US3483848A

US3483848A - Vapor generator with integral economizer

Info

Publication number: US3483848A
Application number: US689747A
Authority: US
Inventors: Richard Allen Green
Original assignee: Babcock and Wilcox Canada Ltd
Current assignee: BWXT Canada Ltd
Priority date: 1967-12-01
Filing date: 1967-12-01
Publication date: 1969-12-16
Anticipated expiration: 1986-12-16

Description

. 16, 1969 v R. A. GREEN 3,483,848

VAPOR GENERATOR WITH INTEGRAL ECONOMIZER Filed Dec. 1, 1967 RICHARD ALLEN GREEN, DECEASED BAMONA BERNICE GREEN, EXEcUTRix.

AT T RNEY United States Patent 0 3,483,848 VAPOR GENERATOR WITH INTEGRAL ECQNOMIZER Richard Allen Green, deceased, late of Gait, Ontario, Canada, by Ramona Bernice Green, executrix, Galt, Untario, Canada, assignor, by mesne assignments, to Babcocir & Wilcox Canada Ltd, Gait, Ontario, Canada, a corporation of Canada Filed Dec. 1, 1967, Ser. No. 689,747 lint. Cl. F221) 1/02 1L8. Cl. 12232 13 Claims ABSTRACT OF THE DISCLOSURE A vapor generator wherein heat is extracted from a primary fluid flowing in tubes extending within a hollow vessel to vaporize a secondary liquid introduced into the vessel, the inflowing secondary liquid being first preheated to a given temperature in an economizer chamber contained within the vessel by heat transferred from a portion of the overall tube surface area, and the preheated liquid is passed into the volume of the vessel outside the economizer and comingled with recirculated secondary liquid for vaporization by heat transferred from the remaining portion of the overall tube surface area. The economizer chamber is arranged within the vessel in separated, spaced-apart relation to the internal boundaries thereof so that substantially no secondary liquid below the given preheat temperature contacts the internal boundaries of the vessel, thereby minimizing thermal shock to the vessel and its tube sheet, and promoting a more effective vaporization of substantially isothermal preheated liquid.

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates in general to vapor generators of the tube and shell type wherein heat extracted from a hot primary fluid flowing in tubes within the shell vessel is used to heat and vaporize a secondary liquid introduced into the shell vessel. More particularly, the invention is directed to a steam generator wherein the inflowing feedwater, which is the secondary liquid makeup, is first passed through an economizer chamber integrally contained within an enclosed hollow vessel for preheating before delivery into the portion of the vessel outside the economizer, where such preheated liquid together with recirculated secondary liquid is further heated and vaporized.

In accordance with the invention, preheating of the feedwater in the economizer is accomplished by heat transferred from a portion of the total effective surface area of the primary fluid carrying tubes, and vaporization of the preheated feedwater outside the economizer, but within the vessel, is accomplished by heat transferred from the remaining portion of the total effective tube surface area. Preferably, the economizer is so constructed and arranged with respect to the tube surface area used for preheating, that the feedwater leaving the economizer is heated to saturation temperature for the pressure existing within the vaporization zone of the vessel.

The economizer chamber is arranged within the vessel in separated, spaced-apart relation to the internal boundaries thereof, and the inflowing feedwater enters the economizer through a thermally shielded conduit that penetrates the vessel for communication with an external feedwater source. This serves to protect the vessel and tube sheet from thermal shock, since all feedwater below the preheat temperature is held within the economizer chamber, and substantially no liquid below the preheat temperature contacts any portion of the internal boundaries of the vessel.

As to the heating of the feedwater in the economizer, it should be noted that the general concept of the invention requires only that such feedwater heating be accomplished by heat transfer from a portion of the available tube surface area. It has been found that by arranging one or more, or even all of the tubes to have length portions extending through the economizer, a relatively economical construction results. However, it should be noted that other tube arrangements can be used to provide the same intended feedwater heating.

For example, a group of tubes can be weld united along similar length portions to define the lateral enclosure wall or walls of the economizer, in which case the inwardly facing semicylindrical portions of such tube lengths will be the feedwater preheating heat transfer surface area, and the outwardly facing semicylindrical portions of the same tube lengths, together with other tube length portions not associated with the economizer will be the vaporization heat transfer area.

The invention is readily adaptable for incorporation into preexisting vapor generator designs, as for example, that represented by US. Patent No. 2,862,479 to Blaser et al.

