US3132691A - Heat exchanger construction and thermal shield therefor - Google Patents

Description

May 12, 1964 s. H. ESLEECK HEAT EXCHANGER CONSTRUCTION AND THERMAL SHIELD THEREFOR 3 Sheets-Sheet 1 Filed Feb. 6, 1959 FIG. 1

SEE F|c.'3

SEE FIG. 5

INVENTOR. Samuel H. Esleeck ATTORNEY s. H. ESLEECK 3,132,691

HEAT EXCHANGER CONSTRUCTION AND THERMAL SHIELD THEREFOR 6, 1959 3 Sheets-Sheet 2 FIG. 3

May 12, 1964 Filed Feb.

INVENTOR.

Samuel H. Esleeck ATTORNEY A CURVE A 1 AL. CURVE B "I FIG.4

CURVE C" INCHES A DEPTH FROM FACE 27 OF THERMAL SHIELD,

MEMBER DISPLACED ONE INCH CLOSER TO TUBE SHEET 14 0 o o O 6 5 4 3 4 CURVE A THERMAL SHIELD EQUIDISTANT FROM THIN CURVE B THERMAL SHIELD WITH WEDGE SHAPED CHAMBER 25 CURVE C THERMAL SHIELD EQUIDISTANT FROM THIN MEMBER May 12, 1964 s. H. ESL EECK 3,132,691

HEAT EXCHANGERCONSTRUCTION AND THERMAL SHIELD THEREFOR Filed Feb. 6, 1959 3 Sheets-Sheet 5 INVENTOR.

Samuel H. Esleeck ATTORNEY United States Patent 3,132,691 HEAT EXCHANGER CONSTRUCTEUN AND THEM-EAL SHEELD THEREFQR Sam-nei H. Esieeek, Lynchhnrg, Va., assignor to The Bahcoclr a Wiicox Company, New York, N.Y., a corporation of New Jersey Filed Feb. 6, 195%, Ser. No. 791,677 4 Claims. (Cl. 165-134) This invention relates in general to heat exchangers and more specifically, it relates to a new and improved means for reducing high thermal gradients in such heat exchangers at the juncture of thick and thin walled members therein.

In a pressure vessel construction wherein a thick and a thin member are joined, such a joint becomes an area subject to stress concentrations due to the reaction of the thick and thin members to pressure differentials. When such a construction, in addition, is subjected to thermal gradients, the resulting thermal stresses are additive to the aforementioned pressure stresses and these cumulative stresses must be used to design the vessel.

The present invention provides a means for reducing the high thermal gradients thereby making such heat exchanger construction commercially more attractive.

The present invention describes means for forming a wedge shaped chamber of quiescent fluid adjacent to the juncture of the thick and thin members of a heat exchanger. Ihis wedge shaped chamber reduces the high thermal gradient in each increment of length by distributing the total temperature change over a greater length of the members.

This invention further provides means, in conjunction with the wedge shaped chamber, to maintain within the chamber the same heat transfer fluid as is present in the main body of fluid within the shell side of the heat exchanger.

Moreover, this invention furnishes means for maintaining the fluid in the chamber in a quiescent state, thereby decreasing its coefficient of thermal conductivity.

The invention also provides means, in conjunction with a tube in the heat exchanger, for forming a wedge shaped annular chamber around the tube with its apex at the.

juncture of the tube and the tube sheet.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a longitudinal section of a shell and tube heat exchanger illustrating the present invention;

FIG. 2 is a cross section of the heat exchanger taken along the line 22 of FIG. 1 with a portion broken away;

FIG. 3 is an enlarged section at the juncture of the tube sheet and shell as generally indicated in FIG. 1;

3,132,691 Patented May 12, 1964 "Ice - adjacent each tube sheet 14- and 15' so as to provide a space to maintain a layer of quiescent fluid 28 between the thermal shield and the tube sheet (see FIGS. 3 and 5) and a layer of quiescent fluid 29 between the thermal shield and the shell (FIG. 3). This thermal shield surrounds each tube and has a means such as surfaces 37 and 24 adjacent the tubes and the shell to form, in conjunction with the. inner surface of the shell, and the outer surface of the tube, wedge shaped annular chambers 25 and 38. These wedge shaped chambers 25 and 38 are further defined by a baflle ring 26 secured to the thermal shield 23 at its outer surface 27, the ring 26 helping to maintain the fluid in chambers 25 and 38 in a quiescent state.

For a further clarification of the invention, a description of the operation of the heat exchanger of FIG. 1 follows. A hot heat transfer fluid flows through the shell 11 and across tubes 16 via inlet and outlet nozzles 21 and 22 respectively, releasing heat by indirect heat transfer relationship to a cooler fluid flowing through the tubes 16, the cooler fluid flowing serially through the inlet'nozzle 19, the inlet chamber 17, the tubes 16, the outlet chamber 18, and the outlet nozzle Ztl.

