Calculating BTU/hr (How Much Propane) to Use for Fire Project

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Kowabunga
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Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Kowabunga » Mon Jan 15, 2018 9:02 am

Hello makers!

The endeavor this year includes creating what is essentially a very long manifold (pipe with jets that will emit flames). As stated, I am trying to calculate the amount of propane that will be consumed.

The length of the manifold is set, but the distance between holes can be scaled. I have found one rough estimate that gives 1280 BTU/hr for a small gas lamp. If I assume each hole to be a small gas lamp that means every hole is 1280 BTU/hr. When I scale for ~800 holes it seems possible that it would take 4 100 lb propane tanks (of 23.6 gallon capacity each) to light up for only 8 hours.

The idea is to have the flames lit continuously each night for roughly 6 hours and have LED strips on for the rest of the time.
If you have experience calculating propane, please enLIGHTen!

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Popeye
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Popeye » Mon Jan 15, 2018 9:11 am

BTU has nothing to do with it. BTU is energy, not gas flow.

What you need to know is the pressure on the pipe, the size and material of the pipe (for friction loss), the number of orfices and their size. If pipe length or diameter is large size and material may be meaningless.
You might want to consider using a long pipe to carry the gas and several small pipes/tube to carry gas to the orfices. Think of a tree.
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cryoguy
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by cryoguy » Mon Jan 15, 2018 9:53 am

Sounds like a fun problem. Is it like a manifold that feeds Bunsen burners is chem lab? Maybe start with this paper.
But ya mass flow rate is what needs to be calculated.

U. S. Department of Commerce National Bureau of Standards
Research Paper RP1991 Volume 42, June 1949
Part of the Journal of Research of the National Bureau of Standards
A Study of Laboratory Bunsen Burners for Natural Gas

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cryoguy
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by cryoguy » Mon Jan 15, 2018 10:15 am

Follow up
It’s been a long time (4 decades to be exact) since my fluid mechanics class, so my advise may not be that helpful. But here is a Wiki page you might look at, if you have not done so already.

Flow distribution in manifolds
From Wikipedia, the free encyclopedia
The flow in manifolds is extensively encountered in many industrial processes when it is necessary to distribute a large fluid stream into several parallel streams and then to collect them into one discharge stream, such as fuel cells, plate heat exchanger, radial flow reactor, and irrigation.

good luck

Kowabunga
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Kowabunga » Mon Jan 15, 2018 10:46 am

Popeye wrote:BTU has nothing to do with it. BTU is energy, not gas flow.

What you need to know is the pressure on the pipe, the size and material of the pipe (for friction loss), the number of orfices and their size. If pipe length or diameter is large size and material may be meaningless.
You might want to consider using a long pipe to carry the gas and several small pipes/tube to carry gas to the orfices. Think of a tree.
I selected 1/2" pipes, the number of holes can be scaled, and the holes would be drilled with a #56 drill bit (0.0465"). This would be done using black pipe. The total length of pipes is just under 340 ft. The plan would be to section it off into maybe four sections of 85 ft. Each section would have its own inlet coming from flexible gas hose.
I have to find the fluid velocity coming from the regulator in order to calculate the head loss over the pipe lengths.

Also:
I haven't calculated the pressure loss yet (even without the holes, but I'm sure the pressure loss from the holes will be significant). I'd plan on having the inlet at the center of the section so the pressure would drop as the gas travels to either end of the section (which is fine, it doesn't have to maintain the same height flame over the entire length).

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Token
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Token » Mon Jan 15, 2018 10:49 am

800 fricking holes!? How the heck are you gonna light that without a big fireball forming somewhere?

So, what you got there is a napkin design with a whole lot of sloppy all over the place.

How precise is your flame orifice? Are you just drilling with a cordless dril and random bit or using precision parts from a gas grill etc??? Drill press?

Are you pre-mixing in a manifold or just pumping propane?

What is in place to prevent wind from blowing out the flame?

Are you diffusing the gas before ignition or burning straight from the hole?

Is the burner pipe horizontal or vertical or in between?

How long is the burn pipe anyway?

OK, so let’s hope you have answers to some of these questions or at least have considered some of them.

++++++++++++

Answer to your question:

Get a 20# tank and a decent regulator with gauges.

Set up a scale model of the burn tube you would like to use. Make it with a nice number of orifices, say 10 or 16 or 32 if you like power of two arithmetic.

Get it lit.

Regulate the flame to the desired size and note your gauges.

Let it burn.

