A METHOD FOR MANUFACTURING A HEAT EXCHANGER

Abstract: The invention relates to a heat-exchanger (10) incor­ porating circulation tubes (16) for conducting a first heat-­ transfer medium, end plates (12), and surface-enlarging plate-like fins (14) secured to the outer peripheral surfaces of the circulation tubes (16) and arranged to be contacted with a second heat-transfer medium, the fins (14) being firmly sec­ ured to the circulation tubes (16), which extend through holes (18) formed in the fins (14). The fins (14) are secured to the tubes (16) by expanding the tube (16) so as to enlarge the periphery thereof. In the region of their contact with each circulation tube (16), the fins (14) are substantially planar and oriented in a plane extending at right angles to the long­ itudinal axis of the circulation tube (16) and are constructed of a single plate-thickness, i.e. with the absence of any collar-­ like bent portion of any other bent portion of the fin plate (14) adjacent the contact surface against the circulation tube (16). The contact surfaces of the fins (14) about the inside per­ iphery of the holes (18) which engage against the circulation tubes (16) extend parallel to the longitudinal axis of the circu­ lation tubes (16) over at least a greater part of the axial extent of the holes, which affords stable ttachment and good heat-­ transfer properties. The whole of the heat-exchanger (10) can be coated with an impervious, protective surface layer (26), for example enamel.

Description: The present invention relates to a method of manufacturing a tube heat-exchanger of the kind which incorporates batteries of heat-exchange fins, i.e., a heat-exchanger which comprises circulation tubes for conducting a first heat-exchange medium and having mounted on the outer peripheral surfaces of the tubes surface-enlarging plate-like fins around which a second heating -exchange medium is intended to flow, the tubes extending through holes formed in respective fins. More specifically, the invention relates to a heat exchanger of the kind in which the heat-transfer fins are secured firmly to the circulation tubes by expanding the tubes radially into firm engagement with the fins. The invention relates primarily to a method of manufacturing tube heat-exchangers of the aforesaid kind in which the medium intended to flow in the tubes is a liquid or optionally a medium which changes phase during a heat-exchange process, and in which the medium intended to flow around the outer surfaces of the tubes is a gas. The heat-exchanger is intended for use in corrosive industrial environments. It is primarily intended for extracting heat from flue gases, e.g. heat from the flue gases of oil and coal fired power stations. Heat-exchangers intended for this purpose need to be robust and powerful. They are therefore made of steel. In corrosive environnents, it is often necessary to coat the surfaces of the heat-exchanger with an impervious corrosion inhibitor, for example, an enamel, unles the heat-exchanger is constructed from a corrosion resistant material throughout. It is generally recognized that in the case of tube heat-exchangers in which liquid flows through the tubes and gas flows around the outer surfaces thereof, the gas transfers heat much less effectively than the liquid. Consequently, it is necessary to enlarge the outer surfaces of the tubes. The two most common ways of achieving this are: a) By providing helical flanges on the outside of the heat-exchanger tubes. The flanges are normally welded to the tubes, so as to eliminate the heat resistance at the juncture between flange and tube. In addition to rotational regenerative heat-exchangers for direct heat exchange between two gases, e.g., regenerative air heaters of the Ljungström type, the most common type of heat-exchanger used industrially in conditions where an enlarged outer tube surface is required are those fitted with helically wound tubes, i.e. with helical fins along the tubes. Otherwise, tube heat-exchangers with smooth tubes are used. Since gas leakages readily occur in said rotating heat-exchangers, they have been replaced progressively with helical-tube type heat exchangers. b) By fitting batteries of flat surface-enlarging fins to the outer surfaces of the heat-exchanger tubes. The fins are often made to a standard design for several heat-exchanger tubes. These fin batteries are mostly used in apparatus intended for general ventilation (comfort) and similar purposes. Consequently, the tubes and fins of such heat-exchangers are given comparatively small dimensions and are also made of a soft material, such as copper or aluminum. One commonly applied method of achieving good heat transfer between the tubes and the fins, i.e. good contact with high contact pressure at the junction therebetween, is to secure the fins to the tubes by expanding the tubes radially into engagement therewith. This can either be effected mechanically with the aid of a mandrel or a spherical body which is drawn through respective tubes, or hydraulically by pumping liquid under high pressure