Abstract:
A plurality of beads protruding from an inner face of the tube are provided in such a manner that the beads are arranged at a predetermined pitch in an axial direction of the tube; and a circumference of the tube is divided at least into thirds, and the beads are aligned in a circumferential direction of the tube; and the beads aligned in the circumferential direction of the tube are provided at plural rows at the predetermined pitch in the axial direction of the tube, and the beads adjoining in the axial direction are shifted by substantially a half of a circumferential length of the bead to one another. Alternatively, the circumference of the tube is divided into parts of an even number of four or more, and the beads are aligned in the circumferential direction so as to be alternately formed in the parts of the circumference.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a tube structure of a multitubular heat exchanger, the heat exchange performance of which is enhanced and the flow resistance in the tube of which is reduced.  
         [0003]     2. Description of the Related Art  
         [0004]     Conventionally, in order to enhance the performance of a multitubular heat exchanger such as an EGR gas cooler or an exhaust heat recovery device for a co-generator in which fluid of low Prandtl Number such as water, air or exhaust gas is used as a medium, for example, in order to enhance the performance of a heat exchanger: in which a large number of tubes for cooling EGR gas are arranged in parallel (This heat exchanger will be referred to as an EGR cooler hereinafter.), as shown in FIGS.  16  to  18 , protrusions protruding to the center of a tube are provided on the inner face of the tube at regular intervals in the axial direction. These protrusions will be referred to as beads in this specification, hereinafter.  
         [0005]     Concerning the form of protruding the beads  2  from the inner surface of the tube  1 , according to the method of press forming the beads  2 , the following two cases are provided. One is a case in which the beads  2  are two-dimensionally protruded from the inner face of the tube on the circumference as shown in  FIG. 16 . The other is a case in which the beads  3  are spirally protruded from the inner face of the tube as shown in  FIG. 17  or Unexamined Japanese Patent Publication No. 2000-345925. There is a small difference between the performance of these two cases.  
         [0006]     The beads  2 ,  3  protruding from the inner face of the tube are bodies for facilitating the generation of a turbulent flow in the fluid flowing in the tube. Therefore, the heat transfer effect of the beads  2 ,  3  is high. However, when a flow rate of the exhaust gas is increased, the pressure loss in the tube is also increased.  
         [0007]     Further, there is provided a tube structure in which the spiral fin  4  is arranged in the tube  1  having the beads  2  so that the heat radiating performance can be enhanced as shown in  FIG. 18 . This spiral fin  4  contributes to the enhancement of the heat radiating performance. However, an increase in the pressure loss in the tube is caused when this spiral fin  4  is arranged in the tube.  
         [0008]     Therefore, it is desired to develop a tube structure capable of satisfying both the enhancement of the heat radiating performance and the reduction of the pressure loss in the tube so that the tube structure can meet the needs in the future.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been accomplished to solve the above problems of the related art. It is a technical task of the invention to provide a tube structure of a multitubular heat exchanger capable of optimizing the heat radiating performance and the pressure loss in the tube even when the regulation of exhaust gas and the regulation of fuel consumption are more intensified.  
         [0010]     As a specific means for effectively solving the above problems, the present invention according to a first aspect of the invention provides a tube structure of a multitubular heat exchanger comprises a tube and a plurality of beads protruding from an inner face of the tube, wherein the beads are arranged at a predetermined pitch in an axial direction of the tube; and a circumference of the tube is divided at least into thirds, and the beads are aligned in a circumferential direction of the tube; and the beads aligned in the circumferential direction of the tube are provided at plural rows at the predetermined pitch in the axial direction of the tube, and the beads adjoining in the axial direction are shifted by substantially a half of a circumferential length of the bead to one another. By virtue of the foregoing, the shape and the arranging method of the bead, which is a body for facilitating the generation of a turbulent flow, are determined in such a manner that the beads are divided into three or more parts in the circumferential direction and the adjoining beads in the axial direction are arranged so that the phases can be shifted from each other. Therefore, when a flow rate in the tube is low, the heat radiating performance can be enhanced by the effect of facilitating the generation of a turbulent flow while the pressure loss is being maintained to be the same as that of the conventional case in which the beads are uniformly formed on the circumference. As the flow rate in the tube is increased, the heat radiating performance is the same as or lower than that of the conventional tube in which the beads are uniformly formed on the circumference. However, concerning the pressure loss, since the beads are divided, a portion of high pressure generated on the downstream side of the bead is decreased when the beads are divided. Therefore, the pressure loss can be greatly reduced.  
