US3841271A

US3841271A - Heat exchanger having a plurality of modular tube bundles

Info

Publication number: US3841271A
Application number: US00120423A
Authority: US
Inventors: W Harris; A Massaro
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1971-03-03
Filing date: 1971-03-03
Publication date: 1974-10-15
Anticipated expiration: 1991-10-15
Also published as: CA1022153A; BE780075A; DE2209119A1; CH538657A; IT949803B; JPS5218416B1; FR2128534B1; FR2128534A1; AT317947B

Abstract

A heat exchanger having a plurality of vertical modular tube bundles which nest at their lower ends in a common pressure vessel, each tube bundle having its upper portion enclosed in a separate individual shell and flange arrangement to provide a heat exchanger having the thermal and hydraulic stability necessary for an evaporator employed with a liquid metal fast breeder nuclear reactor and to provide modules which can be easily and rapidly removed and replaced for inspection and servicing.

Description

United States Patent Harris, Jr. et al.

[ 1 Oct. 15, 1974 [54] HEAT EXCHANGER HAVING A 2,229,554 1/1941 Cummings 122 32 PLURALITY OF MODULAR TUBE 3,251,404 5/1966 Dickinson 122/32 X BUNDLES P E K h w S rimary xaminer ennet prague I73] Assignee: Westinghouse Electnc Corporation, 57] ABSTRACT Plttsburgh, Pa.

p I A heat exchanger having a plurality of vertical modulul 197] lar tube bundles which nest at their lower ends in a [21] Appl. No.: 120,423 common pressure vessel, each tube bundle having its upper portion enclosed in a separate individual shell and flange arrangement to provide a heat exchanger ((jlll. 122/32l;2l26b5/l having the thermal and hydraulic Stability necessary [58] Fie'ld 165/143 for an evaporator employed with a liquid metal fast breeder nuclear reactor and to provide modules which [56] References sited can be easily and rapidly removed and replaced for U E S AT S PATENTS inspection and servicing. 2,220,045 10 1940 Kraft et al 122/32 24 Claims, 4 Drawing Figures T l l 1 1 -41 PNENTED 1 74 sum 2 m 2 FIG.4.

n Flea.

one 000 cacao 0000a QOOobn HEAT EXCHANGER HAVING A PLURALITY OF MODULAR TUBE BUNDLES BACKGROUND OF THE INVENTION This invention relates to heat exchange apparatus for converting water to vapor or heating vapor to a higher temperature, and more particularly to a heat exchanger of this type having modular separately enclosed J- shaped tube bundles. The demand for electrical power in the United States doubles approximately every ten years. Presently, fossil fuels provide the majority of the heat energy for producing the needed electrical power. In the next 30 years it is estimated that over 50 percent of our electrical power will be produced by nuclear energy. The supply of fissionable material is limited, so that the future of nuclear power generation depends on developing a fast breeder reactor, which produces more fissionable material than it consumes. Such a system necessarily requires sound heat exchangers, which can be inspected and serviced with a minimum amount of down time. The present state of the art requires heat exchangers designed to transfer heat from liquid sodium to water, thus such heat exchangers must have provi sions to relieve the sodium water reaction by-products and limit the extent of the damage caused by such reactions as well as to permit rapid servicing of any portions damaged by such reactions.

SUMMARY OF THE INVENTION In general a heat exchanger for transferring heat from a primary to a secondary fluid made in accordance with this invention comprises a plurality of tube bundles having a channel head at each end thereof. The tubes are preferably of J-shape with a long stem portion, a short stem portion and a transverse bent portion joining the stem portions; a secondary fluid port in each head of each tube bundle so disposed that the secondary fluid flows through the tubes of each tube bundle; a separate tubular shell portion for the long stem of each J-shaped tube bundle; a primary fluid port disposed in each of the long stem shell portions; a main vessel having a primary fluid port therein; the lower portions of the tube bundles being disposed in the main vessel; and the tubular shells enwrapping the long stem portion of the tube bundles and being in communication with the main vessel so that primary fluid flows along the outer surface of the tubes forming the long stem portion of the J-shaped tube bundle, when flowing between the primary port in the primay port in the shell portion of the long stems of the J-shaped tube bundles and the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of this invention will become more apparent from reading the following detailed description in connection with the accompanying drawing, in which:

