US3887003A

US3887003A - Bayonet tube heat exchanger

Info

Publication number: US3887003A
Application number: US293957A
Authority: US
Inventors: John Polcer; Robert O Barratt
Original assignee: Foster Wheeler Inc
Current assignee: Foster Wheeler Inc
Priority date: 1972-05-17
Filing date: 1972-10-02
Publication date: 1975-06-03
Anticipated expiration: 1992-06-03
Also published as: CA978181A

Abstract

A bayonet tube heat exchanger in which problems at the upper tube sheet created by high temperatures are eliminated. The bayonet tube assemblies are designed so that the unheated secondary fluid flows through the main tube sheet closer to the tube sheet than the heated secondary fluid which is flowing upwardly after having been heated in the space between the coaxial tubes in the lower portion of the bayonet tube assemblies.

Description

United States Patent [111 3,887,003

Polcer et al. June 3, 1975 154] BAYONET TUBE HEAT EXCHANGER 2.415.025 7/1949 Huff l65/l42 x 3.557.760 l/l97l Romanos [65/32 [75] Inventors: John Polcer. Florham Park. Robert O. Barratt, Parsippany. both of NJ.

Primary bxunmwr-Charles J. Myhre Assignec: 9 wheeler p Am'istzmt Examiner-Thcophil W. Streule. .lr.

Llvmgstom Altor'nt'). Agent. or Firm-John E. Wilson; Marvin A. 221 Filed: on. 2, 1972 Nfllgur [21] Appl. No.: 293,957

[57] ABSTRACT Related U.S. Apphcatlon Data [63] Continuation-impart of Ser. No. 254,l5() May 17. A bayonet tube heat exchanger wmch pmblcms at I972, abandoned, the upper tube sheet created by high temperatures are eliminated. The bayonet tube assemblies are designed 52 '05 CL 1 5 142; 1 5 32. 5 42 50 that th unheated SGCOHdfll) fluid flOWS through Th6 511 lm. Cl. rzsd 7/12 tube Sheet Closer tube Sheet than the of Search H l 2 hfllld secondary which is flowing upwardly flflfil' having been heated in the space between the coaxial [56] References cued tubes in the lower portion of the bayonet tube assem- UNITED STATES PATENTS 1.986.336 1/1935 Hard 165/[42 X 6 Claims, 4 Drawing Figures 44 l l 42 1 i 46" ,T If q 30 0'6 227 1 :2 A: r32 3 7 z; :2 L 1 4- V 1' x 1 J6 PATENTEU JUN a 1915 SHEET SHEET BAYONET TUBE HEAT EXCHANGER CROSS-REFERENCE TO A RELATED APPLICATION This is a continuation-in-part of US. Pat. application Ser. No. 254.150, filed May 17, I972 now abandoned.

BACKGROUND OF THE INVENTION One type of heat exchanger which is presently being used in industry is the bayonet tube type. In the bayonet tube heat exchanger. the bayonet tube assemblies which carry the secondary fluid into indirect heat exchange with the primary fluid are coaxial. The outer tube has a closed end and the secondary fluid is flowed through the inner tube until it reaches the end of the bayonet tube assembly where it impinges against the closed end of the outer tube to reverse its direction and flows back between the inner and outer tubes. It is when the secondary fluid flows between the inner and outer tubes that is picks up the greater part of the heat which is transferred to it by the primary fluid. The heated secondary fluid is collected between the tube sheet on which the outer tubes are mounted and another tube sheet which is connected to the ends of the inner tubes. There is a space between the tube sheets because the inner tubes are longer than the outer tubes.

In heat exchangers which operate at a high temperature, the temperature of the tube sheet to which the outer tubes are mounted will be very high because the secondary fluid when flowing through the tube sheet has received all of the heat it will receive from the primary fluid.

