Metal Heat Exchanger Tubes

Abstract

Develop a highly efficient heat exchanger tube that evaporates liquid on the outside of the tube. [Solution] The present invention relates to a metal heat exchanger tube (1) having integral fins (2) molded on the outside of the tube, the fin bases (3), fin sides (4), and fin tips (5), the fin bases (3) projecting radially from the tube wall (10), and the fins (2) defining flow passages (6) having flow passage bottoms (61) within which spaced-apart additional structures (7, 71, 72) are disposed, dividing the flow passages (6) between the fins (2) into segments (8). The additional structures (7, 71, 72) locally reduce the cross-sectional area available for flow through the flow passages (6) between two fins (2), thereby restricting fluid flow within the flow passages (6) at least during operation. The first additional structures (7, 71) are protrusions (71) extending radially outward from the channel bottom (61), each defined by an end surface (713) located between the channel bottom (61) and the fin tip (5), thereby limiting the radial extent of the protrusions (71). A material protrusion (72) is arranged as a second additional structure (7, 72) facing radially outward at the location of the protrusions (71), and is formed from the material of the fin side portion (4). The material protrusions (72) are arranged radially between the end surface (713) and the fin tip (5), and are formed so that the material protrusions (72) are located above the channel bottom (61) of the channel (6) and are located to the side of the fin side portion (4) around the radial extent of the protrusions (71). The material projections (72) extend axially and radially wider than they do circumferentially.

Description

[Technical field]

[0001] The present invention relates to a metallic heat exchanger tube according to the preamble of claim 1. [Background technology]

[0002] Evaporation occurs in many areas of cooling and air conditioning technology as well as in process and energy technology. Often, tube bundle heat exchangers are used, in which the liquid of a pure substance or mixture evaporates on the outside of the tubes, while salt water or water is cooled on the inside of the tubes.

[0003] By enhancing the heat transfer on the outside and inside of the tubes, the size of the evaporator can be significantly reduced. This reduces the manufacturing costs of such equipment. Moreover, it reduces the required refrigerant volume, which may have become a significant proportion of the total equipment costs with the chlorine-free safe refrigerants that have been mainly used up until now. Moreover, the high-efficiency tubes that are common today are already about four times more efficient than smooth tubes of the same diameter.

[0004] The most efficient commercially available fin tubes for flooded evaporators have a fin structure with a fin density of 55 to 60 fins per inch on the outside of the tube (see U.S. Pat. No. 5,399,411, U.S. Pat. No. 5,499,421, U.S. Pat. No. 5,523,633). This corresponds to a fin pitch of approximately 0.45 to 0.40 mm. It is further known that an evaporator structure with the same fin pitch can be created on the outside of the tube with increased efficiency by inserting additional structural members between the fins in the groove bottom area.

[0005] In US Pat. No. 5,399,436 it is proposed to create undercut secondary grooves at the bottom of the grooves between the fins which run continuously along the primary grooves, the cross section of which may remain constant or vary at regular intervals.

[0006] Further examples of groove bottom structures can be found in US Pat. No. 5,399,623, US Pat. No. 5,499,636 or US Pat. No. 5,499,644. These structures have in common that the structural elements of the groove bottom do not have an undercut shape. Here, recesses in the groove bottom or protrusions in the lower region of the channel are concerned. Relatively high protrusions are explicitly excluded in the prior art, since they would block the fluid flow in the channel to the detriment of the heat exchange.

[0007] Another solution is disclosed in the patent application WO 2005 / 023363, which has a relatively high structure starting from the bottom of the groove. This structure is a protrusion in the flow channel, which causes a division into segments. By dividing the flow channel into segments between two fins, the flow channel is blocked many times in the circumferential direction, so that the generated gas bubbles and the movement of the heat exchange fluid in the flow channel are at least reduced or completely prevented. The exchange of liquid and vapor along the flow channel is further reduced or is no longer promoted by each additional structure. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Pat. No. 5,669,441 [Patent Document 2] U.S. Patent No. 5,697,430 [Patent Document 3] German Patent No. 19757526 [Patent Document 4] European Patent No. 1223400 [Patent Document 5] European Patent No. 0222100 [Patent Document 6] U.S. Patent No. 7,254,964 [Patent Document 7] U.S. Patent No. 5,186,252 [Patent Document 8] European Patent No. 3111153 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention is based on the problem of developing a highly efficient heat exchanger tube that evaporates liquid on the outside of the tube. [Means for solving the problem]

[0010] The invention is set forth by the features of claim 1. The other dependent claims relate to preferred configurations and developments of the invention.

