Book - School of Science and Technology

Boiler firing ± oil fuel 283Pressure atomisationIn this arrangement, the oil supply is fed at a controlled rate to a nozzle, the dischargefrom which meets a stream of air (or sometimes steam) at high, medium or low pressure.The primary air quantity so provided is less than that required for combustion and actsprincipally to atomise the fuel.Mechanical atomisationOil is fed, at a controlled rate, typically to the inside surface of a conical cup which isrotated at a high speed. As the oil leaves the edge of the cup under centrifugal force, it isatomised by a primary air stream supplied concentrically around the cup and contrarotatingvis aÁ vis the oil supply. More details of this type are given later.Pressure jetAt a relatively high pressure, oil is supplied to a fine nozzle which is so designed as toapply a swirling motion to the spray of droplets discharged. An air supply, primary ortotal, is provided in a contra-rotating swirl. Additional information regarding this type isgiven later.EmulsificationThis process may be applied to the principle of either pressure atomisation or pressure jet,the difference being that the oil is pre-mixed with a controlled quantity of primary airbefore it is delivered to the nozzle in the form of an emulsion.Combustion airAs noted above, the total air supply to an oil burner falls into two categories: the primaryair which is intimately involved in the atomisation process and the secondary air whichmakes up the balance necessary to complete the combustion reactions, including anyexcess air which may be required.The secondary air may be induced to flow through pre-set registers at the burner front,either by natural draught from the chimney or as a result of a mechanical induced draughtfan at the boiler exit, in which latter circumstances the boiler combustion chamber isunder suction. Alternatively, and as is now more usual practice for applications ofsubstantial size, the secondary air is supplied by a forced draught fan into what thenbecomes a pressurised combustion chamber.It will be seen that there is a variety of possible combinations of atomising methodswith systems of primary and secondary air supply and hence many possible variations inburner design. When fully automatic operation is required, as in the present application,the choice is generally limited to either pressure jet or rotary cup equipment.Gun-type pressure jet burnersBy far the most adaptable type of burner, the gun-type unit, is available in sizes suitablefor boiler outputs ranging from 10 kW to 2.5 MW. Burners of this pattern consist, in thesimplest form, of a direct electrical drive to a centrifugal fan which produces the whole ofthe air required for combustion, including any necessary excess. A positive displacementoil pump is coupled to the fan drive and supplies oil at an absolute pressure of up to about1 MPa (10 bar) to a fine-calibrated jet. Air from the fan is delivered via swirling vanesdisposed round the nozzle following the general pattern shown in Figure 10.20.