Book - School of Science and Technology

284 Boilers and firing equipmentNozzleIgnition sparkelectrodesFanOil pumpTransformerFigure 10.20 Gun-type pressure jet oil burnerThe air quantity supplied by the fan is set by slots or dampers at the inlet and the oilquantity delivered is adjusted by control of the pump output pressure and the size of thejet orifice. The flame shape produced by the burner may be varied to suit the geometry ofthe boiler combustion chamber by changing the angle of the swirling vanes and byselecting the angle of divergence of the jet orifice.When the fuel used is light oil (class C2 or class D), there is no requirement forpreheating. For heavier grades, an electrical heater is incorporated as part of the burnerunit to raise the oil temperature to between 60 and 70 C. Ignition is by spark between twoelectrodes located near the nozzle tip which have a high tension feed from a transformer.These suffice for smaller size burners and lighter oils but, in other instances, a two-stageprocess for ignition is used via a gas supply, the electrodes igniting a gas pilot and the pilotigniting the oil.Rotary cup burnersThe principal components of this type of burner are illustrated in Figure 10.21, thesecondary air quarl shown being at the point where the burner is mounted to the frontof the boiler combustion chamber. Drive from the motor is not only to the primary air fanand the oil pump but also, via a hollow shaft, to the spinning cup atomiser. Oil is deliveredthrough this shaft, which rotates at about 100 rev/s, to the cup. The primary air supplyfrom the fan passes through an annular chamber having internal swirling vanes to bedelivered by nozzles circumferential to the cup.The secondary air required for combustion is introduced at a lower pressure by afurther forced draught fan, usually an axial flow type, into the burner box where it canbe preheated to some extent by radiation from the quarls. Characterised dampers areprovided for both primary and secondary air and are linked to a metering valve in the oilsupply.