US2719354A - Method of making extended surface heat exchanger - Google Patents

Description

D. DALlN Oct. 4, 1955 METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGER 3 Sheets-Sheet 1 Filed NOV. 13 1950 D. DALlN 3 Sheets-Sheet 2 Oct. 4, 1955 METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGER Filed Nov. 13, 1950 I), Fill/ Oct. 4, 1955 DAMN 2,719,354

METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGER Filed Nov. 13, 1950 5 Sheets-Sheet 3 i p......-.r--.-.../

United States Patent METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGER David Dalin, Stenkullen, Ronninge, Sweden, assignor t0 A/E Svenslra Maskinverken, Sodertalje, Sweden, a corporation of Sweden Application November 13, 1950, Serial No. 195,201

3 Claims. (Cl. 29-1573) This invention relates to heat exchangers and refers more particularly to extended surface for heat exchangers.

It has been conclusively shown that the most effective form of extended surface consists of a multiplicity of closely spaced relatively small diameter rod or wire-like elements joined to and extending from the base wall which separates the two media between which heat transfer is to take place. The advantage of this form of extended surface is discussed at length in Patent No. 2,469,635 issued to David Dalin and Gustav V. Hagby May 10, 1949.

As therein explained the wire or rod-like elements should be made of metal having high thermal conductivity, copper and aluminum being specifically mentioned. A commercially feasible method of joining the element to the base wall is disclosed in the pending application of David Dalin, Serial No. 82,572 filed March 21, 1949, now Patent No. 2,584,189, granted February 5, 1952.

The one disadvantage of the extended surface disclosed in the aforesaid patent and application is that copper exposed to air or gases containing oxygen oxidizes rapidly when its temperature exceeds 260 C., and While under similar conditions aluminum has better resistance to oxidation the improvement is not enough to cope with the elevated temperature prevalent for instance in boiler furnaces and the gas passages thereof, and aluminum is not as satisfactory for the purpose as copper since copper has nearly twice the conductivity of aluminum and not as high a ccefiicient of expansion.

Also in situations involving large and rapid fluctuations in temperature the difference in coefiicients of expansion between steel or steel alloys of which the base wall is preferably made and the copper or aluminum of which the elements are made, placed so great a strain upon the Welded junctions by which the elements are secured to the base wall that these connections crystallized and failed.

Another limitation upon the use of copper for the extended surface elements is its inability to withstand the chemically reactive gases prevalent in certain situations of use, but unless the extended surface elements are made of copper or aluminum, the only commercially feasible metals with the required heat conductivity, the advantages of this ideal extended surface are lost.

The problem to which this invention is addressed is thus clearly delineated, namely to find some way of protecting the extended surface elements against oxidation and corrosion without appreciably impairing their heat conductivity and to provide some means of preventing crystallization and failure of the junctions by which the elements are secured to the base wall. The obvious expedient of plating the elements with appropriate metal to afford resistance to oxidation and corrosion at elevated temperatures is out of the question, if for no other reason than the cost involved since the plating would have to be done after all of the elements are welded to the base wall.

However, notwithstanding the high cost involved, plating with various metals having better oxidation and cor- Patented Oct. 4, 1955 rosion resistance than copper (in air) was tried and found unsatisfactory because of the ditficulty of obtaining a good protective plating at the roots of the elements where they are welded to the base wall and because the plating cracked when the elements were bent into position. Alloying the copper to improve its resistance to oxidation and corrosion is also out of the question for alloying greatly reduces the heat conductivity of the metal.

It was evident, therefore, that unless a solution to these problems was found, this ideal extended surface would be relegated to the less rigorous service of such ancilliary apparatus as economizers and air preheaters where the temperatures involved are not too high, and to service where the gases are not too chemically reactive.

This invention meets and solves these problems.

In broad terms the invention achieves its purpose by making the elements bimetallic with a core of copper or aluminum and a sheath of metal having a resistance to oxidation in air and gases containing oxygen at elevated temperatures, and a resistance to corrosion by chemically reactive gases, better than that of copper. Steel of the same type used for the base wall or tubes to which the extended surface elements are secured is entirely satisfactory for most purposes, but the unequal coefficients of expansion of steel and copper or aluminum introduced another problem which was discovered when the bimetallic elements were first applied to the base wall. It was found that the high heat involved in the resistance welding operation by which the elements are attached to the base wall caused the sheath to crack and burst at the root of the element.

