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 fluing 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 fluing 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 vapour 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 adaptor. 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. The flue 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 fluing. 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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