US3557760A

US3557760A - Vapor generator organization utilizing liquid metal or molten salts

Info

Publication number: US3557760A
Application number: US753185A
Authority: US
Inventors: Nicholas D Romanos; Brice W Kinyon
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1968-08-16
Filing date: 1968-08-16
Publication date: 1971-01-26
Anticipated expiration: 1988-01-26
Also published as: FR2015805A1; SE347060B; DE1941005A1; DE1941005B2; CH496209A; GB1275298A

Abstract

A vapor generator organization for producing superheated vapor wherein a tube bundle is immersed in a heating medium consisting of high temperature liquid metal or molten salts. The tube bundle comprises a plurality of pressure tubes, each one having inserted therewithin, for a length less than the overall pressure tube length, a bayonet tube arrangement. Vaporizable liquid is passed through each bayonet tube arrangement to be vaporized by steam flowing in the annulus formed between the bayonet tube arrangement and its respective pressure tube. The portion of the annulus surrounding the bayonet tube arrangement acts as a low temperature superheater section in which the steam is superheated to a temperature intermediate the temperature of the heating medium and the saturation temperature of the vaporizable liquid, the steam serving as an intermediate heating medium between the primary heating medium and the vaporizing liquid. Additional superheating of the vapor to the final temperature takes place within the pressure tubes over that portion of the length thereof in which the respective bayonet tube arrangements do not extend.

Description

United States Patent [72] Inventors Nicholas D. Romanos;

Brice W. Kinyon, Chattanooga, Tenn. [2]} Appl. No. 753,185 [22] Filed Aug. 16, 1968 [45] Patented Jan. 26, 1971 [73] Assignee Combustion Engineering, Inc.

Windsor, Conn. a corporation of Delaware [54] VAPOR GENERATOR ORGANIZATION UTILIZING LIQUID METAL 0R MOLTEN SALTS 8 Claims, 2 Drawing Figs.

[52] US. Cl 122/32, 122/483 [51] Int. Cl F22b l/04 [50] Field of Search 122/32, 34, 483

[56] References Cited UNITED STATES PATENTS 3,049,105 8/1962 Waldron 122/483 3,097,630 7/1963 Kinyon et al [22/34 Primary ExaminerKenneth W. Sprague Attorneys-Carlton F. Bryant, Eldon H. Luther, Robert L. Olson, John F. Carney, Richard H. Bemeike, Edward L. Kochey, Jr. and Lawrence P. Kessler ABSTRACT: A vapor generator organization for producing superheated vapor wherein a tube bundle is immersed in a heating medium consisting of high temperature liquid metal or molten salts. The tube bundle comprises a plurality of pressure tubes, each one having inserted therewithin, for a length less than the overall pressure tube length, a bayonet tube arrangement. Vaporizable liquid is passed through each bayonet tube arrangement to be vaporized by steam flowing in the annulus formed between the bayonet tube arrangement and its respective pressure tube. The portion of the annulus surrounding the bayonet tube arrangement acts as a low temperature superheater section in which the steam is superheated to a temperature intermediate the temperature of the heating medium and the saturation temperature of the vaporizable liquid, the steam serving as an intermediate heating medium between the primary heating medium and the vaporizing liquid. Additional superheating of the vapor to the final temperature takes place within the pressure tubes over that portion of the length thereof in which the respective bayonet tube arrangements do not extend.

PATENTEB maze mm FIG.

FIG. 2

INVENTOR.

NICHOLAS D. ROMANOS BRICE W. KINYON AT TORN EY BACKGROUND OF THE INVENTION In certain heat exchange applications where adverse chemi- 5 cal reactions would occur upon direct fluid contact, it is necessary to transfer heat from a heating medium to a medium to be heated without the intermixing thereof. This indirect heat exchange is accomplished by passing the mediums along opposite sides of a heating surface which provides an effective heat flow path from one medium to the other. A typical example of a heat exchanger operating on the indirect heat exchange principle is the tube-and-shell arrangement.

Vapor generating organizations using high temperature liquid metal or molten salts as the heating medium and water as the medium to be heated would be particularly subject to the above-noted intermixing problem. As is well known, intermixing of alkali liquid metals and water or molten salts and water produces violent reactions. Organizations using such fluids are, however, becoming more common in view of the increased interest in liquid metals and molten salts for use as coolants in neutronic reactors to permit an increase in the operating temperature thereof in order to improve efficiency.