         [0011]     The invention according to a second aspect of the invention provides a tube structure of a multitubular heat exchanger, wherein the circumference of the tube is divided into parts of an even number of four or more, and the beads are aligned in the circumferential direction so as to be alternately formed in the parts of the circumference. By virtue of the foregoing, the dividing number becomes divisible. Therefore, the tube can be easily manufactured, that is, the tube can be manufactured at a low manufacturing cost although the number of beads is relatively large.  
         [0012]     The invention according to a third aspect of the invention provides a tube structure of a multitubular heat exchanger, wherein the beads are inclined by an angle of not more than 45° with respect to the circumferential direction of the tube. By virtue of the foregoing, the beads formed divided into three or more equal parts in the circumferential direction are appropriately inclined with respect to the circumferential direction. Therefore, the flow passage resistance caused by the beads, which are bodies for facilitating the generation of a turbulent flow for the exhaust gas, can be reduced and the pressure loss in the tube can be effectively decreased.  
         [0013]     The invention according to a fourth aspect of the invention provides a tube structure of a multitubular heat exchanger comprises a tube, wherein the circumference of the tube is divided into parts of an even number of four or more, and the beads are aligned in the circumferential direction so as to be alternately formed in the parts of the circumference. By virtue of the foregoing, the shape and the arranging method of the beads, which are bodies for facilitating the generation of a turbulent flow, are determined in such a manner that the beads are formed being shifted in the circumferential direction by the length of the bead in the circumferential direction between the beads, which are adjacent to each other at the different positions in the axial direction, and the beads, which are provided on the circumference at the intermediate position of the beads. Therefore, a distance between the adjoining beads at different positions in the axial direction can be extended. Accordingly, the heat radiating performance can be enhanced in the case of a low flow rate, and the pressure loss can be effectively reduced in the case of a high flow rate.  
         [0014]     The invention according to a fifth aspect of the invention provides a tube structure of a multitubular heat exchanger, wherein wherein the beads are inclined by an angle of not more than 45° with respect to the circumferential direction of the tube. By virtue of the foregoing, the beads, which are divided into equal parts by an even number in the circumferential direction, are effectively inclined with respect to the circumferential direction. Therefore, the flow passage resistance caused by the beads, which are bodies for facilitating the generation of a turbulent flow of the exhaust gas, can be reduced and the pressure loss in the tube can be effectively decreased.  
         [0015]     The invention according to a sixth aspect of the invention provides a tube structure of a multitubular heat exchanger, wherein inclinations of the beads which are adjacent to each other in the circumferential direction, are made to be opposite. By virtue of the foregoing, the heat transfer facilitating effect can be effectively enhanced without increasing the resisting action of the beads which are bodies for facilitating the generation of a turbulent flow of the exhaust gas. Further, the tube structure of a multitubular heat exchanger, inclinations of the beads which are adjacent to each other in the circumferential direction, may be made to be opposite. Further, the beads may be alternately aligned along the axial direction at substantially a half of the predetermined pitch.  
         [0016]     The invention according to a seventh aspect of the invention provides a tube structure of a multitubular heat exchanger, wherein a bead height e with respect to an inner diameter D of the tube is set at e=0.05 D to 0.2 D and a bead pitch P with respect to the bead height e is set at P= 6   e  to  25   e ; and the inner diameter D is 5 to 30 mm. By virtue of the foregoing, the beads of the most appropriate dimensions for the condition of use, in which a flow rate of the exhaust gas greatly fluctuates, can be formed. Accordingly, the heat radiating performance can be enhanced in the case of a low flow rate of the exhaust gas passing in the tube, and the pressure loss can be effectively reduced in the case of a high flow rate.  
