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MECHANICAL DETAILS OF THE
MODEL X-1 DISTILLING UNITS
 
A. GENERAL DESCRIPTION
 
7A1. Main unit or evaporator. This part of the distilling system is referred to as the evaporator because it is therein that the boiling and condensing, that is, actual distillation, takes place. The evaporator, exclusive of compressor and motor, is cylindrical in shape, with a 12- X 15-inch manhole opening into the space above the tubes of the steam chest. This cylindrical shell is covered with a thick layer of glass wool insulation held in place by a stainless steel jacket.

7A2. Steam chest. Inside the insulated jacket the lower part of the cylinder is the steam chest, where the sea water is vaporized, and distilled water condensed. It consists of 334 admiralty metal tubes, each 16 1/4 inches long with a 3/4-inch o.d. These tubes are set side by side, as shown in Figure 6-1, and enclosed in a shell. At the top and bottom of the shell, the ends of these tubes are expanded into holes in the tube sheets.

Standing among the tubes are two angular sheet brass baffles 14 1/4 inches high. A pipe leads horizontally into the steam chest about 2 1/2 inches from the top and bends down into the corner of the inner baffle extending to 2 inches from the bottom. This section of pipe, called the steam chest vent, is pierced with nineteen 1/16 inch holes, in two rows staggered along the side toward the open part of the baffle. The baffles cause any non-condensable gases such as air to flow to the closed end of the inner baffle, where they pass out through the steam chest vent. See also Section 7A9.

The steam chest also contains the electric heaters, center downtake pipe, and overflow pipe.

7A3. Electric heaters. (See Figure 3-1.) The eight electric heaters are contained in eight 1 3/4inch tubes which are spaced equally around the outer diameter of the tube area of the steam chest. The feed flows through these tubes, coming in contact with the heaters.

  7A4. Downtake. In the center of the bundle of tubes is a 4-inch pipe with ends expanded into the tube sheets. This is called the downtake. The feed enters the evaporator through a pipe above the steam chest, passing down through the downtake where it comes in contact with the concave head which is bolted to the bottom of the steam chest. The concave head is watertight, hence the feed cannot pass beyond it. As more feed is supplied through the downtake pipe, it floods up through the 342 tubes.

7A5. Feed. The feed inlet pipe in the evaporator extends horizontally to the center and there branches into a Y, the two ends of which tun down and extend just below the funnel (Figure 6-1). The Y-ends actually pass through the funnel wall. The newly incoming feed pours into the downtake, mixing with the vaporized portion of the feed already in the steam chest.

7A6. Overflow. The overflow is a 1 3/4-inch o.d. pipe which is inserted inside the downtake (Figure 6-2). At the top of this overflow pipe is brazed a 4-inch funnel. The top of the funnel is 2 inches above the top of the steam chest, maintaining the liquid level in the evaporator at this height. The overflow pipe leads out through the bottom of the evaporator to the heat exchanger, where it gives up its heat, raising the temperature of the incoming feed.

7A7. Portion of sea water distilled. About one-half to two-thirds of the feed is vaporized. The remaining one-third overflows continuously into the funnel, flowing out of the evaporator to the heat exchanger giving up its heat to the incoming feed water. From there it goes to the brine receiver where it is drained to bilges or blown to sea.

7A8. Vapor separator. The vapor separator is an interior compartment, as in the Model S unit,

 
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containing similar baffles (Figure 6-2). The vapor, entering at the top of the separator, descends between the walls, and enters the separator chamber from where it travels upward to the compressor suction.

This circuitous passage of vapor causes any mist of liquid that may be carried up by the vigorous boiling action to separate from the vapor and fall on the separator floor. In the original installations, the separated water drains out through the separator drain pipe, manometer, and a check valve into the evaporator vent pipe and thence through the heat exchanger into the bilges. In the units which have been altered for operation while snorkeling, the drain from the separator runs into a brass seal cup inside the unit and overflows with the brine.

In the original fleet type installations, the water boils at substantially atmospheric pressure inside the vessel. The contact with the atmosphere is through the separator drain pipe and manometer. The manometer (to be described later) is an instrument which indicates the pressure of the steam in the vapor separator. If the pressure indicated is above or below that found in the vessel, adjustment of the heaters and/or the feed must be made.