[0011] The invention encompasses a metal heat exchanger tube with integral fins formed on the outside of the tube, the fin base protruding radially from the tube wall, the fins forming a passage with a passage bottom, in which spaced apart additional structures are arranged, the additional structures dividing the passage between the fins into segments, the additional structures locally reducing the cross-sectional area available for flow in the passage between two fins, at least thereby restricting the fluid flow in the passage during operation. A first additional structure is a radially outward protrusion starting from the passage bottom, which is defined by a terminal surface between the passage bottom and the fin tip, respectively, in the radial direction, thereby limiting the radial extent of the protrusion. A material projection is arranged as a second additional structure facing radially outward at the protrusion, which is made of the material of the fin side. The material projections are disposed radially between the end faces and the fin tips such that the material projections are formed to be laterally adjacent the fin side portions around the radial extent of the protrusions above the flow channel bottom of the flow channel. The material projections extend axially and radially wider than they extend circumferentially.

[0012] The metallic heat exchanger tubes are used in particular for evaporating liquids consisting of pure substances or mixtures on the outside of the tubes.

[0013] Such efficient tubes can be produced using rolled plates on the basis of integrally rolled fin tubes. An integrally rolled fin tube is understood to mean a finned tube in which the fins are formed from the wall material of a smooth tube. A typical integral fin formed on the outside of the tube has a fin base, a fin side and a fin tip, for example in a helical manner, the fin base protruding approximately radially from the tube wall. The number of fins is determined by counting the axially consecutive ridges of the tube. The structure according to the invention is produced by a rolled plate with sharp teeth which processes both the wall material of the channel bottom as well as the material of the fin side in the axial and radial directions.

[0014] Various methods are known here for closing the channels between adjacent fins in such a way that the connection of the channels with the surroundings remains in the form of holes or slits, in particular such nearly closed channels are produced by turning or folding the fins back, by splitting and upsetting the fins or by notching and upsetting the fins.

[0015] The invention starts from the consideration that the spaces between the fins are divided into segments by additional structures to improve the heat transfer during evaporation. This leads to localized overheating in these spaces, enhancing the process of nucleate boiling. The bubble formation then occurs primarily inside the segments, starting at nucleation sites. At these nucleation sites, small bubbles of gas or steam are first formed. When the expanding bubbles reach a certain size, they detach from the surface. As the bubbles detach, the remaining voids in the segments fill up again with liquid, and the cycle begins anew. The small bubbles left behind during the detachment of the bubbles may then form a surface that can be used as nucleation sites for a new bubble formation cycle.

[0016] In order to further form bubbles inside the segments, according to the solution according to the invention, in the region of the first additional structure in the form of a protrusion facing radially outward, there is a further material protrusion as a second additional structure, which is arranged laterally of the fin flank and extends essentially in the axial and radial directions. From the manufacturing process by rolling, a material protrusion is formed from the material of the fin flank, which is preferably placed directly on the protrusion so as to face radially outward. In the structure formed by the material protrusion, a fluid flow of the liquid heat exchanger fluid is promoted, so to speak, from the side to the adjacent segment. Such a fluid induction thus contributes to the formation of bubbles in the segments. The protrusions can extend axially between the respective fin bases of the adjacent fins over the entire channel bottom or only over a part of the channel bottom. These are, so to speak, barriers that extend between two fins, starting from the channel bottom, and extend radially outward and at least partially close the channel in the circumferential direction.

[0017] In other words, the material projections according to the invention, which rest on the preferably solid protrusions of the channel bottom structure, are shaped as second additional structures made of the material of the fin side section and form a flowable transition to both sides of the protrusions below them approximately in the radial direction. They are therefore flow-guiding structures which, so to speak, guide the liquid fluid laterally into the segments. The radially outwardly arranged protrusion end faces can extend over the entire channel width. At the location of the material projections arranged above the protrusions, the liquid fluid is exchanged between the adjacent segments and can reach from one segment to the next. The protrusions on which the material projections rest are therefore thresholds for the passage of the fluid.

