Page 105 - Introduction to Marine Engineering

92 Boilers

To burners
Figure 4.15 Boiler fuel-oil supply system

Fuel burning

The high-pressure fuel is supplied to a burner which it leaves as an
atomised spray (Figure 4.16). The burner also rotates the fuel droplets
by the use of a swirl plate. A rotating cone of tiny oil droplets thus leaves
the burner and passes into the furnace. Various designs of burner exist,
the one just described being known as a 'pressure jet burner' (Figure
4.16(a». The 'rotating cup burner' (Figure 4.14(b)) atomises and swirls
the fuel by throwing it off the edge of a rotating tapered cup. The 'steam
blast jet burner', shown in Figure 4.16(c), atomises and swirls the fuel by
spraying it into a high-velocity jet of steam. The steam is supplied down
a central inner barrel in the burner.
The air register is a collection of flaps, vanes, etc., which surrounds
each burner and is fitted between the boiler casings. The register
provides an entry section through which air is admitted from the
windbox. Air shut-off is achieved by means of a sliding sleeve or check.
Air flows through parallel to the burner, and a swirler provides it with a
rotating motion. The air is swirled in an opposite direction to the fuel to
ensure adequate mixing (Figure 4.17(a)). High-pressure, higb-0i»tput
marine watertube boilers are roof fired (Figure 4.17(b)). This enables a
long flame path and even heat transfer throughout the furnace.