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DESCRIPTION OF THE
MODEL S DISTILLING UNIT
 
A. PRINCIPLES OF DISTILLING ACTION
 
2A1. What is distillation? A ship is always surrounded by limitless quantities of water, except when in drydock; but this is sea water, unfit for human consumption or use in the batteries. If the salt and other substances in sea water could be removed there would be sufficient pure water at hand for all purposes. Such substances can be removed by distillation. The knowledge of distilling liquids comes from ancient days. Distillation is simply the boiling of a liquid and the condensing of its vapor back to the liquid state again. If a teaspoon is held in a cloud of steam rising from a teakettle, the vapor will condense on the spoon and the resulting liquid is distilled water. In the boiling most or all of the impurities are left behind, so that the condensed liquid is relatively pure.

2A2. Purifying action of distillation. Salt and other substances are dissolved, or in solution, in sea water. Sea water does not boil at the same temperature as does fresh water (212 degrees F., at sea level pressure), but at a temperature a few degrees higher. When sea water boils, it is only the water that is vaporized at this temperature, and if this pure vapor is led into another clean container where it may condense, the condensate is pure distilled water. The salt (sodium chloride) and other solid ingredients in the sea water do not vaporize and hence do not appear in the distilled water.

2A3. General explanation of distillation. Figure 2-1 shows a cutaway view of the Model S distilling unit in full detail. Figure 2-2 shows a highly simplified schematic diagram of the working parts of the distilling unit, with arrows indicating the flow of the water and vapor through it.

The distilling process in the Model S distilling unit is a continuous one; sea water is supplied at the rate of about a gallon a minute; part of this is turned to distilled water; the solid residue and concentrated brine flow out separately from the distilled water.

  Inside a casing, a long length of tubing is coiled into a cone, set with its small end down. There are ten such cones nested together. Cold sea water enters at the bottom between the cones, that is, it flows around the outside of the tubing. On its way upward it is heated, so that it is boiling when it emerges from between the cones at the upper end. The vapor is led through a vapor separator into a compressor, where it is compressed and is then discharged down into the inside of the tubing. On the way down through the tubing this vapor is gradually cooled by contact with the colder tubing walls, finally condensing therein and flowing out as pure distilled water to a storage tank. The nested cones of tubing therefore act as heat exchangers. The distilled water is technically known as distillate or condensate. The path of this flow may be easily seen in Figure 2-2.

2A4. Necessity for compressing the vapor. The question may be raised as to why the vapor is compressed in the distilling unit. The explanation involves several considerations, as follows The conical nest of tubes serves four purposes (1) to heat the feed water, (2) to generate the vapor, (3) to condense the vapor, and (4) to cool the condensate to a lower temperature. In the lower part of the nest, the feed water is at the temperature of sea water; the temperature increases during the upward flow, and the feed water leaves the nest boiling. On the downward flow, the vapor is condensed in the upper part of the tube nest, and hot condensed liquid is cooled in the lower part of the tube nest.

Since sea water does not boil at the same temperature, for a given pressure, as does fresh water, but at several degrees higher, the, feed water in the upper part of the nest is actually above 212 degrees F. The vapor from the boiling water is no longer sea water, but fresh water vapor. Fresh water vapor at atmospheric pressure condenses only at 212 degrees F. When a vapor is compressed, its boiling

 
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Figure 2-1. Model S distilling unit (cutaway view).
Figure 2-1. Model S distilling unit (cutaway view).
 
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Figure 2-2. Model S distilling unit (schematic view).
Figure 2-2. Model S distilling unit (schematic view).
 
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point or its condensation point is raised above the temperature of the hot feed water in the upper part of the tube nest. Thus, when the compressed vapor enters the nest on its path downward, it finds a temperature lower than its new condensation point, and so is able to condense. This type of apparatus accordingly is called a vapor compression distilling unit.

2A5. Heat input of the distilling unit. The compression of vapor serves still another purpose. In the starting operation of the unit, the feed water is raised to its boiling point by the electric heaters. After the unit is in normal operation, there will be a steady heat loss of definite amount through the insulation and in the outgoing condensate and brine overflow. This heat loss is balanced by an input of energy from the electric motor, which is transformed to heat by the compression of the vapor. Theoretically, this input of heat by the compressor maintains the heat balance at a constant level, and it is possible to operate the unit with all electric heaters turned off. In actual practice, however, some of the heaters are usually left on after the unit is in normal operation.

