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MECHANICAL DETAILS
OF MODEL S DISTILLING UNIT
 
A. GENERAL DESCRIPTION
 
3A1. Main parts. The Model S distilling unit consists of eight main elements: insulation, shell, heat exchanger, vapor separator, over/low heat exchanger, compressor, motor, and variable pitch drive.

3A2. Insulation. Because of the delicate heat balance on which the unit operates, the insulation must be very efficient, so that as much of the heat as possible may be retained inside the unit to do its proper work.

The whole apparatus, with the exception of the motor and variable pitch drive, is covered with a 2-inch layer of glass wool insulation. This insulation is attached to stainless steel jackets, which form the outer casing of the unit. The jackets with insulation are held in place by clamps and are readily removable.

3A3. Shell. Inside the jacket and insulation is the shell, against which the insulation makes contact. This copper nickel shell encloses only the heat exchanger and vapor separator. It consists of two parts, the cylindrical upper part and the conical lower part; the two parts are bolted together. The upper shell is bolted to the upper head plate, which is the main support of the whole unit. The lower part of the shell consists of two nested conical sections 1/16-inch apart, bolted at the bottom to the lower head plate. The space between these two lower conical sections forms the overflow heat exchanger (Figure 2-2).

3A4. Heat exchanger. Within the lower conical portion of the shell lies the main heat exchanger, projecting part way up into the upper cylindrical portion. The heat exchanger consists of ten cones of copper nickel tubing, nested together and pointed downward. Each cone is made up of eight lengths of 1/4-inch o.d. copper-nickel tubing. Each piece of tubing is 44 inches long. The tubes are wound very tightly against each other in parallel on a cone shaped mandrel. They are tack-brazed

  to prevent their unwinding. The cones measure about 4 inches in diameter at the bottom, 19 inches in diameter at the top, and are a little over 2 feet high. The upper ends of the tubes are connected by unions to eight upper headers placed vertically, and attached to the upper head plate. The lower ends of the tubes in each cone are brazed to a small coil header, horizontally placed, connected by unions to a single lower discharge header.

3A5. Retarders. A 1/8-inch square metal rod is inserted into the lower two-thirds portion of each tube. These rods are called retarders, and serve to decrease the inner area of the tubes through this section so that most of the condensate comes in contact with the walls of the tubes, thereby obtaining maximum heat transfer. The retarders also limit the flow of steam through the tubes, thus maintaining proper compressor discharge pressure (see Figure 2-1).

3A6. Nesting of coils. Five of the ten cones of tubing are wound right hand and five left hand. They are alternated in the assembly.

Between the cones of tubes there are assembled three sheet metal cone spacers made of copper nickel, .020 inch thick. These metal spacers are inserted to form a seal between the cones of tubing. Three are used to provide sufficient flexibility to form a contour to fit the tube cones tightly. If only one spacer were used it would have to be of such thickness that it would require machining to make a tight seal. These spacer cones, acting as seals, insure that the feed water travels around the small passages that exist between the tubes and the spacer cones (Figure 2-1). Having the cone shaped coils wound both left and right and installed alternately prevents their interlocking when forced tightly together. Inside the inner cone of tubes there is another sheet metal cone, the inside of which is sealed off and has no working

 
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purpose. The upper plate of this cone is the floor of the vapor separator. The vent pipe passes out through the bottom of this cone.

3A7. Feed water flow. The incoming water enters from the single feed inlet pipe to the triangular spacers between the tubing and spacers and flows up through a path about 30 feet long before it emerges at the top. During this flow, heat transfer takes place by conduction through the walls of the tubing, (a) heating the feed water gradually to boiling, (b) vaporizing two-thirds of it, (c) condensing the vapor from the compressor, and (d) cooling the condensed liquid.

3A8. Electric heaters. The heat exchanger projects part way up into the upper cylindrical portion of the shell. Here, between the cones of tubes and the shell, is a narrow space into which the water, now at the boiling point, enters (Figure 2-2). The eight electric heaters, spaced equally around the shell, extend into this space. The water level is maintained above the tops of these heaters.