In the converted units (see Figure 6-4) there is no vent from the vapor space inside the unit to the external pressure. The vapor pressure of the boiling water is registered on a compound gage which registers either vacuum (inches of Mercury) or pressure (inches of water). There is a pressure-static switch connected to the gage piping which is set to automatically control four (4) heating elements to maintain the vapor pressure at a positive pressure between six (6) and twelve (12) inches of pressure.

This arrangement minimizes the effect of variable hull pressures on the operation of the unit.

7A9. The steam chest vent. A pipe leads horizontally into the steam chest about 2 1/2 inches from the top and extends down into the steam chest to within 2 inches of the bottom. This section of pipe, called the steam chest vent, is pierced with nineteen 1/16-inch holes, in two rows staggered along the side toward the open part of the baffle, as described in Section 7A2.

In this horizontal branch pipe, just outside the evaporator, is a, union. Inside the pipe at this point is a 100-mesh strainer and a diaphragm with

  a 3/64-inch hole or orifice bored in its center. The purpose of this small orifice is to vent air (which is not condensable) from the steam chest. Normally only a very small amount of air is present, hence the orifice is made small. The diaphragm also prevents any drainage from the vapor separator drain pipe and manometer from entering the distilled water through the steam chest vent, in the ease of the units still not converted. See Section 7A8.

7A10. Steam trap. The condensate in flowing from the evaporator to the heat exchanger passes through the steam trap. The steam trap prevents any steam from flowing from the steam chest into the heat exchanger. In so doing, it automatically keeps the compressor discharge pressure at the required value, sufficient to raise the condensation point of the vapor and thus produce condensation in the steam chest, regardless of the condition of the heat transfer surface. Pressure within the steam chest is necessary to provide a head against which the compressor can work.

The steam trap takes the place of, and serves the same purpose as the retarders in the Model S unit.

The steam trap is of conventional type, roughly spherical in shape, 4 3/4 inches inside diameter. The condensate water enters horizontally at one side and leaves at a point diametrically opposite. The outlet is a short tubular extension with a flat bolted cover, holding the connection and working parts. The inside dimension horizontally from inlet to cover plate is 5 1/8 inches.

Normally the steam trap is about half full of condensate (water), the level of which is at the top of the inlet opening. A spherical float connected by lever action to a pin valve in the outlet, rises and falls with the condensate level, permitting the condensate to flow out only as fast as it flows into the trap.

The action of the steam trap is as follows : The condensate leaving the steam chest contains some uncondensed steam mixed with it. In the steam trap, the condensate fills only the lower half of the enclosed space. The upper half is steam space. When the waster drops below its normal level the float drops with it, shuts the outlet valve, and remains shut until the water level again rises. There is a permanent bypass around the valve of the steam trap, from the vapor chamber to the

 
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outlet pipe. This bypass is 1/32 inch in diameter at its smallest point and serves to prevent the trap from becoming air bound. Before the bypass valve on the distiller is shut, air is discharged through the trap. If the permanent bypass is plugged, the trap must be manually vented before the unit will operate. On the top of the trap there is a small valve which can be opened manually to vent off large quantities of air.

The condensate normally flows from the steam chest as it is formed. When the flow of condensate is restricted, pressure will build up in the steam chest. This may be caused by the float being stuck in the closed position or the orifice in the bypass being plugged. If this condition occurs, vent air from the steam trap by hand; if the pressure comes down and gradually builds up, it is a good indication that the trap is not operating properly. The unit should be secured and the steam trap repaired.

7A11. Pressure gage. A 0- to 15-psi pressure gage (Figure 3-3) is connected in the vapor compressor discharge line. The compressor pressure gage indicates the discharge pressure of the compressor and the pressure at which the compressed steam condenses on the outside of the tubes of the steam chest.

Abnormal compressor discharge pressure is the first indication of trouble. The normal compressor discharge pressure varies with the speed of the compressor and the scale conditions inside the tubes of the steam chest where the sea water is vaporized. At constant compressor speed the compressor discharge pressure is a direct indication of the amount of scale in the evaporator.