FIG. 1 is an elevational view of a heat exchanger made in accordance with this invention;

FIG. 2 is a plan of the heat exchanger;

FIG. 3 is an enlarged vertical sectional view of the heat exchanger taken on line III-III of FIG. 2; and

FIG. 4 is an enlarged cross sectional view taken on line lV-IV of FIG. 3, and showing a single module.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings in detail, FIGS. 1 through 4 show an evaporator or a heat exchanger for a liquid metal fast breeder nuclear reactor power plant (not shown). The heat exchanger 1 is shown to comprise a plurality of (in this instance, three) modular J- shaped tube bundle units 2.

A typical modular J-shaped tube bundle unit 2 comprises a tube bundle 3 having a plurality of tubes 4. The tube bundle 3 has a long stem portion 5 generally vertical, forming the major portion thereof, a generally vertical short stem portion 7 and a base, reverse bend, or arcuate portion 9 joining the generally

parallel stem portions

5 and 7. At the free end of each

stem portion

5 and 7 is a

channel head portion

11 and 13, respectively. The

channel head portions

11 and 13 each have a

tube sheet

15 and 17, respectively, which cooperate with the associated tube sheets to form

head chambers

23 and 25. Each

channel head

11 and 13 has a centrally disposed port or

nozzle

27 and 29, respectively, for secondary fluid, which may be either water or steam depending on whether the heat exchanger is utilized as an evaporator, a superheater or a reheater. Because of its flow characteristic, a heat exchanger made in accordance with this invention will serve equally well in any of the above capacities, the only changes necessary are sizing the various elements 'to correspond to the changes in heat transfer rates, flow rates, temperatures and pressures, which differ extensively for the different services herebefore mentioned.

The long stem 5 has a tubular shell portion 30 extending from adjacent the head portion 1 1 to a location above the lower tube bend 9. A primary fluid or liquid sodium inlet port or nozzle 31 is disposed in the upper end of the shell 30 adjacent the channel head portion 11. In the heat exchanger or evaporator shown in the drawings the tubular shell 30 has an enlarged cross sectional area adjacent the primary inlet port 31 as indicated at 33 to provide sufficient annular area for facilitating the turning of the incoming primary fluid. When the heat exchanger 1 is designed as a superheater, the enlarged portion 33 of the tubular shell 30 may not be required because the channel head 11 may be much thicker due to the higher temperatures in the superheater, therefore there may be sufticient annular area for turning'the incoming primary fluid without providing an enlarged cross section in this area. As shown in FIG. 3, a sleeve or shroud 35 is disposed in the shell 30 of the long stem portion 5 of the tube bundle 3. The sleeve 35 extends from below the tube sheet 15 and above the primary inlet port 31 to a location adjacent the beginning of the bend 9 and below the lower end of the tubular shell 30. The lower end of the tubular shell 30 and sleeve 35 are open.

A seal ring 36 is disposed between the tubular shell 30 and the sleeve 35 below the primary fluid inlet nozzle 31 and below the enlarged area 33 of the tubular shell 30. The seal ring 36 substantially blocks the flow of primary fluid from flowing downwardly in the annular space 37 between the tubular shell 30 and the sleeve 35. A plurality of vibration supports 39 are disposed between the shell 30 and sleeve 35 to center the sleeve 35 within the shell 30 and support the sleeve against vibration. The tubes 4 are supported against vibration and spaced by a plurality of tube supports or flow bafat least minimized the damage to the tube bundle and shell, if such a reaction occurs.

As shown in FIG. 3, the short stem portion 7 of the tube bundle 3 has a tubular shell portion 45 extending partially down the length thereof. The lower ends of the

shells

30 and 45 and the lower end of the tube bundle or arcuate portion 9 nest in a vessel 47. The vessel 47 comprises a tubular wall portion 49, a bottom closure or head 51 and a flat upper head portion 53. The head 51 has 9. depending primary fluid outlet port or nozzle 55 centrally disposed therein. The vessel 47 is supported on a peripheral skirt 57, which forms a base for the entire apparatus 1.