It was also found that when the extended surface elements were subjected to sudden and large temperature rises particularly when often repeated and more so when the heat was supplied via the base wall to the extended surface elements, the sheath would crack and burst along shorter or greater portions of its length.

Upon analysis it was determined that the reason for such cracking and bursting of the sheath was the fact that the core metal expanded so much more rapidly than the steel sheath that it cracked and broke the sheath. This problem is overcome in this invention by annealing the bimetallic elements before welding them to the base wall. By annealing the elements at a temperature at least as high as that encountered in normal service all evidence of cracking and bursting of the sheath was eliminated.

Thus, as will appear more fully hereinafter the present invention completely solves the problems to which it is directed in a practical and simple manner which will be clear from the following description, reference being had to the accompanying drawings which form a part of this specification, and in which:

Figure 1 is a cross sectional view through a heat exchanger constructed in accordance with this invention and especially adapted for use in steam boilers since the extended surface is mounted on the outside of a tube through which boiler fluid may flow;

Figure 2 is an enlarged detail view showing a portion of the tubular base wall of Figure 1 and one of the extended surface elements prior to its attachment to the base wall;

Figure 3 is an enlarged detail view similar to Figure 2 but showing the extended surface element attached to the tubular base wall;

Figure 4 is a detail sectional view at still a larger scale to illustrate particularly the junctions between the core and sheath of the extended surface elements and the junction of the extended surface elements to the base wall;

Figure 5 is a cross sectional view through a portion of another form of heat exchanger constructed in accordance with this invention and adapted particularly for installations wherein the fluid medium at one side of the base wall has substantially the same surface conductance as the fluid medium at the other side thereof so that extended surface should be used on both sides of the base wall;

Figure 6 is a cross sectional view through still another form of heat exchanger constructed in accordance with this invention and distinguished from those of Figures 1 and 5 in that the extended surface elements are applied tangentially to a tubular base wall;

Figure 7 is a plan view, on a reduced scale, of the heat exchanger shown in Figure 6;

Figure 8 is a cross sectional view through a portion of the heat exchanger shown in Figure 1 but illustrating a different construction for the extended. surface elements;

Figure 9 is a cross sectional view through a heat exchanger of the type shown in Figure l but in which all of the exposed surfaces are additionally protected against oxidation and corrosion by a coating of vitreous or other suitable oxidation and corrosion resisting material;

Figure 10 is a. detail sectional view longitudinally through the outer end portion of one of the extended surface elements to show the end of its core covered by a cap' of the same metal as its sheath;

Figure 11 is a cross sectional view through a portion of. a so-called water wall of a furnace illustrating another adaptation of this invention;

Figure 12 is a plan view of a portion of such wall;

Figure 13 is a cross sectional view through Figure 12 on the plane of the line 1313 but on an enlarged scale; and

Figure 14 is a detail cross sectional view through Figure 12 on the plane of the line 14-14.

Referring now more particularly to the accompanying drawings in which like numerals indicate like parts, the numeral 5 designates in all forms of the invention shown the metal base wall which separates the two media between which heat' transfer is to take place and to which the extended surface elements designated generally by' the numeral 6 are attached. Depending upon the service for which the heat exchanger is designed the base wall is either tubular as in Figure l or planar as in Figure 5, but in all instances the base wall is of metal having high tensile strength and a relatively low coefficient of expansion.

For most installations mild steel is ideally suited to thepurpose but where extremely oxidizing or corrosive conditions prevail a suitable steel alloy, as for instance a chromium steel alloy relatively high in chromium but low in carbon content, should be used and, if desired, the base wall may be laminated with only that surface thereof exposed to the oxidizing or corrosive conditions formed. of the more expensive alloy and the other surface formed either of'mild steel or even a cheaper steel or other metal.

For purposes of illustration such a laminated base wall is illustrated in Figure 6 wherein the tube 5 has a core 7 of relatively inexpensive steel or other suitable metal within a shell 8 of metal such as chromium steel alloy which has high resistance particularly to oxidation.

The extended surface elements indicated generally by the numeral 6 are attached to the base wall either endwise, as shown for instance in Figure l, or tangentially as shown in Figure 6. In each instance the extended surface elements are bimetallic and in all embodiments of the invention shown with the exception of that of Figure 8 the extended surface elements consist of a core portion 9 and a sheath portion 10; The core is of metal having high thermal conductivity and a relatively'high coefficient of expansion. Copper is ideally suited for the purpose although aluminum may be used.