  7A12. Relief valve. A relief valve (Figure 3-6) is located on the upper head plate adjacent to the compressor. It connects into the vapor compressor discharge line and prevents 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.

7A13. Feed pump. The main sea water supply to the unit is provided by a centrifugal type motor-driven feed pump, capable, of delivering 3 to 4 gallons per minute at 30 to 40 pounds gage pressure. The feed may also be from the variable ballast tanks, or from the fresh water supply.

7A14. Water tanks. a. Distilled water. The distilled water, from both units, flows into a distilled water receiver or tank (Figure 6-3), made of nonferrous metal, of approximately 46-gallon 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 system, and to the ship's fresh water system.

b. Brine receiver. The overflow of concentrated brine flows from the heat exchanger to a brine receiver or tank, made of copper nickel, of approximately 23-gallon 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.

 
B. THE THREE-LOBED GENERAL MOTORS COMPRESSOR
 
7B1. Description. The General Motors vapor compressor is of the positive displacement type consisting of two rotors enclosed in a special compact housing designed for mounting on evaporators.

Each rotor has three helical lobes designed to produce a continuous uniform flow of vapor. The vapor enters the compressor housing at the bottom and passes upward between the inner and outer walls to the rotor chamber where it fills the spaces between the rotor lobes as they roll apart. This

  vapor is then carried by the rotors in the spaces between the lobes around the cylindrical sides of the housing, producing a pressure at the bottom as the lobes roll together. Clearance is provided to prevent the rotors from touching each other or the surrounding housing.

7B2. Impeller gears. Opposite to the drive end, a pair of one-to-one precision helical gears turns the other impeller. The impeller gears run in an oil bath in an oil-tight housing. The shafts pass through packing glands in the housing.

 
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7B3. Lubrication. The compressor is lubricated from two reservoirs, one at each end. Each oil reservoir is supplied with an oil level indicator, which is attached to the compressor housing. The oil level should be checked every 24 hours and oil added as needed when the compressor is not running. The oil should be changed when the evaporator is cleaned.

Two vertical 1/2-inch nipples closed with pipe caps (Figure 9-1, 89) are provided for filling the oil compartments. Remove the pipe caps and pour the oil into the nipples until the proper level is reached on the gage.

The oil may be drained from the compartments by opening the 1/2-inch plug cocks (Figure 9-1, 17).

Oil is retained in the oil compartments by the use of slinger rings (Figure 9-1, 33 and 34) and steam leakage is prevented by the use of stuffing box glands (51 and 53).

These glands should be adjusted so that they will be just tight enough to prevent leakage. Excessive tightness will damage the packing and shaft sleeves, causing excessive heating and the impellers to stick. The gland nuts must be tightened evenly.

The reservoir of oil at the pulley end of the compressor lubricates the ball bearings by a slinger attached to the driven shaft. The reservoir of oil at the opposite end from the pulley lubricates the timing gears and ball bearings at that end of the compressor by the splashing of the gears.

7B4. Steam packing. When steam leakage cannot be stopped by tightening the glands, new packing must be installed. This may be done without dismantling

  the compressor. First loosen the nuts and back out the gland (Figure 9-1, piece 53).

Each stuffing box is packed with five rings of Johns-Manville No. 350.

Remove all five packing rings, using the packing hook found in the spare parts box. Install five rings of No. 350 packing and install the gland. Tighten gradually and evenly as recommended.

The split packing gland may be removed from the shaft during the packing operation if it is found necessary.

7B5. 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.

7B6. 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 an inch on the slack side. Then tighten the setscrews.

 
C. CONTROL DEVICES ON MODEL X-1 DISTILLING UNIT
 
7C1. Manometer. The manometer (Figure 7-1) is used on the units which have not been converted for operation while snorkeling. It consists essentially of a brass framework holding two glass tubes, one tube within the other. The inner tube is smaller in diameter and approximately twice the length of the outer tube. The outer and larger tube is closed at the bottom and at the top it has two openings, one of which is connected to the drain from the vapor separator, the other being the manometer drain to the heat exchanger. The smaller or inner tube supported by the framework,   is open at both ends. The lower end is inside, near the bottom of the larger tube; the upper end projects out of the top of the outer tube and is open to the atmosphere.