According to a preferred embodiment of the invention, wherein U-shaped tubes are utilized to convey the primary fluid through the interior of the vessel, and length portions of such tubes extend through the economizer, the inlet and outlet of the economizer are arranged to provide a feedwater flow path therethrough which is countercurrent to the primary fluid flow in the tube length portions within the economizer. The economizer preferably provides a fecdwater flow path which is dimensioned in relation to the effective surface area of the tube length portions inside the economizer to establish a net heat transfer rate between the primary fluid flowing through such tube length portions and the feedwater flowing along its preheating path, to raise the temperature of the preheated feedwater delivered at the economizer outlet to a value which exceeds the temperature of the primary fluid exiting the economizer in the tubes.

With the invention, it is thus possible to preheat the feedwater to a saturation temperature that is higher than the exit temperature of the primary fluid, for a given combination of primary fluid and feedwater flow rates, feedwater inlet temperature and primary fluid economizer entrance and exit temperatures.

Another important advantage afforded by the invention is that by reason of the separation of the economizer chamber from the internal boundaries of the vessel, it becomes possible to circulate freely the saturated liquid contained within the vessel along the inside wall surfaces thereof, and along the inside face of such tube sheets as are used in conjunction with the primary fluid tubes.

For example, with a vessel, U-tube bundle, tube sheet and downcomer shroud arrangement similar to that as disclosed by the aforesaid Blaser et a1. patent, saturated liquid recirculated or returned through the downcomer passage is directed to sweep across the tube sheet, flowing in the passage defined by the clearance between such tube sheet and the bottom of the economizer.

By providing a vapor generator with an economizer design which allows such saturated liquid circulation, the thermal stresses acting upon the vessel and the tube sheet are reduced due to the more uniform boundary temperature distribution effected by the circulation, the build-up of deposits on the vessel walls and the tube sheet is greatly reduced, and such circulation tends to promote a more effective vaporization of the saturated liquid in contact with the vaporizing surface portion of the tubes.

It therefore is an object of the invention to provide a vapor generator of the tube and shell type having an economizer integrally contained within the shell vessel for preheating the feed liquid prior to vaporization.

Another object of the invention is to provide a vapor generator as aforesaid wherein the feed liquid does not come in contact with either the vessel wall surfaces or any tube sheet exposed to the hot primary heating fluid until after such feed liquid has been preheated within the economizer.

A further object of the invention is to provide a vapor generator as aforesaid which allows free circulation of saturated liquid along the vessel Wall surfaces and across the tube sheet.

Still another and further object of the invention is to provide a vapor generator as aforesaid whereby the feed liquid can be preheated within the economizer to a saturation temperature higher than the exit temperature of the primary fluid from which has been extracted the heat required to eflect both preheating and vaporization of the feed liquid, as well as vaporization of a portion of the recirculated secondary liquid.

Other and further objects and advantages of the invention will become apparent from the following detailed description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevation view, partly in section, of a vapor generator according to a preferred embodiment of the invention.

FIG. 2 is a plan view of the vapor generator of FIG. 1 as taken looking downward along section line 2-2 therein.

FIG. 3 is an elevation view, partly in section, of an economizer chamber of different construction from that shown in FIGS. 1 and 2, and which can be substituted therefor.

FIG. 4 is a plan view, partly in section, of the economizer chamber shown in FIG. 3, as taken along section line 4-4 therein.

DESCRIPTION OF THE PREFERRED EMBODI- MENTS OF THE INVENTION In FIGS. 1 and 2, the vapor generator includes a vertically elongated pressure vessel 12 having a lower hemispherical head 16 closing the lower end thereof and a dished upper head 14. During normal operation of the vapor generator 10, there is a normal liquid level 22 which forms an upper vapor space 24 and a liquid space 26. Plate means including

plates

28, 30, 32 and 34 in conjunction with the walls of pressure vessel 12 coact to form a vaporization zone 36 and an annular downcomer passage 38, the

plates

32 and 34 functioning as a shroud laterally surrounding vaporization zone 36 externally and being itself surrounded externally by the wall of pressure vessel 12. Extending above the zone 36, cap plate 28 and wall plate form a prismatic extension chamber 40 for the collection of a vapor-liquid mixture.

Within vaporization zone 36 are disposed a multiplicity of U-shaped tubes 42 constituting a vertically elongated tube bundle. A tube sheet 44 located transversely of the longitudinal centerline of pressure Vessel 12 and secured thereto has a multiplicity of tube seats 45 formed therein.

