Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGcompression unit have been described, let ussummarize briefly the sequence of events withinthe unit and consider some of the factors that areimportant in the vapor compression process ofdistillation.The cold sea water feed enters the heatexchanger and is heated there to about 190°or 200 F. From the heat exchanger, the feedgoes into the evaporator. Here it flows down thedowntake and into the bottom of the evaporatorshell, then upward in the tubes. Boiling andevaporation take place in the tubes at atmosphericpressure. About one-half to two-thirds ofthe incoming feed is evaporated; the remainderflows out through the brine overflow, thus maintaininga constant water level within the evaporator.The vapor thus generated rises and enters thevapor separator, where any particles of moisturethat may bepresent are separated from the vaporand drained out of the separator. The vapor goesto the suction side of the compressor. In the compressor,distilled water drips onto the rotors andthus desuperheats the vapor as it is compressed.The vapor is compressed to a pressure of about3 to 5 pounds above atmospheric pressure, andis discharged to the space surrounding the tubesin the steam chest. As the vapor condenses onthe outside of the smaller tubes, the distillatedrops down and collects on the bottom tube plate.Every time a pound of compressed vapor condenses,approximately a pound of vapor isformed in the evaporator section; the compressorsuction is thus kept supplied with the rightamount of vaporThe distillate is drawn off through a steamtrap and flows into the heat exchanger at a temperatureof about 220° F. As it flows through theheat exchanger, the distillate gives up heat to theincoming feed and is cooled to within about 18° Fof the cold feed water temperature. Noncondensablegases, together with a small amount ofvapor, flow into the vent line and then to the heatexchanger.Meanwhile, the sea water which is not vaporizedin the evaporator is flowing continuously intothe funnel, down the brine overflow tube, and intothe heat exchanger. The temperature of this brineis about 214° F. In passing through the heat exchanger,the hot brine raises the temperature ofthe sea water feed that is entering through theheat exchanger.The entire distillation cycle is started byusing the electric heaters to bring the sea waterfeed temperature up to the boiling point and togenerate enough vapor for compressor operation.After the cycle has been started and thecompressor is adequately supplied with vapor,the normal operating cycle begins and the electricheaters are used henceforth only to providethe 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 smallpart of the total heat input.The major part of the heat input comes fromthe compression work that is done on the vaporby the compressor. The major energy transformationsinvolved in normal operation are thusfrom electrical energy (put in at the compressormotor) to mechanical energy (work done bythe compressor on the vapor) to thermal energy.The thermal energy thus supplied is used to boilthe sea water feed and keep the process going.The compression process serves anothervital function in the vapor compression distillingunit. Since the boiling point of sea water is severaldegrees higher than the boiling point of freshwater at any given pressure, the boiling sea waterin the evaporator is actually above 212° F andwould therefore be too hot to condense the freshwater vapor if the vapor were at the same pressureas the boiling sea water. By compressingthe vapor, the boiling pointof the vapor is raisedabove the boiling pointof the sea water at atmosphericpressure. Therefore the compressedvapor can be condensed on the outside of the tubesin which sea water feed is being boiled. Thisprocess would not be possible without the pressuredifference between the evaporating side andthe condensing side of the unit, and this pressuredifference is created by the compression of thevapor.STEAM DISTILLING UNITSSteam distilling plants now in naval use arepractically all of the low pressure type. They are"low pressure" units from two points of view.First, they utilize lowpressure steam (auxiliaryexhaust steam) as the sourceof energy; and second,they operate at less than atmospheric pressure.There are three major types of lowpressure steam distilling units: submerged tubeunits, flash-type units, and vertical basket units.Submerged Tube UnitsSubmerged tube distilling units range from4000 to 50,000 gallons per day in capacity. There456