Principles of naval engineering - Historic Naval Ships Association

CHAPTER 18DISTILLINGPLANTSNaval ships must be self-sustaining as faras the production of fresh water is concerned.The large quantities of fresh water requiredaboard ship for boiler feed, drinking, cooking,bathing, and washing make it impracticable toprovide storage tanks large enough for more thana few days' supply. Therefore, all naval shipsdepend upon distilling plants to meet the requirementsfor large quantities of fresh water of extremelyhigh chemical and biological purity.PRINCIPLES OF DISTILLATIONAll shipboard distilling plants not only performthe same basic function but also performthis function in much the same way. The distillationprocess consists of heating sea water tothe boiling point and condensing the vaporto obtain fresh water (distillate). The distillationprocess for a shipboard plant is illustratedvery simply in figure 18-1.At a given pressure, the rate at which seawater is evaporated in a distilling plant isVAPORdependent upon the rate at which heat is transmittedto the water. The rate of heat transfer tothe water is dependent upon a number of factors;of major importance are the temperature differencebetween the substance giving up heat and thesubstance receiving heat, the available surfacearea through which heat may flow, and the coefficientof heat transfer of the substances and materialsinvolved in the various heat exchangersthat constitute the distilling plant. Additionalfactors such as the velocity of flow of the fluidsand the cleanliness of the heat transfer surfacesalso have a marked effect upon heat transfer ina distilling plant.Since a shipboard distilling plant consists ofa number of heat exchangers, each serving oneor more specified purposes, the plant as a wholeprovides an excellent illustration of manythermodynamic processes and concepts. Practicalmanifestations of heat transfer— includingheating, cooling, and change of phase— abound inthe distilling plant, and the significance of thepressure-temperature relationships of liquidsand their vapors is clearly evident. ^The sea water which is the raw material ofthe distilling plant is a water solution of variousminerals and salts. In addition to the dissolvedmaterial, sea water also contains suspendedmatter such as vegetable and animal growths andbacteria and other micro-organisms. Underproper operating conditions, naval distillingplants are capable of producing fresh water whichcontains only minute traces of the chemical andSEA WATER BRINE DISTILLATEHEAT SOURCE75.284Figure 18-1.— Simplified diagram of shipboarddistillation process.Much of the information given in chapter 8 of thistext has direct and immediate apphcation to the studyof distilling plants. Applicable portions of chapter 8should be reviewed, if necessary, as a basis for thestudy of distilling plants.450