A practical example of such a heat exchanger is a lithium, molten salt, or liquid sodium heated superheater of the bayonet tube type. Such a heat exchanger is likely to have vertically extending bayonet tubes so that the main tube sheet which supports the outer tubes is likely to be lower than the one which supports the inner tubes and closer to the primary fluid. High temperatures in the area of the connection between the outer tubes and the main tube sheet can be problematic when there is an abrupt discontinuation of the flow of liquid sodium coming into the heat exchanger. The temperature in the critical area will change within a very short time. When the temperature drops over a large range abruptly, components which are affected by the change can fail. For instance, if the main tube sheet should suddenly cool, it could become detached from the outer tube because of shrinkage.

Even without abrupt temperature drops, high temperatures in the area of the outer tube and main tube sheet are undesirable because of the limitation on the life of the adjacent components due to creep. Changes in the relative dimensions of components in the area of the main tube sheet are undesirable because tight fits are necessary for safe and efficient operation.

SUMMARY It is the object of the present invention to overcome drawbacks found in the prior art. such as those discussed above. Accordingly, a bayonet tube type heat exchanger having bayonet tube assemblies. each with an outer tube closed at one end and fitted to a main tube sheet at the other end and a coaxial inner tube open at both ends, is provided with means to flow the heated secondary fluid through the main tube sheet inside of the annular space through which the unheated secondary fluid flows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view partly in section of a bayonet tube heat exchanger made in accordance with the present invention;

FIG. 2 is a side view partly in section of one of the bayonet tubes in the vicinity of the main tube sheet to which it is connected and the portion of that tube sheet which surrounds the tube; and

FIG. 3 is a side view partly in section similar to FIG. 2 but showing an alternate embodiment; and

FIG. 4 is a perspective view with parts broken away showing a portion of the heat exchanger of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT There is shown in FIG. I a heat exchanger which is designated generally as 10. The heat exchanger 10 includes a body portion 12 having an upper generally cy- Iindrical portion 14 and an integral inwardly flared bottom portion 16. The heat exchanger 10 is a heat exchanger which is used for superheating steam and is heated by lithium, liquid sodium, molten salt or other suitable primary fluid. The primary fluid. which in the preferred embodiment is liquid sodium, is heated from heat generated in a nuclear reactor which is not shown and which is not part of the present invention.

Liquid sodium enters the heat exchanger 10 through sodium inlets 18 and flows downwardly to a sodium outlet 20 which is centrally located in the bottom portion 16 of the heat exchanger 10.

A centrally located relief pipe 22 has an open lower end and openings 23 in the sidewall adjacent to the upper end. The pipe 22 extends down into the body portion 14 to provide a channel of escape for the prodacts of an inadvertent sodium water reaction.

The flow of sodium is such that the upper level of the sodium pool is maintained at L in the annular area around the relief pipe 22 and at L within it.

The body portion 14 has at its top an annular flange 24, the flange 24 being bolted to an annular flange 26 at the bottom of a tube sheet section 28 which has a flat annular portion 30 which forms a tube sheet and a central portion 31 which serves as a continuation of the relief pipe 22. The portion of the heat exchanger 10 below the main tube sheet 30 is the heat section, i.e the section where the secondary fluid is heated.

The tube sheet 30 which is the main tube sheet in the heat exchanger 10 engages each outer tube 32 of each of the bayonet tube assemblies 34. The outer tubes 32 are each closed at their lower end 35 and encircle the coaxial inner tubes 36 which extend upwardly from a level adjacent to the closed ends of the outer tubes 32 to a level adjacent to the tube sheet 30. In each of the bayonet tube assemblies 34, the inner tubes 36 merge in the vicinity of the tube sheet 30 with a device 37, as shown best perhaps in FIG. 4 the devices 37 directing the upcoming steam between the inner tubes 36 and outer tubes 32 to steam tubes 38 which extend coaxially upward from the device through the outer tube 34 and to an upper tube sheet 40.

