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Linda A

Gas kiln draft question

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Hello

My third attempt at firing my 9 cubic foot downdraft kiln to cone 6 has failed once again.  Stalled around 2000 degrees F.

Throughout the entire firing after warm up I had to increasingly push the damper in in 1/4 inch increments.

At reduction cone 010 I had very little room to push the damper in but I was successful and did have small flame out of the peep.

This did take eleven hours to get to cone 010.  .  I reduced for forty five minutes.

Attempting to get to 2000 degrees I had a very difficult time increasing the temp but did persist for another 7 hours.

The only way I could get temp increase was by slightly pushing the damper in hourly.  By 2000 degrees I had no more room to push damper in as opening in chimney was now only half an inch.   If I pulled out damper at all temp would plummet.

My 2 Venturi burners were burning nicely with a blue flame.  My gas, propane, was open fully throughout entire firing. I do not have a pressure gauge to adjust gas, just a ball valve.  At close to end of firing I did try decreasing gas but this just decreased temp also so I kept it on full.

My question is do I have too much draft (  secondary air)   My chimney, which I added two lengths to,, is  10 feet tall.   Is that to much for a nine cubic foot downdraft  kiln.  The kiln came built with a two foot chimney ..

Thankyou    Linda

 

Thankyou

 

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Decrease gas, increase air flow.  More gas != More heat.  And reduction doesn't mean more heat either.  You will have to fiddle, but fiddle with the gas not at full blast.  Slow and steady is the game, especially once you get past 1800 degrees, any gain in temperature is a win, you will find the winning combo soon, just keep at it.

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1 hour ago, liambesaw said:

Decrease gas, increase air flow.  More gas != More heat.  And reduction doesn't mean more heat either.  You will have to fiddle, but fiddle with the gas not at full blast.  Slow and steady is the game, especially once you get past 1800 degrees, any gain in temperature is a win, you will find the winning combo soon, just keep at it.

THANKYOU for your quick reply. So, you are saying length of my chimney is not the problem.

i experimented before firing with my gas valve and burners by adjusting valve.  It is a simple ball valve.. As I said I don't have a pressure gauge on my system.

i had to turn ball valve to almost off before I noticed any difference in flame.  So would making very small increments increasing gas slowly have some effect on the burner flame that I can't really see.

At beginning of firing would I turn ball valve as low as I can go before it shuts off and then slowly increase hourly.

should I be making damper adjustments each hour also. Would that be increasing air flow.

With my Venturi burners I don't like adjusting the plate at the bottom because I end up getting burnback.  They seem to run best just slightly open.   

Thankyou.   Linda

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Yeah burnback is a clue that air is not moving through the kiln.  You can open your dampers up when that happens or adjust the gas, but you're correct when you notice that very minute adjustments are required when it comes to gas.  In propane a difference of 1 psi is pretty dramatic, in natural gas I think the measurements are different.  When adjusting my propane I keep an eye on the flame coming from the torch, an eye on the chimney and an eye on the pyrometer.  The pyrometer will sometimes show a loss of heat when increasing the airflow, but it catches back up within a minute or two, once you fire a few times you'll start being able to better read the fire.  It sounds like you're really close though at 2000 degrees, just need to defy your better judgement and help it out by letting off the throttle a little and getting some air in there.

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15 hours ago, liambesaw said:

Yeah burnback is a clue that air is not moving through the kiln.  You can open your dampers up when that happens or adjust the gas, but you're correct when you notice that very minute adjustments are required when it comes to gas.  In propane a difference of 1 psi is pretty dramatic, in natural gas I think the measurements are different.  When adjusting my propane I keep an eye on the flame coming from the torch, an eye on the chimney and an eye on the pyrometer.  The pyrometer will sometimes show a loss of heat when increasing the airflow, but it catches back up within a minute or two, once you fire a few times you'll start being able to better read the fire.  It sounds like you're really close though at 2000 degrees, just need to defy your better judgement and help it out by letting off the throttle a little and getting some air in there.

