Oil burners used on auxiliary boilers are the mechanical atomizing type. Two types commonly used are the constant-capacity, pressure-atomizing burner and the variable-capacity, pressure-atomizing burner.

Constant-Capacity, Pressure-Atomizing Burners

Constant-capacity, pressure-atomizing burners are designed for electric ignition and automatic control. Basically, these burners consist of a burner cone or plenum chamber connected to the combustion air fan casing, brackets for nozzle and electrodes, and an oil-atomizing nozzle. Variations of boiler load with this type of burner is accomplished by the automatic off and on cycling of the burner. Burners intended for wide variations of boiler steaming loads are designed with dual or triple nozzle arrangements to eliminate excessive cycling. In such burners, one nozzle is used as a pilot, and additional nozzles are manually cut in as required. Standard commercial nozzles are used with burners of this type.

A typical arrangement of a burner and construction of the oil-atomizing nozzle are shown in figure 18-14.

Figure 18-14.-Constant-capacity, pressure-atomizing burner as installed in the boiler front.

Variable-Capacity, Pressure-Atomizing Burners

The atomizing unit of a variable-capacity, pressure-atomizing burner consists of a burner barrel, a nozzle body or distributor sprayer plate, an orifice plate, and an atomizer nut. The capacity of these atomizers is regulated by an oil control valve placed in the oil return line. While the supply of oil to the atomizer is kept at a constant pressure necessary for proper atomization, amounts of oil returned from the atomizer may be varied by increasing or decreasing pressure at the oil control valve. The amount of oil atomized is always the difference between the oil supplied to and the oil returned from the atomizer. An oil metering valve is linked with a mechanism regulating air admission to the burner so that air and oil may be properly proportioned over the entire range of burner operation. The operating principle of this type of atomizer is illustrated in figure 18-15.