The heaters are 500 watts, 125 volts special chromalox immersion type, of hairpin design (Figure 3-1). They measure 17 5/8 inches over-all in length; 14 inches immersion length; 12 5/8 inches active heating length. Two heaters are wired in series to each switch, requiring four heater switches. Replacement heaters are carried in the spare parts box, with a special wrench for removing and installing them.

CAUTION. The electric heaters should be turned on only when submerged as they will burn out unless covered with water. The large quantity of heat produced is safely carried away by the surrounding water.

3A9. The vapor separator. The vapor separator is enclosed by an open cylinder extending downward from the upper head plate. This cylinder is concentric and inside another open end cylinder extending upward from the conical shaped filler for the heat exchanger. The floor of the separator is formed by the bottom of the outer cylinder and lies about 4 1/2 inches below the topmost coil of the heat exchanger. The vapor separator is thus a separate enclosed chamber. The vapor from the boiling water rises in the narrow space between the shell and the outer separator wall; it then descends between the walls, and enters the separator

  Figure 3-1. Electric heater.
Figure 3-1. Electric heater.

chamber. This circuitous passage of the vapor causes any mist of liquid that may be carried up by the vigorous boiling action to separate from the vapor; hence the name-separator. Such liquid will of course not be distilled, and must be prevented from entering the vapor compressor or it will contaminate the distilled water. The separated liquid collects on the separator floor and drains out through the vent pipe.

3A10. Vapor baffle. On entering the separator, the vapor first strikes against a baffle. This baffle, cylindrical in shape, is attached at the top to the upper head plates. It extends to 1 inch above the separator floor and is located 1 inch inside the outer separator wall. This arrangement insures that the vapor, after passing through the narrow inlet opening at the top, passes down and through the free end of the baffle and into the separator chamber.

 
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3A11. Vent pipe. The vent pipe is a 1/2-inch pipe extending from the hole in the middle of the separator floor to which it is connected, down through the center axis of the unit and out. The external end is open to the atmosphere.

3A12. Water level. The water level in the unit is maintained at about 1/2 inch above the topmost coil of the heat exchanger by means of two overflow pipes, diametrically opposite each other. Figure 2-2, being a schematic view, shows only one of the overflow pipes.

3A13. Overflow pipes. These two pipes, called low overflow pipes, carry the undistilled and concentrated brine down through the overflow heat exchanger. For safety purposes a second pair of high overflow pipes is placed between the regular short overflow pipes; they too drain into the overflow heat exchanger.

3A14. Vent damper. The vent damper is a device connected to the vent pipe of the unit in order to damp out wide and sudden fluctuations of air pressure. Such fluctuations occur on occasion in certain types of submarines when a torpedo is fired or during a quick dive, or under other conditions.

The distilling unit is sensitive to changes of air pressure because the surface of the boiling water is open to the atmosphere inside the submarine through the vent. With rapid changes of pressure, the unit will stop operating since the penetration of the air through the vent, reaching the space where the water is boiling, will cause a sudden increase of compressor pressure. This difficulty is overcome by installing the dampening device on the vent pipe. A functional diagram of this device is shown in Figure 3-2.

The vent damper is a Y-shaped piping arrangement connected into the vent. One upper branch of the Y is open to the air through a 1/16-inch hole in a diaphragm. The small size of this hole causes any wide and sudden changes in hull pressure to be communicated very gradually to the surface of the boiling water in the unit. A stop valve is placed at the end of this branch for a good supply of air at starting. This valve should be opened

  Figure 3-2. Vent damper.
Figure 3-2. Vent damper.

wide when starting the unit, and should be shut after the unit is operating.

The lower branch of the Y leads down into an open top seal cup which is about 4 inches in diameter and 5 inches high. This cup should be filled with water to the level of the overflow connection before starting the unit.

The action of the dampening device is as follows: If the air pressure in the hull decreases, there will be a small discharge of steam into the water in the seal cup with no other apparent changes. If the air pressure in the hull rises, the increased pressure on the water in the open seal cup will force some water up the seal pipe, to balance the difference in pressure between the unit and the hull. Air will gradually pass into the unit through the diaphragm 1/16-inch hole and equalize the pressure at such a rate that the unit will have time to adjust itself to the changed conditions without stopping.

