and Kiln Firing
following article by Mark Brabham on kilns and kiln firing was initially
published on Claynet. Part 1 covers the following areas: Check List,
Natural Gas vs L.P. Gas, Safe Kilns and Firings, Safety Devices
for Kilns and Maintaining Burner Safety Controls. Part 2 covers
Temperature Measurement and Control, Color Scale Chart and Industrial
Please note: The following information relates to atmospheric
gas burners only, i.e. burners that rely on atmospheric air, as
distinct from air from an air fan, for combustion.
The chimney passage must not be restricted and the sizing should
be checked carefully first. A rough rule-of-thumb ratio is 1 square
inch of flue area to 8,000 BTU's of maximum gas input. Tight packing
of the ware without a definite passage for the hot gasses can cause
a problem. Increased firing times and reduction may indicate a problem
here. Ensure the burners are mounted outside the burner port openings
and the correct amount of secondary air (the air pulled in around
the burner tip) is available. This can vary between burner types.
Venturi type burners require less wasteful, secondary air other
burners may need larger ports to enable complete combustion.
Make sure the gas supply components such as the feed line, gas
regulator or gas bottles are of a sufficient size. Freezing of any
of these indicates a problem. It is wasteful to undersize components
and have large pressure drops. It is also important to make sure
the burners are assembled correctly with adequate mixing length
between the air inspirating section and the burner flame retaining
It is important to keep the right ratio between the amount of
gas fed to the burners and total air required for combustion (secondary
and primary air) as close as practicable. This is approx. 24 parts
air to 1 part gas for L.P. Gas and 10 parts air to 1 part gas for
natural gas and will make sure the kiln fires to temperature as
quickly as possible using minimum gas. This is only possible during
those periods when reduction or excess air firing is not necessary
(some excess air can be helpful in the earlier stages of a firing).
Altering the chimney damper changes the pressure condition in the
kiln, thereby controlling the amount of secondary and primary air.
Primary air can also be controlled separately on venturi burners
by altering the "wheel" or shutter on the inspirator casting.
The most common problem is the kiln not reaching temperature.
Although there are many possible reasons, the first thing to check
is the gas supply pressure. It is a benefit with L.P. Gas kilns
to have a gauge on the burner as well as the bottle/tank as this
acts as a check to ensure dirt particles have not been trapped in
the line. The pressure on the cylinder is compared to the pressure
at the burner, a large pressure drop indicates a restriction. Ensure
the gas bottle outlet valve is absolutely clean before refitting
the connection after filling.
Next Page > Natural
Gas vs L.P. Gas > 2