through the tubes. Both methods are based on expanding the tubes radially so that the material of the tube stretches beyond the elastic limit of the tube material, so as to obtain permanent deformation and a high contact pressure. With regard to fin-batteries used with heat-transfer apparatus for general ventilation purposes and like purposes, it is relatively easy to secure the fins by expanding the tubes mechanically or hydraulically in the aforesaid manner. It will be appreciated that in the case of such apparatus, the tubes and fins have small dimensions and are made of soft materials, such as copper or aluminum. In addition, the fins are provided with resilient collars around the holes through which the heat-exchanger tubes pass. This facilitates expansion and ensures that a given contact pressure constantly prevails between the tubes and the fins. The collars also often serve as spacers between the fins. Fin batteries of this kind, however, have not been utilized in the aforesaid industrial applications, despite the advantages to be gained over heat-exchangers equipped with helically wound tubes. These advantages include: greater surface enlargement lower pressure drop more stable heat-exchanger body cheaper heat-exchanger. Thus, the more robust tube-exchanger required in industrial applications has primarily incorporated helically wound tubes, or in some cases smooth tubes. These tube heat-exchangers are mostly made of steel. There are several reasons why fin batteries of the aforesaid construction have not come into use industrially. For example, a number of difficulties and problems arise when fin batteries are to be made of steel, and particularly when they are to be provided with protective surface coatings. These problems are primarily as follows: a) It is more difficult to expand radially heat-exchange tubes which are made of steel. In order to expand the steel tubes hydraulically, it is necessary to use pressures of around 1000 bars in the case of tube thicknesses normally required in such heat-exchangers. b) It is difficult, if not impossible, to provide the steel fins with resilient collars around the holes through which the tubes pass. Among other things, the collars tend to crack. c) When providing the heat-exchanger surfaces with a protective covering, e.g. an enamel covering, it is difficult to ensure that the covering will be fully impervious, which is necessary in order to provide satisfactory protection against corrosion. In order for the enamel surface to be fully impervious, the surfaces of the heat-exchanger prepared to receive the enamel coating must be perfectly smooth and devoid of all cracks and other cavities. These surfaces should also be free of readily dislodged surface materials, such as welding slag or weld beads for example, capable of being knocked-off or otherwise removed when desooting the heat-exchanger or handling the same for some other reason, the removal of such surface materials being liable to leave cavities in the enamelled surface. It is not feasible to use resilient collars around the fin holes through which the tubes pass, since gaps and cracks around the collars would impair the enamelled surface. Such gaps and cracks cause, inter alia, bubbles to form in the enamel, which subsequently rupture and form discontinuities in the enamel as a result thereof. Even if they do not rupture, they are liable to cause imperfect surface covering and as a result, corrosion damage. Neither will this construction enable the fins to be fitted securely enough. It will be appreciated that flexing of the resilient collars creates cracks in the enamel coating. By US A 3 432 905 there is previously known a method of fabricating heat transfer tubing by joining flat plates to the external surface of thin wall tubing by introducing a fluid into a tube and freezing part of the assembly while the tube is surrounded with closely fitting tube sheets (abstract of the disclosure). In performing this method, one end of each of the tubes is capped by rubber caps (column 4, lines 2 and 3). The tubes are then filled with water, which is caused to freeze and expand. By FR-A-1 350 826 there is previously known the basic idea of securing fins to tubing by expanding the latter hydraulically as shown in Fig. 2. Fig.s 5 - 11 indicate, that the fins and their central openings may have arbitrary configuration. Enamelling smooth tubes for heat exchangers is previously known by US-3 268 989. According to this specification, column 3, lines 9 - 10, the flues which have had coating fused thereto are placed in position in the draw necks of the top head. Thus, the flues or tubes are already in an enamelled state when inserted into the head plates. Furthermore, these tubes are not to be provided with any fins. Ullmanns Encyklopädie der technischen Chemie, 1975, Volume 10, page 443, mentions various enamelling temperatures and discloses also, that it is previously known to enamel e.g. water heaters, but fails to mention any tubing with fins. Consequently, the object of this invention is to provide a method of manufacturing