PRINCIPLES OF NAVAL ENGINEERING 



TWO-SHELL DOUBLE-EFFECT UNITS.- 

 Two-shell double-effect units of 20,000 gpd ca- 

 pacity are used on some ships. A typical unit of 

 this kindisshowninfigurel8-7.Asmay be seen, 

 the unit consists of two cylindrical evaporator 

 shells, mounted horizontally, with the long axes 

 of the shells parallel. The first- effect vapor feed 

 heater is built into the upper part of the first- 

 effect shell. The distilling condenser and the dis- 

 tillate cooler are built into separate shells, which 

 are usually mounted between the two evaporator 

 shells. The air ejector condenser is also a sep- 

 arate unit, though it is mounted on one of the 

 shells. 



The operation of the two- shell double- effect 

 unit is almost precisely the same as the opera- 

 tion of the Soloshell double- effect unit. The flow 

 paths of steam, condensate, sea water, brine, 

 vapor, and distillate may be traced out on figure 

 18-7. 



THREE-SHELL TRIPLE-EFFECT UNITS.- 

 Three- shell triple- effect distilling units are 

 similar to the double-effect units previously dis- 

 cussed except that the triple- effect units have an 

 intermediate evaporating stage. 



A triple- effect distilling unit is shown sche- 

 matically in figure 18-8. Although there are sev- 

 eral kinds of triple-effect units, the general re- 

 lationships shown in this illustration hold for any 

 triple- effect plant. 



A standard 20,000 gpd triple-effect unit con- 

 sists of three horizontal cylindrical shells, set 

 side by side with their axes parallel. The first- 

 and second- effect vapor feed heaters are built 

 into the front end of the second- and third- effect 

 evaporator shells. The distilling condenser is 

 contained within the third- effect shell. The air 

 ejector condenser and the distillate cooler are 

 in separate shells and are mounted on the 

 third- effect shell. 



Another 20,000 gpd triple- effect design con- 

 sists essentially of three horizontal shells bolted 

 together end to end, with vertical partition plates 

 between each shell to separate the effects. Vapor 

 separators in independent shells are installed in 

 the vapor piping between effects and between the 

 third effect and the distilling condenser. The 

 first- and second- effect vapor feed heaters are 

 in separate shells and are mounted in the piping 

 at the inlet to the second- effect and third- effect 

 tube nests, respectively. The two sections of the 

 distilling condenser and the distillate cooler are 

 built into a single shell and independently 

 mounted as space and piping arrangements may 



permit. The air ejector condenser is also a 

 separately mounted unit. 



A standard 30,000 gpd triple- effect unit is 

 also in use. This is similar to the standard 20,000 

 gpd unit except that the 30,000 gpd unit is larger. 



There are two types of 40,000 gpd triple- 

 effect units that may be regarded as standard, 

 since both are widely used in naval ships. The 

 first type uses the same arrangement as the 

 standard 20,000 gpd triple- effect unit but has the 

 larger components needed for the increased ca- 

 pacity. The second type consists of three hori- 

 zontal shells, usually mounted side by side, with 

 axes parallel. In this design, both vapor feed 

 heaters and distilling condensers are built 

 as three independent units, each mounted sepa- 

 rately outside the evaporator shells. The air 

 ejector condenser and the distillate cooler are 

 also in independent shells and are separately 

 mounted outside the evaporator shells. 



Triple- effect units operate in virtually the 

 same way as the Soloshell and the two-shell 

 double- effect units previously described, except 

 that the comparable actions in a triple- effect unit 

 are spread out through more equipment and 

 through one more effect. In a triple- effect unit, 

 the sea water feed is piped to the first effect 

 shell, then to the second- effect shell, and then to 

 the third- effect shell. Steam from the auxiliary 

 exhaust line is used to vaporize the feed in the 

 first- effect shell; in the second- effect and third- 

 effect shells, the vapor is generated by the heat 

 given up by vapor generated in the previous shell. 

 In the triple- effect units, as in the double- effect 

 units, this sequence of events is possible because 

 the vacuum is greatest in the shell of the final 

 effect and least in the shell of the first effect. 



Flash- Type Distilling Units 



Some recent ships are equipped with flash- 

 type distilling units. Although these units differ 

 somewhat in design from the submerged tube 

 units, certain operating principles are common 

 to both types. In particular, both the flash-type 

 and the submerged tube type of unit depends upon 

 pressure differentials between the stages (or ef- 

 fects) to generate vapor from the sea water feed. 



Flash-type units consist of two or more 

 stages. Two- stage units of 12,000 gpd capacity 

 are installed on some recent destroyer type 

 ships. Five-stage units of 50,000 gpd capacity are 

 installed on some recent carriers. 



Each stage of a flash-type unit has a flash 

 chamber, a feed box, a vapor separator, and a 



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