Page 46 - Introduction to Marine Engineering

Diesel engines 33

Seawater pumps I Central poolers

[~j Freshwater - low temperature
§§i| Seawater
S2I Freshwater- high temperature
Figure 2.20 Central cooling system

control unit which operates the regulating valve to maintain the desired
temperature setting. A temperature sensor is also used in a similar
control circuit to operate the regulating valve which controls the
bypassing of the central coolers.
It is also possible, with appropriate control equipment, to vary the
quantity of sea water circulated by the pumps to almost precisely meet
the cooler requirements.

Starting air system

Diesel engines are started by supplying compressed air into the cylinders
in the appropriate sequence for the required direction. A supply of
compressed air is stored in air reservoirs or 'bottles' ready for immediate
use. Up to 12 starts are possible with the stored quantity of compressed
air. The starting air system usually has interlocks to prevent starting if
everything is not in order.
A starting air system is shown in Figure 2.21. Compressed air is
supplied by air compressors to the air receivers. The compressed air is
then supplied by a large bore pipe to a remote operating non-return or
automatic valve and then to the cylinder air start valve. Opening of the