[0018] In this case, the material projections may have an axial extension smaller than the extension of the protrusions located below them. Based on the size, shape and arrangement of the material projections, the wettability of the heat exchanger fluid is the primary factor for fluid flow enhancement. In this case, the contours of the material projections extending in the approximate axial and radial directions may be configured to be arcuate or irregular.

[0019] In the present invention, by dividing the flow passage between the two fins into segments in this way, this passage is blocked many times in the circumferential direction, so that the movement of the generated gas bubbles in the passage is at least reduced or even completely prevented, and the exchange of liquid and vapor along the passage is further reduced or no longer promoted by each additional structure.

[0020] A particular advantage of the present invention is that the liquid-vapor exchange is well controlled locally and the filling of the bubble nucleation sites in the segments is localized, in particular laterally, by material protrusions. Overall, by a suitable selection of the segmentation of the flow passages, the evaporator tube structure can be effectively optimized depending on the operating parameters, whereby an improved heat transfer is achieved. Furthermore, structural elements that enhance bubble formation at the groove bottom are particularly effective, since in the groove bottom area the temperature at the fin base is higher than at the fin tip.

[0021] Furthermore, it is advantageous that the additional structure locally reduces the cross-sectional area through which the flow can pass in the flow passage between the two fins. Overall, the segmentation of the flow passages increases the separation of the respective flow passage sections, which allows the evaporator tube structure to be further optimized depending on the application parameters in order to further improve the heat transfer.

[0022] In a preferred embodiment of the invention, the protrusions and the material projections may locally reduce the cross-sectional area available for flow in the flow channel between two fins by at least 30%. In this way, the segment is locally sufficiently restricted for the passage of fluid. The section of the flow channel between two segments is therefore sufficiently or as far as possible isolated on the fluid side from the adjacent section of the flow channel.

[0023] Advantageously, the protrusions and the material projections may locally reduce the cross-sectional area available for flow in the flow channel between two fins by 40 to 70%, the channel section between two segments forming a significant threshold on the fluid side with respect to the adjacent channel section.

[0024] In an advantageous embodiment of the invention, the flow passages may be closed radially outwardly up to the respective local openings. The fins may then have an approximately T- or Γ-shaped cross section, whereby the flow passages between the fins are closed up to the holes as local openings. Through these openings, the steam bubbles produced during the evaporation process can escape. The deformation of the fin tips is carried out in a manner available from the prior art.

[0025] In this regard, the fin tips may be axially folded and even shaped to some extent towards the bottom of the channel. The channel may thus be tapered or completely closed to the desired extent from below and laterally and / or from above by a combination of complementary structural members, in any case such that the channel between the fins is divided into discontinuous segments.

[0026] The combination of the segments according to the invention with the channels closed up to holes or slits results in a structure with a very high efficiency in the evaporation of the liquid over a very wide range of operating conditions, in particular the heat transfer coefficient of the structure remains constant and reaches a high level even when the heat flux or the driving temperature difference changes.

[0027] In a preferred embodiment of the invention, there may be at least one local opening per segment. This minimum condition further ensures that any gas bubbles that may form in a channel segment during the evaporation process can escape to the outside. The local openings are designed in terms of size and shape to allow the liquid medium to pass through and flow into the channel section. Therefore, to be able to support the evaporation process at the local openings, equal amounts of liquid and vapor must be transported through said openings in opposite directions. Usually, a liquid is used that wets the tube material well. Due to the capillary effect, such a liquid can enter the channel through each opening in the outer tube surface, even against an overpressure.

[0028] Furthermore, the ratio of the number of local openings to the number of segments may be between 1:1 and 6:1. More advantageously, this ratio may be between 1:1 and 3:1. The channels between the fins are substantially closed by the material of the upper fin area, and the resulting voids of the channel segments are connected to the surrounding space by openings. These openings may also be formed as holes of the same size or of two or more sizes. In the case of a number of local openings on one segment, holes with two sizes may be particularly suitable. Each small opening is followed, for example, by a large opening along the channel according to a regularly repeated pattern. This structure results in a directional flow in the channel. The liquid is advantageously taken up through the small holes with the help of capillary pressure and wets the channel walls, whereby a thin film is created. The vapor collects in the center of the channel and escapes there due to the smallest capillary pressure. At the same time, the dimensions of the large holes must be determined in such a way that the vapor can escape fast enough and the channel does not dry out. The size and number of the vapor holes relative to the relatively small liquid holes are then adjusted to one another.

