to be less than Maximum Power Dissipation (0.625W in this case). Also watch the temperature, put your finger on it and see its not getting hot at various speed settings.  I'm not suggesting you have a problem, just something to watch out for if failure mid-firing would be a nuisance.

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