In order to prevent the violent reactions noted above, the aforementioned tube-and-shell heat exchange arrangement has been used for vapor generating organizations utilizing these fluids as heat exchange mediums. While preventing the undesirable reactions, there remain several serious problems which are not cured by the use of thetube-and-shell heat exchanger arrangement.

Particularly, problems arise due to the temperature differential existing between the liquid metal or molten salts and water to be vaporized. For example, under reasonable working pressures, the saturation temperature of the water may be 500 F. to 600 F. while the liquid metal or molten salt temperature may be in excess of 1,000 F. This differential temperature results in sharp temperature gradients and high thermal stresses in the pressure parts of the heat exchange arrangement. Moreover, when using certain desirable molten salts as the heating medium, there exists the possibility of the freezing thereof in the feedwater inlet and evaporation sections of the pressure tubes. This is due to the fact that the saturation temperature of the water is below the solidification temperature of the salts.

SUMMARY OF THE INVENTION Our invention is directed to the provision of a tube-andshell heat exchanger for a vapor generator organization utilizing a high temperature liquid metal or molten salt as the heating medium to obtain superheated steam, the organization being free from the difficulties caused by differential temperatures as noted above. According to our invention, a vapor generator organization is provided with a tube bundle located within a shell which receives a high temperature liquid metal or molten salt as the heating medium. The tube bundle is made up of a plurality of pressure tubes each having a bayonet tube arrangement extending concentrically thereinto for a distance less than the overall pressure tube length. The bayonet tube arrangement is made up of an outer tubular member closed at one end having, extending concentrically therewithin from the open end, a central internal tube. Vaporizable liquid, such as water, is passed up through the central internal tube portion of each bayonet tube arrangement and back down the flow annulus formed between the inner and outer concentric tubes thereof. The water is vaporized during its downward pass by steam flowing in the flow annuli formed between the bayonet tube arrangements and their respective pressure tubes; the steam, in turn, is heated by the liquid metal or molten salt heating medium.

The steam, therefore, serves as an intermediate heat transfer medium between the primary heating medium and the vaporizable-liquid. The properties of the steam, being a vapor or gas, are such that the heat'transfer rates from the pressure tube walls do not produce the sharp temperature gradients and resulting extreme thermal stresses. In addition, due to the elevated temperature of the steam, the problem of freezing is eliminated.

High temperature superheating of the steam takes place within the pressure tubes over that portion of the length thereof in which the respective bayonet tube arrangements do not extend. There is a collection portion in the top of the shell into which the superheated steam from the plurality of pressure tubes collects to be taken off to do useful work such as power a turbogenerator set.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional elevation of a vapor generator organization embodying the present invention.

FIG. 2 is a sectional elevation of a typical tube arrangement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. l, numeral 10 designates a steam generator organization oriented vertically on a support means 12. The organization 10 comprises an upper shell 14 and a lower shell 16. The upper shell 14 is capped by a closure head 15 forming a collection chamber 18 having a collection chamber outlet 20. Expansion means (not shown) may be employed with the closure head 15 to allow for differential expansion. Within the upper shell 14 is a heat exchange chamber 22 provided with at least one heating medium inlet 24 and at least one heating medium outlet 26.

Within the lower shell 16 there is an evaporator inlet plenum 28, an evaporator outlet plenum 30, and a downtake plenum 33. The evaporator inlet plenum 28 is formed by a cylindrical vessel 29 closed at its top portion by a tube sheet 62. The evaporator outlet plenum 30 is formed by wall means 31 having portions 32 for mounting of vapor separators 74 of any well-known design. A drying screen 76 is mounted above the separators. Tube-

sheets

58 and 52 serve to close the upper 40 and lower sides, respectively, of the evaporator outlet plenum 30. The downtake plenum 33 is formed by the lower head closure 34 capping the lower shell 16.

The lower shell 16 has a feedwater inlet 35 connected to a feedwater distributor 36 in the form of a ring header having liquid discharge openings 37, from which feedwater is admitted to the lower shell and downtake plenum. At the bottom of the lower shell is a recirculation outlet nozzle 38 connected to a recirculation inlet nozzle 40 by means of a recirculating line 41. Inlet nozzle 40 connects with the evaporator inlet plenum 28 by means of flow conduit 44. Feedwater in the downtake plenum 33 is recirculated from the outlet nozzle 38 to the inlet nozzle 40 and evaporator inlet plenum 28 by means of a recirculation pump 42.

