DISTILLING PLANTS

Fresh water is needed aboard ship for boiler/steam generator feed, drinking, cooking, bathing, washing, and cleaning. Therefore, a naval ship must be self-sufficient in producing fresh water. Space limitations permit only enough storage tank capacity for a couple of days supply. The ship, therefore, depends on distilling plants to produce fresh water of high purity from seawater. As an EN3, you will be required to operate and to maintain various types of distilling plants.

PRINCIPLES OF DISTILLATION

The principle by which distilling plants produce fresh water from seawater is quite simple. There are several different types of distilling plants. Each may appear very com-plicated at first, but they all work on the same basic principles. When water is boiled, it gives off steam vapor that is relatively free of sea salts and minerals. The distillation process heats seawater to the boiling point and condenses the vapor (steam) into fresh water. This leaves behind the impurities of the seawater. The process for a shipboard plant is illustrated very simply in figure 15-1. Notice that the seawater after boiling is identified as brine.

Seawater is a solution of water and various minerals and salts. Seawater also contains suspended matter, such as marine vegetable growths, tiny marine animals, bacteria, and other microorganisms. When properly operated, naval distilling plants produce fresh water that contains only slight traces of chemical salts and no biological contaminants (bacteria that is harmful to humans).

Distilling plants are not effective, however, in removing volatile gases or liquids that have a lower boiling point than water. These dissolved gases and liquids will simply boil into the vapor and be combined with the fresh water (distillate).

Figure 15-1.-Simplified diagram of the shipboard distillation process.