Chapter 18. -DISTILLING PLANT 



biological contaminants which are found natu- 

 rally in sea water. 2 



One of the problems that arises in the distil- 

 lation of sea water occurs because some of the 

 salts present in sea water are negatively solu- 

 ble—that is, they are less soluble in hot water 

 than they are in cold water. A negatively soluble 

 salt remains in solution at low temperatures but 

 precipitates out of solution at higher tempera- 

 tures. The crystalline precipitation of various 

 sea salts forms scale on heat transfer surfaces 

 and thereby interferes with heat transfer. In 

 naval distilling plants, this problem is partially 

 avoided by designing the plants to operate under 

 vacuum or (in the case of one type of plant) at 

 approximately atmospheric pressure. 



The use of low pressures (and therefore low 

 boiling temperatures) has the additional advan- 

 tage of greater thermodynamic efficiency than 

 can be achieved when higher pressures and tem- 

 peratures are used. With low pressures and tem- 

 peratures, less heat is required to make the sea 

 water boil and less heat is lost overboard through 

 the circulating water that cools and condenses the 

 vapor. 



DEFINITION OF TERMS 



The manner in which the various kinds of dis- 

 tilling plants accomplish the distilling process 

 can best be understood if we first become famil- 

 iar with certain terms relating to the process. 

 The terms defined here relate basically to all 

 types of distilling plants now in naval use. Ad- 

 ditional terms that apply specifically to a par- 

 ticular type of distilling unit are defined as 

 necessary in subsequent discussion. 



Distillation. — The process of boiling sea 

 water and then cooling and condensing the result- 

 ing vapor to produce fresh water. 



Evaporation. — The first part of the process of 

 distillation. Evaporation is the process of boiling 

 sea water in order to separate it into fresh water 

 vapor and brine. 



It should be noted that distilling plants are not effec- 

 tive in removing volatile gases or liquids which have 

 a lower boiling point than water, nor are they effective 

 in killing all micro-organisms. These points are of 

 particular importance when a ship is operating in con- 

 taminated or polluted waters, as discussed at the end 

 of this chapter. 



Condensation. — The latter part of the process 

 of distillation. Condensation is the process of 

 cooling the vapor to produce usable fresh water. 



Feed. — The sea water which is the raw ma- 

 terial in the distillation process. 



Vapor.— The product of the evaporation of sea 

 water. The terms vapor and fresh water vapor 

 are used interchangeably. 



Distillate.— The product resulting from the 

 condensation of the fresh water vapor produced 

 by the evaporation of sea water. Distillate is also 

 referred to as condensate , as fresh water , as 

 fresh water condensate, and as sea water distil- 

 late. However, the use of the term condensate 

 should be avoided whenever there is any possibil- 

 ity of confusion between the condensate of the 

 distilling plant and the condensate that results 

 from the condensation of steam in the main and 

 auxiliary condensers. In general, it is best to 

 use the term distillate when referring to the 

 product resulting from the condensation of vapor 

 in the distilling plant. 



Salinity. — The concentration of salt in water. 



Brine. — Water in which the concentration of 

 salt is higher than it is in sea water. 



TYPES OF DISTILLING UNITS 



Distilling units installed in naval ships are 

 of two general types. The vapor compression type 

 of unit is used aboard submarines and small 

 diesel-driven surface craft where the daily re- 

 quirements do not exceed 4000 gallons per day 

 (gpd). The low pressure steam distilling unit is 

 used aboard all steam-driven surface ships and 

 on nuclear submarines. The major difference be- 

 tween the two types of distilling units is in the 

 kind of energy used to operate the unit. Vapor 

 compression units use electrical energy; steam 

 distilling units use auxiliary exhaust steam. 



VAPOR COMPRESSION DISTILLING UNITS 



A vapor compression distilling unit is shown 

 in cutaway view in figure 18- 2 and schematically 

 in figure 18-3. The unit consists of three main 

 components— the evaporator, the compressor, 

 and the heat exchanger— and a number of acces- 

 sories and auxiliaries. 



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