The ends of the U-shaped tubes 42 pass through the tube sheet 44 and are secured in respective tube seats 45 to accommodate the flow of a hot primary fluid through each tube 42 from the inlet end thereof to the outlet end thereof. To aid and simplify the supply and return of the primary fluid, the inlet ends of tubes 42 are inserted into the tube seats 45 grouped together, as are the outlet ends of tubes 42 into tube seats 45a. The hemispherical head 16 is fastened to the outer edge of tube sheet 44 to enclose the outer face thereof and form in conjunction with a dividing baflie 48 a primary fluid inlet chamber 50, and a primary fluid outlet chamber 52, which respectively communicate with the inlet and outlet ends of tubes 42. Inlet and

outlet nozzles

54 and 56 respectively are disposed for connection to external primary fluid supply and return means (not shown) to supply hot primary fluid to inlet chamber and to withdraw spent primary fluid from outlet chamber 52.

Upright whirl chamber type vapor-liquid separators 58 are arranged in a ring in the upper portion of pressure vessel 12 to receive a vapor-liquid mixture which is emitted radially outward from the outlets 60 of the vaporliquid collection chamber 40.

Each of the separators 58 is similar to those described in US. Patent 2,862,479 to Blaser et al., these whirl chambers 58 being arranged so that the normal liquid level 22 is positioned at the upper one-third of their respective heads. Each separator 58 has an outwardly directed extension piece 62 at its liquid discharge end arranged to direct the separated liquid, which ordinarily is at saturation temperature corresponding to the pressure within vessel 12, into the downcomer passage 38 adjacent the vessel 12 wall.

Separated vapor passes out the upper ends of the separators 58 into vapor space 24 and thence through the vapor outlet 68 of vessel 12 to a point of use.

The makeup feed liquid enters the vessel 12 by a thermally shielded inlet pipe 70, which communicates with the lower portion of an economizer chamber 72 disposed within the vessel 12 in separated, spaced-apart relation to the internal boundaries thereof to accommodate preheating of the infiowing feed liquid to a predetermined temperature. T he economizer 72 has an outlet 74 communicating with the interior of vessel 12, or more specifically, with the vapor generation chamber 36 thereof. to deliver thereto preheated secondary liquid at the established preheat temperature, which temperature is preferably the saturation temperature for the pressure prevailing within vessel 12.

The tubes 42 are in general arranged to transfer heat from a portion of their total effective surface area to the feed liquid in the economizer chamber to preheat such liquid to the intended economizer outlet temperature, and the tubes are further arranged so as to transfer heat from the remaining portion of their total effective surface area to the preheated liquid within vaporization zone 36 to vaporize sarne. Consequently, respective portions of the total heat extracted from the primary fluid which flows through tubes 42 are utilized to preheat the feed liquid within the economizer chamber and to vaporize the preheated liquid in vaporization zone 35, and therefore substantially no liquid below the preheat temperature contacts the internal boundaries of the vessel 12 or the zone- 36 side of tube sheet 44. For the embodiment specifically shown by FIG. 1 the U-shaped tubes 42 are disposed in generally parallel relation to one another and convey the primary fluid in respectively similar directions from their individual inlet ends to their outlet ends.

As can be noted from FIG. 1, the economizer chamber 72 has a plurality of transversely extending baffles 76 which serve to define a sinuous feed liquid flow path extending from the entrance of inlet pipe to the outlet 74. The provision of such baffles 76 affords a more cificient utilization of the available tube heating surface allocated to feed liquid preheating.

By way of example, FIG. 1 shows the downcomer shroud plate 34 as defining one of the lateral boundary walls of economizer chamber 72, a vertical plate 78 defining another lateral boundary wall of said economizer chamber, and a plate 80 defining a bottom closure for economizer chamber 72, the downcomer shroud 34, vertical plate 78 and bottom plate 80 being joined together along their respectively intersecting edges to define a generally enclosed economizer chamber 72, the outlet 74 thereof being defined by the uppermost bafiie plates 76. The tubes 42 having length portions 42a within the economizer chamber 72 extend through the bottom plate 30 and their intersections with said plate 80 are sealed to retain the feed liquid within the economizer chamber 72. Bottom plate 80 is disposed in spaced-apart relation to the inside face of tube sheet 44 to define a clearance passage therebetween which allows free circulation across the tube sheet 44 of preheated liquid contained in zone 36 and the saturated liquid as is returned through downcomer passage 38. In this way, the circulation of liquid at saturation temperature along the inside wall surface of vessel 12, and also across tube sheet 44 tends to prevent the accumulation of deposits on such surfaces.