The unit will operate normally during this adjustment period and the only difference noticeable will bean increase in pressure of the compressor. The pressure will gradually drop back to normal. Any liquid running from the vent will pass out of the seal pipe and overflow into the funnel, as it would without the attachment, under all pressure conditions in the submarine.

 
B. THE TWO-LOBED ROOTS-CONNERSVILLE COMPRESSOR
 
3B1. Impellers. The vapor is compressed by the rotating action of the two double-lobed impellers,   each a one-piece bronze casting, accurately machined. They are, in effect, a pair of two-tooth
 
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338450 0-55-2

gears of involute form. The drive is by belt from a motor mounted above the compressor case to a pulley on the shaft of one impeller. Opposite to the drive end, a pair of one-to-one precision gears turns the other impeller. Reference to the circular inset view in Figure 2-1 shows this construction clearly. Figure 5-1 shows an exploded view of the two-lobed compressor.

3B2. Impeller gears. The impeller gears run in an oil bath contained in an oiltight housing. The shafts pass out through packing glands. An oil level indicator is provided on the gear housing. See also Section 5B1.

3B3. Impeller housing. The impellers are enclosed in their own housing which has semicircular ends (Figure 2-2). The vapor enters from the vapor separator, passes through channels to the top of the compressor, is carried around between the impellers and the casing, and is discharged as compressed vapor to the heat exchanger.

3B4. Impellers not lubricated. There is no contact either between the impellers or between the impellers and the impeller housing. There is a slight clearance of a few thousandths of an inch around all faces of the impellers. Therefore no lubrication is needed inside this housing.

3B5. Slip. Since there is higher pressure on the discharge side than on the inlet or suction side, there is a backward slippage of the vapor. This slippage is slight, and reduces the compression only by a very small amount.

3B6. Compressing action. As the impellers rotate in opposite directions, each in turn alternately cuts off a pocket of vapor when it reaches a vertical position, as is shown for the left impeller in Figure 2-2. When this impeller reaches the position where that pocket of vapor may escape, the impeller lobes, continuing to rotate, squeeze or com press the vapor. This type of compressor is very efficient. The fact that no oil is needed inside the compressor housing insures that no oil can get into the distilled water.

3B7. Compressor motor. A 7 1/2-hp motor with necessary starting and protective electrical equipment

  is bolted on top of the compressor casing. The drive to the compressor shaft pulley is by four texrope V-belts.

3B8. Variable pitch drive. The drive pulley on the motor is of the adjustable or variable pitch type. The amount of variation of pitch is small, 5.400 to 6.600 inches' pitch diameter of the pulley, and is intended only to adjust the tension of the belts. The four left-hand sides of the pulley grooves are attached to a sliding sleeve. Rotating this sleeve moves the left-hand sides toward or away from the four stationary right-hand sides. Since the belt grooves are V-shaped in section, this motion increases or decreases the pitch diameter.

Adjusting the variable pitch drive. Loosen the setscrews on the sleeve. Turn the adjustable part of the pulley with the special spanner wrench found in the spare parts box until the belts are at proper tension. The proper tension is that which gives the belts, when running, a bow of about 1 inch on the slack side. Then tighten the setscrews.

3B9. Upper head plate. This heavy copper nickel plate is 13/16 inch thick. It is the main support of the distiller, and is fastened securely to brackets which are bolted to a bulkhead. To it is bolted the shell of the unit. In the bottom of the head plate, inside the shell, is fastened a casing of 3/16-inch thick copper-nickel, forming a separate compartment 3/4 inch high and of nearly the same diameter as the shell.

Four short 1 3/4-inch o.d. tubes are set into the head plate and direct the vapor from the separator to the compressor, without permitting it to enter the head plate compartment (see Figure 2-2). After the vapor is compressed, it is discharged from the compressor down through a 3-inch hole into the upper head plate compartment. The vapor leaves this head plate compartment, or discharge vapor space, at the sides through eight 1-inch o.d. pipes called upper headers, which lead down to the heat exchanger tubes.