a heat exchanger which is not encumbered with the drawbacks of the prior art heat exchangers and which fulfills the aforementioned requirements and goals. These problems are overcome by features of the present invention as set forth in claim 1. The present invention provides a method which avoids the formation of gaps between material of the fins and the tubes, which, in the prior art, provide hidden cavities for oil, moisture or air which when surface treating the fin-tube assembly, e.g. enamelling and firing the surface coverings in kilns at temperatures of around 800°C, give rise to gas bubbles therewith impairing the protective covering. The present invention also provides firm attachment of the fins to the tubes without the need for resilient attachment elements and, in addition, enables reduction in the extent to which the tubes need be expanded radially in order to firmly fix the fins thereto. The present invention provides a firm attachment of the fins to the tubes by forming holes in the fins by machining, by cutting or grinding, and/or with the aid of a fine-punching method or with other methods which cause the fin surfaces in contact with the heat-exchanger tubes to extend parallel to the longitudinal axis of a respective tube along substantially the total axial extent of said holes throughout the thickness of the fin material. This solution ensures thermally conductive contact between the materials of the fin and the tube over the whole surface and about the entire interior periphery of the hole. If the tube accommodating holes are punched in the fins by means of simple conventional hole-punching methods, the wall of the hole obtains a slightly conical configuration. This results in a gap on the tube-wall side, preventing full thermally conductive contact therewith. These gaps are also liable to cause defects when enamelling the heat exchanger. However, by the present invention these and other deleterious effects are avoided, and a secure engagement between the fins and the tubes is obtained with a reduction in the extent to which the tubes need be expanded. The present invention provides an improved method for manufacturing a heat exchanger embodying the advantages set forth above. According to the invention, the fins are fixed securely to the heat exchanger by hydraulically expanding the tubes in a manner to enlarge the outer peripheral surfaces thereof. One particular advantage afforded by this hydraulic expansion of the tube is that the tube is slightly bulged outwards in the fin interspaces. This contributes towards achieving firm securement of the fins while at the same time providing the additional possibility of checking the extent of the expansion, by measuring the free tube-sections between the fins. An embodiment of a heat-exchanger manufactured according to the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a top plan view of the heat-exchanger, the length of which has been shortened for illustration purposes; Fig. 2 is a side view of the heat-exchanger; Fig. 3 illustrates one of the fin plates embodied in the heat-exchanger; Fig. 4 is a sectional view of part of a heat-exchanger tube provided with fins according to the invention, the heat-exchanger tube having been expanded hydraulically in a manner to firmly secure the fins thereto; Fig. 5 is a sectional view of part of a heat-exchanger tube in a fin battery made according to prior art techniques; Fig. 6 illustrates part of a heat-exchanger tube in contact with a fin plate where the hole in the fin plate is formed by means of a conventional punching method; and Fig. 7 illustrates part of a heat-exchanger tube in contact with a fin plate according to the invention, in which the hole in the fin plate has been formed by means of a fine-punching method. In Fig. 1, there is illustrated a heat-exchanger 10 comprising end plates 12, heat-transfer fins 14 and heat-exchange tubes 16. The tubes extend through holes 18 in the fins and in the end plates. The positions of the holes and the tubes in the illustrated embodiment are illustrated in Figs. 2 and 3. In the illustrated embodiment of the heat-exchanger, two of the heat-exchange tubes are provided with connecting sections 20 externally of the end plates, while the remaining tubes are provided with tubular elbows 22 which are curved through 180° and which connect the tubes together in pairs to form a serpentine passage. The tube elbows and the connecting sections may be joined to the heat exchange tubes by weld joints 23. The end plates 12 are provided with right-angle flanges 24 which extend along the longitudinal sides of the end plates. The flanges afford increased stability to the end plates and to the heat-exchanger. If considered suitable, similar flanges can also be arranged on the short sides of the end plates. The flanges are used for mounting the heat-exchanger in an industrial plant, i.e. for connecting the heat-exchanger to duct systems and/or for connecting a plurality of heat-exchange units sequentially one after the other to construct a largewr heat-exchanger battery. Fig. 4 is a cut-away