Venturi burner jet hole question

[timgunn1962]

"Very close" is perhaps a tad optimistic.

The gas flow at any given pressure varies with area, which varies as the square of the diameter (assuming the geometry of the hole entrance and exit remains the same).

.0485"/.043" is 1.128 times the diameter, giving 1.272 times the area. 

On those numbers, the burner would run 27% richer than it would with the #57 drill.

I actually get 3/64 = .046875

.046875/.043 = 1.09 times the diameter, 1.188 times the area and about 19% richer than the #57 drill.

[Latticino]

Actually it is a bit more complicated than that due to boundary layer dynamics and the fact that fluid flow in a tube is not uniform at a certain crossection (as your equation would indicate).  That doesn't even take into account laminar vs turbulent flow, sonic constraints, combustion air induction details and free jet dynamics.  Still, your comment is certainly valid as regards the relative effect of a small diameter difference being somewhat disproportionate for burner design.

[bbderoff]

On 4/24/2014 at 4:27 AM, Frosty said:

Tube length should = 8-9 x throat diameter.

 

Intake area = 2x the throat cross section area.

 

Jet setback should be no less than .5x throat dia.

Awesome! That is the advice most of us would be looking for, I think.
At least those, who - like me - just want a little burner for heating a small forge to shoe horses and adjust tools (forepunch, pritchel, hot set, shoe carrying tongs, etc.)  sometimes, but not going to be forging tool grade steel most of the time. Not having an oxidizing flame is still preferable if I don't want to be brushing scale all the time and a reducing flame will give me less heat on more gas, I guess? 
How detrimental is not having a choke at all? The last set I got from Amazon is so poorly machined that the choke disc does not turn properly on the thread on the jet.
Is there any benefit in having the jet opening further away from the throat than 0.5x the throat diameter(while assuming that the jet/throat diameter ratio is appropriate)?
The previous one (choke disc) I lost (I carry the forge in the trunk), and it was still burning OK, but I wonder if I have used up unnecessarily large volumes of gas?
Cheers, BB 

[Mikey98118]

3 hours ago, bbderoff said:

 

 

 

 

 

I would suggest 2.5 times the mixing tube diameter; this will get the flame close to perfect. Such a flame is easy to fine tune for anything from oxidizing to reducing by adjusting the amount of the flame retention nozzle's overhang past the end of the mixing tube, a slight increase or decrease in the size of the gas orifice, or an adjustable choke plate at the air entrance.

[Frosty]

Yeah but you have a choke on your burners Mike, I tune mine to neutral when I make them and don't worry about fine adjustments later. 

Enough messing with Mike for now.

BB. Plans for a basic 3/4" T burner are pinned in the gas burner section of Iforge. The ratios for all the specified dimensions are consistent through different diameters so it's just arithmetic to make them larger or smaller.

A neutral flame develops the best fuel economy, BTU / volume of fuel gas. An oxidizing flame (Lean) leaves un consumed oxygen in the flame and as you note oxidizes your steel, (black scale). It isn't really hotter than a neutral flame except maybe the BTUs produced by burning the carbon and iron in the steel you're heating. 

A rich, or carburizing flame is cooler as there is unburned fuel in the flame which under some circumstances is okay. It WILL prevent scale production but in return produces carbon Monoxide in the exhaust. That's a B A D thing. 

Sooo, NO a rich flame is not only cooler it's producing the insidious and deadly gas (CO) Carbon Monoxide that's colorless, odorless, tasteless and accumulative in your bloodstream. 

Any propane burner produces CO anyway so you MUST provide good ventilation. Just a LITTLE bit of CO builds in your bloodstream and takes many MANY times as long to flush out. Hospitals treat CO exposure in hyperbaric oxygen chambers and it still takes a long time. Happily CO is a little lighter than air so it tends to drift upwards, especially when more than 2,000f. Still it's not something to make assumptions about, too much ventilation is almost enough.

So, how about some pics of the burners you're using in action. Even the really cheesy ones can usually be made to work. Heck, you should see all the issues guys selling T burners have when they buy one from some knucklehead online.

Frosty The Lucky.