On Sat, 11 Dec 2004 07:23:09 -0800, Eric R Snow
wrote:

On Fri, 10 Dec 2004 21:46:40 -0700, "Bill P" wrote:

SNIP

Well, you use less propane -- the orifice is MUCH smaller. But I always
thought a gas stove with 12kbtu burners had 12kbtu burners whether it
was hooked to NG or propane. Go figger.

Grant

Yes, Grant, the orfices are much smaller for CNG. I've been looking for
a long time to find out those sizes as I want to convert a nice little BBQ
to natural gas and nobody can give me a size for the new hole. I'd
think there's a formula or chart to use to go from a given size propane
orfice to go to natural gas....

can anybody help?

TIA.... bILL

Just take out your set of #60 to #80 drills and start with the
smallest until the flame looks right. I did this for a camp stove that
was made to run on propane at the tank pressure. With a regulator
between the tank and stove the flame was quite small. Just going up
one drill at a time it took about 30 minutes to get the right flame.
ERS

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