time. It is very important that the fuel be evenly
typical combustion chamber drain valve is shown
distributed by the spray to prevent the formation
in figure 4-17.
of any hot spots in the combustion chambers. It
When the fuel collects in the drain lines, the
is of particular value for this reason that the spray
drain valve allows the fuel to drain when pressure
be well centered in the flame area of the liners.
in the combustion chamber manifold is reduced
Fuel nozzle types vary between engines; mostly
to near atmospheric pressure. As shown in figure
4-17, the drain valve is spring-loaded in an open
pressure through small orifices in the nozzles. The
position. It is closed as pressure within the
nozzles generally used are of the vaporizing orifice
manifold and lines to the burners increases above
type and include the simplex and the duplex
that of the spring tension trying to keep the valve
configurations. The duplex nozzle usually requires
open. It is imperative this valve be in good work-
a dual manifold and a pressurizing valve or flow
ing condition to drain accumulated fuel after each
divider. This is to divide primary and secondary
shutdown. Otherwise, a HOT START during the
(main) fuel flow, while the simplex nozzle requires
next starting attempt or an AFTERFIRE after
only a single manifold for proper fuel delivery.
shutdown may occur.
SIMPLEX FUEL NOZZLE.-- The simplex
fuel nozzle was the first type of nozzle used in
In jet engines, the fuel spray nozzles function
turbojet engines, but it was replaced in most
installations with the duplex nozzle, which gives
highly atomized, precisely patterned spray. It then
better atomization at starting and idling speeds.
burns evenly and in the shortest possible space and
The simplex nozzle is still being used to a limited
degree. A simplex nozzle is shown in figure 4-19.
Each of the nozzles of the simplex type consists
of a nozzle tip, an insert, and a strainer made of
a fine-mesh screen and a support.
DUPLEX FUEL NOZZLE.-- The duplex fuel
nozzle is the type nozzle most widely used in
present-day engines. Its use requires a flow
divider, which gives a desirable pattern of spray
for combustion over a wide range of operating
pressures. A nozzle of this type is shown in
Figure 4-19.-Simplex fuel nozzle.
Figure 4-20.-Duplex fuel nozzle.