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Burner to cubic inch relationship

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37 minutes ago, Bill saunders said:

12" and the long flat part of the cylinder is 18".  It is 24" cone to cone.

Based on length to diameter, you will need at least two 1/2" burners or three 3/8" burners for an even internal heat.

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On 3/20/2018 at 2:31 AM, Kuja_torra said:

I think that Frosty just made a simple math error. Lets review the formulas first.

Cylindrical volume is V=A×h

Area of a circle is A=3.14×r^2

Circumference of a circle is C=3.14×2r 

Frosty simply used Circumference in his calculations instead of area to calculate volume.

I just noticed this. Can you explain how such a fundamental error as using the circumference rather than area could conceivably only yield a 4.8 cu/in difference? Did you even stop to think about that before you posted?

A couple  things: First, this isn't rocket science rounding off 4 or 5 cu/in in 1,000 is insignificant and makes calculating simpler.. On top of that bit of silliness using 3.14 rather than 3.1416 would probably cause that much error. Probably because I'm wasting too much time responding to waste more calculating YOUR errors.

Clayton: The ID isn't 8". You're only subtracting the Kaowool, you're forgetting about or . . . whatever the 1/2" hard refractory liner. The ID is actually 7".

I don't calculate the area of a circle using "pi r sq". I use the accurate to 8 decimal places formula ".7854 D sq."

And finally Why are you asking me to do simple math for YOU? In the real world if your calcs don't agree then you double or triple check AND prove. YOUR calcs. none of which you did.

 That's more time than I need to spend here.

Frosty The Lucky.

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Frosty, thanks for the reminder that the refractory liner needs to be included, that dropped my volume about 90 ci.

Now admittedly this is my first propane build, and I was double checking numbers trying to get a good orifice size, right now I'm at .030 mig tip, and found this and the post that is previously linked that had lots of Ron Reil info.

I also came across this page, http://www.joppaglass.com/burner/highp_chart.html. So trying to figure how much btu I might need, I realized that it really depends on the mass of steel I am trying to weld.

Just for example, and prob not 100% accurate, say it takes 100 btu to boil a pint of water in 5 min, works well and heats up my ramen noodles. But that same 100 btu will not the 6 quarts of water for my pasta noodles, it may get there given enough time and insulation, but I'm not waiting that long. 

That all said if I can't get to the amount of heat (btu's) needed I could rebuild with another burner; move up to a larger orifice, until I run into the limits of airflow in an atmospheric burner; increase gas pressure, with the same airflow restrictions.
 

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NA burners tend to have very generous turn-down ranges. Start with a design KNOWN to work well, and then go to the next higher size than you think you might need. You can't put extra heat into a burner, but you can turn it down easily enough. Next, build the burner exactly as the designer recommends. Then install it in a burner port. You can always change it out for a smaller burner, and sell the first burner on eBay.

 

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Two thing right of the bat, I am using a know design and built as designed. Don't assume things.

Now to the can't put extra heat into a burner, well you can by changing orifice size and or pressure. Heat is measured in BTU, temperature is in degrees. Two related but different things. I'd agree that can't effect temp, except for over a small range dependent on the amount of oxygen available. 

Think of heat as volume, we say a large bonfire was really hot compared to a tiny campfire, when it only feels hotter due to a larger volume of heat (more btu's) and not temperature (degrees).

Another way to think of this is your tip size on a oxy/acetylene torch. With welding tips you select the size based on material thickness, because you need more heat to weld thicker material. You change the orifice and sometimes the pressure setting.

Furnace and appliance techs make these types of changes to burner assemblies regularly.

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13 hours ago, ne_smith said:

Two thing right of the bat, I am using a know design and built as designed. Don't assume things.

Try taking your own advice. I write for other people reading this thread too; not just you!!!

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Since this is a sticky discussion with the title "burner to cubic inch relationship", wouldn't it make sense to have some general info about burner size vs. forge size in it? I kind of expected a nice and tidy list, or more than just a discussion about a single burner size at least. :)

 

 

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The burner size to forge volume rules of thumb are laid out in the early posts in I think Both Forges 101 and Burners 101.

Frosty The Lucky.

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Ah, good. 
Now I just wonder why this thread is a sticky, it doesn't seem to contain enough information to deserve the spot.

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Make the list and we will find a place for it to be posted.

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G-son volunteered and we ALL saw you! :)

Frosty The Lucky.

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Yeah, right... :P
I looked here because this seemed like the most logical place to have that info, but no luck. I know the info is out there, hiding in plain sight somewhere in the burners 101/forges 101 threads, but those have a combined length of 113 pages and 25 posts per page. Someone who knows what they're talking about would probably be done long before I've had time to plow through these threads from start to finish yet another time. (I really need to copy & save all the juicy parts when I find them to make the info easier to find again, I can't remember it all. :()

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OhhhhhKay. One last time. It's by ratio based on the area of the burner outlet and forge volume. One well tuned 3/4" NA burner will reliably bring 300-350 cu/in to welding temperature.

A little arithmetic and you can figure out forge/furnace volume per given burner diameter. For example a 1" burner nozzle is twice the area of a 3/4" so it will reliably bring 600-700 cu/in furnace / forge volume to welding temp.

Propane jets follow a similar ratio.

Frosty The Lucky.

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Allrighty! A 3/4" sch 40 pipe has an actual internal diameter of 0.824". that converts to 20,9296mm, I'll round it off to 20,9.

A 20,9mm diameter hole has the area pi * 10,45 ^2 = 343mm2

300cu/i is 4,916l
350cu/i is 5,755l

343/4,916 = 69,8
343/5,755 = 59,6

So, we want a mixing tube diameter that gives us roughly 60-70 mm2 of hole for every liter of forge volume. 

For 1l:
2√(60/pi)= 8,74
2√(70/pi)= 9,44

For 2l:
2√(120/pi)= 12,36
2√(140/pi)= 13,35

Forge volume per burner vs. mixing tube internal diameter (actual diameter, not advertised sizes!)
1l: 8,7 - 9,4mm
2l: 12,4 - 13,4mm
3l: 15,1 - 16,4mm
4l: 17,5 - 18,9mm
5l: 19,5 - 21,1mm
6l: 21,4 - 23,1mm
7l: 23,1 - 25,0mm

And so on. 

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I have been reading several posts.  I am building a gas forge with a volume of 658 after insulation.  It is 18 " long and going by posts I should only need 2 burners.  However Frosty, you said in a post to space no more than 3.5"-4" for burners.  Using that spacing I would be able to use 3 burners and have about 4" from each end to my burners.  The forge is rectangular and I am using 3/4" burners.  Should I do 2 or 3?

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On 6/12/2019 at 4:24 PM, Frosty said:

OhhhhhKay. One last time.

I don't believe you<_<

Pnut

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7 hours ago, DPod said:

I have been reading several posts.  I am building a gas forge with a volume of 658 after insulation.  It is 18 " long and going by posts I should only need 2 burners.  However Frosty, you said in a post to space no more than 3.5"-4" for burners.  Using that spacing I would be able to use 3 burners and have about 4" from each end to my burners.  The forge is rectangular and I am using 3/4" burners.  Should I do 2 or 3?

You could do fine with two properly made and properly tuned burners.

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