Calculating The Propane Usage And Stacking Area For A Kiln

[Mart]

I have found several pages that all repeat the same:

 

Soft brick kiln with 9" walls requires 10,000-16,000 BTUS to reach cone 10 . The higher number will give you a firing of between 6-8 hours, while the lower BTU/CF figure will give you a 14-16 hour firing. Somewhere in between means a 10-12 hour firing.

 

 

 

OK, fine by me  

 

Lets say we have a 250 l kiln (9 cf). We are going to need 16000 x 9= 144000 BTU

We know that 1 Kg of propane packs about 13.97 kWh or 47668 BTU 

So, 144000/47668= 3.02 Kg (6.65 lbs) of propane per firing to cone 10? 

(I am using weight and not the volume because volume can be anything if exact pressure and temperature are unknown)

 

Q 1: How close is this to actual gas usage? 

 

About the downdraft kilns and actual stacking area. 

 

Q 2: Is it safe to say that 250 l total inside volume will give us only about 155 l of stack space?

 

I am asking, because we like to make some firing cost calculations before we start taking bids for building a 250 l kiln... or do we actually need a 350+ l kiln  

 

Thank you.

[Wyndham]

Here's what I have:  a kiln that has 6 raku mr750's burners ,3 on a side. The inside is across the back is about 48 in, front to back is 46 in and floor to top of arch is about 6 ft. the stack is 3 sets of 13x26 in shelves 10-12 shelves high.

It takes 10-12 hrs to fire to cone 10 at 10 o clock with about 3/4 lb gas pressure on each burner at the end of firing.

This takes about 60 to 70 gal of propane per firing at about $1.50/gal

Reduction starts at 1500 deg and goes til the end,heavy to start medium at 1800 deg a bit lighter at 2100 to end.

Hope this helps

Wyndham

[Mart]

Here's what I have:  a kiln that has 6 raku mr750's burners ,3 on a side. The inside is across the back is about 48 in, front to back is 46 in and floor to top of arch is about 6 ft. the stack is 3 sets of 13x26 in shelves 10-12 shelves high.

It takes 10-12 hrs to fire to cone 10 at 10 o clock with about 3/4 lb gas pressure on each burner at the end of firing.

This takes about 60 to 70 gal of propane per firing at about $1.50/gal

Reduction starts at 1500 deg and goes til the end,heavy to start medium at 1800 deg a bit lighter at 2100 to end.

Hope this helps

Wyndham

 

Thank you Wyndham.

Your kiln is 158976 in³ or 92 ft³ ... b.. bu... but this is only! 10x bigger what we are planning to build I am sure we can fit our studio inside of your kiln an have some room left over.

 

Lets say that one gallon gives 91465 * 70= 6 402 550 BTU total This is a long way off from what our "cf³ magic" calculation suggests.

92*16000=1 472 000  or only 4.3 times off.

[Marcia Selsor]

The usage I have seen is for soft brick 14,000 BTUs per cubic foot PER HOUR at maximum consumption temperature.

Marcia

[Wyndham]

mine was to give you a working hand made kiln that uses that amount of fuel over the span of time I've used it. Whether it can be scaled down to what you want, I don't know. Is it more than needed, well maybe but it works well for my purposes. Next door a fellow potter has a 10 cu ft updraft with 4 of the  same type of burners and figures a 9-10 hr firing at around $25.

Hope this helps

Wyndham

[Mart]

The usage I have seen is for soft brick 14,000 BTUs per cubic foot PER HOUR at maximum consumption temperature.

Marcia

 

That what I was thinking too but reading this http://www.claytimes.com/articles/kilnvolm.html i was not sure any more. It sure sounds like the calculated BTU's  are for the total N hours. I a sure I must be missing some nuances of English language here. To me, it sounds like this is all one needs to reach the temperature, in hours listed.

 

Quote from the linked page:

 

For example, you have a 30-cubic-foot soft brick kiln that you want to fire to cone 10 in 16 hours. Multiply 30 by 10,000 for your answer of 300,000 BTUs.

 

 

Same with this page. http://www.wardburner.com/bturequirements.html I see no mention that calculated BTU are required per hour, for 10 or 16 or what ever h you select from the table.

Going from 100C to 600C at 150 C per hour can be done with a fraction of a flame.

 

I looked around and looks like a 250-350 l kilns have 2 x ~20kW (~70K BTU) burners so this is not adding up and that is why I am a bit confused

[Marcia Selsor]

I would never second guess. Marc Ward. He is the Man when it comes to burners!.

