NATURAL GAS Vs L.P. GAS KILN FIRING

Benefits (or otherwise) of natural gas (your town gas supply) firing compared to L.P.Gas (bottled gas).   Natural gas kilns will easily reach stoneware temperatures. Theoretical flame temperatures between these gasses are actually similar but this is not the important point in this case. The heating value of the gasses are different however, allowing more heat to be liberated per volume of L.P. Gas than natural gas. L.P. Gas has a heating value of 95.5 MJ/m3 (2500 BTU/ft3) and natural gas liberates approx. 38 MJ/m3 (1000 BTU/ft3). All this really means is that we must ensure that the burners, valves and the gas piping for natural gas firing are sized to allow sufficient volume of gas to heat the kiln efficiently. Luckily the costs of the gas are based on the heating value and not the volume consumed. Although it depends on local pricing, generally it is possible to obtain natural gas for about half the cost of bottled gas.

CONVERTING L.P.GAS BURNERS

To successfully modify existing L.P. Gas burners to operate on natural gas, the gas pressure would need to be about the same. Usually the town gas supply is reticulated at low pressure and it is therefore not possible to convert the burners. Natural gas burners have larger valves and burner parts to enable sufficient heat to be liberated. If the gas pressures are the same, a simple jet size change should be sufficient.

MATCHING NATURAL GAS BURNERS TO YOUR KILN

Generally for a given kiln volume we would use more natural gas burners than L.P. Gas burners to obtain the required heat input. This can repay the owner with more even firings. Smaller kilns have successful firings with a single burner provided a large preheat pilot is fitted to assist with a low, steaming rate.

ADVANTAGES OF NATURAL GAS COMPARED TO L.P. GAS

Natural gas is "on tap". Consequently there are no worries about empty or freezing gas cylinders or gas leaks during changeover. Natural gas is lighter than air and if a leak occurs the escaping gas will rise, is rapidly diluted with air and easily dispersed. In contrast L.P. Gas is heavier than air and tends to sink to ground level, will accumulate and may cause an explosion. It is important to treat all gasses with utmost caution. The burners are essentially large bunsen type units and if the gas pressure is low are virtually noiseless in operation. Although the venturi L.P. Gas burners are renowned as much quieter burners than others, there is a noticeable difference with natural gas burners.

DISADVANTAGES

Natural gas burners are a little more difficult to adjust. As the control valves need to be larger to handle the increased volume, they tend to not have the same fine control available with the needle type found on L.P. Gas burners.

Natural gas burners are prone to "flash back" if not adjusted correctly. This usually occurs if the primary air supply (the adjustable wheel on the venturi section) is open too far or the gas is turned down to a point where the flame speed exceeds the escape velocity. Natural gas burners have a turn-down ratio (the difference between maximum and minimum flame) of about 4:1. To overcome this larger pilot burners are installed to provide a separate low rate.

It may be necessary to have the gas meter changed to a larger unit. The gas authorities will usually do this free of charge however you will have to pay for the pipe work to the kiln. Make sure it is sized correctly by a competent person to minimise pressure drop.

It is important to ensure there is the proper ratio between the gas and air (primary and secondary) supplied to the kiln to obtain the most rapid temperature rise. This is more critical for natural gas firing where the gas pressure is fixed. The correct primary air is set by matching the burner tip with the air mixing device. Secondary air is set by ensuring the area around the burner tip is not excessive and the flue (chimney) is correctly sized and has adequate draw. With L.P. Gas it is possible to simply increase the gas pressure to obtain a temperature rise although this can be wasteful of gas.

Although not a disadvantage there are procedures that have to be followed as natural gas installations are covered by a safety code. It will be necessary to ensure the burners have approved safety cut outs, a gas regulator and the chimney is of the correct construction and has an approved terminal. The installation may also be inspected by the local authorities but this will ensure the burners operate correctly (and efficiently) and the kiln is safe.

Of course all of this is immaterial if you are lucky to be out in the countryside but unlucky to be miles away from the nearest Natural gas pipe.

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