During the time that the distiller is in operation the manometer contains a small amount of water normally supplied by drainage from the vapor separator. During the warming-up period, some water is drawn out of the manometer into the vapor separator. Just prior to the time that the distiller is ready to be cut in, the manometer should be filled to its normal operating level,

 
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which is at the top of the outer glass, with water from the desuperheater tank. A pipe connection and valve are provided near the bottom of the

Figure 7-1. Manometer.
Figure 7-1. Manometer.

  manometer for this purpose. While the distiller is in operation the upper surface of the water in the outer glass tube is subjected to the pressure from the vapor separator. This pressure may be either above or below atmospheric pressure. Since the upper end of the inner glass tube is open, the water in the inner tube is subjected to two forces: the vapor pressure caused by the boiling water in the distiller, and atmospheric pressure. The reading of the manometer is obtained by determining the difference in water level in the two tubes.

When the level in the small inner tube is above the level in the large outer tube, the pressure in the evaporator is above atmospheric. When the level in the small tube is below the level in the large tube, the pressure in the evaporator is below atmospheric. If the large tube is completely full, its level is at zero and the pressure in the evaporator is indicated directly in inches of water by the level in the small tube on the manometer scale.

The manometer is the primary guide in operating the unit, since it indicates exactly how the unit is balanced. The manometer level remains constant if the unit is in exact balance. When the unit is operating properly, the reading level is behind the collar holding the vapor inlet. If the reading level becomes visible above or below this collar, the unit should be adjusted. On units converted for operation with variable hull pressures, the manometer has been replaced with a pressure actuated switch and a compound gage. (See paragraph 7A8.) (See also Figure 6-4.)

7C2. The rotameter. Two of these devices are used for each distilling unit (Figure 7-2). One is inserted in the incoming sea water line into the heat exchanger to measure the rate of the feed flow, which is normally 70 to 90 gph. The other is in the outlet pipe from the heat exchanger to the brine receiver to measure the rate of overflow, which is approximately 1/3 to 1/2 of the total feed.

The rotameter is an upright pyrex glass tube about 14 inches long (exclusive of end fittings) through which the water flows. A metal casing with a plexiglas window protects it. The tube is tapered, with the small end at the bottom. Inside, a small metal rotor with a central hole slides up and down on a guide rod, and is caused to spin for free sliding by small vanes cut into its sides. Since the tube is tapered, the space between the

 
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rotor and the tube wall increases as the rotor rises, permitting more water to flow through that space. Therefore, the rotor will always rise to a height corresponding to the rate of flow at any particular time. A scale on the tube reads directly in gallons per hour.

7C3. Bypass valve. This valve (Figure 7-3) on the Model X-1 unit is located on top of the upper head plate. It is not built into the head plate as in the Model S unit, but is an individual device, bolted on. It is a stop valve and when open, it permits the compressor discharge to return directly into the vapor separator. It is opened when starting, and closed during distillation.

7C4. Feed regulating and flow control valves. In the feed line, just after the water filters leading to the heat exchanger, is the feed regulating valve (Figure 7-4) and the flow control valve (Figure 3-9). These valves control the flow of incoming feed water. The flow control valve is set to give any desired flow through the feed rotameter. The regulating valve then maintains the feed constant at this rate regardless of feed pressure changes.

The flow control or feed valve is used as a variable orifice. Any similar type valve could be substituted but the adjustment is much finer if a flow control type is used.

The regulating valve is a spring-loaded diaphragm type valve, similar to an ordinary reducing valve, having two watertight compartments separated by the diaphragm. It is installed with the spring and diaphragm below the feed line. Connecting into the feed line is a ball type valve with the lower or discharge side open to the compartment on top of the diaphragm and open to the feed line just before the flow control valve. The diaphragm acts on the ball to open or close the ball valve. The water pressure present just before the flow valve, is exerted downward on the diaphragm and tends to close the ball valve by displacing the diaphragm downward. The compartment below the diaphragm is connected into the feed line just after the flow control valve. A coil spring in this lower compartment exerts a pressure upward on the diaphragm equivalent to 5 psi in addition to whatever pressure is present in the feed line just after the flow control valve. Thus the pressure in the feed line just before the flow control valve will always be exactly 5 psi more than the pressure in the feed line just after the

 

Figure 7-2. Rotameter.
Figure 7-2. Rotameter.