The sheath 10-is of metal having high tensile strength and a relatively low coefficient of expansion but most important possessing good resistance to oxidation and corrosion. at. elevated temperatures. The metal of which the sheath 10 is formed must possess better resistance to oxidation in air at elevated temperatures than copper and the metal should be highly resistant to corrosion regardless of temperature. This means that the sheath metal should be proof against oxidation and scaling to temperatures of at least 400 C. and in some cases as high as 800 C.

Another requirement of the metal used for the sheath is that it have substantially the same coefficient of expansion as the metal of the base wall or at least that surface of the base wall to which the element is attached. Thus, if the base wall is of mild steel the sheath metal likewise preferably should be of mild steel and if the base wall is made of a steel alloy the sheath likewise preferably should be made of a steel alloy.

Another metal that can be advantageously used for the. sheath is Admiralty bronze which is highly resistant to oxidation andcorrosion and has an expansion coelficient close to that of steel.

The wall thickness of the sheath may vary but generally should be no greater than required by the manufacturing process used to produce the wire from which the element is made, and/or no greater than necessary to secure a good. and durable bond between the sheath and the base wall and generally assure durability of the sheath under. prevailing service conditions. For elements of average diameter which may range between 2 mm. and 8 mm. the wall. thickness of the sheath should be approximately between .2 mm. and .7 mm.

The specific manner in which the elements are produced, that is the manner in which the core is enshroudedwith the sheath, forms no part of this invention and may be done in any of the ways known to the art of producing bimetallic wire. Thus, for instance, an ingot may be prepared having a core of copper or aluminum, depending upon which of these metals is to be used for the core, and then progressively reduced in diameter and elongated as by rolling and wire-drawing until. a wire of the desired diameter is obtained. Such diameter reduction and elongation, of course, forces the sheath into firm intimate gripping relation with the core.

The specific way in which the ingot is prepared also is of no direct concern to this invention. Thus, the ingot may be produced by casting the core metal within a tube of the sheath metal, or the sheath metal may be wrapped about a bar of the core metal with its longitudinal seam secured by welding.

Regardless of how the wire from which the extended surface elements are formed is manufactured it has been found to be extremely important that before the wire is attached to. the base wall either as rod-like elements secured endwise to the wall or as wire-like members secured tangentially to the base wall, the wire must be annealed. Experience has demonstrated that unless it is annealed the sheath cracks and bursts during the welding operation, where the heat of the welding operation is concentrated. This no doubt follows from the fact that because of the difference in expansion coeflicients between the core metal and sheath metal the rapid temperature rise so stresses the sheath metal as to cause its failure- Thev gradual increase and decrease in temperature of the annealing operation, however, does not set up these strains but on the contrary conditions the wire to preclude rupture or failure of the sheath during subsequent temperature changes in use.

While the exact explanation of how this conditioning is achieved by annealing. the wire is not known it is quite probably that during the annealing operation the greater expansion of the core expands the sheath beyond its elastic limit, and since the temperature employed in the annealing should be at least that encountered in the average use for which the heat exchanger is designed, the. diameter to which the sheath is expanded during the annealing operation is not exceeded in service. During the cooling step of the annealing operation the core, of course, contracts more than the sheath and thus tends to pull away from the sheath. This creates a zone in which the molecules of the metal of the core and sheath pull in opposite directions, and this zone apparently has suificient elasticity to accommodate any subsequent expansion and contraction of the core without imposing bursting stresses upon the sheath.

It is also possible that a minute clearance between the core and the sheath may be the result of the annealing operation. However, there has been no noticeable reduction in the heat transfer from the sheath to the core.

Hence, this zone which lies at the contiguous surfaces of the core and sheath can be considered a cushioning zone to cushion the stresses involved in subsequent expansion and contraction of the element.

The explanation of why annealing conditions the wire to preclude bursting of its sheath in service may also lie simply in the fact that annealing renders both the core and sheath ductile, but whatever the explanation may be, as pointed out before, it is important that the wire be annealed before it is attached to the base wall.

In cases where the extended surface elements during use are subjected to extremely high temperature or where the lack of affinity between the metals in the core and the sheath tend to lessen the bond between the core and the sheath a thin layer of a third metal or other material having suitable bonding, flexibility or firmability properties may be introduced between the core and the sheath in order to thereby increase the effectiveness of the cushioning zone.