The tube sheet 40 is annular in configuration and is flanged at its outer marginal portion 42 and its inner marginal portion 44, where it is bolted between the lower tube sheet section 28 and an annular cover 46.

The upper tube sheet 40 and annular cover 46 define a steam chamber 48 in which the steam is collected and then exhausted through steam outlets 50.

Incoming cold stream enters through steam inlets 52 and collects at a cold steam chamber 54 which is annular and which is defined by the tube sheets 30 and 40 and the portion of the lower tube sheet section between the tube sheets 30 and 40. The cold steam in the cold steam chamber 54 moves downwardly between the steam tubes 38 and outer tubes 32 until it is directed into the inner tubes 36 by the same devices 37 which direct the upcoming steam to the steam tubes 38 and which will now be described.

FIG. 2 shows in enlarged scale one of the bayonet tube assemblies 34 in the area adjacent to its connection with the tube sheet 30. Each of the present devices 37 comprises a generally cylindrical upper section 66 which is coaxial with an inner cylindrical section 68 which merges with one of the steam tubes 38 and which at its lower end is curved outwardly to place the steam tube 38 in communication with the space between the inner tube 36 and the outer tube 32 of the bayonet tube assembly 34 through an opening 70. With this arrangement. cold steam in the cold steam chamber 54 will flow downwardly between the

cylindrical sections

66 and 68 and then through 1 of the inner tubes 36 until it impinges against the closed lower end 35 (FIG. 1) of the associated bayonet tube assembly 34. The steam then flows upwardly in the annular spaces between the outer tubes 32 and the inner tubes 36. An annular layer forms about each device 37 because of annular welds 72 which seal the upper ends of the cylindrical sections to annular lips 74. The annular layer thus extends downwardly from the welds 72 around the devices 37.

With the present arrangement, it is cold steam rather than hot steam which is closest to the tube sheet 30 when flowing through it. Thus, the problems associated with the transfer of large quantities of heat to the main tube sheet 30 are avoided. in the event of a sudden discontinuance of the supply of liquid sodium, the tube sheet 30 would not be subjected to an abrupt temperature drop because it would not have been previously heated to an excessively high temperature.

FIG. 3 shows an alternate embodiment in which a shroud 76 spaced above the main tube sheet 30 has a plurality of upwardly extending sleeves through which the upper portions of the outer cylindrical sections extend and to which they are welded. in the embodiment of FIG. 3. the outer cylindrical sections 67 are longer than the corresponding sections 66 in the FIG. 2 embodiment so that they extend upwardly to the shroud 76. in the embodiment of FIG. 3. stagnant steam will extend around the outside of the outer cylindrical sec tions 67 and between the shroud 76 and tube sheet 30. ln FIG. 3, the stagnant steam layers from each of the bayonet tube assemblies communicate with the stagnant steam between the shroud 76 and the tube sheet 30.

In both embodiments the purpose of the stagnant steam layer around the devices 37 is to insulate the tube sheet 30 from the heat of the steam passing through the devices 37.

The foregoing describes but one preferred embodiment of the present invention. other embodiments being possible without exceeding the scope of the present invention as defined in the following claims.

What is claimed is:

l. A bayonet tube type heat exchanger comprising:

an upper tube sheet;

a main tube sheet having a plurality of holes;

a secondary fluid inlet between said upper tube sheet and said main tube sheet;

a secondary fluid outlet above said upper tube sheet;

a heat section below said main tube sheet sealed off from said secondary fluid by said main tube sheet;

a plurality of bayonet tube assemblies each one comprising:

an outer tube open at the top thereof where it is connected to the main tube sheet at one of said tube sheet holes and closed at its bottom, said outer tube extending into said heat section;

an inner tube coaxial with said outer tube the bottom of said inner tube being adjacent to the closed end of said outer tube the top of said inner tube being in said heat section adjacent to and below said main tube sheet;

a steam tube extending from within said outer tube to said upper tube sheet;