Located within the heat exchange chamber 22 is a tube bundle comprising a plurality of pressure tubes 46 connected between an upper tube sheet 48 and a lower tube sheet 50, the tubes 46 being in fluid communication with the lower shell 16 and the collection chamber 18. While only several tubes have been shown for simplicity, it should be understood that the tube bundle may in fact have as many as 2000 pressure tubes. The upper tube sheet 48, which extends laterally of and is secured to the wall of the upper shell 14 serves to separate the heat exchange chamber 22 from the collection chamber 18; the lower tube sheet 50 is connected to the upper shell 14 and separates it from the lower shell 16. Support means 52 serve to brace the pressure tubes 46 at several points between the

tube sheets

48 and 50, and are of such configuration to permit axial flow of the heating medium along the tube length. A vertically elongated, cylindrical flow guiding baffle 54 surrounds the pressure tubes 46. Also within the heat exchange chamber 22 are

baffles

72, and 82 for the purposes to be described hereinbelow.

The particular structure of the individual pressure tubes within the t ube bundle may best be seen in FIG. 2. Each pressure tube 46 has an internal bore 46'. Extending within the bore 46' for a portion of the length thereof is a bayonet tube arrangement. The arrangement includes an outer portion 56 and a concentric central internal tube portion 60. The outer tube 56, connected to the evaporator outlet plenum tube sheet 58 at one end (top of the evaporator outlet plenum 30) and closed at the other, has an internal bore 56'. The central internal tube portion 60, with one end extending within bore 56' of the outer tube portion 56, is connected at the other end to the evaporator inlet plenum tube sheet 62 (top of the evaporator inlet plenum 28). The central tube portion 60 has an internal bore 60'. Mounted on the end of outer tube portion 56 is a bayonet tube extension 64 extending within the pressure tube 46 and terminating approximately at the level where the tube 46 joins the tube sheet 48. This extension 64 may be connected to the outer tube portion 56 by any appropriate method, such as by welding or bolting by means of a central stud (not shown). Bayonet tube extension 64, outer tube portion 56 and central internal tube portion 60 are supported in the respective tubes within which they are positioned by

spiral fin spacers

66, 68 and 70, which additionally serve to guide the flow through the respective annuli, or flow passages, formed between the tubes as explained hereinbelow. lt should be noted that only pressure tube 46 is structurally subjected to high pressure; the outer tube portion 56, central tube portion 60, tube sheet 58, wall means 31, tube sheet 62, and vessel 29 are not subjected to high pressure but act merely as fluid flow channels.

The operation of the steam generator organization 10 is as follows. High temperature liquid metal or molten salts are ad mitted to heat exchange chamber 22 through heating medium inlet 24 to serve as a primary heating medium. The baffle 72, connected between flow guiding baffle 54 and upper shell 14 at a point below heating medium inlet 24, causes the incoming heating medium to flow within the flow guiding baffle 54 washing the outside diameters of the plurality of pressure tubes 46. After performing its heating medium function (i.e., giving up heat through the tubes 46), the liquid metal or molten salts exit through the heating medium outlet 26.

Vaporizable liquid is admitted to the downtake plenum 33 from distributor 36 to be recirculated by the recirculating pump 42 from recirculating outlet nozzle 38 through recirculating inlet nozzle 40 and conduit 44 to the evaporator inlet plenum 28. In the instant embodiment this vaporizable liquid is feedwater (the entering feedwater is supplied at a relatively cold temperature so as to prevent cavitation in the pump 42). The feedwater flows up the bore 60' of the central internal tube portions 60 and then down the flow passages formed by the annuli between the outer tube portions 56 and their respective central tube portions 60. The flow in the annuli is heated by the steam flowing about the bayonet tube arrangements, as explained below, and exits into the evaporator outlet plenum as a mixture of steam and vaporizable liquid. The partially vaporized mixture is then passed through steam separators 74 (FIG. 1), the separated liquid returning to the downtake plenum 33 to be recirculated and the vapor passing through a drying screen 76.