[0029] In an advantageous embodiment of the invention, the protrusion as the first additional structure may be molded at least from the material of the channel bottom between two integrally circumferential fins, so that a material-bonded connection for good heat exchange from the tube wall to the respective structural element remains. Moreover, the protrusion may also consist of the material of the fin side. Dividing the channel into segments from a single material of the channel bottom is particularly advantageous for the evaporation process.

[0030] In a particularly advantageous embodiment, the protrusion as the first additional structure may have a height of 0.15 to 1 mm, the dimensioning of this additional structure being particularly well adapted to high-efficiency fin tubes, which represent structural sizes of the external structure advantageously in the sub-millimeter to millimeter range.

[0031] Preferably, the protrusions may have an asymmetric shape. The asymmetry of the structure is manifested here in a cut surface extending perpendicular to the longitudinal axis of the tube. The asymmetric shape may further contribute to the evaporation process, especially if a relatively large surface is formed. The asymmetry may be formed both in the additional structure at the bottom of the channel and also in the fin tips.

[0032] In an advantageous embodiment of the invention, the protrusions may have a trapezoidal cross section in a cut plane perpendicular to the tube longitudinal axis. The trapezoidal cross section is a structural element that is technically well controllable for integrally rolled fin tube structures. Slight manufacturing-induced asymmetries of the normally parallel trapezoidal bases may appear here.

[0033] Advantageously, at the location of said protrusions, opposing material projections may be formed in the direction of the longitudinal axis of the tube. A protrusion with opposing material projections is therefore a threshold for the passage of a fluid. [Brief description of the drawings]

[0034] [Figure 1] 1 shows a schematic partial cross-sectional view of a heat exchanger tube having segments separated by additional structures. [Diagram 2] 1A and 1B are schematic perspective views of a portion of the external structure of a heat exchanger tube having folded fin tips; [Diagram 3] 1 shows a schematic detailed view of the material projection at the location of the protrusion. [Figure 4] 13 shows a schematic detailed view of another embodiment of a material projection at the location of the protrusion. [Diagram 5] 1 shows a schematic perspective view of a portion of the external structure of a heat exchanger tube having material protrusions facing each other at the locations of the protrusions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Exemplary embodiments of the invention will now be described in more detail with the aid of schematic drawings, in which corresponding parts are provided with the same reference numerals in all figures.

[0036] 1 shows a schematic partial cross-sectional view of a heat exchanger tube 1 according to the invention with segments 8 separated by additional structures 7. The integrally rolled heat exchanger tube 1 has fins 2 which run helically around the outside of the tube with primary grooves as channels 6 between them. The fins 2 run continuously along a helix on the outside of the tube. The fin bases 3 protrude approximately radially from the tube wall 10. The fin height H is measured in the finished heat exchanger tube 1 from the deepest point of the channel bottom 61, over the fin side 4 to the fin tip 5 of the fully formed fin tube.

[0037] A heat exchanger tube 1 is proposed in which additional structures 7 in the form of radially outwardly directed protrusions 71 are arranged in the region of the channel bottom 61, which protrusions 71 are each defined in the radial direction by an end face 713 between the channel bottom 61 and the fin tip 5. These protrusions 71 are called first additional structures and are formed from the material of the tube wall 10 from the channel bottom 61. The protrusions 71 are advantageously arranged at regular intervals on the channel bottom 61 and extend transversely to the extension of the channel from the fin base 3 of the fin 2 at least partially or completely towards the next fin base beyond, which is not shown in the drawing. At the location of the protrusions 71, material projections 72 are arranged as second additional structures 7, which are radially outwardly directed and are formed from the material of the fin side 4. The material projections 72 are arranged radially between the end faces 713 and the fin tips 5, respectively, so that they are formed laterally around the radial extent of the projections 71 above the channel bottoms 61 of the channels 6 and on the sides of the fin flanks 4. The material projections 72 extend axially and radially wider than in the circumferential direction. The primary grooves as channels 6 are thus at least partially tapered at regular intervals. The resulting segments 8, together with the material projections 72 as guiding structures for the fluid flow, are particularly favorable for bubble nucleation. The direct exchange of liquid and vapor between the respective segments 8 is at least reduced.

