Chapter 18. -DISTILLING PLANTS 



EQUALIZER 



SKIRT 



CORRUGATED 

 BASKET TYPE 

 HEATING SECTION 



EVAPORATOR 

 SHELL 



E'g??^ STEAM 



SEA WATER 



47.127 

 Figure 18-12.— Sectional view of evaporatorand 

 basket in vertical basket distilling plant. 



DISTILLING PLANT OPERATION 



Although a detailed discussion of distilling 

 plant operation is beyond the scope of this text, 

 certain operational considerations should be 

 noted. The factors mentioned here apply primar- 

 ily (although not exclusively) to low pressure 

 steam distilling units. 



Naval distilling plants are designed to pro- 

 duce distillate of very high quality. The chloride 

 content of distillate discharged to the ship's tanks 

 must not exceed 0.065 equivalents per million. 

 Any distilling unit which cannot produce distillate 

 of this quality is not considered to be operating 

 properly. 



Steady operating conditions are essential to 

 the satisfactory operation of a distilling unit. 

 Fluctuations in the pressure and temperature of 

 the first- effect generating steam will cause fluc- 

 tuations of pressure and temperature through- 

 out the entire unit. Such fluctuations may cause 

 priming, with increased salinity of the distillate, 

 and may also cause erratic operation of the feed 

 and brine pump. Rapid fluctuations of pressure 

 in the last effect tend to cause priming. 



To achieve satisfactory operation of a distil- 

 ling unit, it is necessary to maintain the designed 

 vacuum in all effects. When the unit is operated 

 at less than the designed vacuum, the heat level 

 rises throughout the unit and there is an 

 increased tendency toward scale formation. 

 Scale fprmation is highly undesirable, since 

 scale interferes with heat transfer and thus 



reduces the capacity of the unit. Excessive scale 

 formation may also impair the quality of the dis- 

 tillate. 



Various methods have been used to retard 

 scale formation in distilling units. In the past, 

 a common method was the continuous injection 

 of a solution of Navy boiler compound and corn- 

 starch into the distilling unit. The boiler com- 

 pound tends to minimize the formation of scale, 

 and the cornstarch tends to minimize priming. 



The boiler compound and cornstarch method 

 of treatment is not fully effective in preventing 

 scale formation, however, and daily removal of 

 scale is required when this method is used. The 

 removal of scale is accomplished by a procedure 

 called chill shocking. For chill shocking, the unit 

 is secured and pumped dry while it is still hot. 

 Then cold sea v/ater is introduced, and the result- 

 ing thermal shock causes scale to flake off and 

 fall tothebottomof the tube nest. The unit is then 

 pumped dry, the loose scale is removed, and the 

 unit is filled with water and started up again. 



Chill shocking is an effective way of removing 

 scale, but it is somewhat laborious and time- con- 

 suming. A particular disadvantage of the chill 

 shocking process is that it requires each operat- 

 ing distilling unit to be out of production for an 

 hour or more each day. This can lead to serious 

 water shortages under some circumstances. 



A new chemical compound called HAGEVAP 

 has been adopted as the standard compound for 

 evaporator feed treatment. This compound has 

 proved superior to the boiler compound and corn- 

 starch previously used, and is now authorized 

 for use in submerged tube, vertical basket, and 

 five-stage flash-type distilling units; it does not 

 appear to be necessary for two- stage flash-type 

 units. Where the HAGEVAP treatment is used, 

 chill shocking is not necessary because there 

 is no scale formation (or practically none). The 

 use of this compound requires the installation of 

 certain equipment, including special pumps, 

 tanks, and piping; authorization for such installa- 

 tion has been issued, and the alteration has been 

 or will soon be made for all classes of ships 

 having low pressure steam distilling plants 

 (other than two-stage flash-type units). 



The concentration of brine (or brine density , 

 as it is called) has a direct bearing on the quality 

 of the distillate. If the brine concentration is too 

 low, there will be a loss in capacity and economy. 



469 