After being dried by the screen 76, the vapor enters the pressure tubes 46 passing in the flow passages formed by the annuli between the outer tube portions 56 and of the bayonet tube arrangements and their respective pressure tubes 46. For the length of the outer tube portions 56, the steam in the second-mentioned series of annuli, while being superheated, serves as an intermediate heating medium between the primary heating medium and the feedwater being vaporized; for the additional length of the pressure tubes 46, the steam is further superheated by the heating medium to a high temperature superheat condition. Restated, the steam flow in the second series of annuli results in each pressure tube 46 becoming a superheated steam generating module having a low temperature superheater portion coaxial with an evaporator portion extending for the length of the respective bayonet tube arrangements and a high temperature superheater portion extending for the remainder of the pressure tube length. The bayonet tube extension 64 serves to direct the steam flow to the inside diameter of the pressure tubes 46 so that proper heat exchange will occur to accomplish the desired superheating of the steam. The high temperature superheated steam is then passed to the collection chamber 18 from which it may be withdrawn through outlet 20 to be applied to do useful work.

The flow paths for the feedwater, the partially vaporized mixture, and the steam for each individual tube is indicated by the arrows in FIG. 2.

In a particular embodiment where certain high temperature liquid metals are used as the heating medium, it is a general practice to place a blanket of inert gas, such as argon, beneath the upper tube sheet 48 (between tube sheet 48 and baffle plate by means of an inlet nozzle (not shown). The inert gas blanket serves as an additional separation means between the liquid metal and the steam, and to reduce the thermal stress and thermal shocking of the upper tube sheet 48. In this embodiment the baffle plate 80 is connected to the upper shell 14 above the heating medium inlet 24. Openings 81 (FIG. 2) in the plate 80 surrounding the pressure tubes 46 allow the liquid metal to flow above the plate 80. The plate 80 serves to reduce turbulence in the incoming high temperature liquid metal flowing thereabove to prevent the liquid from entrapping large quantities of inert gas while at the same time permitting superheating of the steam to continue above the level of the plate. The additional baffle 82 is connected to the upper shell 14 at a point below the heating medium outlet 26 and extends thereabove to cause a reversal of the flow of the heating medium before it exits through outlet 26. The effect of this flow reversal will be to separate any of the inert gas which may be entrapped in the liquid metal by causing the bubbles of the lighter gas to rise to the surface 84 of the liquid metal and the space thereabove. I

Another expedient which may be used in particular instances for effective heat transfer conditions with certain molten salts, is the use of the baffles to produce crossflow of the heating medium across the heat exchange chamber 22. With this baffle arrangement, the flow guidingbaffle 54 and

baffle plates

80 and 82 are not required.

In view of the particular organization of the instant invention, it is apparent that the heretofore-mentioned problems of severe stress gradients and molten salt freezing resulting from large temperature differentials will be eliminated.

The heating medium, i.e. the high temperature liquid metal or molten salts, is no longer in direct heat exchange contact with the substantially lower temperature incoming feedwater. It is rather in an indirect heat transfer relationship therewith as a result of the intermediate steam passing between the heating medium and the incoming vaporizable feedwater. By having the steam in heat exchange contact with the heating medium, more uniform metal temperatures exist through the pressure containing members reducing the possibility of thermal shocking. Moreover, the higher temperature of the steam (relative to feedwater) prevents freezing of the heating medium. This arrangement additionally protects against contamination of the heating medium in that in the event of a minor leak of steam into either liquid alkali metal or molten salt, there is no violent reaction, and there is time after determination of the defect to permit an orderly shutdown of the equipment to make repairs.

Further "advantages of the proposed invention derive from the downward flow of the heating medium through the heat exchange chamber 22. The entering heating medium, at its highest temperature, is against the leaving superheated steam, and the leaving heating medium is against the entering saturated steam, which is at its lowest temperature, thus minimizing the temperature difference between the two fluids while maximizing the effective utilization of the heat transfer surface. The changing density of the heating medium, increasing as the temperature decreases during downward flow, aids the flow. With reverse flow, hot fluid entering at the bottom and cold fluid leaving at the top, there would be a tendency for the denser cold fluid to counterflow causing thermal shock.