[0038] In order to form the protrusions 71 with the radially outward material projections 72 at the channel bottom 61, the fin tip 5 as the end region of the fin 2 is advantageously further deformed in such a way that the fin 2 radially partially closes the channel 6 at the axially folded fin tip 51. In order to allow the steam bubbles to escape from the channel 6, the connection of the channel 6 with the surroundings is made in the form of holes 9 as local openings. The deformation of the fin tip 5 is carried out in the manner of a rolling technique available from the prior art. The primary groove 6 is thus an undercut groove. By combining the protrusions 71 and the material projections 72 as additional structures 7, segments 8 in the form of voids are obtained, which furthermore are distinguished by a very high efficiency in the evaporation of the liquid over a very wide range of operating conditions. The liquid evaporates inside the segments 8 supported by the material projections 72 as additional fluid-guiding structures. The resulting steam leaves the channel 6 at the local openings 9 through which the liquid fluid also flows. For the fluid to flow, a well-wettable tube surface may also be of help.

[0039] The solution according to the invention concerns a structured tube with an increased heat transfer coefficient on the outside of the tube. In order not to transfer most of the thermal resistance to the inside, the heat transfer coefficient on the inside can be further enhanced by a suitable internal structure 11. Heat exchanger tubes 1 for tube bundle heat exchangers usually have at least one structured area as well as smooth end pieces and possibly smooth intermediate pieces. These smooth end pieces or intermediate pieces define the structured area. In order to be able to mount the heat exchanger tube 1 in the tube bundle heat exchanger without any problems, the outer diameter of the structured area must not be larger than the outer diameter of the smooth end pieces and intermediate pieces.

[0040] FIG. 2 shows a schematic perspective view of a part of the external structure of a heat exchanger tube 1 with a folded fin tip 51. For a better illustration, only the structural elements of the external structure that are most important for understanding are shown. In order to form a protrusion 71 with a radially outward material projection 72 at the channel bottom 61, the fin tip 5 as the end region of the fin 2 is again deformed so that the fin 2 radially partially closes the channel 6 at the axially folded fin tip 51. The connection of the channel 6 with the surroundings is made as a local opening 9 for the escape of steam bubbles from the channel 6 and for the inflow of liquid fluid into the channel 6. The primary groove 6 is thus again an undercut groove. The axially folded fin tip 51 is formed from the fin 2 and extends axially beyond the channel 6. The additional structure 7 reduces the cross-sectional area available for flow in the channel 6 between the two fins 2 locally in a particularly effective manner, so that the fluid flow in the channel 6 is restricted during operation.

[0041] FIG. 3 shows a schematic detailed view of the material projection 72 at the location of the projection 71. The material projection 72, which rests radially on the preferably solid projection 71 of the channel bottom structure, is produced from the material of the fin side 4 by means of a toothed rolling plate which processes both the wall material at the channel bottom 61 and the material at the fin side 4. Although the projection 71 and the material projection 72 are formed from different regions of the tube wall, the material projection 72 may mainly form a radially flowing transition to the two side surfaces 711 of the projection 71 below it. In this case, the projection 71 extends only in a portion of the channel bottom 61 and terminates in the tube axial direction with a front surface 712. The material projection 72 is formed in the shape of a partition wall and extends approximately radially and in the direction of the tube longitudinal axis A, for example almost to the center of the channel in this axial direction. Here, the end surface 713 of the projection 71 may extend further in the direction of the tube longitudinal axis A or over the entire channel width between the opposite fins. From within this range the fluid flow can be more precisely controlled and contribute to bubble formation within two circumferentially adjacent segments 8. The protrusion 71 on which the material projection 72 rests is therefore also a threshold for the passage of fluid.

[0042] 3, the axial extent of the material projection 72 is slightly smaller than the axial extent of the underlying projection 71. This creates an opening for the heat exchanger liquid fluid relative to the tube longitudinal axis A, which can be relatively easily guided laterally into the adjacent segment 8, promoting bubble formation.

