Page 321 - Practical Ship Design

Machinery Selection 279

Assist power for . - General service
-
- M.P. drum

Automatic speed
changing clutch
mechanism
--- -_
Exhaust gas
-- Charging air
economizer

/
Reduction gear

Fig. 9.5. Schematic for electrical generation and propulsion power augmentation by waste heat
utilisation.

small sea load of this type of ship. Under these circumstances the exhaust waste
heat may become a scarce resource and “other heat” demands should, if possible,
be met by the use of the heat in cooling water or lubricating oil systems.
When the electrical power which can be developed from waste heat is more than
that needed to meet the normal sea load, it becomes possible to use this power
either to increase the ship’s speed or to reduce the power drawn from the main
engine, thereby reducing the fuel consumption. A schematic for this is shown in
Fig. 9.5.
It may be worth noting that improvements in the efficiency of diesel engines
have reduced, and will no doubt continue to reduce, the waste heat available and
will in general also tend to reduce its quality by increases in the mass flow and
reductions in the temperature, which can now be as low as 150°C. That there is still
a large quantity of energy available is shown in Fig 9.6.

9.5.3 Shaft-driven alternators

If the electrical power required cannot be produced from waste heat, the next best
thing is to produce it by burning fuel oil rather than diesel oil, but unfortunately the