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2 L EARNING O BJECTIVES Understanding of the distillation process for producing freshwater from seawater. Identify temperature and pressure relationship associated with efficient operation of evaporator units. Define water quality standards before and after desalination. Examine and compare the various terms associated with freshwater makers. Discuss the advantages and disadvantages of each freshwater making device. 15:48 2

3 WATER PRODUCTION A considerable amount of fresh water is consumed in a ship. The crew uses on average about 70 litre/person/day and in a passenger ship, consumption can be as high as 225 litre/person/day. Water used in the machinery spaces as make up for cooling system losses may be fresh or distilled but distilled water is essential for steam plant where there is a water tube boiler. 15:48 3

4 The overall principle used to produce freshwater from seawater is distillation. Distillation is the process of boiling seawater. Condensing the vapors into distillate. The salt is not carried over with the vapor; it is left behind with remaining seawater. The concentrated seawater is known as brine. Engine cylinder cooling water works as heat source. 15:48 4

5 P RINCIPLES OF D ISTILLATION At any given pressure, the rate of evaporation of the seawater is dependent on how much heat is transferred. It is dependent on several factors Temperature difference between the heat source and the seawater Available surface area Coefficient of heat transfer Velocity of the fluids involved Cleanliness of the heat transfer surfaces As the seawater boils, salt and other minerals in the water are left behind Some of the salts are more soluble at low temperatures than at high temperatures. 15:48 5

6 P RINCIPLES OF D ISTILLATION A distiller is an excellent example of a thermodynamic process. Heat exchanges Phase changes Almost all distillation processes requires the use of chemicals (pumped in with the seawater) to: Help prevent the formation of scale Keep foaming to a minimum 15:48 6

7 D ISTILLATION T ERMS Distillation Boiling of seawater to produce vapor, then condensing the resulting vapors into fresh water which is used for potable water, distillate, and make-up feed water. Evaporation The first stage of distillation, where steam or vapor is produced from boiling seawater. 15:48 7

8 D ISTILLATION T ERMS Condensation The second part of the process is condensing the vapors. This is usually accomplished with a shell and tube or plate type heat exchanger Salt water, which becomes the evaporator feed water, is used to take away the latent heat of evaporation. This also preheats the feed water prior to entering the evaporator. 15:48 8

9 D ISTILLATION T ERMS Feed The seawater that will be fed into the unit for evaporation Vapor The product of the boiling process Distillate The product resulting from the condensation of the fresh water vapor. Most distiller units will produce about 25% fresh water from the feed water furnished to the unit. 15:48 9

10 D ISTILLATION T ERMS Salinity The concentration of salt in fresh water. Most evaporators produce fresh water in the 1–3 ppm range. Amounts at or over 5 ppm will cause the evaporator to dump the water to the bilge. Brine Water in which the concentration of salt is higher than that of seawater. The brine density will normally be kept at 1 ½ times that of seawater Brine is what is left over, and is pumped over the side 15:48 10 Brine

11 D ISTILLATION T ERMS Salinity Indicator – conductivity device. It works by passing low level electrical signal through distillate. The amount of electricity conducted is directly proportional to the concentration of salt minerals in the distillate. It is usually combined with a 3 way valve To discharge fresh water to tanks or dump the water to the bilge if the salinity concentration exceeds 5ppm. 15:48 11

12 D ISTILLATION T ERMS Demisters A material that removes water droplets which may have salt contained within. Stainless steel wool is often used to form the demister pads inside the Nyrex unit. During maintenance of the unit, these pads are removed and cleaned with fresh water. Demisters are located between the evaporator and condenser. 15:48 12

13 O THER CONSIDERATIONS Water boils quicker in a vacuum or at a lower temperature. Seawater has a solid concentration of 35,000 ppm or 35 ‰. The boiling point of seawater at atmospheric pressure is o F Water must boil at least 165 o F to insure that bacteria are killed. Water output / volume of seawater input is about 20-25%. About 20-25% of the seawater is converted to fresh water. 15:48 13

14 Older types High pressure Evaporators Single shell with heat coils. Steam was used as the source of heat. The units operated under high pressure. The coils became easily fouled with scale. They did not produce a lot of water, and were not efficient. T YPES OF D ISTILLERS 15:48 14

15 Solo-shell double effect evaporator They were an improvement over the high pressure evaps. Steam was used as the heating medium, but the shell was kept under a vacuum, so the boiling temperature was lower. The units had two effects which meant the seawater boiled in one chamber; the brine from that effect drained to the second chamber. Vacuum was kept lower in the second effect so the brine was pulled over to the second effect. 15:48 15

16 T YPES OF D ISTILLERS Newer types of distillers Vapor compression Small units that are used on submarines, oil rigs, where average daily water consumption is less then 4000 gpd. The vapor compression evaporator uses the vapor produced as the heat source to boil the seawater. 15:48 16