The flat head 53 has a plurality of oblong openings 59, which receive the lower portion 9 of the tube bundle 3 and the lower portion of the tube bundle stems and 7 and the

shells

30 and 45. Covers 61 form closures forthe openings 59 and encompass the

shells

30 and 45 of the

stems

5 and 7, respectively. The covers 61 are bolted and seal welded to the flat head portion 53 of the vessel 47 to form a positive seal and in such a manner that the tube bundles 3 can be easily and rapidly removed for inspection, repair and/or replacement. The vessel 47 and modular J shaped tube bundle units 2 can be shipped separately and installed in the field, eliminating the need for special handling required, when such heat exchangers are shipped in one piece.

A rupture disc 63 is provided in the flat head portion 53 of the vessel 47 to relieve the pressure in the vessel in event of a reaction between the sodium and water. To protect the tubes 4 adjacent the tube sheet and 17 from corrosion and thermal stresses, a blanket of inert gas is provided as indicated at 65 and 66 below each

tube sheet

15 and 17. Inert gas also blankets the flat head portion 53 of the vessel 47, as indicated at 67, to maintain the level of the liquid sodium, or primary fluid in the vessel 47 below the lower open end of the shell 45 of the short stern portion 7.

Perforated sheets 69 are disposed inside the vessel 47 between adjacent reverse bend portions 9 of the tube bundles to direct the primary fluid in downward direction and to stop a reaction between sodium and water from damaging adjacent tube bundles, if there is a malfunction in one tube bundle in the area not protected by a shell or sleeve portions.

As shown in FIG. 3 the oblong covers 61 are so disposed relatively to the

tubular shells

30 and 45 of the tube bundles and the vessel 47 that the modular tube bundle units 2 can be removed from the vessel by removing a seal weld 75 and bolts 77 which fasten the oblong cover to the flat heads 53. The tube bundles 3 are, so disposed within the

shells

30 and 45 that when the J-shaped modules are removed from the vessel 47 the tube bundles 3 may be easily removed from the

shells

30 and 45. The sleeves 35 enwrapping the long stem portions 5 of the tube bundles have a plurality of longitudinal seams 79 as shown in FIG. 4 so that they may be removed from the large stem portions to in spect the outer surfaces of the tubes 4.

The tube bundle units 2 and vessel 47, as hereinbefore noted, may be utilized as an evaporator, a stem superheater or a steam reheater in a liquid metal fast breeder nuclear reactor turbine generator power plant. A heat exchanger having such modular tube bundle units advantageously can be manufactured with any number of modular units so that the tube length in each modular can be limited to to feet, which is the length which can be presently supplied from tube manufacturers without special facilities and without butt welding the tubes together to provide the required length. Tube welds increase the danger of tube failure from sodium reacting with the tube material in the weld area and are therefore undesirable.

The modular J-shaped tube bundle units provide easy access to the tube sheets, and smaller diameter tube sheets which can be made thinner and manufactured to closer tolerances utilizing presently known shop procedures.

By utilizing a long radius reverse bend the tubes in each modular may be inspected inside and out when the unit is fabricated and when it is overhauled. The J- shaped design with the long radius reverse bends also provide free linear expansion of the tubes to occur in the main vessel 47 without undue stresses at the tube bends or the tube sheet welds.

Having a plurality of J-shaped modular tube bundle units which may be easily and rapidly removed from the vessel makes it economical to provide a spare module so that a malfunctioning or a damaged module can be removed and the spare module inserted in its place with a minimum amount of unit down time.

Although heat exchangers made in accordance with this invention are well suited to operate as a superheater or a reheater, they function exceptionally well as a once through steam generator or evaporator as their thermal and hydraulic characteristicsprovide a very stable once through steam generator.

When operating as an evaporator, primary fluid, liquid sodium, enters the primary inlet port .31 in the upper end of each shell 30 of the long stern portion 5 of each J-shaped module 2 and flows upwardly in the annular space 37 between the sleeve 35 and the shell 30 as the seal ring 36 prevents any substantial downward flow. The primary fluid flows over the upper end of the sleeve 35 and down the inside of the sleeve passing over the outside surfaces of the tubes 4 forming the long stem portion 5 of the tube bundle 3. The primary fluid then flows out the lower open end of the sleeve, over the reverse bends, and out the primary outlet port 55 centrally disposed in the lower closure 51 of the vessel 47, as indicated by the arrows with the solid shafts. As the primary fluid passes through the heat exchanger 1, it transfers its heat to the secondary fluid and becomes cooler and more dense exhancing natural circulation of the primary fluid through the heat exchanger.

