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Many designers of oil burners prefer to provide a good refractory lined throat which provides the right kind of flame quality. This is in addition to the spinner vanes provided in the burner to give a spin to the primary air. The secondary air also comes out through the throat and mixes with the flame at such a location that the hydrocarbons will be ensured of oxygen to burn. The main functions of the burner throat are to provide proper shape to the flame, to mix the fuel and air, and to provide the stabilizing heat back to the flame.
Construction & Materials
The burner throat is normally made of high alumina plastic refractory or castable refractory of Grade A is used. This refractory must have a maximum permanent linear change of plus/minus 6 % at 1550 degree C. Crushing strength must be more than 300 kg/cm2 after drying at 110 degree C and cooled to ambient temperature. After heating to 1200 degree C to 3 hours and cooling to ambient temperature the crushing strength should be more than 210 kg/cm2 and at 1750 degree C 275 kg/cm2 . It should be able to with stand a service temperature of not less than 1750 degree C. The bulk density must be 2900 kg /m3 after drying at 110 degree C and cooling to ambient temperature. The Grog size must be maximum 5mm and Ferric Oxide (Fe2O3) should be only 5% max. These properties are only a typical one, however the designers do specify the refractory best suited to their design. Some burner designers use pre-formed ceramic shaped tiles for the burner throat.
While making the burner throat from refractory the following will have to be ensured to achieve maximum life of the burner throat.
- Check the shelf life of the refractory being use, normally refractory material have a shelf life of one year, some may even have as low as three months. Use the refractory only if it is within the shelf life period.
- Mix the refractory with the correct quantity of water specified by the refractory supplier. Using higher or lower quantity of water will not ensure the required life of burner throat.
- Use only soft water of the quality specified by the refractory supplier.
- Use thin plywood while making the throat at the specified intervals and shape to allow for the required expansion gaps.
- Allow the setting time and dry the throat at the specified temperature – normally 110 degree C – and time. Many practice this by doing the throat job just before boiler light up and continue to operate the boiler after drying operation is completed rather than cooling it to ambient temperature.
- It is a good practice to make a template of the throat shape on a thick wood with a center pivot leg for keeping it at the center of the burner and rotating the template to achieve the correct shape of the burner throat. Care must be taken to make the template with the correct inside diameter and curve profile.
The burner throat must be inspected periodically and closely during all boiler outages. The longitudinal cracks seen on the burner throat surface is not uncommon and is not severe. However circumferential cracks are generally an indication that the burner throat has to be repaired or replaced. It is advisable to replace the burner throat if the circumferential cracks are more and deep.