As an example of a specific embodiment of the present invention, the following design is given. In the heat exchanger of FIG. 1, the components, including the thermal shield, are fabricated of austenitic stainless steel. At the location where the shell 11 joins the tube sheet and hemispherical head, the shell has a thickened section as indicated by 11A. Referring to FIG. 3, the thermal shield, in association with the tube sheet 14 forms a gap 28, inch wide, and in association with the thickened part of the shell 1 1A a gap 29, /8 inch wide both of which serve as a repository for the quiescent fluid. The wedge shaped chamber 25 in the thermal shield is circular in section, having a diameter at surface 27 of 1% inches less than the inside diameter of the shell 11, and with a uniform tapered surface 24 such that, at a distance of 2 /2 inchesfrom the surface 27, the diameter of the thermal shield 23 is but /8 inch less than the inside 1 diameter of the shell 11 thereby forming an included FIG. 4 is a graph comparing the thermal gradient with three modifications of a thermal shield, including the present invention, drawn to the same scale and in relation with the section shown in FIG. 3;

'FIG. 5 is an enlarged section of the present invention at the juncture of the tube and tube sheet as generally angle of 12% degrees between the shell and the surface 24. Thereafter, the thermal shield forms a constant gap 29 adjacent to the thickened section 11A of the shell. The A3 inch thick baflie ring 26, in this example, restricts the opening to the chamber 25 by leaving a inch clearance with the shell 11.

Referring to FIG. 5, the thermal shield 23, in association with the tube sheet 14, forms a continuation of the gap 28 which is A inch wide and serves the same function as described above. The wedge shaped chamber 38 formed in the thermal shield has an opening, circular in section, 1% inches larger in diameter than the outside diameter of the tube 16 and arranged symmetrically thereabout at the surface 27 of the thermal shield 23. Following the conical surface 37, the circular section reduces in diameter to only A; inch larger in diameter than the tube 16, at the gap 28. The baflie ring 26 is for restricting the opening to the chamber 38 and is concentric to each tube 16.

In this specific illustration, liquid sodium is the hot heat transfer fluid flowing outside thetubes entering through nozzle 21 and leaving through nozzle 22. Pressurized water is the cooler fluid entering through nozzle 19 then flowing through the tubes 16, thence through outlet nozzle 20.

While specific examples of materials, fluids, and critical dimensions have been given, it is to be understood that other materials, fluids and dimensions may be employed in producing the invention; for instance, other alloys of steel, carbon steel, or non-ferrous metals may be used. Also other fluid combinations may be used, such as Water to water, water to steam, steam to steam, liquid metal to stearrn, and organic fluid to water, organic fluid to organic fluid, etc. The liquid metal used would not be limited to liquid sodium, but may be any liquid metal with the desiuable thermd properties.

In considering a heat exchanger constructed and operated as described above but Without the thermal shield, the thick tube sheet has the hotter fluid flowing adjacent to one face thereof, i.e. the tube side, and the cooler fluid flowing adjacent the opposite face. The hotter fluid causes the face with which it is in contact to expand to an extent dictated by the temperature of the hotter fluid. The cooler face of the tube sheet tends to expand to the extent dictated by the temperature of the cooler fluid flowing on that side, thereby causing diflerent degrees of expansion in a single piece of material. These expansion differentials pnoduce diiferential forces which act on the two faces, setting up a stress within this member, this stress being commonly referred to as the thermal stress. In the case of a thin member being joined to a thick member, as a tube joined to a tube sheet, or the thinner section of the shell joined to the thicker section, these thermal stresses are accentuated due to the further difference of thermal expansion of the thin member simultaneously in contact with both a hot fluid and a cold fluid and the mutual restraining forces exerted by each member upon the other.

The thermal shield 23 as illustrated in FIGS. 1, 2, 3 and 5 is not fixedly attached to either the thick or thin members and thus it is free to expand in all directions thereby transmitting no thermally caused stresses to these members. This shield is made of material having a low thermal conductivity as compared to the heat transfer fluid in which it is located. The quiescent layer of fluid between the thermal shield 23 and the tube sheets 14 or 15 or the shell 11 and 11A in combination with the low thermal conductivity of the thermal shield, as compared to that of the fluid, limits the heat flow from the hot fluid into the tube sheet or shell section, thereby reducing the thermal stresses imposed thereon. However, if the thermal shield were everywhere equidistant from the tube 16, or the shell 11 as illustrated by the dotted lines 36 and 39 in FIGS. 3 and 5, the thermal gradient in the thin member would not be reduced in degree but would only be displaced along the thin member away from the thick member as illustrated by curves A and C in FIG. 4. In this instance, curve C illustrates the thermal gradient with a thermal shield 3" thick and curve A illustrates the thermal gradient with a thermal shield 4" thick as shown in FIG. 3. If this thermal gradient were only displaced along the shell away from the tube sheet the high gradient would occur at the juncture of the thick and thin sections of the shell, and could make the high stress imposed upon this point prohibitive from a design standpoint.