If the wire is attached endwise as in Figure 1 it is, of course, cut into rod-like elements of the appropriate length and these elements are then resistance welded in the manner described in the aforesaid pending application, Serial No. 82,572. In conformance with the method there described the elements and base wall are connected in an electric circuit and the elements are pressed against th base wall while connected in a resistance welding circuit; and if the exchanger is of the type shown in Figure 1 the elements are applied radially to the tube as indicated by construction lines in Figure 1 and then subsequently bent into parallelism.

As explained at length in the aforesaid copending application, the welding time, welding current, conductivity of the element, conductivity of the base wall and the welding force or pressure are critical factors. However, since the sheath of the elements is of the same metal as the base wall or at least has substantially the same coefiicient of expansion as the base wall, the securement of the element to the base wall is considerably easier than when the junction depends solely upon the welding of dissimilar metals as for instance copper to steel. As a result crystallization and failure of the junctions which has been experienced with units in which the elements were entirely of copper welded to a steel base wall is entirely overcome by this invention, since in welding the elements to the base wall the sheath metal fuses perfectly with the metal of the base wall and no disruptive forces exist in service to cause crystallization and failure.

Whether or not the core of the element also fuses with the base wall or becomes securely welded thereto is, therefore, no longer of importance since the junction of the sheath to the wall is fully capable of holding the element in place and it should be observed that whether the core is fused to the base wall or not it is in such intimate contact therewith as to assure good heat conduction therebetween.

A comparison of Figures 2 and 4 illustrates the before and after conditions of the element and base wall and attention is directed particularly to Figure 4 which shows how the metal of the sheath is fused to the metal of the base wall. This view also indicates more or less 6 diagrammatically the cushioning zone between the sheath and core by which expansion and contraction of the core without imposing bursting stresses upon the sheath is made possible.

In that form of the heat exchanger illustrated in Figures 6 and 7 wherein the extended surface is better defined as a series of wires secured tangentially to the tubular base wall, the present invention makes this form of heat exchanger commercially possible since it obviates the difficulties that were experienced in the past due to the close proximity of the welded connections. Whereas in the past it was necessary to individually weld each junction it is now possible to simultaneously weld a great number of the junctions so that an entire mat of wire elements can be secured to a plurality of tubes in one welding operation.

As has been seen, one of the problems solved by this invention is that which caused the crystallization and failure of the junctions by which the elements are secured to the base wall. As shown in Figure 8, this advantage can be attained even though no steel sheath is employed. Situations where it is desirable to have the exposed surfaces of the elements of copper thus can be accommodated. In this case the elements are provided with a small diameter core of steel or other metal high in tensile strength and having substantially the same coefficient of expansion of the base wall, within a body of copper. When such an element is welded in position the steel core fuses with the metal of the base wall to achieve the desired strength at the junction even though the body of the element is very nearly all copper.

Though the art has generally understood the advantages of coating metal surfaces with a layer of vitreous or other material possessing good resistance to oxidation and corrosion to protect the same against oxidation at high temperatures, before the advent of the extended surface heat exchanger of the aforesaid Dalin et al. patent with its great reduction in tube requirements and hence far fewer connections to be made between tubes and headers, the use of such protective coatings was not practically feasible.

The obviously greater difficulty of connecting a vitreous enamel or glass covered tube to a header and the attendant cost ruled out this practice, but while the improved extended surface of the aforesaid patent eliminated the reason for not using a protective coating of vitreous or other protective material, it substituted an even greater barrier to its adoption. Such protective materials can be produced with a coefiicient of expansion which matches that of the metal to which it is to be applied but it cannot be rendered compatible in this respect with more than one metal.

Therefore, since the extended surface of the new heat exchanger was copper and the base wall steel, a vitreous or similar coating was impossible. This invention, however, for the first time makes it feasible and possible to coat the entire surface of the heat exchanger with a vitreous or other suitable protective covering since all the exposed surfaces which are to be covered have the same coeflicient of expansion. Thus where the added protection of such a coating is desired, it may be applied, and Figure 9 shows such a coating 12 extending continuously over the entire exposed surface, including the ends of the extended surface elements.

Heating surface so protected has the tremendous advantage of being practically universal in its utility. With it there is no need for special temperature controls to pre- Vent condensation of chemically reactive gases upon the heating surfaces which means, of course, that the dew point problem disappears. Also such a coating adapts the structure to use in acid and strong alkaline liquids.

