mortar. Figure 4-11 shows a refractory-lined
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
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
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
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
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
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.