In accordance with the general concept of the invention, it is not absolutely necessary that the tube length portions allocated for preheating the feed liquid extend through economizer chamber 72, as is shown by FIG. 1, but it has been found convenient and relatively economical to provide an arrangement of tubes 42 wherein at least one and preferably a plurality of tubes 42 has a length portion 42a extending through the economizer chamber 72. If desired, one or more of the tubes 42 can extend through the interior of vessel 12 with all portions of its length being completely outside the economizer chamber 72 as exemplified by tubes 42 shown in phantom, How ever, it is recommended that all of the tubes 42 have similar length portions 42a extending through economizer chamber 72, so as to provide a reserve preheating surface capability in the event that during the service life of the vapor generator it should become necessary to plug one or more of the tubes 42. It should be noted that where all the tubes extend through the economizer chamber 72, the eflluent primary fluid from the several tubes will be more uniform in temperature than in arrangements where some of the tubes are completely outside of the economizer chamber 72.

According to a preferred embodiment of the invention,

it is desired to achieve a liquid preheat temperature at the economizer outlet 74 which is higher than the temperature of the primary fluid as measured at the tube 10- cations where the primary fluid flow exits the economizer chamber 72. This can be achieved by the economizer inlet 70 and outlet 74 arrangement shown in FIG. 1 whereby the economizer chamber has a feed liquid flow path which is countercurrent to the flow of primary fluid in the tube length portions extending through the economizer chamber 72.

The vapor generator 10 can be used to eflect vaporization of a wide variety of different feed or secondary liquids by means of heat extracted from any of a variety of hot primary fluids that flow through tubes 42. Such primary fluid can be organic liquids, liquid metal, water or gas, and can be derived from any suitable source having the heat transport characteristics and capability required to produce the desired vapor conditions when using a feed liquid of given thermal conditions. In general, the vapor generator capabilities of vapor generator 10 will, for a fixed physical configuration, be dependent upon the flow and inlet temperatures of the primary and secondary fluids, and the internal pressure of vessel 12.

For a typical application of the invention as a steam generator, the primary fluid can be heavy water with an inlet temperature of 560 F. pressurized to 1250 p.s.i.g., and the secondary liquid can be feedwater at an inlet temperature 340 F, at 579 p.s.i.g., the internal pressure of the vessel 12. In view of the fact that the liquid in the vaporization zone 36 will be in the equilibrium with the gen erated steam vapor, and hence is at the saturation temperature 484 F. for 579 p.s.i.g., and such separated liquid as is returned through downcomer passage 38 will be at substantially the same saturation temperature, it is advantageous to construct the economizer chamber 72 so that the feedwater delivered at its outlet 74 and into zone 36 is also at substantially saturation temperature. In this way, all of the liquid contacting the wall of vessel 12 and the inside face of tube sheet 44 will be at a uniform temperature, namely the saturation temperature of 484 F. As can be appreciated by the artisan, maintaining the inside of vessel 12 and tube sheet 44 at a uniform temperature is advantageous in reducing thermal stresses that would otherwise exist if the inside boundary surface temperature were allowed to vary appreciably from pointto-point.

The economizer chamber 72 and its baflles 76 are preferably so dimensioned and arranged to provide a feedwater flow path therein of cross-sectional and length dimensions selected in relation to the effective surface area of the tube length portions 42a inside economizer chamber 72 to establish a net heat transfer rate between the primary fluid flowing through such tube length portions 42a and the feedwater traveling along its flow path as to raise the temperature of the preheated feedwater delivered at outlet 74 to the desired temperature, which can be any temperature above the feedwater inlet temperature up to saturation temperature, and which is preferably the saturation temperature. Such designing of the economizer chamber 72 incorporating tube length portions 42a and the baflles 76 can be accomplished by the straightforward application of routine engineering calculations and formulas, all of which are presumed to be well known to the artisan. The inlet pipe and economizer outlet are so arranged that the feedwater flow during preheating is countercurrent with respect to the primary fluid flow in the tube length portions 42a extending through economizer chamber 72, or whatever tube length portions are utilized for preheating purposes. With countercurrent flow, the feedwater can be readily preheated to an outlet temperature above the temperature at which the primary fluid exits the economizer 72, as measured at the tube locations intersecting with bottom plate 80.