Where the surface is not coated with vitreous or other protective material, the ends of the elements, if desired, may'be pinched to close the sheath material over the end of the core, or a separate cap 13 may be applied as 7 shown in Figure 10, but since the cross sectional area of the core'is very small this precaution is ordinarily not required.

Another application of this invention, illustrated in Figures 11 to 14, inclusive, is for a so-called water wall for'the furnaces of steam boilers and the like. As brought out, for instance, in the patent to T. E. Murray No. 2,220,579 issued November 5, 1940, it has been customary in the prior art to line the walls of furnaces with water tubes, and to reduce the number of tubes required laterally projecting fins have been welded thereto. This invention is, therefore, admirably suited to this purpose since it enables far greater spacing of the water tubes than has been heretofore possible.

To this end the tubes through which the boiler fluid circulates have fins 6' welded thereto in the same fashion as the extended surface elements are attached to the base Wall in the embodiments of the invention described. The fins 6 comprise a core 9' of copper or other metal of high thermal conductivity and a sheath 10 of steel or other suitable metal having the same coefficient of expansion as the tubes. It is, of course, important here as in the case of the wire-like elements that the fins be annealed before their attachment to the tubes.

The advantage of this construction over the steel fins is illustrated by comparison of the length of the bimetallic fins with the approximate length of steel fins (indicated by the broken lines L in Figure 13) as used for this purpose in the prior art. Obviously with this construction the same protection for the furnace walls is obtained with far less tubes than was heretofore necessary.

It is, of course, to be expected that enshrouding the elements in the manner described will detract somewhat from their ability to transfer heat to and from a medium flowing thereover, but this slight reduction in surface conductance is negligible compared to the tremendous advantage which this invention achieves. By actual test it was found that as between elements formed entirely of copper and those constructed in accordance with this invention, there is a reduction in heat transfer of only six percent.

From the foregoing description taken in connection with the accompanying drawings, it will be readily apparent to those skilled'in the art that this invention is of tremendous importance to the heat exchange art generally and especially to that branch of the art dealing with steam boilers and ancilliary apparatus since it provides the means by which all of the advantages of the aforesaid Dalin et al. patent and the copending application may be achieved in a commercially economical'and entirely practical manner.

What 1 claim as my invention is:

1. In the method of permanently attaching extended surface elongated elements of metal having heat conductivity and coetficient of expansion values on the order of those of copper, to heat exchanger tubes having a base wall of ferrous metal of considerably lower heat conductivity and coefiicient of expansion values than those of the extended surface elements, without the necessity of fusing the elements to the base wall, the steps of: tightly encasing each extended surface element in a thin attaching sheath of ferrous metal which is corrosion resistant at high temperatures and has substantially the same coefficient of expansion as that of the base wall; annealing the thus encased extended surface elements; and by resistance welding securing the attaching sheaths of the annealed elements to the base wall to provide an uninterrupted direct all metal heat conducting path between the base wall and the areas of the elements closest thereto and with the encased elements projecting outwardly from the base wall, so that the desired good heat conducting and 7 mechanically secure connection between the elements and the tubes is effected without the need for actual fusion ofthe elements themselves to the base wall as a consequence of the welded joints between their attaching sheaths .and the base wall, regardless of whether theelments are applied radially endwise to the heat exchanger tubes or tangentially of said tubes.

2. In'the method of permanently attaching extended surface elongated elements of metal having heat conductivity and coefficient of expansion values on the order of those of'copper, to heat exchanger tubes having a base wall of ferrous metal of considerably lower heat conductivity and coefiicient of expansion values than those of the extended surface elements, without the necessity of fusing the elements to the base wall, the steps of: tightly encasing the entire length of each extended surface element in a thin tubular attaching sheath of ferrous metal which is corrosion resistant at high temperatures andhas substantially the same coetficiento'f expansion as that of the base wall; annealing the thus encased extended surface elements; and'by resistance welding securing one end of each attaching sheath to the base wall with the adjacent exposed end of the annealed element directly abutting the base wall in intimate heat conducting relationship therewith, so' that the desired good heat conducting and me chanically secure connection between the elements and the tubes is effected without the need for actual fusion of the elements themselves to the base'wall as a consequence of the welded joints between their attaching sheaths and the base wall. i

