Page 96 - Introduction to Marine Engineering

Boilers 83

Other boiler arrangements

Apart from straightforward watertube and firetube boilers, other steam
raising equipment is in use, e.g. the steam-to-steam generator, the
double evaporation boiler and various exhaust gas boiler arrangements.

The steam-to-steam generator

Steam-to-steam generators produce low-pressure saturated steam for
domestic and other services. They are used in conjunction with
watertube boilers to provide a secondary steam circuit which avoids any
possible contamination of the primary-circuit feedwater. The arrange-
ment may be horizontal or vertical with coils within the shell which heat
the feedwater. The coils are supplied with high-pressure, high-
temperature steam from the main boiler. A horizontal steam-to-steam
generator is shown in Figure 4.9.

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SHELL

Figure 4.9 Steam-to-steam generator

Double evaporation boilers

A double evaporation boiler uses two independent systems for steam
generation and therefore avoids any contamination between the
primary and secondary feedwater. The primary circuit is in effect a
conventional watertube boiler which provides steam to the heating coils
of a steam-to-steam generator, which is the secondary system. The
complete boiler is enclosed in a pressurised casing.