Forge burner orifice issues

[Awalker]

I just changed my forge from a single burner at an angle in the side to a two burner from the top. The side burner was a Ron Reil 2 x 3/4" bell reducer burner. When I changed to top burners I used a 1 1/2" x 3/4" swage into a 3/4" elbow then into a 4 1/2" nipple and I used a forged 3/4" coupler that I flared the end on. I started out with a #72 drill size orifice but found it to be unstable and wouldn't stay lit. I bored it out to a #65 and it is much more stable but still not as hot as I'd like I am running it around 7 to 12psi. I started out with the #72 orifice as that was what was in a forge I had used previously that ran fine, and would run all day on about 4 gallons of LP. My single burner would use every bit of a a 5 gallon tank in 5 hours (it had a #56 orifice in it running at 17psi). I think I may go back and rebuild the venturi portion of the burner with the 2x3/4 bell reducer, as that seemed to work well on my other burner (I didn't have 2 of them handy or I would have used them on these burners).
After I had it running I played around with injecting some air into it to see if it would burn noticably hotter, and it did. So, I made an additional air injection manifold that stands off the burner a few inches and has a valve I can throttle that is connected to my compressor. It doesn't take a lot of 175# compressed air to make a big difference in the flame characteristics and I was very impressed. I think when I change the swages out I am going to change to a blower as well, the compressor works but adds even more noise and a 7.5 horse motor coming on and off all the time is annoying. With the air injection on I can run the LP pressure around 3 to 7 psi and it gets good and hot in the.
BTW the forge is made of 12" pipe that is 12" long. I added Kaowool when I moved the burners and that made a big difference (it had a firebrick liner before), and it has a brick closure in front and back.
Any thoughts on those burners would be much appreciated.

[Ken Kelley]

I switched from venturi to forced air burners and will never go back. Forced air burners use less fuel and are capable of a wide range of adjustment for flame temperature and forge atmosphere. I keep the sidearm burner I started out with around for the odd fire brick pile heating chore but that's about it. For info on forced air burners go to Bladesmith Forum or check out Indian George's site.

Ken

[ThomasPowers]

That's funny; I have been taking my two propane forges to teach with now about 10 times and the forced air forge goes through more than entire BBQ tank in a day while the aspirated forge still has gas left at the end of the day in it's tank every single time.

I'd say you can't make such comparisons without normalizing *all* the variables!

The forced air is sure a lot easier to use and tune and control atmosphere---it's the one I use for billet welding when I want to weld with propane.

If you want to go with forced air, scrap the air injection and make a real forced air burner; they are simpler to build and can use fairly small/cheap blowers rather than expensive air compressors.

[Awalker]

I really like the forced air, and the compressor solution is temporary as I didn't have a blower around to try.

[looper567]

Forced air burners commonly have larger orifices than Atmospheric burners. A Forced air burner with a 1/16" (0.0625) orifice will use a little over 3lbs. of propane an hour at 2 PSI. An Atmospheric burner with a .040 orifice needs to run at a pressure of 12 PSI to flow that much propane. From most of the designs I've seen, forced air burner orifices are even larger than a 1/16", I've seen some as large as 1/8" (0.125).
PSI means NOTHING, you need to know what the flow rate of the gas is to make any kind of reasonable determination. Forced air systems have their place, when you want very precise control of the fuel/air mixture, but for any Blacksmithing/Bladesmithing, Atmospheric burners with a good choke are all anyone really needs. You never need to worry about the danger of explosions when the blower quits either!