detail view of a section of the tube-fin arrangement and illustrates how fins 14 are firmly secured to a heat-exchanger tube 16 by hydraulically expanding the tube. At least the marginal portions of the plate-like material forming the fins 14 which surround the holes 18 are oriented in a plane perpendicular to the longitudinal axis of the tube section passing through the hole, so as to provide an interior surface confronting the tube which has an extent or depth corresponding to the thickness of the fin material. Fig. 4 shows that the tube and the fins are coated with a protective enamel layer 26. It will also be seen from the figure that the wall of the tube in the space between mutually adjacent fins is slightly bulged, as shown at 28, these bulges being formed when expanding the tube hydraulically. The radial extent of the bulges depends on the individually prevailing circumstances, such as the material used and the dimension thereof. In the case of a tube having a diameter of 18mm, the expansion is roughly 0.8mm. The bulges thus formed assist in firmly securing the fins while affording, at the same time, an additional possibility of checking the expansion achieved, by measuring the diameter of the tube between the fins. For reasons of comparision, Fig. 5 illustrates a similar detail view of a conventional prior art finned heat-exchange tube used in conjunction with general ventilation apparatus (comfort ventilation). In this conventional construction, the fins 30 are provided with resilient collars 32 around the holes through which the heat-exchanger tubes 34 pass. Since the fins of this construction are thin and made of a soft material, e.g. aluminum, it has been possible to form the collars in a simple fashion from the fin material itself.In the illustrated case, the collars also serve as spacers between respective fins. The major purpose of the collars, however, is to ensure that a sufficient contact surface is obtained to provide satisfactory contact pressure between the heat-exchanger tube and the fins, so as to obtain satisfactory heat-transfer conditions. The fins have been secured in position by expanding the heat-exchanger tube. The expansion required to provide sufficient contact pressure is facilitated by the fact that the heat-exchanger tube has a small wall thickness and is made of a soft material, e.g. copper, and also by the fact that the collars provide a certain degree of resilience in the connection between the fins and the heat-exchanger tube. When a comparison is made between a construction according to the invention as illustrated in Fig. 4 and the prior art construction as illustrated in Fig. 5, it will be seen that the known construction cannot suitably be used in heat-exchangers which are to be provided with a protective surface coating, such as an enamel coating. Gaps, cracks and cavities around the collars 32 of the Fig. 5 construction would constitute obstacles to obtaining a fully satisfactory enamel surface. Similarly, the resiliency in the joint between the fins and respective tubes would result in the formation of cracks in the enamel. No such cracks, etc. are to be found between fin collars and heat-exchanger tubes in the heat-exchanger construction according to the invention illustrated in Fig. 4. The surfaces of the fins and tubes of the heat-exchanger illustrated in Fig. 4 are substantially completely smooth, which when surface coating the surfaces with a corrosion-resistant protective coating, for example enamel, can result in an extremely durable and completely impervious surface layer. In addition, the fins are so firmly secured that no resilience capable of damaging the enamel layer is to be found in the location where the fins join respective tubes. Another advantageous result of the rigidity of this attachment is that the extent to which the heat-exchanger tubes need to be expanded radially in order to firmly secure the fins is much smaller than that to which the tubes of known heat-exchangers need to be expanded, either hydraulically or in sone other way, in order to firmly secure the fin batteries to respective tubes. Fig. 6 illustrates how a hole punched in a fin in accordance with a conventional punching technique will produce a slightly conical wall surface 36. This conical hole-wall surface defines a gap 38 with the heat-exchanger tube 16 which can deteriorate the surface coating, e.g. an enamel coating in a manner readily understood. With the aid of a more accurate fine-punching method, or some other accurate method, it is possible to provide holes having hole-walls 40 according to Fig. 7 which are parallel to the longitudinal axis of the tube, and therewith parallel to the original cylindrical surface of the tube along practically the whole depth of hole. A slight deviation 42 at the immediate location where the punch passes through the fin can be accepted, however. No gap, which may adversely affect the surface coating, namely the enamel covering, is formed between the tube wall and the hole walls of the fins when forming the holes more accurately in accordance with