Principles of naval engineering - Historic Naval Ships Association

-Chapter 18. -DISTILLING PLANTSare three kinds of submerged tube distillingunits: (1) the Soloshell double-effect unit,(2)thetwo-shell double-effect unit, and (3) the threeshelltriple-effect unit.The difference between double- effect unitsand triple- effect units is merely in the numberof stages of evaporation. Two stages of evaporationoccur in a double- effect unit, and three in atriple- effect unit.SOLOSHELL DOUBLE-EFFECT UNITS.Most Soloshell double- effect units have capacitiesof 12,000 gallons per day or less. However,some Soloshell units of 20,000 gpd capacity arein use.A Soloshell double- effect unit is shown schematicallyin figure 18-5 and in cutaway view infigure 18-6. The unit consists of a single cylindricalshell which is mounted with the long axisin a horizontal position. A longitudinal verticalpartition plate divides the shell into a first- effectshell and a second- effect shell. The first- effectshell contains the first effect tube bundle, avapor separator, and the vapor feed heater. Thesecond- effect shell contains the second- effecttube bundle, a vapor separator, and the distillingcondenser. A distillate cooler, not a part of themain cylindrical shell, is mounted at any convenientlocation, as piping arrangements permit.Another separate unit, the air ejector condenser,is mounted on brackets on the outside of the evaporatorshell. The air ejector takes suction on thesecond- effect part of the shell, maintaining itunder a vacuum of approximately 26 inches ofmercury. A lesser vacuum— about 16 inches ofmercury— is maintained in the first- effect shell.Steam for the distilling unit is obtained fromthe auxiliary exhaust line through a regulatingvalve. This valve is adjusted to maintain a constantsteam pressure of 1 to 5 psig in the line betweenthe regulating valve and a control orifice.The size of the opening in the control orifice determinesthe amount of steam admitted to thedistilling unit and hence controls the output ofdistilled water.steam pressure is reduced by theWhen theregulating valve, the steam becomes superheated.Since superheat has the undesirable effectof increasing the rate of scale formation,provision is made for desuperheating the steam.This is done by spraying hot water into the steamline between the control orifice and the pointwhere the steam enters the first- effect shell. Thehot water for desuperheating the steam is takenfrom the first- effect drain pump discharge.After being desuperheated, the steam passesinto the first- effect tube nest, where it heats thesea water feed that surrounds the first-effecttubes. The sea water boils, generating steamwhich is called vapor to distinguish it from thesteam which is the external source of energy forthe unit. The condensate that results from thecondensation of the supply steam is dischargedby the first- effect drain pump to the low pressuredrain system or to the condensate system and isthus eventually used again in the boiler feed system.Although the vapor generated in the firsteffectshell is pure water vapor, it does containsmall particles of liquid feed. As the vapor rises,a series of baffles above the surface of the waterbegins the process of separating the vapor andthe water particles.After passing through the baffles, the vaporenters the vapor separator. As the vapor passesaround the hooked edges of the baffles and vanesin the separator, it is forced to change directionseveral times; and with each change of directionsome water particles are separated from thevapor. The hooked edges trap particles of waterand drain them away, discharging them back intothe feed at a distance from the vapor separator.After passing through the first- effect vaporseparator, the vapor goes to the vapor feedheater. Sea water feed passes through the tubesof the vapor feed heater, and part of the vapor iscondensed asitflowsover the tubes of the heater.This distillate, together with the remaining uncondensedvapor, goes through an externalcrossover pipe and enters the tube nest of thesecond- effect shell. The remaining vapor is nowcondensed as it gives up the rest of its latent heatto the sea water feed in the second- effect shell.Since the pressure in the second- effect shellis considerably less than the pressure in thefirst- effect shell, the introduction of the vaporand the distillate from the first- effect shellcauses the sea water feed in the second- effectshell to boil and vaporize.The vapor thus generated in the second- effectshell passes through baffles just above the surfaceof the water and then goes to the secondeffectvapor separator. From the vapor separator,it passes to the distilling condenser. Thecondensing tubes nearest the incoming vapor areutilized as a feed heating section; the vapor condenseson the outside of the tubes and thus heatsthe incoming sea water feed which is circulatingthrough the tubes. The remainder of the vapor is457