[TheForge] Building a propane forge

[theforge@mailman.qth.net]

In a message dated 2/16/2004 1:28:34 PM Pacific Standard Time, 
schade@acegroup.cc writes:
What is a "gas accelerator"?

Bob
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Accelerators
Called by many an injector, the gas accelerator typically consists of a gas 
tube, and a MIG tip, screwed together and sealed. The gas tube has standard 
male pipe thread on one end in order to be easily connected to gas appliance 
fittings. On the "business end" it has internal thread to match the MIG tip 
chosen. The best tips are Tweco 14T series, because they are full length (1 1/2"), 
have a tapered funnel at the orifice entrance (threaded end) and an exterior 
taper for laminar airflow.
  The job of the accelerator is to trade gas pressure for momentum in the 
escaping gas molecules the long narrow tunnel provides extended time to 
accelerate the gas molecules, which the hole in a plate does not. Furthermore, just 
like a rifle barrel it improves the "aim" of the accelerated molecules into a 
coherent high speed gas stream. This is ideal for entraining air molecules with 
the venturi effect. The ideal stoichometric mixture for propane and air is 
twenty-four parts air to one part gas. Older gas burners barely reach this level. 
With twenty to one being considered acceptable. An accelerator can so far 
exceed this mixture that the burner must be choked back to the ideal mix.
However, entrainment is only the beginning of the picture. Straight pipe 
burners do something that the classic old wasp-waist burners hadn't 
considered--they swirl the gas air mixture, promoting better mixing, and allowing high 
mixture speeds at the nozzle. Swirl and speed are important to remember; we will 
come back to them.
   The first requirement of any hydrocarbon burner is to achieve a neutral 
flame. Both reducing (fuel rich) and oxidizing (oxygen rich) flames waste heat 
potential, and create hazardous amounts of carbon monoxide.
    But there are other requirements too. The flame has to be great enough to 
do useful work, and the burner needs a reasonable turndown range. The 
turndown range is the amount that a burner can be backed off from its highest setting 
without losing the ability to achieve a neutral flame, or even worse, blowing 
out.
    Finally, total or near total primary flame combustion of propane has been 
an unfilled desire but will be considered a requirement from now on. 
Ordinarily, propane combusts as much or more in the secondary flame as in the primary. 
The primary flame is the one which is considered important to industry, as is 
demonstrated by the fact that you will find all fuel gas heating charts list 
the primary flame temperature first, and very few of them list the secondary 
flame temperature at all. This brings us back to swirl and speed of the burner 
gases.
   Propane is a vapor, not a true gas. It forms miniature clumps that must be 
broken down into individual molecules for proper mixing and combustion. When 
air is entrained into a straight pipe, it begins to swirl at the intakes and 
moving down the tube that swirl is increased in direct proportion to the 
mixture's speed, which is ultimately a function of the gas stream speed and force.
  A really good accelerator can be combined with other factors to create a 
gas/air speed at the nozzle that is capable of feeding a very fierce flame. Such 
a fierce flame that you can watch the explosions ripping back and forth along 
an extended wave front, which is continuously ignited from the center 
outward, which is normal, and at the same time from its periphery inward. You can see 
the accelerated and superheated oxygen and fuel molecules (that normally 
escape the primary flame, and which then spread out as they mix with air to form a 
very problematic secondary flame) being caught up with and ignited in an 
expanded and jagged looking primary flame. Result -- total primary flame ignition. 
Well, it would be more accurate to say you can see their trail as an after 
image on your retina. Your mind then forms a conglomerate image out of the 
collage of incoming data.
  So, unless you're the mad scientist type, who glories in such a sight for 
its own sake, what could this mean to you? It means an added temperature input 
in any ceramic forge capable of turning a zirconia coating incandescent and 
causing a secondary heat source to recycle the flame heat as infrared radiation, 
instead of seeing CO rich blue flames exiting your forge. The forgoing has 
been the result of my observations and reading, However, a combustion engineer 
might very well take exception to my theories (but not the results). No doubt I 
will receive much scolding during the coming months. That's OK, Larry Zoeller 
has already forced a modification of my most dearly held belief. He managed 
to make a gas tube good enough get around my insistence on the full 1 1/2" MIG 
tip length, at least in the larger burners. It won't work in the 1/2" hand 
torch size.
Mikey


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