17 T YPES OF D ISTILLERS Vapor compression To start the process, the unit has electric heaters. Once the unit is producing vapor, the electric heaters will only be used to supplement heat source. When the seawater starts to boil, the vapors are drawn off the shell by a vapor compressor, which compresses the vapors between 3 – 5 psi. The compressed vapors are discharged to a space surrounding tubes which contain the seawater. The distillate is drawn off via a steam trap, to insure that all of the vapors are condensed. The freshwater leaves the unit at about 220 o F and flow through a heat exchanger that also acts as a preheater for the incoming seawater. 15:48 17

18 Flash Evaporators Distilled water is produced by passing preheated seawater into a low pressure chamber; lower than the vapor pressure of the incoming seawater. No heating takes place inside the shell. The seawater is atomized or broken down into very small water droplets within the shell. The internal energy of the atomized water will cause a portion of it to flash, producing distillate vapor. 15:48 18

19 T YPES OF D ISTILLERS Flash Evaporators The vapor flows through a demister into a vapor condenser. The distilled water from that section combines with the output of the second effect and is pumped to storage tanks. Brine from the first effect flows to the second effect chamber, which is at a lower vacuum. By decreasing the pressure, the temperature at which the brine flashes is lowered. The brine still has enough internal energy to cause flashing to occur. Advantage of the flash distiller is that no heating takes place inside the shell. Scale build up is not as sever. 15:48 19

20 N YREX S YSTEMS P LATE T YPE E VAPORATORS The figure is a diagrammatic arrangement of a two stage plate-type evaporator. The plates are made of titanium and the other parts are made of materials that resist saltwater corrosion. The energy source is usually the main propulsion diesel engine jack cooling water. 15:48 20

21 N YREX S YSTEMS P LATE T YPE E VAPORATORS The incoming seawater (5) is preheated in the second stage distillate condenser (13) then flows to the first stage evaporator plate heat exchanger (11). A portion of the seawater is evaporated at approximately 10 in of Hg or 160 o F The vacuum is maintained with an eductor that uses saltwater from the supply pump discharge as the operating medium. 15:48 21

22 N YREX S YSTEMS P LATE T YPE E VAPORATORS The steam generated in the first stage travels through a vapor demister and becomes the heating medium for the second stage. Steam condenses on one side of the heat exchanger (12), and brine evaporates on the other. The brine evaporates at approximately 113 o F and 2.8 inches of Hg. 15:48 22

23 N YREX S YSTEMS P LATE T YPE E VAPORATORS Vapor flows from the second stage plate evaporator through a demister to heat exchange (13). The distillate produced in the second stage combines with the distillate from (12) and is pumped to storage tanks. Brine is pumped overboard with the same eductor used to create the unit vacuum. Chemicals are used to minimize foaming and help keep the plates from scaling. 15:48 23

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26 R EVERSE O SMOSIS D ESALINATION Used extensively in small boats and yachts. Osmosis is a naturally occurring phenomenon in which the less concentrated solution of two solvents separated by a semi-permeable membrane will pass through the membrane to dilute the more concentrated solution until equilibrium is obtained. 15:48 26

27 R EVERSE O SMOSIS D ESALINATION The process can be reversed by applying pressure to the high concentrated solution. The pressure forces freshwater to flow through the membrane leaving concentrated seawater on the other side of the unit. The operating temperatures are 60 to 80 o F. As the temperature decreases, the output of the unit decreases. 15:48 27

28 R EVERSE O SMOSIS D ESALINATION Unit pressure must be maintained. Pressures vary from 400 to 1000 psi. At 350 psi no fresh water will be produced. The osmotic pressure for seawater is 350 to 400 psi. Unit safety features Salinity indicator which will dump the fresh water when the quality get to a certain point. Differential pressure cutouts, which stop the unit at a preset differential. Output quality is 300 to 400 ppm Which is ok for human consumption, but not good enough for boiler feed water or for diesel engine jacket water cooling. 15:48 28

29 Membranes typically need to be replaced every 18 to 24 months. Periodic cleaning of the membranes with chemicals helps extend the life of the membrane. Lay up chemicals are required to be introduced into the unit if it will not be used for a few months. 15:48 29

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31 T HREE OPERATING CONDITIONS THAT AFFECT R/O PRODUCTION Osmotic pressure across membrane (ΔP) Flow rate of seawater across R/O Seawater temperature 15:48 31

32 Never operate RO units while in port, as grease and oils may foul the membrane. Never run potable water through the unit that has been chlorinated as this will damage the cellulose acetate membrane and ruin the elements. Water should be chlorinated or purified using UV as the potential for bacteria from an RO unit is more likely than the fresh water produced by a distiller unit. 15:48 32

33 K EY POINTS ABOUT R / O UNITS Advantage Less expensive water production than distillers No heat source required Disadvantage Lots of maintenance issues - (3) chemicals Water quality is less than distiller units Not pasteurized water supply needs chlorides or UV light Less % of production than distiller unit RO 5-10% Ngrex 25% 15:48 33

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35 THANK YOU 15:48 35


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