 

15 hours ago, neilestrick said:

I agree. If the gas was full open, you probably had too much. More gas does not equal more heat.

Ok. Thanks.  I'll try with less gas next firing.  Need to put a different pressure gauge and valve on the system so I can monitor the gas closely.

i assume I should start lower at the beginning and increase gas in increments as I am firing.

linda

 

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14 hours ago, Mark C. said:

I'm guessing you are using to much gas. keep the damper open until 1,800 then start reduction you should not have to have burners full on.

Thanks Mark

linda

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15 hours ago, liambesaw said:

Yeah burnback is a clue that air is not moving through the kiln.  You can open your dampers up when that happens or adjust the gas, but you're correct when you notice that very minute adjustments are required when it comes to gas.  In propane a difference of 1 psi is pretty dramatic, in natural gas I think the measurements are different.  When adjusting my propane I keep an eye on the flame coming from the torch, an eye on the chimney and an eye on the pyrometer.  The pyrometer will sometimes show a loss of heat when increasing the airflow, but it catches back up within a minute or two, once you fire a few times you'll start being able to better read the fire.  It sounds like you're really close though at 2000 degrees, just need to defy your better judgement and help it out by letting off the throttle a little and getting some air in there.

I just want to clarify what I call burnback.

Burnback to me is when the gas is igniting in the Venturi tube not at the top of the burner.  Venturi makes a loud noise and little gas flame pops come out of the bottom of the Venturi at the metal wheel which is for air adjustment.

is that what you are talking about?

thanks.   Linda

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5 minutes ago, Linda A said:

 

Ok. Thanks.  I'll try with less gas next firing.  Need to put a different pressure gauge and valve on the system so I can monitor the gas closely.

i assume I should start lower at the beginning and increase gas in increments as I am firing.

linda

 

Also should I just leave the chimney at 10 feet.

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8 minutes ago, Linda A said:

I just want to clarify what I call burnback.

Burnback to me is when the gas is igniting in the Venturi tube not at the top of the burner.  Venturi makes a loud noise and little gas flame pops come out of the bottom of the Venturi at the metal wheel which is for air adjustment.

is that what you are talking about?

thanks.   Linda

Yes thats the flame backing into the burner because it's trying to push too much gas into the kiln.  It's an airflow issue.  Your chimney length should be based on your kiln.  There's a free PDF that discusses chimney length here: https://ceramicartsnetwork.org/wp-content/uploads/2010/05/gaskilnsfreemium.pdf

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14 minutes ago, Linda A said:

 

Ok. Thanks.  I'll try with less gas next firing.  Need to put a different pressure gauge and valve on the system so I can monitor the gas closely.

i assume I should start lower at the beginning and increase gas in increments as I am firing.

linda

 

Once you get the right mix and the ramp up is uncontrolled (max temp rise), you will probably not need to fiddle with the mix very much, it doesn't burn more or less gas the hotter it gets, you are passing heat through the kiln, not filling the kiln with heat.  With that mindset you can see that the proper mix of gas and air will not need to change as the kiln gets hotter.  The only time that changes is when you are changing the rate of heating.  If you are on propane, having the valve all the way open can freeze your lines which can also lead to some frustrating gas\air issues.

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leave the chimney (stack) at 10 feet . the ball valve is what I use on my 8 burner kiln. they are just fine for fine adjustments. You just need to only turn up very slowly as its very easy to just turn it full on which is not what you want.I would open the air adjustment full open the whole fire(the spinning cast piece at rear of burner)leave the damper open until body reduction (1800) then close it up enough to get some back pressure.

get a pressure gauge  as it will help you.

'

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13 hours ago, Mark C. said:

leave the chimney (stack) at 10 feet . the ball valve is what I use on my 8 burner kiln. they are just fine for fine adjustments. You just need to only turn up very slowly as its very easy to just turn it full on which is not what you want.I would open the air adjustment full open the whole fire(the spinning cast piece at rear of burner)leave the damper open until body reduction (1800) then close it up enough to get some back pressure.

get a pressure gauge  as it will help you.

'

Thanks.  I will be getting a pressure gauge

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