a device within said tube sheet hole between said inner tube and said steam tube, said device having a generally cylindrical outer tubular section flared inward at its bottom and a generally cylindrical inner tubular section extending eoaxially within said outer tubular section, said inner tubular section at its bottom communicating through a first opening in said outer section with the space between said inner tube and said outer tube, said inner tube communicating with the space between said outer tubular section and said inner tubular section through a second opening in said outer tubular section;

whereby, secondary fluid travelling upward through said bayonet tube assembly in the space between said inner tube and said outer tube toward said steam tube when passing through said main tube sheet. will pass through said first opening and then through said inner tubular section and unheated secondary fluid passing down said bayonet tube assembly through said main tube sheet will pass downward through the space between said inner tubular section and said outer tubular section and then through said second opening into said inner tube.

2. The heat exchanger defined in claim 1 wherein there is an annular space between said outer tube and said outer tubular section over at least a portion of the circumference thereof and said outer tubular section is joined to said main tube sheet to provide a circumferential seal between said outer tubular section and said tube sheet so that a stagnant layer of secondary fluid will form below the seal around said outer cylindrical section.

3. The heat exchanger defined in claim 2 wherein said steam tube and said inner tube have a common longitudinal axis.

4. The heat exchanger defined in claim 1 wherein there is a space between said outer tube and outer tubular section over at least a portion of the circumference thereofand the outer tubular sections of the bayonet tube assemblies are joined by a shroud which is spaced from said main tube sheet so that there is a stag nant layer of secondary fluid between said shroud and said main tube sheet which is in communication with 6 the secondary fluid between said inner tube and said 6. The heat exchanger defined in claim 5 wherein outer tube. said bayonet tubes extend vertically in said heat section 5. The heat exchanger defined in claim 4 wherein with said main tube sheet forming the top of said heat said steam tube and said inner tube have a common section. longitudinal axis. 5

Claims

1. A bayonet tube type heat exchanger comprising: an upper tube sheet; a main tube sheet having a plurality of holes; a secondary fluid inlet between said upper tube sheet and said main tube sheet; a secondary fluid outlet above said upper tube sheet; a heat section below said main tube sheet sealed off from said secondary fluid by said main tube sheet; a plurality of bayonet tube assemblies each one comprising: an outer tube open at the top thereof where it is connected to the main tube sheet at one of said tube sheet holes and closed at its bottom, said outer tube extending into said heat section; an inner tube coaxial with said outer tube the bottom of said inner tube being adjacent to the closed end of said outer tube the top of said inner tube being in said heat section adjacent to and below said main tube sheet; a steam tube extending from within said outer tube to said upper tube sheet; a device within said tube sheet hole between said inner tube and said steam tube, said device having a generally cylindrical outer tubular section flared inward at its bottom and a generally cylindrical inner tubular section extending coaxially within said outer tubular section, said inner tubular section at its bottom communicating through a first opening in said outer section with the space between said inner tube and said outer tube, said inner tube communicating with the space between said outer tubular section and said inner tubular section through a second opening in said outer tubular section; whereby, secondary fluid travelling upward through said bayonet tube assembly in the space between said inner tube and said outer tube toward said steam tube when passing through said main tube sheet, will pass through said first opening and then through said inner tubular section and unheated secondary fluid passing down said bayonet tube assembly through said main tube sheet will pass downward through the space between said inner tubular section and said outer tubular section and then through said second opening into said inner tube.

4. The heat exchanger defined in claim 1 wherein there is a space between said outer tube and outer tubular section over at least a portion of the circumference thereof and the outer tubular sections of the bayonet tube assemblies are joined by a shroud which is spaced from said main tube sheet so that there is a stagnant layer of secondary fluid between said shroud and said main tube sheet which is in communication with the secondary fluid between said inner tube and said outer tube.

5. The heat exchanger defined in claim 4 wherein said steam tube and said inner tube have a common longitudinal axis.