Chapter 18. -DISTILLING PLANTS 



are three kinds of submerged tube distilling 

 units: (1) the Soloshell double-effect unit,(2)the 

 two-shell double-effect unit, and (3) the three- 

 shell triple-effect unit. 



The difference between double- effect units 

 and triple- effect units is merely in the number 

 of stages of evaporation. Two stages of evapora- 

 tion occur in a double- effect unit, and three in a 

 triple- effect unit. 



SOLOSHELL DOUBLE-EFFECT UNITS. - 

 Most Soloshell double- effect units have capac- 

 ities of 12,000 gallons per day or less. However, 

 some Soloshell units of 20,000 gpd capacity are 

 in use. 



A Soloshell double- effect unit is shown sche- 

 matically in figure 18-5 and in cutaway view in 

 figure 18-6. The unit consists of a single cylin- 

 drical shell which is mounted with the long axis 

 in a horizontal position. A longitudinal vertical 

 partition plate divides the shell into a first- effect 

 shell and a second- effect shell. The first- effect 

 shell contains the first effect tube bundle, a 

 vapor separator, and the vapor feed heater. The 

 second- effect shell contains the second- effect 

 tube bundle, a vapor separator, and the distilling 

 condenser. A distillate cooler, not a part of the 

 main cylindrical shell, is mounted at any conven- 

 ient location, as piping arrangements permit. 

 Another separate unit, the air ejector condenser, 

 is mounted on brackets on the outside of the evap- 

 orator shell. The air ejector takes suction on the 

 second- effect part of the shell, maintaining it 

 under a vacuum of approximately 26 inches of 

 mercury. A lesser vacuum— about 16 inches of 

 mercury— is maintained in the first- effect shell. 



Steam for the distilling unit is obtained from 

 the auxiliary exhaust line through a regulating 

 valve. This valve is adjusted to maintain a con- 

 stant steam pressure of 1 to 5 psig in the line be- 

 tween the regulating valve and a control orifice. 

 The size of the opening in the control orifice de- 

 termines the amount of steam admitted to the 

 distilling unit and hence controls the output of 

 distilled water. 



When the steam pressure is reduced by the 

 regulating valve, the steam becomes super- 

 heated. Since superheat has the undesirable ef- 

 fect of increasing the rate of scale formation, 

 provision is made for desuperheating the steam. 

 This is done by spraying hot water into the steam 

 line between the control orifice and the point 

 where the steam enters the first- effect shell. The 

 hot water for desuperheating the steam is taken 

 from the first- effect drain pump discharge. 



After being desuperheated, the steam passes 

 into the first- effect tube nest, where it heats the 

 sea water feed that surrounds the first-effect 

 tubes. The sea water boils, generating steam 

 which is called vapor to distinguish it from the 

 steam which is the external source of energy for 

 the unit. The condensate that results from the 

 condensation of the supply steam is discharged 

 by the first- effect drain pump to the low pressure 

 drain system or to the condensate system and is 

 thus eventually used again in the boiler feed sys- 

 tem. 



Although the vapor generated in the first- 

 effect shell is pure water vapor, it does contain 

 small particles of liquid feed. As the vapor rises, 

 a series of baffles above the surface of the water 

 begins the process of separating the vapor and 

 the water particles. 



After passing through the baffles, the vapor 

 enters the vapor separator. As the vapor passes 

 around the hooked edges of the baffles and vanes 

 in the separator, it is forced to change direction 

 several times; and with each change of direction 

 some water particles are separated from the 

 vapor. The hooked edges trap particles of water 

 and drain them away, discharging them back into 

 the feed at a distance from the vapor separator. 



After passing through the first- effect vapor 

 separator, the vapor goes to the vapor feed 

 heater. Sea water feed passes through the tubes 

 of the vapor feed heater, and part of the vapor is 

 condensed asitflowsover the tubes of the heater. 

 This distillate, together with the remaining un- 

 condensed vapor, goes through an external 

 crossover pipe and enters the tube nest of the 

 second- effect shell. The remaining vapor is now 

 condensed as it gives up the rest of its latent heat 

 to the sea water feed in the second- effect shell. 



Since the pressure in the second- effect shell 

 is considerably less than the pressure in the 

 first- effect shell, the introduction of the vapor 

 and the distillate from the first- effect shell 

 causes the sea water feed in the second- effect 

 shell to boil and vaporize. 



The vapor thus generated in the second- effect 

 shell passes through baffles just above the sur- 

 face of the water and then goes to the second- 

 effect vapor separator. From the vapor sepa- 

 rator, it passes to the distilling condenser. The 

 condensing tubes nearest the incoming vapor are 

 utilized as a feed heating section; the vapor con- 

 denses on the outside of the tubes and thus heats 

 the incoming sea water feed which is circulating 

 through the tubes. The remainder of the vapor is 



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