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Ribbon Burner gas jet size


Cast and Forge

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Hey community,

I often read in here that the size of the gas orfice should be 1 mm in diameter or even bigger in blown burners. Doesnt this contradicts with the whole "blown burners are more efficient" theme? 

Furthermore its often adviced to use larger gas orfices when experiencing burner problems like not getting up to temperature.

I run my blown ribbon burner with a 0,6 mm mig tip on about 0,2 bar (propane) for general forging. I encountered backfire problems due to overheating of the burner block (hope to solve this soon and coat it with itc100 as adviced here) but besides that it runs perfectly smooth.

So where does that "bigger is better" attitude come from?

Dont get this as an offend. Iam just curious.

Tim

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The pressure in a Naturally Aspirated burner is needed to impart speed to the gas issuing from the jet. There is some quite complex, and not particularly intuitive, Physics going on, but basically there is a conservation of momentum thing happening which draws in air and mixes it with the gas. 
 

In a blown burner, the air is forced in, rather than being drawn in by the fast-moving gas stream, so there is no need to have high pressure and a high-speed gas stream. A bigger jet will give a lower speed for a given gas flow and the lower speed is obtained with a lower pressure. High speeds tend to be associated with laminar flow, while low speeds tend to be associated with turbulent flow. Turbulence is needed to mix the gas and air effectively, so lower gas speeds are preferable where high speed is not needed to get the air in the first place.

Flow through a jet varies as the square root of the pressure: half the flow needs a quarter of the pressure. Double the flow needs 4 times the pressure. If you are going to use a “jet”, it and regulate the gas flow on pressure, the “jet” needs to be sized to give a useful range of flows across the pressure range you can set with your regulator. Finding an appropriate jet size for a NA burner is the difficult part. Not needing to do it for a blown burner is probably the main reason blown burners are usually considered easier to build.
 

If you use a needle valve (which can be thought of as an adjustable-area “jet”) to control gas flow, you don’t need to worry about regulator pressure overmuch.

 

 

 

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Alright.

I first built the ribbon burner with a gas injector made from a copper tube with 3 1mm holes drilled in it. Iam using a regulator with a needle valve. As I first fired the burner it worked well. But I noticed that the gauge on the outlet side of the regulator shows 0,0 bar regardless of how far i cranked it up. I reduced the 3 holes to just 1. Just a slightly difference on the gauge. 

I switches to the 0,6 mm mig tip it was fine. 

The thing is when I use a big diameter gas orfice that there is not really back pressure building up. So you cant really judge how efficient your burner is running. When using the 0,6 mm tip i can compare it to my venturi burner which also uses the same diameter tip.

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Efficiency is a function of BOTH orifice size and gas pressure. It doesn't make any sense talking about just one.  What would make the most sense would be to put the gas tank on a scale  and  measure how fast the weight decreases when the forge is running at a set, measured, temperature.   

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5 hours ago, timgunn1962 said:

High speeds tend to be associated with laminar flow, while low speeds tend to be associated with turbulent flow

Not to be pedantic or anything, but I'm pretty sure you have this backwards per the typical engineering definition of turbulence.  Specifically laminar flow is usually defined as fluid flow with a Reynolds number of 2,000 or less and the same fluid flow starts to become turbulent above a Re of 2,000 and is fully turbulent at Re above 4,000.  Since Reynolds number is directly related to fluid velocity, as the fluid velocity increases (all else being equal) the Reynolds number should also increase.

That being said, one non-intuitive thing is that friction acting against a fluid in a pipe or duct may be greater for fluids in laminar flow than in turbulent flow.  This has to do with the boundary layer as the fluid approaches the sidewall of the pipe.  

I'm sequestered, and don't have access to all my reference manuals at home, but I'm pretty sure about this.

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Tim in Saxony: You're making a common mistake, you have been lead to believe pressure actually means something as far as forge performance is concerned. It doesn't. The only real use a gauge serves on a forge furnace at our level is to make it easy to duplicate specific temperatures. 

If your gauge doesn't read at any setting of the regulator it's probably defective. The gauge on mine reads when on a weed burner which uses propane many times as fast as a forge burner. I'm betting your gauge is defective or something is blocking the inlet. Did you use thread TAPE when you installed it? If so that's probably the problem, a tiny shred of tape plugging something in the regulator. I highly recommend using gas rated thread paste SPARINGLY.

If anything is preventing your burner developing sufficient heat it is not having a proper fuel air ratio. When running a gun (blown) burner you can NOT just turn the gas up or down without adjusting the air flow. A gauge is useless UNLESS you have an air flow meter that is calibrated to the gas flow. You can buy this level control system but they are NOT cheap. 

Your realistic alternative is to learn to read the burner's flames and sound and hand adjust every temperature change. 

Frosty The Lucky.

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53 minutes ago, Frosty said:

The only real use a gauge serves on a forge furnace at our level is to make it easy to duplicate specific temperatures.

I've also discovered that the volume gauge on my twin tank assembly is pretty worthless: with two full tanks, it reads "LOW".

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11 hours ago, Latticino said:

Not to be pedantic or anything, but I'm pretty sure you have this backwards per the typical engineering definition of turbulence.  Specifically laminar flow is usually defined as fluid flow with a Reynolds number of 2,000 or less and the same fluid flow starts to become turbulent above a Re of 2,000 and is fully turbulent at Re above 4,000.  Since Reynolds number is directly related to fluid velocity, as the fluid velocity increases (all else being equal) the Reynolds number should also increase.

That being said, one non-intuitive thing is that friction acting against a fluid in a pipe or duct may be greater for fluids in laminar flow than in turbulent flow.  This has to do with the boundary layer as the fluid approaches the sidewall of the pipe.  

I'm sequestered, and don't have access to all my reference manuals at home, but I'm pretty sure about this.

You are right and I am wrong. Thanks for correcting me.

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9 hours ago, Frosty said:

A gauge is useless UNLESS you have an air flow meter that is calibrated to the gas flow. You can buy this level control system but they are NOT cheap. 

Research zero pressure regulators, that's what I use in my residential presure natural gas forge.  As Frosty said, not cheap.  Fortunately I got mine dumpster diving when a university glass blowing shop was being upgraded.

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Learning to adjust "by eye and by ear" is a valuable skill.  I was once teaching the intro to smithing class at the University and some members of the "advanced" class asked if they could use my forge instead of the Fine Arts Metal's forge.  They told me it wasn't working right, it's a blown propane forge hard to not be adjustable to "right".  Anyway I told them to turn it on.  3' of dragon's breath!  I told them it was turned up too high.  They said "No the line on the regulator handle was lined up with the line on the regulator body."  So I dialed it 3 complete turns down and the line was again aligned, I adjusted the air and the forge was usable.  Remember folks these are the people who will be running the world when we get old and feeble!

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  • 3 months later...

I've installed a low side refrigeration gage on my blown bobbin burner set up, installed before the needle valve and measuring the pressure drop to judge the gas flow, 3/8" supply up to the needle valve reduced to a 1/4" copper tubing sealed at the end and drilled orifice of 1/32 drill bite.

LP gas boils off to a vapor/gas at a given rate producing a given line pressure, by measuring the pressure drop you can determine your gas feed rate.

Tried to upload a 114.5mb GIF File but it failed returning a -200 error code.

20200703_202631.jpg

20200703_202641.jpg

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