Before getting into the discussion of the process of distillation, you should familiarize yourself with the terms defined in the following paragraphs. These terms apply basically to all types of distilling plants now in naval service.

DISTILLATION-The process of boiling seawater and then cooling and condensing the resulting vapor to produce fresh water.

EVAPORATION-The process of boiling seawater to separate it into freshwater vapor and brine. Note that evaporation is the first half of the process of distillation.

CONDENSATION-The process of cooling the freshwater vapor produced by evaporation to produce usable fresh water. Note that condensation is the second half of the process of distillation.

FEED-Seawater, which is the raw material of the distilling unit; also called SEAWATER

FEED or EVAPORATOR FEED. Be careful how you use these terms. Do not confuse the feed (water) for the distilling plants with the feed (water) for the boilers. Feed for a distilling plant is nothing but raw seawater. Feed for the boilers or steam generators is distilled water of very high purity.

VAPOR-The product of the evaporation of seawater feed.

DISTILLATE-The product (also called fresh water) that results from the condensation of the steam (vapor) produced by the evaporation of seawater.

POTABLE WATER-Water that is safe for human consumption. Potable water is fresh water (distillate) that has been treated with a chemical disinfectant to kill harmful bacteria while the water is in the storage tanks.

BRINE-As seawater feed is evaporated in the distilling plant, the concentration of chemical salts in the remaining seawater feed becomes greater. Any water in which the concentration of chemical salts is higher than it is in seawater is called brine.

SALINITY-The concentration of chemical salts in water is called salinity, which increases the ability of water to conduct electrical current. This conductivity is measured by electrical devices, called salinity cells; salinity is indicated in units of either epm (equivalents per million), ppm (parts per million), or micromhos/cm.

EFFECT-In a distilling plant, an effect is that part of a unit where a distillation process occurs. For example, the first place where boiling (or evaporation) of feed into vapor occurs is in the first effect. Distilling plants may be of the single effect design or the multiple effect types (two, three, four, or five effects). This means that the feed is boiled more than once within the plant. An effect may also be referred to as a STAGE.

SATURATED STEAM-Water in a gaseous state that is in contact with the liquid from which it was boiled at a given pressure and temperature. The properties of saturated steam are defined in table 15-1 according to pressure and temperature.

SUPERHEATED STEAM-Vapor that is not adjacent or next to its liquid source and that has been heated to a temperature above its saturation temperature.

DEGREE OF SUPERHEAT-The temperature difference of a superheated vapor between its saturation temperature and its existing temperature for a given value of pressure.

Lets take an example where the steam pressure at the outlet of an orifice in the auxiliary exhaust steam piping system is 16 in.Hg and the temperature of the steam is 240F (116C).

Table 15-1 gives the properties of saturated steam. Look in the column labeled Vacuum Inches of Hg Gauge and find 16.69 and 15.67. By interpolation (estimation), we find the saturation temperature (at the right) to be 176F (80C).

However, the auxiliary exhaust steam is approximately 240F (116C). In this case, then, there is about 64F (36C) of superheat in the incoming steam (240F - 176F = 64F).