Propane orifice size

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cp4449
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Location: Granada Hills

Propane orifice size

Post by cp4449 » Wed May 23, 2018 4:54 pm

What is the oriface size for propane burner that most of you using? My oriface on my burner got damaged, I got a new one, but they sent one with a guide hole. I normally run at 5-8 psi, slot burner
C.P. Mahony
Apparently reinstated 1" Foreman
Los Angeles Live Steamers

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ChipsAhoy
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Re: Propane oriface size

Post by ChipsAhoy » Wed May 23, 2018 5:34 pm

The Marty burners that I'm playing with use a #71 which is what the instructions call for. He goes on to say .....as close to .026 as possible. Now, he is working with 1 psi and up to 5 psi. One nozzle. The two that I have made so far appear way too rich, but I suspect that is poor machining and the design is fine.

The Wendy has a slot burner, I have not built one yet.
It has one #40 orifice feeding 4 ea. 10 inch tubes with 21 slots per tube.
I intend to build one and see what it does.
Scotty

rkcarguy
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Re: Propane oriface size

Post by rkcarguy » Wed May 23, 2018 5:55 pm

I always recommend trying some carburetor jets from something, so you have some adjustment. For example, the jets for the 6.5HP clone engines are around $1 each. If you drill and tap a brass pipe cap so the jet can be threaded in from the inside, you'll get a really nice dispersion of "gas" that will burn very well. # drill bits have some gaps in that size range, so I had the same problem racing go karts and trying to drill the jets....one size is too lean, next size is pig rich.

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ChuckHackett-844
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Re: Propane oriface size

Post by ChuckHackett-844 » Wed May 23, 2018 7:48 pm

(I assume we are talking about a 'furnace' type burner, i.e.: one orifice, one mixing tube feeding a single manifold)

To calculate the orifice size you need to know how many BTU your burner can produce. From that, and knowing the pressure you want to feed it, you can calculate the orifice size that will deliver the given BTU at that pressure. .. or ...

Start with a small orifice and work your way up (adjusting the primary air inlet as needed) until, at full fire, the flame starts lifting from the burner - that's your limit.

That limit can be reached with many pressure/orifice size combinations. A large orifice at a low pressure will deliver the same BTUs as a small orifice at high pressure. This is why, asking someone "what burner pressure do you use" is meaningless unless you are using the same size orifice.

It is more desirable to use a smaller orifice at higher pressure because it does a better job of drawing in primary air and promoting mixing. There is a limit however because once the gas velocity through the orifice reaches the speed of sound a pressure wave is formed and the flow will not increase with a further increase in gas pressure.

For each burner configuration there is a maximum gas flow volume it can handle and thus a maximum BTU. The propane/air "flame velocity" (how fast the flame front propagates) is fixed for any given propane/air mixture ratio. When the gas velocity coming out of the holes/slots exceeds the "flame velocity" the fire will lift from the burner and, if you keep going, go out.

Hope that helps ...
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

cp4449
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Re: Propane oriface size

Post by cp4449 » Wed May 23, 2018 9:10 pm

OMG, the best explanation of this subject I have read. I never thought of carb jets. Good idea. Chuck, your write up was great, and I have two orifices I can drill. They are currently .71, I will start with that and work my way up until at a set pressure, lift the flame.
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Berkman
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Re: Propane oriface size

Post by Berkman » Thu May 24, 2018 10:29 am

Have a photo of the burner you use in your northern Chuck ? Thank you in advance.

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SteveM
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Re: Propane oriface size

Post by SteveM » Thu May 24, 2018 12:06 pm

cp4449 wrote:
Wed May 23, 2018 9:10 pm
OMG, the best explanation of this subject I have read.
I was thinking the exact same thing as I read Chuck's post.
ChuckHackett-844 wrote:
Wed May 23, 2018 7:48 pm
To calculate the orifice size you need to know how many BTU your burner can produce. From that, and knowing the pressure you want to feed it, you can calculate the orifice size that will deliver the given BTU at that pressure
What is the calculation, or are there some tables or an online calculator we can use to get a starting point?

Steve

RickBarb
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Re: Propane oriface size

Post by RickBarb » Thu May 24, 2018 1:55 pm

Hi, I hesitate to join in on this thread as my attempt to gas fire my 3-1/2 scale Rob Roy have for the most part been unsuccessful. My problem is that I think I am trying to put 10# of stuff in a 5# sack. The firebox is 2"x3-9/32". I am guessing I need approximately 16,000 btu. I am close, but not yet successful. Now as for orifices, I have used "misting nozzles". They have a .012" hole that can be enlarged to whatever diameter you want. They are brass construction and are dirt cheap. You can buy on eBay 20 pieces for $6 to $8 from china, free shipping. How do they do it? For calculating heat output for pressure and orifice size I use an interactive calculator by: Thomas Vincent @ http://ronreil.abana.org/BTU_Calculator.xls. You just enter orifice diameter and coefficient and pressure and it calculates the output in btu's. Hope this helps. Any suggestions for me to try.

Rick

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ChuckHackett-844
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Re: Propane orifice size

Post by ChuckHackett-844 » Fri May 25, 2018 9:55 pm

I have drawings, photos, etc. for the burner in my Northern but I don't know how to post such here. If you send an email to misccdh@whitetrout.net I can email you a copy.
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

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