I'm trying to learn, in a methodical way, about the vaporizing burner. It's clear that it's a complex system, with lots of variables. From what I've learned so far, they include at least the following:

1. fuel volatility
2. vaporizer heat input (dependent on length, tube material, cable size & material)
3. vapor temperature at jet
4. pressure at jet
5. jet size
6. distance from jet to mixing tube opening
7. profile of mixing tube opening
8. mixing tube diameter
9. grate opening (total area of holes)
10. grate temperature
11. stack performance - draft

And it seems like there’s a number of items that wouldn’t be used for tuning, but can’t be allowed to be wrong, like alignment of fuel streams with mixing tubes and sealing the burner from air leaks.

Rather than trying to get my arms around the whole system at once, I’d like to start by investigating just one variable. And I’m hoping the following thought experiment will help (especially with input from folks with hands-on experience!).

Suppose a burner is perfectly set up. The flame is the best color, rising the right height from the grate. Now, changing nothing else, increase the diameter of the mixing tubes. What happens to the fire? Is the venturi effect in the tubes weakened, mixing less air with the fuel and richening the mixture? Or does more air get mixed because the tube is wider, and the mixture leans out? Does mixture pressure to the grate change, altering the flame rise?

If the diameter is reduced, with no other changes to the system, will the result be the exact opposite of whatever happens when it's increased?

And a separate, but surely related question - in how much of the mixing tube length does air entrainment occur? I understand that the tube needs to extend to the rear of the burner, under the baffle plates, to promote mixture distribution under the grate. But I’d think the actual mixing occurs only near the mouth of the tube - once you get back far enough, it doesn’t seem like new air could be added. Is the mixing tube diameter important only near the mouth?

Kelly
Mount Joy, PA

Dear Kelly, Making a larger diameter mixing tube without changing anything else will lean out the fuel mixture by allowing more air to enter. The opposite would occur with smaller mixing tubes. The cubic feet of air per minute would be reduced with the smaller tubes. The mixing of air/fuel only occurs in about the first three inches of the mixing tubes, the rest of the tube is for proper air/fuel mixture distribution in the plenum below the burner grate. The mixing tube diameter should remain free from obstructions and no reduction in size for proper flow.

The fuel jets have to be perfectly centered in the mixing tubes to get a proper mixing of the air/fuel mixture and proper distribution. It they are not centered, an uneven burner grate fire, and/or mixing tube fires can result. Burner grate temperature is not critical unless it gets too hot from a mixing tube fire, then heat damage can be done to the grate. The burner grate doesn't get much hotter than the temperature of the air fuel mixture passing through it. The walls of the burner liner get hot enough from radiant heat to melt low temperature metals.

I painted my burner grate in my 606 Stanley with an aluminum plastic spray paint two years ago, and I have put about 1,500 miles on since then. I dropped my burner yesterday and the plastic spray paint looks almost as good as when I sprayed it on the grate two years ago. I had sprayed the aluminum paint on to reflect any radiant heat to keep my burner grate cool. I think that about the only thing that the spray paint did was restrict my slotted burner's flow with it's paint, and it kept the grate from rusting. I cleaned every slot with a broken hacksaw blade that had it's set of the teeth ground off of it.

On a tour on a cold rainy day in September, I added a summer grade heavy sulfur base diesel to my fuel tank because I couldn't find kerosene. The heavy sulfur based diesel had a hard time passing through the burner grate where earlier I didn't have any problem with the kerosene. It was probably the rusty burner grate slots with the added silver paint on top restricting the fuel mixture flow. When I turned on the diesel with my main fuel control valve, only about 3/4 of the diesel fuel mixture would go into the mixing tubes when it was cool. The excess mixture flooded the outside damp air around the outside of the burner. I have to make sure that I do not buy any more of the summer grade diesel for my Stanleys. I then added straight unleaded gasoline and finished the rest of the tour without any more problems.

The distance of the fuel nozzels/jets from the mixing tubes will greatly affect the heat produced by the burner grate fire. Moving them out makes a hotter fire and at highway speed, the steamer can really go. The problem about leaving them out there is when you are standing still, the fuel mixture can easily miss the mixing tubes and find a source for ignition resulting in a tube fire. I have found that for reliable running, the nozzels should not be located any further out than the diameter of the mixing tube. The closer the nozzels are to the mixing tubes, the more reliable the fire is. The closest distance that would be practical is for the nozzels to be located outside just flush with the outside opening of the mixing tubes.



Edited 1 time(s). Last edit at 03/05/2007 04:53PM by JW.