While we have illustrated and described preferred embodiments of our invention, it is to be understood that such are merely illustrative and not restrictive and variations and modifications may be made therein without departing from the spirit and scope ofthe invention. We therefore do not wish to be limited to the precise details set forth therein but desire to avail ourselves of such changes as fall within the purview of our invention. We claim: 1. in a vapor generator organization wherein a heating medium is used to produce a superheated vapor by the indirect transfer of heat to a vaporizable liquid, the combination comprising:

a. a vertically elongated vessel; b. a pair of spaced tube sheets disposed transversely of the axis of said vessel and dividing the interior thereof into an inlet chamber a heat exchange chamber, and a vapor collection chamber; c. means for circulating heating medium through said heat exchange chamber; and d. a tube bundle disposed within said heat exchange chamber, said tube bundle including: i. a plurality of pressure tubes disposed in heat exchange relation with said circulating heating medium, said pressure tubes being connected between said tube sheets and having their ends in fluid communication with said inlet and vapor collection chambers; and ii. means forming a bayonet tube arrangement extending axially within each of said pressure tubes, said means including: aa. an outer tube concentrically spaced from the wall of said pressure tube to define a first annular flow passage therebetween;

bb. an inner tube concentrically spaced from the wall of said outer tube to define a second annular flow passage therebetween, and

cc. means at one end of said inner tube for maintaining said inner tube and said second annular flow passage in fluid communication for effecting a double pass of fluid through said bayonet tube arrangement means;

e. means for passing vaporizable liquid through said inner tube of each of said bayonet tube arrangements whereby said liquid absorbs heat in flowing through said inner tubes and said second annular flow passages to transform a portion thereof into vapor;

f. means within said inlet chamber for collecting the vaporliquid mixture discharged from said second annular flow passages;

g. means within said inlet chamber for passing the vapor collected in said last named means through said first annular flow passages to said vapor collection chamber whereby it is superheated; and

h. means for withdrawing superheated vapor from said vapor collection chamber.

2. The vapor generator organization of claim 1 wherein said heat exchange chamber is located intermediate said inlet and said vapor collection chambers.

3. The vapor generator organization of claim 1 wherein the means for passing the vapor collected in said vapor collection means through said first annular flow passages includes separating means for separating said vapor-liquid mixture so as to allow passage of the separated vapor through said first annular flow passages while returning the separated liquid so as to be recirculated through said inner tubes. 4. The vapor generator organization of claim 1 wherein said bayonet tube arrangement means extend within each of said pressure tubes for a length less than the full length of the respective pressure tubes, each of said bayonet tube arrangement means having a flow guiding extension on the end thereof extending the remaining pressure tube length.

5. in a vapor generator organization wherein a heating medium such as liquid metal or molten salts is used to produce a superheated vapor by the indirect transfer of heat to a vaporizablc li uid. the combination comprising:

a. a vertica y elongated vessel. said vessel having an inlet chamber, a heat exchange chamber and a collection chamber axiallyspaced therein;

a first tube sheet secured to said elongated vessel located between said inlet chamber and said heat exchange chamber and a second tube sheet secured to said clongated vessel located between said heat exchange chamber and said collection chamber; and

c. a tube bundle located within said heat exchange chamber.

said tube bundle comprising:

i. a plurality of pressure tubes connected between said first and second tube sheets and in fluid communication with said inlet and collection chambers; and

ii. a plurality of bayonet tube arrangements. each having a central internal tube portion extending concentrically within an outer tube portion, closed at one end for a length less than the full length of the outer tube portion and forming a flow annulus therebetween, one of said bayonet tube arrangements concentrically located within each of said plurality of pressure tubes extending therewithin for a distance less than the total length of the respective pressure tube and forming a flow annulus therebetween;

d. means supplying vaporizable fluid to said inlet chamber;

e. means within said inlet chamber to direct the flow of vaporizable liquid to each of said central internal tube portions;

f. means within said inlet chamber connecting the annuli formed between said central internal tube portions and their respective outer tube portions to the annuli formed between said bayonet tube arrangements and their respective pressure tubes;

g. means for circulating the heating medium through said heat exchange chamber about said plurality of pressure tubes; and

h. a conduit for withdrawing vapor collected in said collection chamber to do useful work.

6. The vapor generator organization of claim 5 wherein the means connecting said two annuli includes a means for separating the vapor and vaporizablc liquid received from said first-mentioned annuli and passing the vapor to said secondmentioned annuli while returning the vaporizablc liquid to said inlet chamber for recirculation.

7. The vapor generator organization of claim 5 wherein each of said bayonet tube arrangements has an extension attached to the closed end thereof, said-extension being of a length so as to extend from its respective bayonet tube arrangement the remainder of the length of its respective pressure tube.

