Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGTWO-SHELLDOUBLE-EFFECT UNITS.-Two-shell double-effect units of 20,000 gpd capacityare used on some ships. A typical unit ofthis kindisshowninfigurel8-7.Asmay be seen,the unit consists of two cylindrical evaporatorshells, mounted horizontally, with the long axesof the shells parallel. The first- effect vapor feedheater is built into the upper part of the firsteffectshell. The distilling condenser and the distillatecooler are built into separate shells, whichare usually mounted between the two evaporatorshells. The air ejector condenser is also a separateunit, though it is mounted on one of theshells.The operation of the two- shell double- effectunit is almost precisely the same as the operationof the Soloshell double- effect unit. The flowpaths of steam, condensate, sea water, brine,vapor, and distillate may be traced out on figure18-7.THREE-SHELL TRIPLE-EFFECT UNITS.-Three- shell triple- effect distilling units aresimilar to the double-effect units previously discussedexcept that the triple- effect units have anintermediate evaporating stage.A triple- effect distilling unit is shown schematicallyin figure 18-8. Although there are severalkinds of triple-effect units, the general relationshipsshown in this illustration hold for anytriple- effect plant.A standard 20,000 gpd triple-effect unit consistsof three horizontal cylindrical shells, setside by side with their axes parallel. The firstandsecond- effect vapor feed heaters are builtinto the front end of the second- and third- effectevaporator shells. The distilling condenser iscontained within the third- effect shell. The airejector condenser and the distillate cooler arein separate shells and are mounted on thethird- effect shell.Another 20,000 gpd triple- effect design consistsessentially of three horizontal shells boltedtogether end to end, with vertical partition platesbetween each shell to separate the effects. Vaporseparators in independent shells are installed inthe vapor piping between effects and between thethird effect and the distilling condenser. Thefirst- and second- effect vapor feed heaters arein separate shells and are mounted in the pipingat the inlet to the second- effect and third- effecttube nests, respectively. The two sections of thedistilling condenser and the distillate cooler arebuilt into a single shell and independentlymounted as space and piping arrangements maypermit. The air ejector condenser is also aseparately mounted unit.A standard 30,000 gpd triple- effect unit isalso in use. This is similar to the standard 20,000gpd unit except that the 30,000 gpd unit is larger.There are two types of 40,000 gpd tripleeffectunits that may be regarded as standard,since both are widely used in naval ships. Thefirst type uses the same arrangement as thestandard 20,000 gpd triple- effect unit but has thelarger components needed for the increased capacity.The second type consists of three horizontalshells, usually mounted side by side, withaxes parallel. In this design, both vapor feedheaters and distilling condensers are builtas three independent units, each mounted separatelyoutside the evaporator shells. The airejector condenser and the distillate cooler arealso in independent shells and are separatelymounted outside the evaporator shells.Triple- effect units operate in virtually thesame way as the Soloshell and the two-shelldouble- effect units previously described, exceptthat the comparable actions in a triple- effect unitare spread out through more equipment andthrough one more effect. In a triple- effect unit,the sea water feed is piped to the first effectshell, then to the second- effect shell, and then tothe third- effect shell. Steam from the auxiliaryexhaust line is used to vaporize the feed in thefirst- effect shell; in the second- effect and thirdeffectshells, the vapor is generated by the heatgiven up by vapor generated in the previous shell.In the triple- effect units, as in the double- effectunits, this sequence of events is possible becausethe vacuum is greatest in the shell of the finaleffect and least in the shell of the first effect.Flash- Type Distilling UnitsSome recent ships are equipped with flashtypedistilling units. Although these units differsomewhat in design from the submerged tubecertain operating principles are commonunits,to both types. In particular, both the flash-typeand the submerged tube type of unit depends uponpressure differentials between the stages (or effects)to generate vapor from the sea water feed.Flash-type units consist of two or morestages. Two- stage units of 12,000 gpd capacityare installed on some recent destroyer typeships. Five-stage units of 50,000 gpd capacity areinstalled on some recent carriers.Each stage of a flash-type unit has a flashchamber, a feed box, a vapor separator, and a460