search  current discussion  categories  kilns & firing - burners 

burner pressure and firing(long)

updated tue 30 apr 96 on tue 30 apr 96

Hey folks,

Vince wanted me to jump in on this thread....I'll apologize in advance if
I've missed some relevant post concerning this subject...haven't been reading
most of ClayArt lately.....gratefully, I'm very busy.
I see some of the posts in this thread talking about pressure-how much one
uses-what one sets the pressure to on their gauges, but nobody mentions what
orifice size they use with these pressures. There are two parts (literally)
to the BTU equation. One deals with orifice size, the other part with
pressure. If you don't know what orifice size you have in your burners, then
pressure changes are just so much guess work. Okay, I'm going to get a little
preachy here;
Potters don't usually guess about how much cobalt they're dumping in a
glaze. They don't guess about what temperature the kiln is at. They don't
guess at what temperature their clay matures. They don't guess when to send
their slides in for a jury. They don't guess...well, you get my point. So why
do most potters guess about the amount of BTU's they need to fire a kiln?
What pressure they're using? What size orifice is in the burner? Here comes
the preachy part; They aren't taught the first thing about it in school.
Their teachers weren't taught either! Many art programs teach the equivalent
of eating, but don't teach the first thing about cooking. There, I've vented.
Let me say that there are lots of good schools with conscientious teachers
who cover ALL the basic things their serious students should know about clay.
There are far too many, though, that act as if the technical side of clay is
of no importance. That's fine if you want a feel good, big vocabulary, art
education, just don't then hold that the next step is to go out and make a
living from clay. Students from this kind of environment are not giving a
full deck of cards to use and soon realize they didn't get the education they
thought they or their parents were paying for.... I guess I wasn't finished
Now, potters don't need to be able to give a dissertation on orifice
coefficient or explain how a quadratic equation affects gas pressure, but
they should be able to use an orifice chart, know the gas pressure they are
using, and determine/know what the BTU output for their burners. If you drive
a car, you should know how to change a flat tire, but you don't need to know
how to rebuild the transmission.
Back to the matter at hand.... Vince is right when he says that high
pressure doesn't automatically mean greater BTU output. It's how big the hole
(orifice) is AND what pressure is running through it that means something.
I've designed and built burners that fired with an output of 4 million BTU's
operating on 7" wc. (1/4 PSI). I also sell Venturi burners that put out
250,000 BTU @ 25 PSI. That's 16 times LESS BTU's with 100 times MORE
pressure. Without boring everyone with technical gobbledygook (I know I'm
already running that particular risk), increases in pressure do not change
BTU output as much as changing orifice size. Doubling the area of an orifice
will double the BTU's. Doubling the pressure will not double the output. This
is because of the two parts to the BTU equation I mentioned earlier. The part
of the equation that concerns orifice size is linear (straight
multiplication), the part that involves pressure is quadratic (involves a
square root).
OK, if you've read this far, your scroll button is stuck or you're
desperate. If your desperate, chances are the above might not mean a whole
lot because it doesn't answer your problem. If you kiln is stalled or is just
too damn slow, the usual problem is not enough BTU's going into the kiln.
Feel free to request a catalog (no charge). In it you'll find a data guide
that shows you how many BTU's per cubic foot you need to reach a certain
temperature in different types of kilns. (Some of the basics that should be
taught and a lot of times aren't). After you have that info., whether you get
it from me or not, then you can tackle the question of just how much heat
you're getting from your burners and how to remedy the situation of a stalled
or slow kiln. I feel it's prudent to spend as much time on knowing what your
burners are doing as you spend on a few glaze tests. I also have orifice
charts (not free- $8.00 - I crunched lots'o numbers to devise it) that has
pressure ranges from 4"wc to 25 PSI, orifice sizes in numbered, lettered and
fractional from #80 to 1/2". It also has an altitude adjustment between sea
level and up to 11,000 ft. I stuck the crass commercialism at the bottom
hoping to throw off the net police.

Marc Ward
Ward Burner Systems
PO Box 333
Dandridge, TN 37725
423.397.2914 voice
423.397.1253 fax