[0043] FIG. 4 shows a schematic detailed view of another embodiment of the material projection 72 at the location of the projection 71. The material projection 72, which rests radially on the projection 71 of the channel bottom structure, is produced from the material of the fin side 4 by a toothed rolling plate which processes both the wall material at the channel bottom 61 and the material at the fin side 4. The contour of the material projection, which extends approximately in the axial and radial directions, can then be configured as an arc or irregular shape. In this embodiment, the material projection 72 has an axially varying extent. In other words, when viewed radially outward, a flowing transition to the fin side 4 is realized. Overall, the surface of the material projection 72 is slightly curved inwards. This shape is a certain modification of a normally flat surface and is particularly advantageous with regard to the surface properties and wettability of the heat exchanger liquid fluid. With such a structure, the heat exchanger liquid fluid is particularly advantageously guided laterally into the adjacent segments 8, which promotes bubble formation.

[0044] FIG. 5 shows a schematic perspective view of a part of the external structure of a heat exchanger tube 1 with material projections 72 facing each other at the projections 71. For a clearer explanation, only the structural parts of the external structure that are most important for understanding are shown. In order to form the projections 71 with the radially outward material projections 72 at the channel bottom 61, the fin tip 5 as the end region of the fin 2 is also modified in this case so that the fin 2 radially partially closes the channel 6 at the axially folded fin tip 51. The connection of the channel 6 with the surroundings is made as a local opening 9 for the escape of steam bubbles from the channel 6 and for the inflow of liquid fluid into the channel 6. The projections 71 and the material projections 72 as additional structures 7 reduce the cross-sectional area available for flow in the channel 6 between the two fins 2 locally and particularly effectively, so that the fluid flow in the channel 6 is restricted during operation.

[0045] In this case, the protrusions 71 extend in the direction of the tube longitudinal axis A over the entire flow path width between adjacent fins 2. At the location of the protrusions 71, opposing material projections 72 are formed facing radially outward. The projections 71 on which the material projections 72 rest are therefore thresholds for the passage of the fluid. [Explanation of symbols]

[0046] 1 Heat Exchanger Tube 2 Fins 3 Fin base 4 Fin side 5 Fin tip, fin end region 51 Axial bent fin tip 6 Flow path, primary groove 61 Bottom of channel 7 Additional structures 71 Protrusion as the first additional structure at the bottom of the channel 711 Side of protrusion 712 Front of protrusion 713 End surface of protrusion 72 Material protrusion 8 Segments 9 Local opening, hole 10 Tube wall 11 Internal structure A Tube vertical axis H Fin Height

Claims

1. A metal heat exchanger tube having integral spirally circumferential fins formed on the outside of the tube, the fin bases projecting radially from a tube wall, channels being defined between the fins and having channel bottoms, and spaced apart additional structures disposed within the channels; the additional structure divides the flow passage between the fins into segments and locally reduces a cross-sectional area through which the flow passage can pass between two of the fins, thereby at least restricting fluid flow in the flow passage during operation; the additional structure includes a first additional structure and a second additional structure; the first additional structure being a solid protrusion extending radially outwardly from the flow channel bottom, the protrusion being defined by terminal faces radially between the flow channel bottom and the fin tip, the definition limiting the radial extent of the protrusion; a material projection is disposed as the second additional structure so as to face radially outward at the location of the protrusion; the material projections are formed from the material of the fin side portions and are radially disposed between the end faces and the fin tips, respectively, such that the material projections are formed laterally of the fin side portions around the radial extent of the protrusions above the flow channel bottom of the flow channel; A metallic heat exchanger tube, characterized in that the material protrusions extend wider in the axial and radial directions than in the circumferential direction, and the protrusions and the material protrusions locally reduce the cross-sectional area available for flow in the flow passage between two of the fins by 40% to 70%.

2. 2. The metallic heat exchanger tube of claim 1, wherein the flow passages are closed radially outwardly to their respective local openings, and the fins have a T-shaped or Γ-shaped cross section.

3. A metal heat exchanger tube as described in any one of claims 1 to 2, characterized in that there is at least one local opening for each segment.

4. 4. The metallic heat exchanger tube according to claim 1, wherein the protrusion is molded at least from the material of the bottom of the channel between two integrally circumferential fins.

5. 5. The metallic heat exchanger tube of claim 4, wherein the protrusions have a height of 0.15 to 1 mm.

6. 6. A metallic heat exchanger tube according to claim 1, wherein the protrusions have a trapezoidal cross section in a cut plane extending perpendicular to the longitudinal axis of the tube.

7. 7. A metallic heat exchanger tube according to claim 1, characterized in that at the location of the protrusions, the material projections are formed facing each other in the direction of the tube longitudinal axis.

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