The secondary fluid, water, enters the inlet ports 29 centrally disposed in the spherical dome 21 at the upper end of the short stem portion 7 of each tube bundle 3 and flows downwardly through the tubes 4 forming the short stern portion 7, then flows through the reversed bend or arcuate portions 9 of the tube bundle 3 and starts to vaporize as it begins to flow up the tubes 4 forming the long stem portion 5 of the tube bundle 3. As the secondary fluid continues up the tubes 4, the secondary fluid changes its state from water to steam and in the upper end of the long stem portion 5, the steam is superheated. The flow of the secondary fluid is indicated by arrows having dotted shafts. The J- shaped modular tube bundles thus provide for natural circulation of the secondary fluid flowing through the tubes and a counter flow relation between the primary and secondary fluid as the fluids flow through the long stem portion 5 of the modules. The secondary fluid may then flow through another heat exchanger (not shown) having one or more J-shaped tube bundles, this other heat exchanger serving as a superheater. In a heat exchanger 1 utilized as a superheater the steam preferably may enter the secondary fluid ports 29 in the short stern portion of ,the tube bundles and flow through the tubes to the secondary fluid ports 27 in the long stern portion of the tube bundles receiving heat from primary fluid, liquid sodium, flowing on the outside of the tubes of the tube bundles and through the vessel 47 in the same manner as herebefore described for the steam generator.

As well known in the art, after leaving the superheater, the steam flows through a high pressure turbine (not shown) and then it may flow through a reheat heater, very similar to the superheater with a flow path of primary and secondary fluid generally similar to the flow path of these fluids through the superheater.

Because of the heat exchangers hydraulic and thermal characteristics it is suited to be employed as an evaporator, a superheater or a reheater, thus providing reliable heat exchangers for a liquid metal fast breeder reactor, which are economical to produce, ship and erect and service in the field.

What is claimed is:

1. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising at least one tube bundle having a head at each end thereof, a long stem portion, and an arcuate portion extending from said long stem portion,

a secondary fluid port in each head of said tube bundle so disposed to permit said secondary fluid to flow through said tubes of said tube bundle,

a separate tubular shell portion enwrapping the long stem portion of said tube bundle,

a primary fluid port disposed in said long stem shell portion,

a vessel having a primary fluid port therein,

said arcuate portion of said tube bundle being disposed in said vessel, and

said tubular shell being in communication with said vessel and arranged to permit flow of primary fluid along the outer surface of said tubes in the long stern portion of said tube bundle, when flow between said primary port in said vessel and said primary port in said shell portion is established.

2. A heat exchanger as set forth in claim 1, wherein the vessel has at least one opening for receiving the arcuate portion of the tube bundle and cover means for said opening adapted to seal said opening and form a closure encompassing the tubular shell.

3. A heat exchanger as set forth in claim 1, wherein the primary fluid port in the tubular shell is adjacent the head at the end of the long stem portion.

4. A heat exchanger as set forth in claim 1, wherein the tubular shell has an enlarged cross sectional area adjacent the primary fluid port.

5. A heat exchanger as set forth in claim 1, wherein the ports for primary and secondary fluids are so disposed to permit the primary and secondary fluid to flow in counterflow relation in the long stem portion of the tube bundle.

6. A heat exchanger as set forth in claim 1, wherein the tube bundle has a short stem portion, on the end of the tube bundle opposite the end having the long stem portion, the long and short stem portions of the tube bundle each having a shell portion extending from adjacent the heads at the ends of the stems to a location within the vessel, the shell portions within the vessel being in communication with the vessel.

7. A heat exchanger as set forth in claim 6 and further comprising a supply of inert gas to maintain a level of primary fluid in the vessel, the tube bundle entering the vessel above the primary fluid level and extending downwardly into the primary fluid, the shell portion of the long stern portion of the tube bundle terminating in the primary fluid, the shell portion of the short stem terminating above said fluid level, and the terminal end of each shell portion being open to the vessel.

8. A heat exchanger as set forth in claim 6, wherein the shell portion of the short stem portion terminates a short distance from the location where it enters the vessel and the shell portion of the long stem portion terminates a greater distance from the location where it enters the vessel.

