and Kiln Firing
Safe Kilns And Firings
Many potters have expressed concerns regarding the safety aspect
of gas kilns and the firing procedures. This section looks at ventilation
and flueing requirements. Remember there are regulations that must
be adhered to and you should always check with your local authorities
prior to any installation.
Potentially the most dangerous aspect of firing a gas kiln is insufficient
fresh air and inadequate exhausting of combustion products.
Never locate kilns inside sealed rooms or where access to sufficient
fresh, outside air is not available.
Gas burners require large volumes of oxygen to complete combustion.
Typically a natural gas burner requires for every 1 cubic meter
(cubic foot) of gas burned, 10 cubic meters (cubic feet) of fresh
air. L.P. Gas burners require 2.5 times the air volume. Fresh air
for the kiln should be available using fixed venting at a low level
in the door or wall or through a powered fan. The vent size is sized
by determining the MJ/Hour total burner rating. If the air is drawn
from outside, the rate in our area is 160mm2 per MJ/Hour, therefore
a typical gas kiln with 2 burners with an input of 400MJ/Hour would
require fixed ventilation of at least 250 x 256 mm. If the kiln
is drawing air from an adjacent room the area should be doubled.
Incidentally the maximum hole size if mesh is used is 6mm square.
Note: This is for natural gas burners only, L.P. Gas burners
require 2.5 times the area.
Please note: All gasfitting or flueing work should be undertaken
by a licensed gasfitter.
The products of combustion can contain hazardous gasses and should
be safely exhausted to atmosphere. A natural gas burner using 1
cubic meter of gas will produce 1 cubic meter of carbon dioxide,
2 cubic meters of water vapor and 8 cubic meters of nitrogen. If
the fresh oxygen supply is restricted and the combustion process
is incomplete carbon monoxide will result. This is a lethal gas
even in small quantities. Of course, this is a common occurrence
in reduction firings and the need to be absolutely sure the gasses
are safely removed from the work area is vital.
The chimney (or flue) should be of the correct construction. It
is possible to construct a brick chimney but is usually more economical
to use a metal type with the large variety of connections and accessories
available. A draft terminal can be fitted on top of an existing
brick flue with the right adapter. A metal flue will also reach
operating temperature, or a temperature to create sufficient draw,
quicker than a brick chimney. Generally it is more economical over
the longer term to specify a stainless steel flue. It is not necessary
with gas kilns fitted with venturi burners to have a chimney that
is excessively long. This used to be essential with oil burners
or gas burners that relied on large volumes of primary air (air
required around the burner tip to complete combustion) for combustion
as the chimney height determined the draw available. Flues on modern
downdraft gas kilns are generally terminated near the top of the
kiln, a canopy is fitted over this, a pipe is continued through
the roof and an approved terminal is connected to the outlet. The
canopy is necessary for two main reasons:
The canopy will slow down the draw from the kiln. Too much draw
will make the kiln inefficient through pulling the heat through
the stack before it has completed its task and can contribute to
uneven temperatures. The air the canopy pulls in helps to dilute
the flue gasses, cooling them to ensure the chimney lasts longer
and saving our environment.
It is important the canopy is larger in area than the outlet from
the kiln. This ensures that there is no spillage of gasses. The
canopy should also be a certain shape so that there are not sharp
edges or steps to cause turbulence. This can induce spillage and
effect the passage of gasses. It is a good idea to buy a ready made
unit from companies that have experience in gas flueing. If the
pipe is passing through a roof it is necessary to have a minimum
clearance from any combustible material of 600mm however this can
be reduced if the material is protected with insulation. An approved
flue cowl should be fitted to the end of the stack. This will help
to ensure the wind does not affect the firing, birds stay out of
the chimney and there is not excessive draw from the flue.
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Devices for Kilns > 4