Whatever he says.yes!

 

Marcia

[Mart]

I would never second guess. Marc Ward. He is the Man when it comes to burners!.

Whatever he says.yes!

 

Marcia

 

I am not second guessing (actually, I am) because I just do not understand, what he is telling me.

English is not my first language. Reading his article, I can not figure out, what those BTU's are for. Is he talking about 16000 x ft² = BTU's per every (8)hours of firing (16000*9*8=) or are those BTU's for the whole session (from ambient to final cone 10 in 8 hours 16000*9= ).

You see the difference? Is it going to cost X or 8*X per firing of the 9 ft² kiln?

[Marcia Selsor]

 

The usage I have seen is for soft brick 14,000 BTUs per cubic foot PER HOUR at maximum consumption temperature.

Marcia

 

That what I was thinking too but reading this http://www.claytimes.com/articles/kilnvolm.html i was not sure any more. It sure sounds like the calculated BTU's  are for the total N hours. I a sure I must be missing some nuances of English language here. To me, it sounds like this is all one needs to reach the temperature, in hours listed.

 

Quote from the linked page:

 

 

For example, you have a 30-cubic-foot soft brick kiln that you want to fire to cone 10 in 16 hours. Multiply 30 by 10,000 for your answer of 300,000 BTUs.

 

Same with this page. http://www.wardburner.com/bturequirements.html I see no mention that calculated BTU are required per hour, for 10 or 16 or what ever h you select from the table.

Going from 100C to 600C at 150 C per hour can be done with a fraction of a flame.

 

I looked around and looks like a 250-350 l kilns have 2 x ~20kW (~70K BTU) burners so this is not adding up and that is why I am a bit confused

 

I use to use a guide from an industrial manual Pyrotechnic engineering. It also included in the calculations what the outside temperature was on the cold face, wind etc. I also had Paul Soldner's little kiln building book. 140-300k seems about right depending on the insulation factor. Marc gave a nice simple chart.

Marcia

[neilestrick]

"For example, you have a 30-cubic-foot soft brick kiln that you want to fire to cone 10 in 16 hours. Multiply 30 by 10,000 for your answer of 300,000 BTUs."

I wouldn't go that route. 16 hours is ridiculously slow. Use 14,000-16,000 btu/hr per cubic foot and you'll be much happier. First figure out what size shelves you will use. Then design the kiln around that. Your total volume, not just stacking space, is what you use to calculate the needed btus/hr. You will not need the max btu's during the early part of the firing, but might towards the end. The total cost will vary depending on how fast you decide to fire, how quickly you turn up the kiln, how tight the load is, etc. And just because you may only use 7 pounds of propane, that doesn't necessarily mean you can use a 20 pound grill tank to fire. It may freeze up.

[Marcia Selsor]

I built a catenary arch kiln in upstate NY and had 4 100 pound tanks to fire it. They still would tend to freeze up in the winter, not too bad in the warmer months.

I think you need to consider how much gas you'll need to climb in temperature. For a 30 cu. ft. well -insulated kiln at say 14,000 but x 30= 420000 btus from all your burners. I built other kilns, mostly for natural gas or fuel oil. That catenary was my first propane experience.1971

 

Marcia

[Mart]

This is exactly what I am trying to figure out but it's still unclear, is this calculation 16000 BTU x 9 ft³= 144000 BTU per hour or for the whole run from ambient to cone 10.

Reading Neilestrick's reply, it sounds like the result of the calculation is actually per hour and not the whole 8 h firing.

1152000/47668=24.17 Kg (total ~50 EUR) of propane to fire a 250 l kiln with only 155 l of usable stacking space... huh...Lets not forget that 155 l will never-ever be filled 100% so we actually get only about 75-80 l of billable liters. Add all the other cost and we are looking at seriously absurd price per liter of kiln space.

 

I wrote to info@wardburner.com. Hopefully I can get a straight answer

I rather not spend more of you valuable time on cost estimates with ~800% variance.

[Marcia Selsor]

Yes it is per hour as far as I ever understood BTU ratings for firings kilns.But is is for the maximum needed like after red heat and the kiln is cranked up to climb.

Marc makes good burners. Talk to him. He'll give you a good answer.

Marcia

[neilestrick]

Yes, BTUs per hour. And make sure you're calculating off the total interior volume, not just stacking space.