Fig. 7. A highly durable and tough enamel surface can thus be obtained. The important heat transfer between the fins and the tubes is ensured since the contact surface therebetween, which has a uniformly distributed high contact pressure, is even greater subsequent to the hydraulic expansion of the tubes. In addition, the extent to which the heat-exchanger tubes need to be expanded in order to firmly secure the fins has been further reduced. Examples of other accurate metlods for the making of holes 14 with cylindrical walls are various machining methods, such as drilling, cutting or grinding. However, these methods are more time consuming and especially for long manufacturing runs more expensive. Therefore, the fine-punching method identified above is preferred. The heat exchanger's protective coating made of enamel is most durable and resistant. The application of an enamel coating on a heat exchanger comprises the following operative steps: cleaning application of enamel material (submersion in enamel material or float coating with fluid enamel material) drying firing cooling In order to avoid bubble and crack formation in the enamel on the heat exchanger, special care must be taken of the drying and cooling steps in order to obtain an impermeable coating. The drying is normally done from the outside at increased surrounding temperature or in a radiant heat oven. The surface layer will then dry out first and form a "skin", which impedes or inhibits the removal of the last remains of moisture at the root or base of the fins. This moisture may be surface-bonded to the surface of the enamel material particles or may be retained by capillary action between the fins and the tubes. Such retention further delays the moisture removal. The result is that bubbles are formed during the firing operation in the enamel layer. This is caused by the violent volume increase of the water when it is transformed to high temperature steam. (The firing temperature is above 800°C). According to a further feature of the invention, the drying of the float coating of enamel material is performed from the inside out using the circulation tubes of the heat exchanger. A heated medium, for instance a hot gas, is passed in (arrow A in Fig. 1) through one of the circulation sections 20 or tube openings, passes through the circulation tubes emitting its heat to the tubes 16 and fins 14 and passes out (arrow B) at the other connection section 20 or tube opening. In this way, a reverse temperature gradient is obtained and the moisture is removed starting from the surface to which the coating is applied. All moisture is driven out, also from the unavoidable capillary passages between the fins 14 and the tubes 16. The hot gas may suitably be supplied through a collector pipe or manifold to several circulation tube loops simultaneously. The cooling of the heat exchanger must be slow, otherwise cracks will occur at the roots of the fins where they are connected to the tubes. According to the invention, the heat exchanger is cooled slowly (from a firing temperature of 800-840°C to 500°C in 15 minutes). This corresonds to a cooling rate of about 20° a minute. It is important that the tube elbows 18 are welded to the tubes 16 after the hydraulic expansion operation. Otherwise, there may be created built-in stress in the tubing which is released during the firing and causes crack formation in the dried enamel material during the heating-up period. The described embodiment illustrates one single tubular loop through the heat exchanger, with the inlet and outlet of mutually the same size. It will be understood, however, that the tubular loop can be divided into a plurality of loops, by connecting more connectors 20 in parallel instead of tube in series by the elbows 22. Such connectors may, of course, also be mounted on both end walls. A heat exchanger of the aforedescribed kind can be given extremely large dimensions. The tube length may be up to about 10 m, and the tubes can have a diameter up to about 75 mm. The tubes may have a wall thickenss of at least up to approximately 5 mm. The thickenss of the flanges of fins can also be up to 5mm. The end walls are preferably thicker than the fins. For example, the end wall thickness may be 5mm and a corresponding fin thickness of about 1 mm. The heat exchanger is manufactured from steel, in order to fulfill requirements of temperature resistance, wear resistance and to obtain suitable properties for enamelling processes or other surface processes. Preferably, the slow cooling of the heat exchanger is done by passing a first cooling medium through the interiors of the circulation tubes, and gradually reducing the temperature of the medium. Preferably the exteriors of the tubes and the fins are also exposed to a cooling medium, which may be ambient air or a cooling spray, simultaneously with the interior cooling. For most applications, the wall thickness for steel tubes should be 0.5 to 5.0 mm, preferably about 2 mm, while the thickness of steel fins mounted thereon should be 0.4 to 5.0 mm, preferably about 1.25 mm.

