[TheForge] hot portable atmospheric forges may need propylene

[Keporter at aol.com]

Yes, atmospheric burners are self limiting, but no, they aren't  limited by 
the amount of oxygen they can draw in (any of these designs can  be tuned to an 
oxidizing flame). The limit is the amount of nitrogen--not  oxygen--found 
within the intake air. Nitrogen contributes nothing to  combustion, and is 
therefore a drag on the flame's energy density. This is  the sole reason that 
oxy/fuel flames are hottest. However, the theoretical limit  of an air/propane flame 
is listed as 3600 degrees, so you might not want to  dismiss them out of 
hand. 
 
Liquid oxygen costs money to use. Quite a bit more money than the cost of  
many fuels. If you want to take the temperature of an existing forge higher  
than you can get by using the best of tube burners (correctly tuned), the  best 
IR coating (water separated ITC #100), and the best exhaust system  (larger 
openings with exterior baffles), then the next logical step is the use  of 
propylene fuel gas, rather than oxygen enrichment. Propylene will cost you  about 40 
percent more than small amounts of propane, but it is still only  about 60 
percent of the cost of MAPP gas; at least in Seattle.  Propylene was rated 
between 25 and 50 degrees of the temperature of MAPP.  However, Air Liquide has 
replaced the propane content in its formula with  propylene, so this can no 
longer hold true. In fact, I'll have to write them now  and find out what the new 
flame temperature of MAPP is supposed to be.
 
I have done a study of oxygen enrichment, and it works great as an  
economical means of raising natural gas flame temperatures.  With twenty percent 
enrichment, you receive about a 1000 degree boost.  That's an excellent investment, 
when you factor in the low cost of NG. The  reason enrichment works so well is 
two fold: First, you get the biggest  temperature increase for oxygen 
enrichment at the lower end of a sliding scale;  the upper end of that scale shows 
only a 50 degree increase for the final  twenty percent oxygen invested. 
Secondly, the fuel it's commonly used  with (natural gas) requires the least amount 
of added oxygen (1.5 to  1) of any fuel but acetylene (1.1 or 1.2 to 1 
depending on whose  figures you use). Using it with propane, you would need 3.5 to 1 
of added  oxygen. Therefore, you can multiply the needed percent of added 
oxygen by the  difference between these figures; at this point switching to 
propylene is  not only looking simpler, but cheaper too. Propylene uses the same 
fittings and  regulators as propane. I have used it during burner tests, and it 
will give the  same comparative heat increase over propane with top flight 
burners or  ancient designs. On the other hand, my interest in propylene is for 
hand torches  and burners being used as hand torches, where ambient air can help 
cool  the SS nozzle. When you start running internal forge temperatures up to 
the  higher ranges for very long, you well most certainly melt the SS nozzles 
right  off your burners, so you want to use a good grade of ceramic to build  
a burner port, with the nozzle shape incorporated,  instead. 
 
I have not bothered with recuperative schemes, because the temperature  boost 
simply isn't worth the added building expense and lack of design  
flexibility, for ordinary forge work. Since, these higher temperatures are  needed for 
some projects, one of you might give them a second look.  There is the Sandia 
forge with plans already existing, but I think an all  ceramic version would be 
the best design. It would probably also work best  with the ceramic nozzles 
everyone will need to operate at the desired  temperatures. However, it will 
either need a tall stack or fan to suck hot air  down into the furnace, and 
neither scheme sounds very compatible with the word  portable. In the meantime 
propylene out on a demonstration site is a very  minor cost compared to say a 
hot-work permit, or even just your  time and travel expenses :-)
Mike P.