Will this propane burner work?

[Rainbows]

 

So here is my theorised burner. 

to the left is 8mm tube to take a compression fitting leading to a 1.1mm tip and a threaded plate that will run up and down for air adjustments.

Then a venturi going from 40-21.4mm (this halves the area) with a 12-1 taper or a 24:1 maybe depending how you look at it. 

Then a 25.4 OD, 21.4 ID 170mm long tube. 

Then the flare is made as amal does so the internal diameter and length is 42.8mm. 

 

Does this look like it would theoretically work? I wasn't sure where to put the jet in relation to the venturi. Other designs looked to have it just on the lip of the cone so I copied that.

 

 

[ThomasPowers]

I don't think any aspirated burner would work with that forge as shown.

Which leads me to the question: if you don't know the basics; why are you designing a burner rather than using a known good design?

[timgunn1962]

As long as you are able to change the jet size to tune it, yes the burner will work or can be made to work. Most likely quite well if you tune it well.

However, this is true of almost any NA burner and it does not look as if it will actually perform any better than most of the much simpler burner designs out there.

The jet looks to have a step-change in section, rather than a smooth transition from large to small diameter. This suggests to me that you are not familiar with the fundamental principles that apply to NA burners.

There is a very good reason to maximize the discharge coefficient of the gas jet, namely to maximize the kinetic energy of the gas. I am pretty sure the reason most of the better homebuild designs use MIG tips is because of their smooth transition into the minor diameter. The MIG tips are designed to smoothly feed wire, but the shape is coincidentally very good for feeding gas. 

If a smooth transition can achieve a discharge coefficient of 0.8 and a square-edge gives about 0.64, the smooth transition gives 25% higher velocity and 56% more kinetic energy. It's effectively the KE that draws in the air.

Although you have included a 1:12 machined bore, it is usually the expansion section after the Venturi throat in the commercial designs.

In fact, there is no Venturi throat to speak of, so there is no Venturi.

Many (most?) of the homebuild burner designs dispense with a Venturi because, although it can make a significant difference to performance, it's difficult to make one without machining facilities. Your design seems to require the machining facilities but does not make use of a Venturi.

It looks like a minor change would make the gas jet location axially adjustable: a no-brainer since you mention that you are unsure where it should be.

Unless it's a college project or similar, it would seem much cheaper and easier just to cough up the 65-ish quid (including VAT and delivery) for a 3/4" Amal atmospheric injector. 

The Amal-style flame retention cup should work fine if you are intending to run the burner outside the forge. For use in a forging or welding forge, it is wholly unnecessary. It can also get expensive if the plan is to use materials for the cup that will stand up to welding temperatures.

I use retention cups in dedicated HT forges which run very rich and with flame temperatures around 800 degC (1472 degF) or a little less. In forging or welding forges, I just use straight pipe and an Amal injector. 

The exhaust area of the forge seems very small with just the round opening(s?). I think you'd probably need the small door(s?) open to run at all.

The vertical burner aimed at the centre of the forge floor seems to me to be about the worst arrangement commonly seen: prone to chimneying on shutdown and with a high probability of the flame hitting the metal before combustion is complete, Oxidizing the workpiece.

[VainEnd84]

That burner design looks very similar to these ones https://forgeburners.com, some people love them, some have had a difficult time getting them tuned, from what I've read.

[timgunn1962]

I think the Forgeburners burners are probably imported Aussie Gameco/Artisan Supplies burners.

It's pretty easy to find pictures of the outside of them online, but it's what is going on inside that makes the difference to performance. I'd really love to see one taken apart.

[Rainbows]

20 hours ago, ThomasPowers said:

I don't think any aspirated burner would work with that forge as shown.

Which leads me to the question: if you don't know the basics; why are you designing a burner rather than using a known good design?

Confused as to why, its just a forge with a hole in the front and back to make the 7:1 output input ratio (think that was from anvilfire) + some doors. The outside is square cause I don't have rolling equipment but the inside will be semi circle/ellipse. Seems like same sort as forge as others have?

 

16 hours ago, timgunn1962 said:

snipped for length

Thanks for the tip on the tapered jet. I guess in real life there would be some taper just from the end of the drill bit but I will see about grinding a D reamer to make a fine taper or check if I have any tiny boring bars. 

Bit confused on the ventrui part. Doesn't the tube count as the venturi throat? I might be getting my terminology mixed up. Should I add a parallel pipe section before the reducing venturi? Also yeah I have access to a DMG MORI so I can blast these out pretty fast.

Originally I was gonna lock the jet holder in place with two thin nuts but since then realised it makes way more sense to just use a grub screw in the burner to hold it in place. Will make it adjustable at the same time I make it grub screw held. 

The material cost (for the part that would be replaced by the Amal) is only going to be £5 but that's without considering labour cost. If time was a cost factor I would go for the Amal but for me the blacksmithing is just a fun thing on the side and I like the idea of having something I made making all the heat. 

The retaining cup was just gonna be mild steel for any miscellaneous blow torch applications. I was thinking that if the forge did need one (now know it doesn't) I would build the retaining cup into the refractory.  

Theoretically the two windows should follow the rule from here on having a 7:1 exhaust - burner pipe ratio. Will recheck all my maths before I finally put refractory cement to mould though. 

not much thought went into the placement. Could easily change the burner so it comes in from the side with the top of the burner tube being tangent to the top of the forge cavity.  If it works better could also get it on a 45 degree angle like propane bottle forges do but the square body would make that a bit more of a pain. 

 

15 hours ago, VainEnd84 said:

That burner design looks very similar to these ones https://forgeburners.com, some people love them, some have had a difficult time getting them tuned, from what I've read.

Yeah you got me, they were more or less the design inspiration. I remember people on a australian forum seemed to like them so I went with them. 

 

8 hours ago, timgunn1962 said:

I think the Forgeburners burners are probably imported Aussie Gameco/Artisan Supplies burners.

It's pretty easy to find pictures of the outside of them online, but it's what is going on inside that makes the difference to performance. I'd really love to see one taken apart.

Your right on them either being imports or maybe just a copy. Alec Steele mentions them being based off an australian burner. 

[ThomasPowers]

The ratio looked off to me and I thought that too much back pressure would cause problems---you did allow for having stock running through the hole right, so it's 7 + stock size to 1 right?

[Mikey98118]

I have written about mounting gas jets on linear burners via a saddle, in Burners 101. That much of your burner makes sense; everything else about them is wrong.