NOTE. The Roots-Connersville two-lobe compressor has been replaced on most submarines by the General Motors three-lobe compressor. This compressor is described in Section 7B.

 
C. CONTROL DEVICES
 
3C1. Pressure gage. A 0- to 15-psi pressure gage (Figure 3-3) is connected into the discharge vapor   space of the upper head plate, which, for operating control, provides continuous reading of the
 
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Figure 3-3. Pressure gage.
Figure 3-3. Pressure gage.

pressure of the vapor going into the heat exchanger.

3C2. Vent thermometer. A distant reading dial thermometer indicates the temperature in the vent pipe. The bulb of the thermometer (Figure 3-4), inserted in the vent pipe, is connected by a 9-foot armored capillary tubing to the dial which is graduated from 30 degrees F. to 240 degrees F.

3C3. Weir. The weir (Figure 3-5) measures the rate of flow of the overflow brine discharge. The overflow pipe leads out at the bottom of the unit,

  then turns vertically upward along the side to such a height that the interior overflow heat exchanger is always full of liquid. The top of the pipe is open, and also very near the top is an open vertical slot 3 inches long and 1/16 inch wide. This slot is the weir, through which the liquid flows. The weir has a scale alongside it, and the height of the liquid pouring through the weir indicates the rate of flow in gallons per hour (gph), the maximum reading being 50 gph.

Just below the weir slot is a cup, 3 1/4 x 6 x 2 inches high, surrounding the weir pipe and silver

Figure 3-4. Vent thermometer.
Figure 3-4. Vent thermometer.

 
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brazed to it, into which the liquid falls. From the bottom of the cup the brine flows through a drain pipe, to a temporary brine receiver tank, and finally to the sea.

Care of the weir. The weir slot must be kept clean and free of any deposit at all times, otherwise the readings will be in error.

Figure 3-5. Weir.
Figure 3-5. Weir.

Reading the weir. The liquid flows out of the slot and down into the cup in a curve. Care should be taken in reading the scale not to sight this outside curving part of the flow against the scale, or the reading will be too low. One should sight through the slot, reading the highest level of the liquid inside the weir against the scale. With sea water feed, there should always be a minimum of 20 gph flowing.

3C4. Relief valve. This valve (Figure 3-6) is located on the upper head plate adjacent to the compressor. It connects through the head plate into the compressor discharge space, to prevent overloading of the compressor motor. The valve is normally closed under spring pressure set at 7 1/2 psi. It can also be manually opened at any time by lifting the lever. It is a safety valve, not a control valve.

 

Figure 3-6. Relief valve.
Figure 3-6. Relief valve.

3C5. Bypass valve. The bypass valve (Figure 3-7) is not a separate valve, nor connected into the system as ordinary valves are. It is, instead, an integral part of the upper head plate. The bypass valve opening connects the compressor discharge space and the vapor chamber above the boiling sea water (Figure 2-1). The round part at the bottom is a bale and is open at both ends (Figure 3-7). The bypass valve is normally closed during distillation, but it is temporarily opened at starting, as described in Section 4B1.

 
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Figure 3-7. Bypass valve.
Figure 3-7. Bypass valve.

3C6. Pressure reducing valve. The pressure reducing valve (Figure 3-8) is connected into the sea water feed line between the feed pump and the feed water strainers. The incoming pressure through the pump may vary from 35 to 150 psi.

This reducing valve measures 9 3/4 inches in height. There are two separate airtight compartments in the valve, divided by a rubber diaphragm. In the upper compartment is a spring, which may be set to provide a given reduced pressure by means of the adjusting screw. The cover cap over the adjusting screw is secured by a padlock to prevent tampering.

The lower compartment is further divided into two separate spaces by a small piston attached to the middle of the stem, the piston sliding in a cylinder (Figure 3-8). The stem has whole drilled through from its lower end to just above the piston, where a port leads out into the space above the piston. Figure 3-8 shows how the feed water bears both upward against the piston and downward against the valve disk, thus balancing. The water in the outlet side of the valve also flows up through the stem and bears against the diaphragm, keeping the spring in balance at its set pressure.