2A6. Vent to atmosphere. Since the process of boiling the sea water takes place inside the shell of the unit, it is necessary to prevent any increase of pressure on the boiling water, for increased pressure would raise the boiling point and unbalance the whole system, and probably stop its operation. The situation is different in the compressor. When the vapor goes into the compressor, it is sealed off from the boiling liquid and may then be compressed without affecting the boiling point. In order that the boiling may always take place at atmospheric pressure as found within the submarine, a pipe called the vent leads down from the vapor separator (Figure 2-2) out through the bottom of the unit. This vent, being open to the atmosphere, insures that the pressure in the vapor separator is always the same as the pressure of the surrounding atmosphere. A distant reading dial thermometer is connected to the vent by a flexible tubing and gives the temperature in the vent pipe.

Although this open vent pipe leads downward out of the unit, the steam will not flow out when in normal amount inside, because the outer atmosphere exerts pressure upon it through the vent. However the interior and exterior pressures are so

  maintained that there is a very small excess of pressure inside the unit, which causes a slight feather of steam to appear at the vent. This is an indication that the unit is operating satisfactorily. Any excess steam which the compressor cannot hold, however, will be able to pass out through the vent, which thus acts as a safety device.

This vent pipe also serves to permit drainage into the bilge of any slight amount of liquid carried into the vapor by the violent boiling action, and prevents it from gathering on the floor of the vapor separator.

2A7. Sea water not distilled. All the incoming sea water cannot be distilled, for some of it must remain as a vehicle to carry away the concentrated salt content left from the distilled portion. The undistilled portion, which is concentrated brine, is maintained at a level just above the top coil of the heat exchanger by overflow pipes. It flows down through these overflow pipes into a separate conical passage, called the overflow heat exchanger, located around the nest of tubes (Figure 2-2), where it gives up some of its heat by conduction through the metal walls, thus helping to heat the incoming feed water.

2A8. Overflow weir cup. The overflow pipe after leading out of the overflow heat exchanger at the bottom of the unit casing, rises again for a short distance. At the top of the upright overflow pipe, the brine flows out through an opening called the weir, which meters or measures the quantity of brine overflow in gallons per hour. The overflow brine passing out of the weir falls into an open cup and then drains down into a storage tank called the brine receiver, from which it is discharged to the sea.

Since water in any open vertical U-shaped container must always be at the same level in both arms of the U, the open weir is located at such a height as to insure that the interior overflow heat exchanger (Figure 2-2) is always full of liquid, which thereby exerts its full heating effect on the sea water inside.

2A9. Time required to start sea water boiling in the distilling unit. When starting the unit, it takes from 60 to 90 minutes to bring the sea water to the boiling point.

2A10. Heat balance in the distilling unit. It is important to know the heat flow through the various

 
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Figure 2-3. Piping arrangement, Model S distilling plant (two units).

parts of the unit in actual quantities. The following example is typical.

Heat input. The total input of heat is 16,125 Btu per hour, when the unit is fully operating with 10,185 Btu per hour from the compressor and 5,940 Btu per hour from the electric heaters.

Heat loss. The total quantity of heat loss that flows out through the four separate paths, is as follows:

1,825Btu per hour in the condensate.
11,600Btu per hour in the overflow.
400Btu per hour through the vent.
2,300Btu per hour by radiation from hot metal parts.

This represents a total heat loss of 16,125 Btu per hour. The heat balance is not always at this exact number of Btu per hour, because various momentary changes of rate of feed and temperature of sea water, voltage fluctuations in the motor, and other operating conditions, naturally will cause it to shift around.

The heat balance of the unit is very sensitive and all changes which may be necessary in the operating conditions should be made slowly.

2A11. Purity of distilled water. If no leaks are present in the system, the distilled water will contain not over four parts of salt to a million parts of water. The distiller cannot, of course, remove any volatile liquids, that is, liquids which boil at or below the boiling temperature of water. For example, in badly polluted harbors or streams, a trace of ammonia may be present in the distilled water; and in improperly chlorinated waters, a trace of chlorine may likewise come over in the distilled water.

