Chapter 18. -DISTILLING PLANTS 



EVAPORATOR. — The cylindrical shell in 

 wnich vaporization and condensation occur is 

 commonly called the evaporator. The evaporator 

 consists of two principal elements: the steam 

 chest and the vapor separator. 



The steam chest^ includes all space within 

 the evaporator shell except the space that is oc- 

 cupied by the vapor separator. The steam chest 

 is considered to have an evaporating side and a 

 condensing side. The evaporating side includes 

 the space within the tubes of the tube bundle 

 (which is located in the lower part of the evapo- 

 rator shell) and the space which communicates 

 with the inside of the tubes. The condensing side 

 includes the space which surrounds the external 

 surfaces of the tubes; this space communicates 

 with the discharge side of the compressor by 

 means of a pipe, as shown in figure 18-3. 



The tube bundle is enclosed in a shell. At 

 top and bottom of the bundle the tube ends are 

 expanded into tube sheets. Most of the tubes are 

 small; but a few, set near the periphery, 

 are larger. Each of the larger tubes contains an 

 electric heater. As the sea water feed flows 

 through these larger tubes, it is heated to the 

 boiling point by the heaters. 



The feed inlet pipe extends horizontally to the 

 center of the evaporator, where it branches into 

 a Y. The two ends of the feed pipe turn downward 

 into the downtake, as shown in figure 18-?. Sea 

 water feed enters the evaporator through the 

 horizontal inlet pipe, pours into the downtake, and 

 passes down to the bottomheadof the evaporator 

 shell; from there, the feed flows upward through 

 the tubes. 



A funnel is installed inside the downtake, at 

 the top. The top of the funnel is about 2 inches 

 above the top of the evaporator tubes, and the 

 brine level in the evaporator shell is thus main- 

 tained at this height. About one-half to two- 

 thirds of the feed is vaporized; the remaining 

 brine overflows continuously into the funnel and 

 then into the brine overflow tube which is 

 installed inside the downtake. The overflow tube 

 leads the brine out through the bottom of 

 the evaporator shell, to the heat exchanger. In 

 the heat exchanger, the brine gives up its heat and 

 raises the temperature of the incoming feed. 



The vapor separator is an internal compart- 

 ment located at the top of the evaporator shell. 



3 "Steam chest" is a somewhat misleading term for a 

 unit which is not operated by steam and which has no 

 steam coming into it from an external source. Although 

 called a steam chest, it might more accurately be 

 thought of as a "vapor chest." 



The separator consists of two cylindrical baf- 

 fles. One cylinder extends downward from the 

 upper head plate of the evaporator; the other ex- 

 tends upward, and is fitted around the upper cyl- 

 inder to form a baffle. The floor of the separator 

 is formedby the bottom of the outer cylinder. The 

 space between the two cylinders provides a pas- 

 sage for the vapor flowing from the evaporating 

 side of the steam chest to the suction side of the 

 compressor. 



The vapor from the boiling sea water rises 

 up through the space between the shell wall and 

 tlie outer cylinder of the separator; it then flows 

 downward through the space between the cyl- 

 inders of the separator and enters the separator 

 chamber. From the separator chamber, the 

 vapor travels upward to the intake side of the 

 compressor. 



In the course of this roundabout passage 

 through the vapor separator, the vapor is separa- 

 ted from any entrained particles of water. The 

 water drops to the floor of the separator and is 

 continuously drained away. This water has a high 

 salt concentration, and must be continuously 

 drained in order to keep it from entering 

 the compressor and thus getting into the condens- 

 ing side of the evaporator, where it would 

 contaminate the distillate. 



VAPOR COMPRESSOR.- The vapor which 

 flows upward from the separator is compressed 

 by a positive-displacement compressor. The 

 type of compressor discussed here has two 

 three-lobe rotors of the type shown in the insert 

 on figure 18-2 and in figure 18-3. Two- lobe com- 

 pressors of the type shown in the main part of 

 figure 18-2 were an earlier design. 



The two rotors are enclosed in a compact 

 housing which is mounted on the evaporator. The 

 three lobes on each rotor are designed to produce 

 a continuous and uniform flow of vapor. The vapor 

 enters the compressor housing at the bottom and 

 then passes upward between the inner and outer 

 walls of the housing to the rotor chamber, where 

 it fills the space between the rotor lobes. The 

 vapor is then carried around the cylindrical sides 

 of the housing, and a pressure is developed at the 

 bottom as the lobes roll together. Clearances are 

 provided so that the rotor lobes do not actually 

 touch each other and do not touch the housing. 



The shaft of one rotor is fitted with a drive 

 pulley on one end and a gear on the other end. 

 This gear meshes with a gear on the shaft of the 

 other rotor, to provide the necessary drive for 

 the second rotor. 



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