AIR REGISTERS

Figure 4-13.—Fuel-oil mortar. Figure 4-11 shows a refractory-lined furnace. The refractory lining protects the furnace steel casing and prevents the loss of heat from the furnace. The lining also retains heat for a relatively long time and helps to maintain the high furnace temperatures that are needed for complete fuel combustion. Combustion Air Air is forced into the furnace by a forced draft blower. The forced draft blower is a large volume fan that can be powered by an electric motor or a steam turbine. The forced draft blower blows air into the outer casing of the boiler (fig. 4-12). The air then travels between the inner casing and outer casing to the boiler front where it is forced into the furnace through the air registers. The air registers are part of the fuel-oil burner assembly that consists of four main parts: air doors, a diffuser, air foils, and the atomizer assembly. Figure 4-13 shows a side view of a fuel-oil burner assembly. AIR REGISTERS.— The air entering the furnace through the air registers mixes with a fine fuel-oil spray through the atomizer. Figure 4-13 shows the arrangement of an air register in a fuel- oil burner assembly. The air doors are used to open or close the register, as necessary. They are usually kept either fully opened or fully closed. When the air doors are open, air rushes in and is given a whirling motion by the diffuser plate. The diffuser plate causes the air to mix evenly with the atomized oil in such a way that the flame will not blow away from the atomizer (atomizers are burner assembly. discussed in the next paragraph). The air foils guide the major quantity of air so it mixes with the larger particles of fuel oil spray beyond the diffuser. ATOMIZERS.— Atomizers (devices for pro- ducing a fine spray) break up the fuel oil into very fine particles. In the following paragraphs we will briefly discuss the three types of atomizers. These three types are the return-flow atomizer, the steam-assist atomizer, and the vented-plunger atomizer. Return-Flow Atomizer.— The return-flow atomizer provides a constant supply of fuel-oil pressure. Any fuel oil not needed to meet steam demand is returned to the fuel-oil service tank. This is accomplished by the return control valve installed in the piping between the boiler front and the service tank. As the return control valve is closed, more fuel oil is forced through the sprayer plate into the furnace. The return-flow atomizer is shown in figure 4-14. Figure 4-14.—Return-flow atomizer. 4-13