9. A heat exchanger as set forth in claim 6, and further comprising a sleeve disposed in the long stem portion of the tube bundle said sleeve extending'from adjacent the head at one end of the long stem portion of the tube bundle to adjacent the other end of the long stem of the tube bundle.

10. A heat exchanger as set forth in claim 9 and further comprising a ring disposed between the shell and the sleeve adjacent the primary port disposed in the shell portion, the ring being adapted to cause the primary fluid to flow through the center of the sleeve and across the outer surface of the tube disposed in the long stem portion of the tube bundle.

11. A heat exchanger as set forth in claim 9, wherein the tube bundle and shell are removable from the vessel and the tube bundle and heads are removable from the long and short stern shells.

12. A heat exchanger as set forth in claim 11, wherein the sleeve has lengthwise seams so disposed that when the tube bundle is removed from the short and long stem shells the sleeve can be removed from the long stem portion of the tube bundle.

13. A heat exchanger as set forth in claim 1, wherein the secondary fluid ports are so disposed that the secondary fluid enters the head at one end of the tube bundle in a liquid state and leave the head at the end of the long stem portion of the tube bundle in a gaseous state.

14. A heat exchanger as set forth in claim 1, wherein neither the primary nor the secondary fluid changes state within the heat exchanger.

15. A heat exchanger as set forth in claim 1, wherein there are a plurality of tube bundles and each tube bundle has a separate shell portion enwrapping the long stem portion thereof.

16. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein,

a plurality of modular tube bundles having a head at each end thereof,

a separate tubular shell enwrapping the major portion of each tube bundle,

a secondary fluid port so disposed in each head to permit secondary fluid to flow through the tubes of said tube bundles,

a portion of each tube bundle being disposed in said vessel,

a primary fluid port being disposed in each tubular shell,

said tubular shells being in communication with said vessel and disposed to permit primary fluid to flow over the outer surface of said tubes in said major portion of said tube bundle when a flow between said primary ports in said shells and said primary ports in said vessel is established, and

said vessel having a plurality of openings for receiving the shells and tube bundles and closure means for forming a seal between the shells and the vessel.

17. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein,

a plurality of modular tube bundles having a head at each end thereof,

a separate tubular shell enwrapping the major portion of each tube bundle,

a portion of each tube bundle being disposed in said vessel,

a primary fluid port being disposed in each tubular shell,

a sleeve disposed between each shell and tube bundle and extending into the vessel, and

said tubular shells being in communication with said vessel and disposed to permit primary fluid to flow over the outer surface of said tubes in said major portion of said tube bundle when a flow between said primary ports in said shells and said primary ports in said vessel is established.

18. A heat exchanger as set forth in claim 17, wherein the shells are removable from the vessel, the tube bundles are removable from the shells end the sleeves are removable from the tube bundles.

19. A heat exchanger as set forth in claim 17 and further comprising a ring so disposed between each shell and sleeve to cause primary fluid to flow through the center of each sleeve when a flow of primary fluid is established between the primary ports in the shells and the primary port in the vessel.

20. A heat exchanger as set forth in claim 17 and further comprising a plurality of sheets disposed in the vessel between adjacent tube bundles.

21. A heat exchanger as set forth in claim 17 and further comprising a plurality of perforated sheets disposed in the vessel between adjacent tube bundles.

22. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein,

a plurality of modular tube bundles having a head at each end thereof,

a separate tubular shell enwrapping the major portion of each tube bundle,

each tube bundle having a reverse bend portion which is disposed in said vessel,

a primary fluid port being disposed in each tubular shell, and

23. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein,

a plurality of modular tube bundles having a head at each end thereof,

a separate tubular shell enwrapping the major portion of each tube bundle,

each tube bundle having a short portion and a reverse bend joining the short portion and the major portion said reverse bend being disposed in said vessel,

a primary fluid port being disposed in each tubular shell, and said tubular shells being in communication with said vessel and disposed to permit primary fluid to flow over the outer surface of said tubes in said major portion of said tube bundle when a flow between said primary ports in said shells and said primary ports in said vessel is established. 24. A heat exchanger as set forth in claim 23 wherein the major portion of each tube bundle extends in the vertical direction and the lower portion of each tube bundle is disposed in the vessel.