Without the present invention, but using a thermal shield having a boundary as indicated by line 36 (or 39), this thermal gradient occurs in the thin member due to the fact that while it it not mechanically attached to the thermal shield, it is in good thermal contact therewith due to the heat transfer fluid between them. This fluid gap offers very little resistance to heat flow as compared to the thermal shield. As heat flows into the face 27 of the thermal shield in contact with the hot fluid, it meets with an increasing amount of thermal resistance in the form of the thermal shield itself. Since a flow of heat through a solid is analogous to the flow of current through an electrical circuit, it may be understood that the heat flow will seek the path of least resistance. Thus, a major portion of the heat entering the face 27 of the thermal shield will soon cross the small fluid gap adjacent surface 36 and enter the thin member. In essence, the thermal energy enters the thermal shield in an axial direction, but soon the major portion is diverted in a radial direction towards the thin member. The rate of heat input into the thin member is then quickly reduced because of the high resistance of the thermal shield to heat flow. Temperature gradients are a function of the change in thermal flow; therefore, if the thermal flow is quickly reduced, a high temperature gradient will result as illustrated in FIG. 4, curve A, where the temperature change from O to 1 inch in depth is the major portion of the temperature dif ferential from 0 to 4 inches in depth.

By beveling the thermal shield adjacent to the thin member, i.e. the tube or thin part of the shell, as taught by this invention, a tapered gap 25 exists between the thermal shield and the thin member. The volume of this wedge shaped chamber, formerly filled with a material of high thermal resistivity, is now filled with a material of relatively high thermal conductivity, i.e. the heat transfer fluid. The high radial heat flow described above still exists but now an appreciable axial flow of heat occurs in the fluid wedge which gradually decreases as the volume of the fluid chamber decreases. This axial heat flow pattern reduces the temperature gradient in the gap along the axial direction and thus reduces the gradient along the thin member as compared to the gradient existing in the former example.

In FIG. 4, curves B and A illustrate the difference in the thermal gradient in the thin walled member with the present invention of a wedge shaped chamber 25 as compared to the use of a thermal shield 23 without the wedge shaped chamber and using a thermal shield shaped as shown by the dotted 'line 36. In the specific heat exchanger example given above, with the liquid metal at 670 F. and the pressurized water at 297 F., a temperature drop of 334 took place in the shell in a distance of one inch from the face of the thermal shield as shown in curve A. With the use of the present invention, this same temperature differential was modulated so it occurred over a distance of 2 /2 inches from the face of the thermal shield as shown in curve B.

Where a thermal shield is not required to protect a tube sheet, the thermal gradient in a tube may be reduced by the alternate arrangement as illustrated. in FIG. 6 which shows a section through a tube sheet 30 in which a tube 31 is firmly seated. The inner face 32 of the tube sheet 30 is provided with a conical surface 33 concentric to tube 31 and extending approximately one-half the depth of the tube sheet from the face 32. This conical surface 33, in conjunction with the tube 31, forms an annular wedge shaped chamber 34 around the tube which serves to reduce the thermal gradient present at the juncture of the tube sheet and tube. This wedge shaped chamber operates in the same manner as described above and may be provided with a bafiie ring 35 to keep the fluid in the chamber 34 in a quiescent state.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of operation of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used toadvantage without a corresponding use of other features.

I claim:

1. A heat exchanger comprising a tube sheet having opposed first and second faces, a tube extending through the tube sheet and contacting the second face of the tube sheet, means for passing a cooling fluid in contact with the first face of the tube sheet and through the tube, means for passing a heating fluid having a higher temperature than the cooling fluid in contact with the outer surface of the tube and the second face of the tube sheet causing heat to flow through the tube to the tube sheet resulting in a thermal gradient along the tube, means forming a frusto-conically shaped fluid chamber surrounding the portion of the tube subject to said thermal gradient and including an outer face in contact with the heating fluid, the small end of said chamber located at said point of contact of the tube and the tube sheet and the large end of said chamber located at said outer face, said chamber being in communication with and occupied by said heating fluid, and a baflie means associated with said chamber forming means and said tube located at the large end of said chamber to substantially close the large end of said fluid chamber to maintain said fluid therein in a substantially quiescent state to reduce said thermal gradient along said tube.