  The total resultant pressure of these opposing forces is the desired reduced pressure asset by the spring. The piston-and-stem arrangement further tends to damp out vibrations caused by pressure surges of the feed water.

Figure 3-8. Pressure reducing valve.
Figure 3-8. Pressure reducing valve.

3C7. Flow control valve. A flow control valve (Figure 3-9) is installed in each feed line going to the two units. This valve, sometimes called a feed valve, is a conventional globe valve, installed just after the feed water strainers. A scale alongside the handle stem indicates the number of turns which have been given, and a dial on the

 
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Figure 3-9. Flow control or feed valve.
Figure 3-9. Flow control or feed valve.
  stem shows the amount of any one turn. Thus any position of the valve may be precisely read, and exactly repeated at a later time. The valve is so designed that equal openings give equal increases in the rate of flow.

3C8. Feed pump. The main sea water supply to the unit is fed in by a centrifugal type motor-driven feed pump, bulkhead mounted, capable of delivering 3 to 4 gallons per minute of water at 30 psi gage pressure. The feed may also be from auxiliary salt water supply, or from fresh water supply.

3C9. Water tanks. a. Distilled water. The distilled water, from both units, flows into a distilled water receiver or tank (Figure 2-3), made of nonferrous metal, of approximately 46 gallons capacity. Air at 10 psi is admitted at the top of the tank to give a head pressure. A petcock is provided for sampling. There is also a vent and a drain to the bilge. Piping connections lead to the desuperheater tank, to the battery water tanks, and to the ship's tanks.

b. Brine receiver. The overflow of concentrated brine flows from the weirs to a brine receiver or tank, made of copper nickel, of approximately 23 gallons capacity. Air at 30 psi is admitted at the top of the tank to provide a head when discharging overboard. There is a vent and a drain to the bilge. The drain to the bilge has a side-swing connection leading either overboard or to fresh water storage when feeding fresh water.

 
D. THE DESUPERHEATER
 
3D1. Desuperheater. An 8-gallon desuperheater tank, fed by a pipe from the distilled water tank (Figure 2-3), is supported above the units. A water level gage is attached to the desuperheater tank, and an overflow pipe leads to the bilge. From the bottom of the desuperheater tank, a 1/4-inch tube leads to each of the compressors and into the impeller housings above the impellers. Valves in these tubes are adjusted to cause the distilled water to flow as drops, not its a steady stream on the impeller lobes. Since the drip is inside the compressors and hence not visible, a sight feed glass is inserted in each tube just outside the compressor with a glass window through which the water drops may be seen to pass. In   normal operation of the units the desuperheater flow is at a rate of 200 drops or more per minute. This is a very rapid flow and is the rate that exists just before the flow becomes a steady stream in the sight glass.

3D2. Need for desuperheater. When steam generated by boiling liquid at atmospheric pressure and a temperature of 212 degrees F. is compressed mechanically to a pressure between 3 to 6 psi, the steam is superheated and reaches a temperature of 285 degrees to 400 degrees F. in the compressor. If this compression is carried on in the presence of water, the water removes the superheat from the steam and allows it to pass into the distiller at a temperature of saturated steam, which is 222 degrees F. at 3 psi and

 
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230 degrees F. at 6 psi gage. Desuperheating is needed for two purposes

a. Water from the desuperheater tank dripping on the impellers keeps the impellers and their shafts cooled. This cooling action prevents too great an expansion of the impellers by heat, thus retaining the required clearance of the impellers. It also prevents the shaft packing from getting too hot, which would cause rapid deterioration of the packing.

b. Better heat transfer is obtained from saturated steam than from superheated steam. A

  rapid rate of heat transfer is necessary to assist in keeping the feed water boiling; the quicker the steam condenses, the lower the pressure on the discharge side of the compressor will be.

Distilled water must be used for this desuperheating process. Ordinary fresh water contains various minerals and chemical compounds. These substances, while harmless to human beings, would be deposited on the impellers (since only the water vaporizes) and would gradually build up to a thickness that would cause the impellers to bind.

 
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