Hi Bill, I`m not sure either, but it sounds like you^re on the right track. Hopefully, some more input will disipate any doubts. Thanls! Nelson.

Rich, you`ve explained quite well the physics behind a venturi gas burner.After researching some,I realize that in my case,there`s no need for such a sofisticated device as an accelerator and other fancy tech features,considering the low gas price here.Neverthess, I cannot help considering basic improvements over a basic design, as possible. For instance, the spring idea I like. Obviously reducing the speed some before the air-gas mix reaches the combustion end, increases burner performance. Do you think a expansion chamber half way the burn tube may slow down the flow too much? Probably, would depend on how much expansion, or how much texture, for the speed reduction principle would apply anyway! It just crossed my mind, most venturi designs use a T-reduction. What about using a Y-reduction? The initial flow speed at the venturi spot should be higher for Y than for a T due to the angle, so if I start off with a little higher speed then slowing down the flow for mixing won`t be a concern...Just thought too,that the little air speed gained may canceled out with the little reduction of the gas speed at that little turn of the Y (just things to wonder huh!).Again, this may be just another detail NOT TO WORRY ABOUT ? I do have a whole bunch of refractory bricks, and wonder if it would not be good to have a little ineficiency due to some heat absorption by the bricks, but the convinience of getting a piece hot faster with an increase in gas flow from idling heat ???Hope some of my assumptions aren`t way off, but the could. Nelson.

Nelson,

There are so many variables in any given burner design that you could spend countless hours experimenting. You might come up with something new that works better.

 One reason that some people use the reducing tee for the venturi is that certain tees have a nearly perfect taper where they change size, resulting in a more efficient Bernoulli (venturi) effect. The Ward brand reducing tee is one that is very good, while most of the cheap Chinese and Thai tees that I've seen had an almost hemispherical reduction step, or a very short ramp, either of which is far less efficient than the longer ramp of the Ward tees. I get my Ward tees, and a number of other burner parts, from Larry Zoeller, by the way.  Good guy to do business with.

I'm dubious about the success of using a "Y" instead of the tee, as they don't have the right configuration to create a good venturi.  The venturi area is the place where you do want everything to be shiny and smooth, and the gases traveling quickly and with little turbulence.  Taking a few minutes with a die grinder and shining up the ramp in your reducing tee is one way to improve the efficiency of the burner.

Including a mixing chamber in the burner tube is not a new idea, and it is one that has some real merit.  You 'll probably want to bring the tube back down to the original diameter after the mixing chamber, to get the gas flow to the right speed for proper flame control.

Keep in mind, too, that backpressure in the forge chamber itself has an effect on burner performance.  Too little backpressure and the burner is running too quickly and may not retain the flame at its end, as well as running too lean due to inducing too much air.  Too much backpressure and the burner is choked, just as if you restrict the intake side of the burner.  That can cause backfires and too rich a mixture.

Forge and burner design are an art as much as a science.  It takes a lot of experimentation andsome good luck to end up with a forge that works really well, particularly when you're using venturi burners.  With blown burners youcan get away with a lot more errors in design, since you can adjust both the air and gas to overcome problems.  I'm about to have to build a new forge for the shop and it will most likely be a blown burner design, because I need to have a lot of featuers in the forge that will make it impossible to have just one burner design that will work with all the different configurations that the forge will be capable of.  Blown burners will solve that issue easily and simply.  The only drawback of blown burners is that they don't readily lend themselves to the use of an "idle" circuit to conserve fuel.  Our fuel here is not cheap, so that is a consideration I'm trying to work out a way around.

Rich I realize there are so many variables, and combinations of them that make the issue rather interesting, and worthwhile trying one own`s contraption to see how it works. I do have a nice fan, but having read some of the drawbacks, seems appropiate to try the venturi type, again this is a matter of personal preference. All that said, I will start my experiment out, based on those basic principles you mentioned as well as a bunch more not to overlook. I`ll post the ongoing task and hope that when I light it up, the last step will be worth showing too.
Rich, thanks you for your highly tech input. Nelson.