Vince Pitelka on mon 29 apr 96
I had hoped that Marc Ward would jump in on this one, and he still might, but
here's my slant on gas pressure and firing. For your four 75,000 BTU burners
you may need 3.0 lbs. of gas pressure, but other systems run fine on far less.
I had a 100 cu.ft. soft-brick car kiln with six tube burners with 3/16"
orifices, and initially PG&E gave me the standard household 7 W.C.I. (water
column inches) natural gas. The first bisque was slow, and I could tell I'd
have trouble high-firing, so PG&E cranked the pressure up to 10 W.C.I., and
after that my ^10 firings never took more than 10 hours. In comparison to that
kiln, your 3.0 lbs. of pressure is pretty high.
In contrast, our 60 cu.ft. hardbrick salt kiln used to have four #4 Ransome
venturis operating on 15 lbs. of propane pressure, and was severely
underpowered. Firings were very long, and the hot-face suffered. Now the kiln
has four humongous Gas Appliance Company venturis, still operating on max. 15
lbs. propane, and firing times are reasonable.
Another example. Last summer we built a 50 cu.ft. soft-brick downdraft, and
used two Dedell/Ward (I don't know which) power burners that we had on hand,
operating on the 11 W.C.I. we have in the kiln room. The first firing was by
the seat of our pants, and after body reduction I turned the gas up to 8 W.C.I.
An hour later I walked past the kiln and could tell by the burner ports that
things were much hotter than I would have expected. I checked the cones and
^9 was bending. I immediately turned the gas back to 4 W.C.I., but the firing
still finished much too fast and the results sucked. Now, for an eight to
ten-hour ^10 reduction firing, we never turn the gas above 2 W.C.I. It is
obvious that as they are now, these burners are grossly oversized for this
kiln. This summer I will tear apart the burners and replace the orifice tube
and drill smaller orifices, so that we can use more of the line-pressure-range,
giving us more sensitivity in adjustments.
That 100 cu.ft. car kiln was one of the best kilns I have ever fired. Those
tube burners, salvaged from a bank of 15 of them on an industrial boiler,
operated with just a soft hiss, and during glaze-reduction you could hear a
very gentle rumbling within the kiln. Everyone tends to go for venturis or
power burners, but in a natural draft kiln with good stack-height and
appropriate burner-port inserts, guaranteeing adequate secondary air velocity,
a simple set of tube burners with flame-retention tips can be mighty fine.
Vince Pitelka - firstname.lastname@example.org
Appalachian Center for Crafts - Tennessee Technological University