While with a fixed vapor generator 10 configuration, the feedwater outlet temperature, and primary fluid exit temperature will be dependent upon the flow rates and inlet temperatures of such two fluids, it also follows conversely that the economizer chamber 72 can be sized together with the tubes 42 so as to produce steam vapor having saturation temperature and pressure properties within an acceptable range for cases wherein the steam demand varies, and/or other operating parameters vary, assuming that suitable external control means (not shown) are provided to regulate the associated pressure as well as the feedwater flow rate, primary fluid flow rate, feedwater inlet temperature, and primary fluid inlet temperature to the extent necessary to compensate for such variations.

As a guide to the artisan in understanding and practicing the invention, the typical design parameters of a steam generator constructed in accordance with the FIGS. 1 and 2 embodiment of the invention are set forth in the following Table I.

TABLE I Steam generation flow rate lb./hr 540,000 Steam conditions (saturated):

Pressure p.s.i.g 579 Temperature F 484 Enthalpy B.t.u./lb 1203 Feedwater conditions:

Inlet temperature F 340 Pressure (same as steam) p.s.i.g 579 Primary heating fluid operating conditions:

Primary fluid Heavy water Pressure p.s.ig 1250 Temperature Inlet F 560 Outlet F 480 Flow rate lb./hr 5.111 10 Primary fluid tubes (0.496 OD 0.049 wall):

Number of tubes 2600 Total heating surface sq. ft 20,000 Economizer heating surface sq.ft 2650 vaporization zone heating surface sq. ft 17,350

In connection with the foregoing example, it should be realized that in the event that it should be desired to construct a steam vapor generator in accordance with the invention to produce steam at a different flow rate, or outlet thermal conditions, or to utilize different primary fluid operating conditions, such can be done by modifying the dimensions of the basic vapor generator 10 as dictated by conventional engineering practice.

It has been found that one of the advantages afforded by the integral economizer 72 provided by the invention, as compared with direct contact feedwater heating, lies in the fact that with the economizer 72, about 20 percent less overall heating surface is required than for direct contact feedwater heating, i.e. without an economizer 72. Furthermore, with direct contact feedwater heating, the primary fluid outlet temperature establishes the maximum saturation temperature and hence the maximum steam operating pressure, whereas with the invention, the countercurrent feedwater flow through the economizer allows a feedwater preheat temperature higher than the primary fluid outlet temperature, and therefore simply by sizing the economizer tube heating surface area to preheat the feedwater to saturation, a sizeable boost in attainable steam pressure can be realized.

FIGS. 3 and 4 serve to illustrate that the concept of the invention is not necessarily limited to the economizer 72 construction specifically shown in FIGS. 1 and 2 but that other economizer configurations can be substituted. From FIGS. 1 and 2, it will be recalled that the lateral boundaries of the economizer 72 are defined by a portion of the downcomer shroud 34 and a vertical plate, since such members are conveniently available. However, as shown by FIGS. 3 and 4, the lateral boundaries of the economizer chamber 172 can be formed by a plate 178 and an array of circumferentially spaced primary fluid carrying tubes 150 which are weld-united along portions 150a of their lengths by filler bars 143, the inward semicylindrical surfaces of the tube portions 150a, together with similar length portions 142a of the tubes 142 passing through the interior of economizer 172, constituting the surface area allocated to feedwater preheating. The outward semicylindrical surfaces of tube portions 150a, together with such other tubes 142 and length portions thereof as are outside of economizer chamber 172 serve as the vaporization or boiling heating surface area.

The bottom of economizer 172 is closed by a plate 180 which is similar to the plate 80 shown in FIGS. 1 and 2, and a plurality of bafiies 176 are supported by the tubulous wall defined by tube length portions 150a and filler bars 143, and/or by plate 178.

As can be appreciated by the artisan from the foregoing, the invention is susceptible of numerous modifications and variations as will become obvious therefrom. However, the invention is intended to be limited only by the following claims in which I have endeavored to claim all inherent novelty.

What is claimed is:

1. A vapor generator which comprises a substantially enclosed hollow vessel having an outlet for the discharge of vapors, a plurality of tubes extending through the interior of said vessel to convey therethrough a hot primary fluid from which heat is extracted to heat and vaporize a secondary liquid, and means defining a generally enclosed economizer chamber disposed within said vessel to accommodate preheating of an inflowing secondary liquid to a predetermined temperature, said economizer chamber having an inlet disposed to receive secondary liquid from an external source thereof, and an outlet communicating with the interior of said vessel to deliver thereto preheated secondary liquid at said temperature, said economizer chamber being spaced-apart from the internal boundaries of said vessel to define therewith flow spaces for the free circulation of preheated secondary liquid in contact with the entire exterior surface of the economizer chamber, said tubes being disposed to transfer heat from a portion of their total effective surface area to the secondary liquid in said economizer chamber to preheat said liquid to said temperature, and to transfer heat from the remaining portion of their total effective surface area to the preheated secondary liquid in said vessel to vaporize same, whereby respectively corresponding portions of the total heat extracted from the primary fluid are utilized to preheat the secondary liquid within the economizer chamber, and to vaporize the preheated secondary liquid within that portion of the vessel outside the economizer chamber, and substantially no secondary liquid below said reheat temperature contacts the internal boundaries of said vessel.