Claims

1. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising at least one tube bundle having a head at each end thereof, a long stem portion, and an arcuate portion extending from said long stem portion, a secondary fluid port in each head of said tube bundle so disposed to permit said secondary fluid to flow through said tubes of said tube bundle, a separate tubular shell portion enwrapping the long stem portion of said tube bundle, a primary fluid port disposed in said long stem shell portion, a vessel having a primary fluid port therein, said arcuate portion of said tube bundle being disposed in said vessel, and said tubular shell being in communication with said vessel and arranged to permit flow of primary fluid along the outer surface of said tubes in the long stem portion of said tube bundle, when flow between said primary port in said vessel and said primary port in said shell portion is established.

7. A heat exchanger as set forth in claim 6 and further comprising a supply of inert gas to maintain a level of primary fluid in the vessel, the tube bundle entering the vessel above the primary fluid level and extending downwardly into the primary fluid, the shell portion of the long stem portion of the tube bundle terminating in the primary fluid, the shell portion of the short stem terminating above said fluid level, and the terminal end of each shell portion being open to the vessel.

9. A heat exchanger as set forth in claim 6, and further comprising a sleeve disposed in the long stem portion of the tube bundle said sleeve extending from adjacent the head at one end of the long stem portion of the tube bundle to adjacent the other end of the long stem of the tube bundle.

11. A heat exchanger as set forth in claim 9, wherein the tube bundle and shell are removable from the vessel and the tube bundle and heads are removable from the long anD short stem shells.

16. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein, a plurality of modular tube bundles having a head at each end thereof, a separate tubular shell enwrapping the major portion of each tube bundle, a secondary fluid port so disposed in each head to permit secondary fluid to flow through the tubes of said tube bundles, a portion of each tube bundle being disposed in said vessel, a primary fluid port being disposed in each tubular shell, said tubular shells being in communication with said vessel and disposed to permit primary fluid to flow over the outer surface of said tubes in said major portion of said tube bundle when a flow between said primary ports in said shells and said primary ports in said vessel is established, and said vessel having a plurality of openings for receiving the shells and tube bundles and closure means for forming a seal between the shells and the vessel.

17. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein, a plurality of modular tube bundles having a head at each end thereof, a separate tubular shell enwrapping the major portion of each tube bundle, a secondary fluid port so disposed in each head to permit secondary fluid to flow through the tubes of said tube bundles, a portion of each tube bundle being disposed in said vessel, a primary fluid port being disposed in each tubular shell, a sleeve disposed between each shell and tube bundle and extending into the vessel, and said tubular shells being in communication with said vessel and disposed to permit primary fluid to flow over the outer surface of said tubes in said major portion of said tube bundle when a flow between said primary ports in said shells and said primary ports in said vessel is established.

22. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein, a plurality of modular tube bundles having a head at each end thereof, a separate tubular shell enwrapping the major portion of each tube bundle, a secondary fluid port so disposed in each head to permit secondary fluid to flow through the tubes of said tube bundles, each tube bundle having a reverse bend portion which is disposed in said vessel, a primary fluid port being disposed in each tubular shell, and said tubular shells being in communication with said vessel and disposed to permit primary fluid to flow over the outer surface of said tubes in said major portion of said tube bundle when a flow between said primary ports in said shells and said primary ports in said vessel is established.

23. A heat exchanger for transferring heat from a primary to a secondary fluid, said heat exchanger comprising a vessel having a primary port therein, a plurality of modular tube bundles having a head at each end thereof, a separate tubular shell enwrapping the major portion of each tube bundle, a secondary fluid port so disposed in each head to permit secondary fluid to flow through the tubes of said tube bundles, each tube bundle having a short portion and a reverse bend joining the short portion and the major portion said reverse bend being disposed in said vessel, a primary fluid port being disposed in each tubular shell, and said tubular shells being in communication with said vessel and disposed to permit primary fluid to flow over the outer surface of said tubes in said major portion of said tube bundle when a flow between said primary ports in said shells and said primary ports in said vessel is established.

24. A heat exchanger as set forth in claim 23 wherein the major portion of each tube bundle extends in the vertical direction and the lower portion of each tube bundle is disposed in the vessel.