One reason for the non match is Propane is very often Not Propane.

Liquefied Petroleum gas or LPG - is a mixture of a number of gases
to make a general heat content of real Propane.

In warm states Butane is the word and they switch to LPG / Propane
in the winter as it won't liquefy like Butane will.

My bet is the mixture.

Martin
Loren Amelang wrote:

snip

I like this plan. Since the above question was how to go from propane
to NG, it should work well. Going the other way, it is easy to reach a
point where the amount of air you can get into the burner and mixed
with the gas limits how much you can increase the orifice size.

Pure methane (the major component of NG) has a stoichiometric air-gas
ratio of 9.53 to 1 on a volume basis (the weight ratio is 17.2 to 1).
For air-propane, you need 23.82 to 1 by volume, or 15.25 to 1 by
weight.
http://www.process-heating.com/CDA/ArticleInformation/Energy_Notes_Item/0,3271,84749,00.html

The BTU/cuft for NG is 1012, and for propane is 2516, so you need only
40% as much propane by volume for the same output. Taking 40% * 23.82
/ 9.53 gives almost exactly 1 - theoretically the same amount of air
through the burner, with 40% as much propane, should give the same
output.
http://www.altenergy.com/propaned.htm

Somehow in real life I've never found that to be true. NG is lighter
than air and mixes well with it, but propane is heavier than air and
doesn't want to flow up and out of the burner. Even though propane is
delivered at much higher pressure, it doesn't seem to encourage
airflow as well. The sizes of the holes where the mixture actually
emerges at the base of the flame are as critical as the orifice - too
small and the flame lifts off the burner and goes out; too large and
the flame pops back through the holes and burns at the orifice making
enough black soot to clog the burner.

Not only have I never reached equal heat output converting from NG to
propane, I've usually run into serious usability issues that forced me
to back off to even lower than maximum possible output.

Anybody out there understand why?

Loren


--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder