PRINCIPLES OF NAVAL ENGINEERING 



compression unit have been described, let us 

 summarize briefly the sequence of events within 

 the unit and consider some of the factors that are 

 important in the vapor compression process of 

 distillation. 



The cold sea water feed enters the heat 

 exchanger and is heated there to about 190° 

 or 200 F. From the heat exchanger, the feed 

 goes into the evaporator. Here it flows down the 

 downtake and into the bottom of the evaporator 

 shell, then upward in the tubes. Boiling and 

 evaporation take place in the tubes at atmos- 

 pheric pressure. About one-half to two-thirds of 

 the incoming feed is evaporated; the remainder 

 flows out through the brine overflow, thus main- 

 taining a constant water level within the evapo- 

 rator. 



The vapor thus generated rises and enters the 

 vapor separator, where any particles of moisture 

 that may bepresent are separated from the vapor 

 and drained out of the separator. The vapor goes 

 to the suction side of the compressor. In the com- 

 pressor, distilled water drips onto the rotors and 

 thus desuperheats the vapor as it is compressed. 

 The vapor is compressed to a pressure of about 

 3 to 5 pounds above atmospheric pressure, and 

 is discharged to the space surrounding the tubes 

 in the steam chest. As the vapor condenses on 

 the outside of the smaller tubes, the distillate 

 drops down and collects on the bottom tube plate. 

 Every time a pound of compressed vapor con- 

 denses, approximately a pound of vapor is 

 formed in the evaporator section; the compres- 

 sor suction is thus kept supplied with the right 

 amount of vapor 



The distillate is drawn off through a steam 

 trap and flows into the heat exchanger at a tem- 

 perature of about 220° F. As it flows through the 

 heat exchanger, the distillate gives up heat to the 

 incoming feed and is cooled to within about 18° F 

 of the cold feed water temperature. Nonconden- 

 sable gases, together with a small amount of 

 vapor, flow into the vent line and then to the heat 

 exchanger. 



Meanwhile, the sea water which is not vapor- 

 ized in the evaporator is flowing continuously into 

 the funnel, down the brine overflow tube, and into 

 the heat exchanger. The temperature of this brine 

 is about 214° F. In passing through the heat ex- 

 changer, the hot brine raises the temperature of 

 the sea water feed that is entering through the 

 heat exchanger. 



The entire distillation cycle is started by 

 using the electric heaters to bring the sea water 

 feed temperature up to the boiling point and to 



generate enough vapor for compressor opera- 

 tion. After the cycle has been started and the 

 compressor is adequately supplied with vapor, 

 the normal operating cycle begins and the elec- 

 tric heaters are used henceforth only to provide 

 the heat necessary to make up for heat losses. 

 After the unit has become fully operational, then, 

 the heat input from the heaters is only a small 

 part of the total heat input. 



The major part of the heat input comes from 

 the compression work that is done on the vapor 

 by the compressor. The major energy transfor- 

 mations involved in normal operation are thus 

 from electrical energy (put in at the compres- 

 sor motor) to mechanical energy (work done by 

 the compressor on the vapor) to thermal energy. 

 The thermal energy thus supplied is used to boil 

 the sea water feed and keep the process going. 



The compression process serves another 

 vital function in the vapor compression distilling 

 unit. Since the boiling point of sea water is sev- 

 eral degrees higher than the boiling point of fresh 

 water at any given pressure, the boiling sea water 

 in the evaporator is actually above 212° F and 

 would therefore be too hot to condense the fresh 

 water vapor if the vapor were at the same pres- 

 sure as the boiling sea water. By compressing 

 the vapor, the boiling pointof the vapor is raised 

 above the boiling pointof the sea water at atmos- 

 pheric pressure. Therefore the compressed 

 vapor can be condensed on the outside of the tubes 

 in which sea water feed is being boiled. This 

 process would not be possible without the pres- 

 sure difference between the evaporating side and 

 the condensing side of the unit, and this pressure 

 difference is created by the compression of the 

 vapor. 



STEAM DISTILLING UNITS 



Steam distilling plants now in naval use are 

 practically all of the low pressure type. They are 

 "low pressure" units from two points of view. 

 First, they utilize lowpressure steam (auxiliary 

 exhaust steam) as the sourceof energy; and sec- 

 ond, they operate at less than atmospheric pres- 

 sure. There are three major types of low 

 pressure steam distilling units: submerged tube 

 units, flash-type units, and vertical basket units. 



Submerged Tube Units 



Submerged tube distilling units range from 

 4000 to 50,000 gallons per day in capacity. There 



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