2A12. Two-unit plant. In the complete submarine distilling plant there are two separate units, each with its necessary control devices, connected in parallel. They are normally operated at the same time, not alternately. A schematic diagram of the complete system, with piping connections, valves, and tanks is shown in Figure 2-3.

Two units are necessary, not only as a safety factor, but also to provide sufficient distilled water. These units may normally be run 300 to 350 operating hours without cleaning, each giving 40 gallons per hour. This means a total of 24,000 to 28,000 gallons of distilled water. The consumption of distilled water is about 500 gallons per day

  for all purposes. On a war patrol lasting 60 days, the total consumption will be about 30,000 gallons, and may run higher in the tropics.

2A13. Water for storage batteries. Water distilled from sea water is entirely fit for human consumption and for storage batteries. In the event fresh water is taken aboard from shore, such fresh water has to be distilled before it is suitable for storage battery use. Only fresh water taken aboard at a United States port and definitely known to be pure may be used without distilling or boiling for drinking, cooking, or other human use. In distilling fresh water that may be taken aboard, the operation of the distilling unit is practically the same as when distilling sea water; the only difference being that the overflow is returned to the ship's fresh water tanks from the brine tank, instead of being discharged overboard.

2A14. Testing storage battery water. Water that has been distilled must be tested for purity before it is used in the storage batteries.

a. Test for purity. The instrument used to make this test is the Kleinschmidt Water Tester, Type No. A. It is self-contained in a bag 6 x 6 x 4 3/4 inches, operated by three flashlight cells. A small open top container, called a cell, has two fine platinum wire electrodes inside, which lead through the base of the container to prongs. When these prongs are inserted in the socket, the electrodes become part of the electrical circuit. The metered scale is graduated from 0 to 100 in microamperes. A reading of 40 microamperes indicates that the water contains 0.29 grains of salt per gallon. Any reading above 40 shows that the water contains too much salt for storage battery purposes. Detailed instructions for using the tester follow:

CAUTION. Do not touch the inside of the cell for any reason. To do so will contaminate the surfaces and may ruin the fine platinum electrodes in the cell. Clean only by rinsing with the liquid to be tested.

1. Set switch (beneath the meter) to CHECK and turn knob (lower right) until the meter reads 50 (red line). Then turn the switch to the READ position.

2. Remove the cell and remove the cell cover; rinse the cell and fill it with the water to be tested.

3. Do not replace the cell cover while the cell is full of water. The testing should be conducted

 
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without the cover in place. The cell cover should be in place only when the cell is empty and not in use.

4. Wipe moisture from the outside of the cell, particularly the prongs in the base, with a clean cloth. Care should be taken to prevent the cloth or anything else from coming in contact with the water in the cell being tested.

5. Insert the cell in the socket carefully so as not to spill any water, and read the meter immediately. Do not wait for the meter needle to steady since it may drift either up or down from the correct initial reading.

6. The Model S distilling unit, when working properly, will produce water having a reading between 10 and 20. Water giving readings less than 40 is suitable for use in storage batteries; however, readings as high as 40 indicate that leakage is occurring within the still.

7. If a reading is obtained which indicates contaminated water, this should not be taken as the final test. Five or six check tests should be run to make sure the water itself and not the tester is faulty.

8. The tester serves to protect the batteries

  against an excess of sodium chloride and thereby serves to indicate whether the distillate is being contaminated by sea water. It will not protect against an excess of iron, copper, or nickel. Samples of battery water should be given a complete chemical analysis to determine conformity to the Bureau of Ships instructions when shore testing facilities are available.

9. The tester is shipped from the factory without the flashlight cells necessary for its operation. Three Navy Type O flashlight cells should be installed for operation. If the tester is to be stored for any length of time, especially in tropical climates, the flashlight cells should be removed from the tester.

10. When check readings cannot be adjusted to 50, the flashlight cells should be renewed.

b. Silver nitrate test. Silver nitrate is sometimes used as a quick test of the condensate. Pour some of the condensate in a glass. Drop in a few drops of silver nitrate solution or a small crystal of silver nitrate. If the water remains perfectly clear, it is safe for storage battery use. If any cloudiness or precipitate shows, the water is not fit for storage battery use.

 
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