2. The vapor generator according to claim 1 wherein said tubes are arranged to provide a surface area portion disposed in heat transferring relation to said economizer chamber to preheat said inflowing secondary liquid to a temperature corresponding to the saturation temperature for the internal pressure of the vessel, for a given combination of secondary liquid flow rate, secondary liquid inlet temperature, primary fluid flow rate, and primary fluid tube inlet temperature conditions.

3. The vapor generator according to claim 1 wherein at least one of said tubes has a length portion extending through said economizer chamber.

4. The vapor generator according to claim 1 wherein at least one of said tubes extends through the interior of said vessel with all portions of its length being completely outside said economizer chamber.

5. The vapor generator according to claim 1 wherein said tubes are disposed in generally parallel relation to one another and convey said primary fluid in respectively similar directions from their individual primary fluid inlet ends to their respective primary fluid outlet ends, and wherein a plurality of said tubes having correspondingly similar length portions extending through said economizer chamber.

6. The vapor generator according to claim 5 wherein the inlet and outlet of said economizer chamber are arranged, in relation to the primary fluid inlet and outlet ends of the tubes having length portions extending through the economizer chamber, to define a secondary liquid flow path through the economizer chamber which is countercurrent to the flow of primary fluid in said tubes.

7. The vapor generator according to claim 6 wherein said economizer chamber has a secondary liquid flow path dimensioned in relation to the effective surface area of the tube length portions inside the economizer chamber to establish a net heat transfer rate between the primary fluid flowing through said tube length portions and the secondary liquid along said flow path to raise the temperature of the preheated secondary liquid delivered at the outlet of said economizer chamber to a value which exceeds the temperature of the primary fluid as measured at the tube locations where the primary fluid flow exits the economizer chamber, for a given combination of primary fluid flow rate, secondary liquid flow rate, primary fluid economizer chamber entrance and exit temperature, and secondary liquid inlet temperature conditions.

8. The vapor generator according to claim 6 wherein said tubes are U-shaped and extend in generally vertical planes, and including a shroud disposed within said vessel in external laterally surrounding relation to said tubes and in spaced-apart internally surrounding relation to the internal boundary wall surface of said vessel to define therewith an annular downcomer passage extending along said wall surface a portion of said downcomer shroud defining a boundary portion of said economizer chamber.

9. The vapor generator according to claim 8 including a tube sheet disposed within said vessel to receive and support said tubes at their primary fluid inlet and outlet extremities, said tube sheet being disposed in spacedapart relation to said economizer chamber and in spacedapart relation to the lower end of Said downcomer shroud J to allow free circulation over said tube sheet of preheated secondary liquid contained in said vessel and saturated secondary liquid returned through said downcomer passage.

Til. The vapor generator according to claim 9 wherein said economizer chamber has a bottom closure plate disposed in adjacent spaced-apart relation to said tube sheet, and a vertically extending lateral closure plate disposed within said downcorner shroud and connected edgewise thereto, said bottom closure plate being connected along its edges to said downcomer shroud and lateral closure plate to define an economizer chamber enclosed at its bottom portion and laterally and having an outlet at its upper portion.

11. The vapor generator according to claim 10 including a thermally shielded secondary liquid inlet conduit extending through the wall of said vessel and said downcorner shroud for communication with said economizer chamber at the lower portion thereof.

12. The vapor generator according to claim 11 wherein it? the tubes having length portions extending within said economizer chamber pass through the bottom closure plate thereof in sealing engagement therewith.

13. The vapor generator according to claim 12 including a plurality of bafile plate members disposed within said economizer chamber to increase the eifective secondary liquid preheating flow path therein.

References Cited UNITED STATES PATENTS 3,141,445 7/1964 Bell 122-34 3,356,135 12/1967 Sayre 12232 XR 2,220,045 10/1940 Kraft et al. 122-32 FOREIGN PATENTS 84l,800 1/1960 Great Britain.

KENNETH W. SPRAGUE, Primary Examiner