Steam spray burners of the older types had disadvantages in that they were so designed that there was a tendency for the nozzle to clog with sludge or coke formed from the oil by the heat, without means of being readily cleaned. This has been overcome in the more modern types.
Steam spray burners, as now used, may be divided into two classes: 1st, inside mixers; and 2nd, outside mixers. In the former the steam and oil come into contact within the burner and the mixture is atomized in passing through the orifice of the burner nozzle.
Fig. 28. Peabody Oil Burner
In the outside mixing class the steam flows through a narrow slot or horizontal row of small holes in the burner nozzle; the oil flows through a similar slot or hole above the steam orifice, and is picked up by the steam outside of the burner and is atomized. Fig. 28 shows a type of the Peabody burner of this class, which has [Pg 219] given eminent satisfaction. The construction is evident from the cut. It will be noted that the portions of the burner forming the orifice may be readily replaced in case of wear, or if it is desired to alter the form of the flame.
Where burners of the spray type are used, heating the oil is of advantage not only in causing it to be atomized more easily, but in aiding economical combustion. The temperature is, of course, limited by the flash point of the oil used, but within the limit of this temperature there is no danger of decomposition or of carbon deposits on the supply pipes. Such heating should be done close to the boiler to minimize radiation loss. If the temperature is raised to a point where an appreciable vaporization occurs, the oil will flow irregularly from the burner and cause the flame to sputter.
On both steam and air atomizing types, a by-pass should be installed between the steam or air and the oil pipes to provide for the blowing out of the oil duct. Strainers should be provided for removing sludge from the fuel and should be so located as to allow for rapid removal, cleaning and replacing.
Mechanical burners have been in use for some time in European countries, but their introduction and use has been of only recent occurrence in the United States. Here as already stated, the means for atomization are purely mechanical. The most successful of the mechanical atomizers up to the present have been of the round flame type, and only these will be considered. Experiments have been made with flat flame mechanical burners, but their satisfactory action has been confined to instances where it is only necessary to burn a small quantity of oil through each individual burner.
This system of oil burning is especially adapted for marine work as the quantity of steam for putting pressure on the oil is small and the condensed steam may be returned to the system.
The only method by which successful mechanical atomization has been accomplished is one by which the oil is given a whirling motion within the burner tip. This is done either by forcing the oil through a passage of helical form or by delivering it tangentially to a circular chamber from which there is a central outlet. The oil is fed to these burners under a pressure which varies with the make of the burner and the rates at which individual burners are using oil. The oil particles fly off from such a burner in straight lines in the form of a cone rather than in the form of a spiral spray, as might be supposed.