3. In the method of permanently attaching extended surface elongated elements of metal having heat conductivity and c'oefiicient of expansion values on the order of those of copper, to a plurality of spaced apart heat exchanger tubes having a base wall of ferrous metal of considerably lower heat conductivity and coefiicient of ex pansion values than those of the extended surface elements, without the necessity of fusing the elements to the base wall, the steps of: tightly encasing the entire length of each extended surface element in a thin attaching sheath of ferrous metal which is corrosion resistant at high temperatures and has substantially the same coefiicient of expansion as that of the base wall; annealing the thus encased extended surface elements; and by resistance welding securing the attaching sheaths of the annealed elements to the base wall, with the encased elements substantially parallel to one another and extending transversely across the tubes tangentially thereof, to provide an uninterrupted direct all'metal heat conducting path between the base wall and the areas of the elements closest thereto so that the desired good heat conducting and mechanically secure connection between the elements and the tubes is effected without the need for actual fusion of the elements themselves to the base wall as a consequence of the welded'joints between their attaching sheaths and the base wall.

References Cited in the file of this patent UNITED STATES PATENTS 47,940 Farmer et a1. May 30, 1865 1,140,136 Eldred May 18, 1915 1,512,961 Weil Oct. 28, 1924 1,603,491 Osnos Oct. 19, 1926 1,607,968 Spire Nov. 23, 1926 1,653,378 Steel Dec. 20, 1927 1,802,695 Bennett Apr. 28, 1931 1,821,702 Freeman Sept. 1, 1931 1,884,741 Kleffel; Oct. 25, 1932 1,929,444 Murray Oct. 10, 1933 2,064,141 Askin Dec. 15, 1936 2,088,446 Specht July 27, 1937 2,149,696 Holmes Mar. 7, 1939 2,154,448 Hotfer Apr. 18, 1939 2,308,319 Stanton Jan. 12, 1943 2,337,294 Cooper Dec. 21, 1943 2,433,687 Durst a. Dec. 30, 1947 2,450,203 Morgan Sept. 28, 1948 2,584,189 Dalin Feb. 5, 1952

Claims (1)

1. IN THE METHOD OF PERMANENTLY ATTACHING EXTENDED SURFACE ELONGATED ELEMENTS OF METAL HAVING HEAT CONDUCTIVITY AND COEFFICIENT OF EXPANSION VALUES ON THE ORDER OF THOSE OF COPPER, TO HEAT EXCHANGER TUBES HAVING A BASE WALL OF FERROUS METAL OF CONSIDERABLY LOWER HEAT CONDUCTIVITY AND COEFFICIENT OF EXPANSION VALUES THAN THOSE OF THE EXTENDED SURFACE ELEMENTS, WITHOUT THE NECESSITY OF FUSING THE ELEMENTS TO THE BASE WALL, THE STEPS OF: TIGHTLY ENCASING EACH EXTENDED SURFACE ELEMENT IN A THIN ATTACHING SHEATH OF FERROUS METAL WHICH IS CORROSION RESISTANT AT HIGH TEMPERATURES AND HAS SUBSTANTIALLY THE SAME COEFFICIENT OF EXPANSION AS THAT OF THE BASE WALL; ANNEALING THE THUS ENCASED EXTENDED SURFACE ELEMENTS; AND BY RESISTANCE WELDING SECURING THE ATTACHING SHEATHS OF THE ANNEALED ELEMENTS TO THE BASE WALL TO PROVIDE AN UNINTERRUPED DIRECT ALL METAL HEAT CONDUCTING PATH BETWEEN THE BASE WALL AND THE AREAS OF THE ELEMENTS CLOSEST THERETO AND WITH THE ENCASED ELEMENTS PROJECTING OUTWARDLY FROM THE BASE WALL, SO THAT THE DESIRED GOOD HEAT CONDUCTING AND MECHANICALLY SECURED CONNECTION BETWEEN THE ELEMENTS AND THE TUBES IS EFFECTED WITHOUT THE NEED TO ACTUAL FUSION OF THE ELEMENTS THEMSELVES TO THE BASE WALL AS A CONSEQUENCE OF THE WELDED JOINTS BETWEEN THEIR ATTACHING SHEATHS AND THE BASE WALL, REGARDLESS OF WHEATHER THE ELEMENTS ARE APPLIED RADIALLY ENDWISE TO THE HEAT EXCHANGER TUBES OR TANGENTIALLY OF SAID TUBES.

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