 
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Figure 7-3. Bypass valve.
Figure 7-3. Bypass valve.
flow control valve. Once the flow control is set at any given opening, the flow through it will remain constant because of the constant pressure drop through it, regardless of pressure fluctuations in the feed line before the regulating valve or after the flow control valve.

The small spring on top of the ball serves to keep the ball in place.

7C5. Other valves in the feed line. Between the regulating and flow control valves, a relief valve

  set at 50 psi is connected into the feed line of each unit. A feed pump discharge gage is set into the line beyond the feed water filters and indicates the feed pressure just before the regulating valve.

7C6. Order of devices in the feed line. The devices of the feed line, starting from the hull, are as follows: stop valve, Macomb strainer, feed pump, filters (two), feed pump discharge pressure gage, feed regulating valve, relief valve, flow control valve, and feed rotameter.

 
D. THE DESUPERHEATER
 
7D1. Desuperheater. An 8-gallon desuperheater tank, fed by a pipe from the distilled water tank (Figure 6-3), 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 as a steady stream on the impeller lobes. Since the drip is inside the compressors and hence not visible, a sight feed glass with a glass window through which the water drops may be seen passing, is inserted in each tube just   outside the compressor. In normal operation of the units, the desuperheater flow is at a rate of 200 drops or more per minute. A rate of 200 drops per minute is a very rapid one. It is the rate that exists just before the flow becomes a steady stream in the sight glass.

7B2. Need for desuperheating. 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 pounds, the steam is superheated and reaches a temperature of 285 degrees to 400 degrees F. in the compressor. If this compression takes place in the presence of water, the water removes the superheat from the steam and

 
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Figure 7-4. Feed regulating valve.
Figure 7-4. Feed regulating valve.
allows it to pass into the distiller at a temperature of saturated steam, which is 222 degrees F. at 3 pounds and 230 degrees F. at 6 pounds 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.
 
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b. Better heat transfer is obtained from saturated steam than from superheated steam. A fast 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 health, 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.
 
E. HEAT EXCHANGER
 
7E1. Heat exchanger. The feed heat exchanger is a preheater which warms the incoming sea water feed. The sea water enters at ocean temperature, which varies according to location and season. It leaves the heat exchanger at about 200 degrees F. Hence, when the feed enters the evaporator, it needs to be raised only a few degrees more to reach the boiling point. The construction of this heat exchanger, and a diagram of the flow paths through it are shown in Figure 7-5.

The distilled water or condensate leaving the steam chest reenters the heat exchanger, at about the same temperature as the condensing steam in the steam chest (220 degrees F. at 3 psi), passing countercurrent (that is, in the opposite direction) to the feed flow, in order to assure the best heat transfer from the hot outgoing distilled water to the cool incoming feed, and thus warming the feed and cooling the distillate or condensate. The heat exchanger is of the double tube type. It consists of fifty 1 1/4-inches i.d., straight tubes, 45 inches long; and fifty 3/4-inch o.d., straight tubes, 48 inches long. The 3/4-inch tubes are externally finned with No. 18 gage copper nickel wire. There are also six tube sheets, three on each end. In the assembly of the heat exchanger, the large tubes are arranged in a bank and inserted in the inner tube sheets (Figure 7-5); the tubes are packed into these tube sheets with fiber and metallic packing to prevent leakage.

CAUTION. The metallic packing should not be installed so that it is in contact with the fresh water or condensate side of the heat exchanger.

The small tubes have their ends silver-soldered into a bushing at each end. This assembly is inserted inside the larger tubes. The ends of the small tubes with the bushing extend out of the larger tubes about 1 1/2 inches on each end and through the outer tube sheets. The bushing is packed into these tube sheets. The tube sheet

  cover plates seal the ends of the heat exchanger. The outer tube sheets and the tube sheet cover plates contain milled passages which direct the flow of water. The entire tube bundle is enclosed in a brass casing for protection.

