Principles of naval engineering - Historic Naval Ships Association

Chapter 18. -DISTILLING PLANTbiological contaminants which are found naturallyin sea water. 2One of the problems that arises in the distillationof sea water occurs because some of thesalts present in sea water are negatively soluble—thatis, they are less soluble in hot waterthan they are in cold water. A negatively solublesalt remains in solution at low temperatures butprecipitates out of solution at higher temperatures.The crystalline precipitation of varioussea salts forms scale on heat transfer surfacesand thereby interferes with heat transfer. Innaval distilling plants, this problem is partiallyavoided by designing the plants to operate undervacuum or (in the case of one type of plant) atapproximately atmospheric pressure.The use of low pressures (and therefore lowboiling temperatures) has the additional advantageof greater thermodynamic efficiency thancan be achieved when higher pressures and temperaturesare used. With low pressures and temperatures,less heat is required to make the seawater boil and less heat is lost overboard throughthe circulating water that cools and condenses thevapor.DEFINITION OF TERMSThe manner in which the various kinds of distillingplants accomplish the distilling processcan best be understood if we first become familiarwith certain terms relating to the process.The terms defined here relate basically to alltypes of distilling plants now in naval use. Additionalterms that apply specifically to a particulartype of distilling unit are defined asnecessary in subsequent discussion.Distillation. — The process of boiling seawater and then cooling and condensing the resultingvapor to produce fresh water.Evaporation. — The first part of the process ofdistillation. Evaporation is the process of boilingsea water in order to separate it into fresh watervapor and brine.Condensation. — The latter part of the processof distillation. Condensation is the process ofcooling the vapor to produce usable fresh water.Feed.— The sea water which is the raw materialin the distillation process.Vapor.— The product of the evaporation of seawater. The terms vapor and fresh water vaporare used interchangeably.Distillate.— The product resulting from thecondensation of the fresh water vapor producedby the evaporation of sea water. Distillate is alsoreferred to as condensate , as fresh water ,asfresh water condensate, and as sea water distillate.However, the use of the term condensateshould be avoided whenever there is any possibilityof confusion between the condensate of thedistilling plant and the condensate that resultsfrom the condensation of steam in the main andauxiliary condensers. In general, it is best touse the term distillate when referring to theproduct resulting from the condensation of vaporin the distilling plant.Salinity. — The concentration of salt in water.Brine. — Water in which the concentration ofsalt is higher than it is in sea water.TYPES OF DISTILLING UNITSDistilling units installed in naval ships areof two general types. The vapor compression typeof unit is used aboard submarines and smalldiesel-driven surface craft where the daily requirementsdo not exceed 4000 gallons per day(gpd). The low pressure steam distilling unit isused aboard all steam-driven surface ships andon nuclear submarines. The major difference betweenthe two types of distilling units is in thekind of energy used to operate the unit. Vaporcompression units use electrical energy; steamdistilling units use auxiliary exhaust steam.VAPOR COMPRESSION DISTILLING UNITSIt should be noted that distilling plants are not effectivein removing volatile gases or liquids which havea lower boiling point than water, nor are they effectivein killing all micro-organisms. These points are ofparticular importance when a ship is operating in contaminatedor polluted waters, as discussed at the endof this chapter.A vapor compression distilling unit is shownin cutaway view in figure 18- 2 and schematicallyin figure 18-3. The unit consists of three maincomponents— the evaporator, the compressor,and the heat exchanger— and a number of accessoriesand auxiliaries.451