.8. The vapor generator organization of claim 5 wherein the means for circulating the heating medium through said heat exchange chamber about said plurality of pressure tubes includes:

a. inlet nozzle means connected to said heat exchange chamber of said elongated vessel;

b. outlet nozzle means connected to said heat exchange chamber of said elongated vessel below said inlet nozzle means; and baffle means for directing the flow of heating medium from said inlet nozzle means to said outlet nozzle means, said baffle means comprising:

i. a first baffle surrounding said tube bundle and extending for a length therealong from above the level of said nozzle inlet means to below the level of said nozzle outlet means;

ii. a second baffle sealingly connecting said first baffle to said elongated vessel, said second baffle located below the level of said inlet nozzle means; and

iii. a third baffle located between said first baffle and said elongated vessel, said third baffle connected to said elongated vessel below the level of the nozzle outlet means and extend above the level thereof.

Claims

2. The vapor generator organization of claim 1 wherein said heat exchange chamber is located intermediate said inlet and said vapor collection chambers.
3. The vapor generator organization of claim 1 wherein the means for passing the vapor collected in said vapor collection means through said first annular flow passages includes separating means for separating said vapor-liquid mixture so as to allow passage of the separated vapor through said first annular flow passages while returning the separated liquid so as to be recirculated through said inner tubes.
4. The vapor generator organization of claim 1 wherein said bayonet tube arrangement means extend within each of said pressure tubes for a length less than the full length of the respective pressure tubes, each of said bayonet tube arrangement means having a flow guiding extension on the end thereof extending the remaining pressure tube length.
5. In a vapor generator organization wherein a heating medium such as liquid metal or molten salts is used to produce a superheated vapor by the indirect transfer of heat to a vaporizable liquid, the combination comprising: a. a vertically elongated vessel, said vessel having an inlet chamber, a heat exchange chamber and a collection chamber axially spaced therein; b. a first tube sheet secured to said elongated vessel located between said inlet chamber and said heat exchange chamber and a second tube sheet secured to said elongated vessel located between said heat exchange chamber and said collection chamber; and c. a tube bundle located within said heat exchange chamber, said tube bundle comprising: i. a plurality of pressure tubes connected between said first and second tube sheets and in fluid communication with said inlet and collection chambers; and ii. a plurality of bayonet tube arrangements, each having a central internal tube portion extending concentrically within an outer tube portion, closed at one end, for a length less than the full length of the outer tube portion and forming a flow annulus therebetween, one of said bayonet tube arrangements concentrically located within each of said plurality of pressure tubes extending therewithin for a distance less than the total length of the respective pressure tube and forming a flow annulus therebetween; d. means supplying vaporizable fluid to said inlet chamber; e. means within said inlet chamber to direct the flow of vaporizable liquid to each of said central internal tube portions; f. means within said inlet chamber connecting the annuli formed between said central internal tube portions and their respective outer tube portions to the annuli formed between said bayonet tube arrangements and their respective pressure tubes; g. means for circulating the heating medium through said heat exchange chamber about said plurality of pressure tubes; and h. a conduit for withdrawing vapor collected in said collection chamber to do useful work.
6. The vapor generator organization of claim 5 wherein the means connecting said two annuli includes a means for separating the vapor and vaporizable liquid received from said first-mentioned annuli and passing the vapor to said second-mentioned annuli while returning the vaporizable liquid to said inlet chamber for recirculation.
7. The vapor generator organization of claim 5 wherein each of said bayonet tube arrangements has an extension attached to the closed end thereof, said extension being of a length so as to extend from its respective bayonet tube arrangement the remainder of the length of its respective pressure tube.
8. The vapor generator organization of claim 5 wherein the means for circulating the heating medium through said heat exchange chamber about said plurality of pressure tubes includes: a. inlet nozzle means connected to said heat exchange chamber of said elongated vessel; b. outlet nozzle means connected to said heat exchange chamber of said elongated vessel below said inlet nozzle means; and c. baffle means for directing the flow of heating medium from said inlet nozzle means to said outlet nozzle means, said baffle means comprising: i. a first baffle surrounding said tube bundle and extending for a length therealong from above the level of said nozzle inlet means to below the level of said nozzle outlet means; ii. a second baffle sealingly connecting said first baffle to said elongated vessel, said second baffle located below the level of said inlet nozzle means; and iii. a third baffle located between said first baffle and said elongated vessel, said third baffle connected to said elongated vessel below the level of the nozzle outlet means and extend above the level thereof.