7E4. Flaw paths in the heat exchanger. There are four distinct flow paths through the heat exchanger. The condensate flows through 48 large tubes, around the smaller tubes. Steam from the vent pipe and drainage from the vapor separator flow through two large tubes around the smaller tubes. Feed water flows through 36 of the small tubes. The brine overflow from the steam chest flows through 14 of the small tubes. The flow in the large tubes is in the space left around the small tubes. The small tubes are connected by return headers at each end, and in such away as to provide two separate longer paths of water flow. The large tubes are likewise connected by return headers at each end in such a way as to provide two separate longer paths of water flow, each completely separate from the others.

The flow diagrams in the lower right corner of Figure 7-5 show these four separate paths. The diagrams show the pipes as viewed from the inlet and looking toward the rear or return end; that is, the pipes are represented as circles. In order to describe the flow paths in a simple manner, the tubes are designated in accordance with the following plan.

The rows are designated by letters starting with the top row. Capital letters refer to the large tubes. Small letters refer to the 3/4-inch o.d. tubes. The tubes in each row are designated by numbers from 1 to 6 starting at the right. In the diagram only the top row has the numbers; each row is similarly designated, starting from the right. Two diagrams are used in order to simplify the presentation of the four paths. In the following description, the term goes forward

 
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Figure 7-5. Heat exchanger.
Figure 7-5. Heat exchanger.
means that the flow is away from the observer as he looks at the flow diagrams in Figure 7-5; that is, from the inlet and toward the return end. The term comes back means that the flow is in the opposite direction; that is, toward the observer, or from the return end to the inlet end. When it is said that the flow goes forward in a1, comes back in a2, it is naturally understood that the flow crosses from a1 to a2 in the return end header. When it is said that the flow comes back to a2, goes forward in a5, it is understood that the flow crosses from a2 to a5 in the inlet end header.

Feed flow. The feed flows through 36 small tubes, entering cold at the top of the heat exchanger. It goes forward in a1, comes back in a2, goes forward in a5, comes back in a6, goes forward in b6, comes back in b5, goes forward in b2, and comes back in b1. Continuing, it follows the same route in rows c, d, e, f, g, h, and i, emerging hot from tube i6 at the inlet end of the heat exchanger, as may be seen in Figure 7-5.

Overflow path. The hot brine overflow uses the remaining 14 small tubes, entering hot at the bottom of the heat exchanger. It goes forward in h4, comes back in h3, goes forward in g3, and comes back in g4. Continuing, it follows the same route, as may be seen in Figure 7-5, in rows f, e, c, b, and a, emerging cool from tube a4 at the inlet end of the heat exchanger.

Condensate path. The condensate flows in 48 of the large diameter tubes, entering hot at the bottom

  of the heat exchanger. It goes forward in I2, comes back in I1, goes forward in H1, comes back in H2, goes forward in H3, comes back in H4, goes forward in H5, comes back in H6, goes forward in G6, comes back in G5, goes forward in G4, comes back in G3, goes forward in G2, comes back in G1, and goes forward in F1. Continuing, it follows the same route, as may be seen in Figure 7-5, through the remaining rows. The condensate emerges cool from tube A1 at the inlet end of the heat exchanger.

Vent flow. Steam and noncondensable gases from the manometer and steam chest flow inside the remaining two large tubes in the bottom row, where the; steam is condensed and the gases cooled. They enter hot and go forward in 16; and come back in 15, emerging somewhat cooled from tube 15 at the inlet end of the heat exchanger.

7E3. Step-wise heat transfer. It may be noted that the heat transfer takes place in a step-wise manner in the heat exchanger. The hot condensate gives up some heat to the feed in tubes I2, I1, H1, and H2; picks up a little heat from the overflow in G4, and G3; gives up heat to feed in G2, G1, F1, and F2; and continues this step-wise heat transfer throughout its path to A1. The loss of heat from condensate to feed, however, is always much greater than the gain of heat from overflow to condensate, so that the total result is a large heat transfer and overflow to feed.

 
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