Claims: 1. A method of manufacturing a heat exchanger (10) for use in corrosive industrial environments, comprising circulation tubes (16) of steel conducting a first heat-transfer medium, namely a liquid or optionally a medium which changes phase during a heat-exchange process, attaching surface-enlarging throughout flat platelike fins (14) of steel to the outer surface of said circulation tubes for contact with a second heat-transfer medium, namely a gas, by extending said tubes through holes (18) provided in said fins and securing the latter firmly to the periphery of said tubes by expanding the tubes into engagement with said periphery while having the fins oriented in a plane at right angles to the longitudinal axis of the circulation tubes, forming the holes provided in the fins for accommodating and securing the circulation tubes by fine-punching or some other similarly accurate machining method, such as drilling, cutting or grinding, for achieving contact surfaces of the holes against the circulation tubes in accurate parallel relation to the longitudinal axis of the latter along substantially the entire axial extension of the holes, thereby enabling reducing the extent to which the tubes need to be expanded radially in order to firmly fix the fins thereto, securing the tubes to said fins by controlled hydraulic expansion of the tubes and expanding the tubes radially so that the material of the tubes stretches beyond the elastic limit of the tube material, so as to obtain permanent deformation and a high contact pressure and at the same time providing the additional possibility of checking the extent of expansion by measuring the diameter of the tube between the fins, reduced in its extent by the said coaxial hole surface, by pumping liquid under high pressure to the tubes, and covering the assembly of circulation tubes with secured fins and optionally end plates with an impervious protective layer (26) of enamel material by the operative steps of cleaning the heat exchanger assembly, applying a float coating of liquid enamel material thereon, drying the enamel material by firing by subjecting the surface covering to firing in kilns at temperatures of around 800 °C, preferably above 800 °C, and cooling the assembly slowly and simultaneously by subjecting the so enamelled surface to slow cooling at a cooling rate of about 20 °C a minute. 2. A method according to Claim 1, characterized in that any weld joints (23) for the connection of tube bends (22) and/or connecting sections (20) to circulation sections (20) to the circulation tubes are effected outside of end plates (12) and after that the fins (14) and end plates (12) have been secured to the circulation tubes. 3. A method according to Claim 1, characterized in that the drying operation is performed by passing a heating medium, suitably a hot gas, in through one connecting section (20) or tube opening (arrow A) through the circulation tubes (14) and out through another connecting section (20) or tube opening (arrow B), and that the cooling is performed slowly and simultanously in the entire heat exchanger (10).

Inventors: JAKOBSSON, PER-OLOF, JANSSON, GOESTA, WADELL, BERNDT, Jansson, Gösta
Applicants: ABB FLAEKT AB
Application year: 1987
Main classification: F28F1
IPC codes: F28F19, F28D7, B21C37, B21D53, F28F1