ENGINEMAN 1 & Cline of the brine overboard pump. In plantsequipped with wire control valves and in baskettype plants, the brine density is controlled by ad-justing the first-effect feed valve. Increasing therate of feed decreases the brine density, anddecreasing the rate of feed increases the brinedensity.Frequent changes of brine density have atendency to disrupt steady performance of theplant; therefore, only very small changes shouldbe made. The proper setting for a specific plantshould be learned from experience, and thissetting should be maintained as practicable.Use of the SalinometerThe salinometer is an instrument (on theprinciple of a float) for measuring the degree ofsalinity or the concentration of the brine. It is ahollow, metal vessel weighted at the bottom, andhaving a projecting stem which is graduated infour scales to read the salinity for varioustemperatures of the brine. The graduations aremarked in thirty-seconds. When the salinity of asample of brine is to be measured, the temperatureof the sample should be brought to a temperaturecorresponding to that of one of the scales on theinstrument in order that an accurate reading maybe obtained. The accuracy of the salinometershould be checked occasionally by placing it indistilled water; if it is accurate, it should sink tothe zero mark on the scale corresponding to thetemperature of the water.A pot is provided for holding the sample ofbrine. The pot must be amply deep so that thereis no danger of breaking the bottom of thesalinometer when it is placed in the pot. To usethe salinometer, proceed as follows:1. Draw off a sample of brine, from the testcocks on the discharge side of the brine pump,then insert the thermometer into the sample.2. Allow the sample to cool to thetemperature of one of the scale temperatures.3. Put the salinometer in the pot and read thedegree of salinity.4. Remove the salinometer and wipe off allmoisture since accumulations of salt or dirt willresult in false readings.FLASH TYPE DISTILLINGPLANTSThe flash type evaporator, like all distillingplants, removes salts and other impurities fromraw seawater by the process of evaporation andcondensation. This is accomplished by boiling thewater to convert it to steam, and condensing thissteam to form distilled water. The flashevaporator is different from other distilling plants,because evaporation takes place at temperatureswell below the normal boiling point of water andwithout the use of submerged heat transfersurfaces.In the flash type distilling plant, thetemperature of the water is never raised beyond175 °F, and is only raised to this temperaturewithin the last pass of tubes of the saltwaterheater. Flash evaporation takes place attemperatures as low as 104°F. In addition, noboiling occurs on heat transfer tube surfaces; asa result, the scale formation is greatly reduced andoperation at maximum efficiency is prolonged.The term “flash evaporation” means thatwater is converted to steam as it enters anevaporating chamber, without further addition ofheat. Flashing at low temperatures is possible onlywhen a vacuum is maintained in the chamber,since the boiling point of water decreases as thepressure in the chamber is reduced. As in othermethods of distillation, a portion of the water re-mains behind in the evaporating chamber and istaken off as a concentrated waste (brine).Principal ComponentsThe unit discussed in this section is a five-stageplant in which feedwater is flashed to vapor infive evaporator stages at successively lowerpressures.Connections (or passages) that exist betweenthe evaporator stages are the feedwater and itsdistillate loop seals, which permit the flow of feed-water and distillate from stage to stage whilepreserving the varying degrees of vacuum in eachstage.Condensers are mounted on top of each stagebetween the front and rear water boxes. Feedwaterflows through the tubes in six passes, entering at7-28
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