Page 155 - Introduction to Marine Engineering

Auxiliaries 141
cooling water is shut off and the end covers removed. Any seepage into
the tubes will indicate the leak. It is also possible to introduce fluorescent
dyes into the shell-side liquid: any seepage will show under an ultraviolet
light as a bright green glow. Leaking tubes can be temporarily plugged
at each end or removed and replaced with a new tube.
Plate-type coolers which develop leaks present a more difficult problem.
The plates must be visually examined to detect the faulty point. The
joints between the plates can present problems in service, or on assembly
of the cooler after maintenance.
Where coolers are out of use for a long period, such as during surveys
or major overhauls, they should be drained on the sea water side,
flushed through or washed with fresh water, and left to dry until
required for service.

Heaters
Heaters, such as those used for heavy oil, are shell and tube type units,
similar in construction to coolers. The heating medium in most cases is
condensing steam.

Distillation systems

Distillation is the production of pure water from sea water by
evaporation and re-condensing. Distilled water is produced as a result of
evaporating sea water either by a boiling or a flash process. This
evaporation enables the reduction of the 32000 parts per million of
dissolved solids in sea water down to the one or two present in distilled
water. The machine used is called an 'evaporator', although the word
'distiller' is also used.

Boiling process
Sea water is boiled using energy from a heating coil, and by reducing the
pressure in the evaporator shell, boiling can take place at about 60°C.
The sea water from the ship's services is first circulated through the
condenser and then part of the outlet is provided as feed to the
evaporation chamber (Figure 7.6). Hot diesel engine jacket water or
steam is passed through the heater nest and, because of the reduced
pressure in the chamber, the sea water boils. The steam produced rises
and passes through a water separator or demister which prevents water
droplets passing through. In the condensing section the steam becomes
pure water, which is drawn off by a distillate pump. The sea water feed is
regulated by a flow controller and about half the feed is evaporated. The