[Awalker]

Looper, I have worked in the gas industry for 15years and PSI means as much as orifice size. For each orifice size you will get a certain amount of btus per pound of pressure. So if I wanted to get a certain amount of btus I could use a small orifice size and higher pressure or a larger orifice size and lower pressure. There is a chart around here somewhere with the btu rating and pressures of various orifices. Volume (your flow rate), and pressure are inversley related to one another as per the ideal gas laws, so pressure does mean something! The main reason I see people putting 1/8" or 1/16" orifices in blown burners is that the burners are adjustable by installing either a needle valve upstream or adjusting the pressure regulator upstream to change the flow rate of the burner for their desired characteristics. I have also heard more variable fuel usage reports from people with blown burners for that very reason, they just have them adjusted to use too much fuel. A well designed forced air burner won't explode with the forced air stops either. Now if you are running to rich and you stop of the air supply the flame will want to exit the forge looking for air to burn but it won't explode because it is already on fire.
Also, just a point to ponder, how many people have changed to blown forges, then gone back to atmospheric?

[looper567]

What I mean by PSI meaning nothing is how it relates to fuel use and efficiency. So many people assume a blown burner running at 2 PSI is more economical than a Venturi burner running at 7 PSI, when the fact is, the Venturi burner may be only using half the propane of the blown burner.
How many "well designed" blown burners have you seen while looking through many of the homemade forge building sites? I surly haven't seen many. When the forced air pressure drops, the flame no longer wants to burn at the mouth of the burner opening. Sometimes the chamber will fill with gas and then ignite, shooting a fireball from the forge openings, and other times the flame will burn back into the burners mixer, I've seen both with a commercial unit (Mankel). So the point I was making was PSI has absolutely NO bearing on fuel use and efficiency, as so many imply.
A blown burner with a large enough orifice can run off of a BBQ regulator with only 11 Water Column Inches, but still flow a huge amount of LP, it is dependent on the Forced air and Mixing chamber design to create the proper fuel mixture. A Venturi burner depends on pressure in the Bell to create the Venturi effect (high pressure in the bell results in low pressure above the bell). But even with that, PSI still has no bearing WHATSOEVER on efficiency and economy.

[Awalker]

I see what you are saying about the blown burners, and no I havn't see many well designed homemade ones. I was assuming the blown and venturi burners to have the same size orifice. If the orifices are different sizes then no, pressure is no basis for comparison. So, assuming that, PSI has everything to do with fuel economy, as the higher the pressure the more gas can be crammed through the orifice until such a point as the orifice goes sonic and no more gas will go through no matter the pressure.
BTW, my burners are venturi burners that I am forcing air into, when the air stops on mine they still run just fine. When I am running my burners (with #65 orifices) at 7psi they are more economical than running them at 12psi (uses less fuel at lower pressure), but I like the hotter forge running at the higher pressure. So if I add the forced air I get the hotter forge at a lower pressure. The thing I am curious about is the other pipe forge I used had #72 orifices, and would run all day long on 4 gallons of LP but I can't get mine to run on #72 orifices.

[Candidquality]

Not sure how well you designed your intake for the venturi, but it can make a massive difference. Simply put, the best end design is a doughnut (torus). Split it in half like a bagle. Then lay it cut side down on the table. Looking down at it now, the air going into the center of that is going to flow better and have the least problems. And since you are only concerned with what goes into the hole, you only need the inside 1/2 of that form to make the air flow properly. The typical bell reducer is a poor choice, but can be made to work as you've already seen. You can look at any commercial central heat system using gas and see the same form. Now there is also a slight taper to the tube at times which you can research if really want to get a great burner design. In the meantime, eat the doughnut and ponder the possibilities.

If I can remember later I'll send out a sheetmetal version that can be bent to tube for the proper flame holder and intake for a venturi. Just need to know the inside dimensions of the forge you are trying to heat and the insulation thickness. Really should make that into a program to save time.

[piglet_74]

I've been wondering if any design ideas could be pulled from these.

http://www.structint.com/tekbrefs/sib96138/fig3.gif

It's a picture of a reactor jet pump. I used to do the instrumentation for them. The drive flow from the recirc pump would come up the middle and split in half in the rams head. From there the drive flow would entrain 2 times the volume of water from around the nozzles. Then they would build pressure in the diffuser to be able to force the water through the core. Normal core flow at 100% power was 102.5 million lbs/hr with only 30 million lbs/hr from the pumps themselves.