2. A heat exchanger comprising a fluid-tight container having at least one tube sheet having opposed first and second faces, a tube seat formed through said tube sheet, a tube extending Within said container and having an end thereof fitted into said tube seat and securely fastened to said tube sheet, means for passing a first heat transfer fluid in contact with the first face of the tube sheet and through said tube, means for passing a second heat transfer fluid at a temperature different than that of said first fluid through said container in contact with the second face of said tube sheet and about said tube causing heat to flow between said tube and said tube sheet resulting in a thermal gradient along said tube, means forming a frusto-conically shaped fluid chamber in said second-face of said tube sheet surrounding said tube, the small end of said chamber located at said point of contact between said tube and said tube sheet and the large end located at the second'face of the tube sheet, said chamber being in communication with and occupied by said second heat transfer fluid, and a baflie ring surrounding said tube at the large end of said wedge-shaped chamber and substantially closing the end of said chamber to maintain the fluid therein in a substantially quiescent state to reduce the thermal gradient along said tube.

3. A heat exchanger comprising a fluid-tight container having at least one tube sheet having opposed first and second faces, a tube seat formed through said tube sheet, a tube extending within said container and having an end thereof fitted into said tube seat and securely fastened to said tube sheet, means for passing a first heat transfer fluid in contact with the first face of said tube sheet and through said tube, means for passing a second heat transfer fluid at a temperature different than that of said first fluid through said container in contact with the second face of said tube sheet and about said tube causing heat to flow between said tube and said tube sheet resulting in a thermal gradient along said tube, a thermal block disposed around said tube and having a first face adjacent said second face of said tube sheet and an oppositely disposed second face, means forming a frusto-conically shaped fluid chamber in said thermal block around and contiguous to the outer surface of said tube, said chamber having the small end located at the first face of said thermal block and its large end at the second face of said thermal block, said chamber being in communication with and occupied by said second heat transfer fluid, and a baflie ring surrounding said tube at the widest portion of said wedge-shaped chamber and substantially closing the end of said chamber to maintain the fluid therein in a substantially quiescent state to reduce said thermal gradient along said tube.

4. A heat exchanger comprising a fluid-tight container having at least one tube sheet having opposed first and second faces, a tube seat formed through said tube sheet, a tube extending within said container and having an end thereof fitted into said tube seat and securely fastened to said tube sheet, means for passing a first heat transfer fluid in contact with the first face of said tube sheet and through said tube, means for passing a second heat transfer fluid at a temperature different than that of said first fluid through said container in contact with the second face of said tube sheet and about said tube causing heat to flow between said tube and said tube sheet resulting in a thermal gradient along said tube, a thermal block disposed around said tube and having a first face adjacent said second face of said tube sheet and an oppositely disposed second face, means forming a frusto-conically shaped fluid chamber in said thermal block around and contiguous to the outer surface of said tube, said chamber having the small end located at the first face of said thermal block and its large end at the second face of said thermal block, said chamber being in communication with and occupied by said second heat transfer fluid, and a baflie ring surrounding said tube at the widest portion of said wedge-shaped chamber and substantially closing the end of said chamber to maintain the fluid therein in a substantially quiescent state to reduce said thermal gradient along said tube, said thermal block being immersed in said second heat transfer fluid' and having a lower thermal conductivity than said fluid in which it is immersed.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A HEAT EXCHANGER COMPRISING A TUBE SHEET HAVING OPPOSED FIRST AND SECOND FACES, A TUBE EXTENDING THROUGH THE TUBE SHEET AND CONTACTING THE SECOND FACE OF THE TUBE SHEET, MEANS FOR PASSING A COOLING FLUID IN CONTACT WITH THE FIRST FACE OF THE TUBE SHEET AND THROUGH THE TUBE, MEANS FOR PASSING A HEATING FLUID HAVING A HIGHER TEMPERATURE THAN THE COOLING FLUID IN CONTACT WITH THE OUTER SURFACE OF THE TUBE AND THE SECOND FACE OF THE TUBE CAUSING HEAT TO FLOW THROUGH THE TUBE TO THE TUBE SHEET RESULTING IN A THERMAL GRADIENT ALONG THE TUBE, MEANS FORMING A FRUSTO-CONICALLY SHAPED FLUID CHAMBER SURROUNDING THE PORTION OF THE TUBE SUBJECT TO SAID THERMAL GRADIENT AND INCLUDING AN OUTER FACE IN CONTACT WITH THE HEATING FLUID, THE SMALL END OF SAID CHAMBER LOCATED AT SAID POINT OF CONTACT OF THE TUBE AND THE TUBE SHEET AND THE LARGE END

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