While it’s burning, you can then tackle some of the other considerations like wrapping the burn pipe in stainless-steel wool for wind resistance, maybe a thermal element safety valve from a furnace ...

Bottom line: the math and science behind all this is well known and published, but the learning curve is steep. Focus first on the safety angle, then do a trial run using your materials to get the data. You will not be able to get reliable data unless you reuse an existing furnace/grill/fireplace burner that has reliable specs and materials already designed into the thing.

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Token
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Token » Mon Jan 15, 2018 10:55 am

Sweet mother of Dog, you got some hairy peaches on you!

That sounds like a whole lot of fun!

Buy life insurance before you test fire it.

Kowabunga
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Kowabunga » Mon Jan 15, 2018 11:04 am

Gonna try something out based on your suggestions. I'll get back to you all shortly. If you don't hear from me, you'll know what happened.

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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Token » Mon Jan 15, 2018 11:31 am

100 meter burner, flame every 10 cm.

I’d seriously consider 34 x 10’ sections.

Unless you’re going to put a speaker on the cap and make dancing flames to the beat of the music ... which is real fun but you don’t need more than 30’ for the full audio spectrum.

Design goals should be:

1. Detection and cutoff for flameout.
2. How to light without boom.
3. How to keep burning in wind and dust.

Starting a scale test with a standard length of black pipe, figure all the safety, then figure how to scale.

For something this big, you’ll need to work with the FAST team and they will require a failsafe system.

https://burningman.org/event/art-perfor ... allations/

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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Popeye » Mon Jan 15, 2018 2:21 pm

When you do your mock up you could set the 20# tank on an accurate scale and have someone record the weight every 30 seconds or so. This would give you mass flow over time. If you can also record the pressure at each end of the 1/2" pipe and flame height it might be helpful.
Adjustable regulators: https://www.artisansupplies.com.au/prod ... cessories/
Barbecue grills seem to have a regulator preset to between 10-20 pounds. Forges seem to start at about 5 pounds but with greater mass flow of course.
Just a thought, if the pressure drops so the end orfices go out could it suck air and pop when restarted?

Oh, don't test the mock up in your living room or garage :twisted: :roll: .
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Kowabunga
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Kowabunga » Mon Jan 15, 2018 6:47 pm

I made it to the big box and selected some lengths and fittings for a 10’ mock-up. Unfortunately the big box didn’t have the proper adaptor to go from the flared gas hose connector to the tapered black pipe :? And yes, all the regulators were 10 PSI. A needle/gate valve will control volume flow from there.

Popeye, I will use those suggestions for flow rate. I took fluid dynamics not too long ago and will dig up the equations as well as check the resources mentioned. Did you have any more information on the dangers of the “pop”? Sounds like a risk of negative pressure in the pipes.

Token, I’ll take the 10 cm interval suggestion with this 10’ mock-up. The holes should keep me busy until the rest of the parts arrive. This test apparatus is gonna be one big ol’ flambeau.
I’ll also reach out to FAST.

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Popeye
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Popeye » Tue Jan 16, 2018 10:24 am

Kowabunga wrote: Popeye, I will use those suggestions for flow rate. I took fluid dynamics not too long ago and will dig up the equations as well as check the resources mentioned. Did you have any more information on the dangers of the “pop”? Sounds like a risk of negative pressure in the pipes.
If the fuel air mixture in the end of the pipe becomes explosive it will "pop", a mini explosion and could damage the orfices. I very much doubt the pipe itself could be damaged. Think of the damage that can be done to an Oxy-Acetelyne tip if pressures are not right or it is not turned off properly.
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Popeye
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Re: Calculating BTU/hr (How Much Propane) to Use for Fire Project

Post by Popeye » Thu Feb 01, 2018 12:26 pm

http://www.poofersupply.com/Training_c45/
I've seen this site reccomended before. Might be helpful.

Kowabunga wrote:Hello makers!

The endeavor this year includes creating what is essentially a very long manifold (pipe with jets that will emit flames). As stated, I am trying to calculate the amount of propane that will be consumed.

The length of the manifold is set, but the distance between holes can be scaled. I have found one rough estimate that gives 1280 BTU/hr for a small gas lamp. If I assume each hole to be a small gas lamp that means every hole is 1280 BTU/hr. When I scale for ~800 holes it seems possible that it would take 4 100 lb propane tanks (of 23.6 gallon capacity each) to light up for only 8 hours.

The idea is to have the flames lit continuously each night for roughly 6 hours and have LED strips on for the rest of the time.
If you have experience calculating propane, please enLIGHTen!
nobody wants to live in a world with only one flavor...

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