         [0017]     Further, A tube structure of a multitubular heat exchanger comprising a tube, an inner surface of which is divided into parts of an even number of four or more; and beads aligned along the axial direction at a predetermined pitch in each part of the inner face, wherein the beads are alternately arranged in the adjacent parts of the inner face of the tube can be provided. The above aspects can be applied to this structure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     In the accompanying drawings:  
         [0019]      FIG. 1  is a schematic illustration showing a tube structure of a multitubular heat exchanger of the first embodiment of the present invention, wherein  FIG. 1A  is a front view,  FIG. 1B  is a side view and  FIG. 1C  is a development showing a bead pattern;  
         [0020]      FIG. 2  is a cross-sectional view of a bead forming portion in the tube structure of the multitubular heat exchanger of the first embodiment of the present invention, wherein  FIG. 2A  is a sectional view taken on line I-I in  FIG. 1B , and  FIG. 2B  is a sectional view taken on line II-II in  FIG. 1B ;  
         [0021]      FIG. 3  is a schematic illustration showing a tube structure of a multitubular heat exchanger of the second embodiment of the present invention, wherein  FIG. 3A  is a front view,  FIG. 3B  is a side view and  FIG. 3C  is a development showing a bead pattern;  
         [0022]      FIG. 4  is a cross-sectional view of a bead forming portion in the tube structure of the multitubular heat exchanger of the second embodiment of the present invention, wherein  FIG. 4A  is a sectional view taken on line I-I in  FIG. 3B , and  FIG. 4B  is a sectional view taken on line II-II in  FIG. 3B ;  
         [0023]      FIG. 5  is a schematic illustration showing a tube structure of a multitubular heat exchanger of the third embodiment of the present invention, wherein  FIG. 5A  is a front view,  FIG. 5B  is a side view and  FIG. 5C  is a development showing a bead pattern;  
         [0024]      FIG. 6  is a sectional view taken on line I-I in  FIG. 5B  showing a bead forming portion in the tube structure of the multitubular heat exchanger of the third embodiment;  
         [0025]      FIG. 7  is a schematic illustration showing a tube structure of a multitubular heat exchanger of the fourth embodiment of the present invention, wherein  FIG. 7A  is a front view,  FIG. 7B  is a side view and  FIG. 7C  is a development showing a bead pattern;  
         [0026]      FIG. 8  is a cross-sectional view of a bead forming portion in the tube structure of the multitubular heat exchanger of the fourth embodiment of the present invention, wherein  FIG. 8A  is a sectional view taken on line I-I in  FIG. 7B , and  FIG. 8B  is a sectional view taken on line II-II in  FIG. 7B ;  
         [0027]      FIG. 9  is a schematic illustration showing a tube structure of a multitubular heat exchanger of the fifth embodiment of the present invention, wherein  FIG. 5A  is a front view,  FIG. 5B  is a side view and  FIG. 5C  is a development showing a bead pattern;  
         [0028]      FIG. 10  is a cross-sectional view of a bead forming portion in the tube structure of the multitubular heat exchanger of the fifth embodiment of the present invention, wherein  FIG. 10A  is a sectional view taken on line I-I in  FIG. 9B , and  FIG. 10B  is a sectional view taken on line II-II in  FIG. 9B ;  
         [0029]      FIG. 11  is a schematic illustration showing a relation between the tube structure of the multitubular heat exchanger of the sixth embodiment of the present invention and the dimensions of the bead;  
         [0030]      FIG. 12  is a cross-sectional view showing the first variation of the tube structure of the multitubular heat exchanger of the second embodiment of the present invention;  
         [0031]      FIG. 13  is a cross-sectional view showing the second variation of the tube structure of the multitubular heat exchanger of the second embodiment of the present invention;  
         [0032]      FIG. 14  is a cross-sectional view showing the third variation of the tube structure of the multitubular heat exchanger of the second embodiment . . . of the present invention;  
         [0033]      FIG. 15  is a graph showing the heat radiating performance and the pressure loss resistance index of the second to the fifth embodiment of the present invention;  
         [0034]      FIG. 16  is a schematic illustration showing a tube having two-dimensional protrusion beads in the conventional tube structure of the multitubular heat exchanger, wherein  FIG. 16A  is a front view and  FIG. 16B  is a side view;  
         [0035]      FIG. 17  is a schematic illustration showing a tube having spiral protrusion beads in the conventional tube structure of the multitubular heat exchanger, wherein  FIG. 17A  is a front view and  FIG. 17B  is a side view; and  
         [0036]      FIG. 18  is a schematic illustration showing a tube having protrusion beads attached with spiral fins in the conventional tube structure of the multitubular heat exchanger, wherein  FIG. 18A  is a front view and  FIG. 18B  is a side view.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     An embodiment of the present invention will be specifically explained as follows.  
         [0038]     However, it should be noted that this embodiment is explained for the better understanding of the present invention. Therefore, the present invention is not limited to this embodiment as long as specific remarks are not made.  
         [0039]     Like reference marks are used to indicate like parts in the related art and this embodiment, and the explanations are omitted here.  
       First Embodiment  
       [0040]     As shown in  FIGS. 1 and 2 , the tube structure of the multitubular heat exchanger of the first embodiment is composed as follows. The beads  12  are provided at the same position in the axial direction in the tube  10  and protruded from the inner face of the tube  10  in such a manner that the circumferential length is divided into three equal parts so as to form the beads  12 . The beads  13 , which are adjacent to the beads  12  at a different position in the axial direction, are provided in such a manner that the positions of the beads  13  with respect to the positions of the beads  12  are shifted in the circumferential direction by a half of the length of the bead  12  formed in such a manner that the circumference is divided into three equal parts, that is, the positions of the beads  13  with respect to the positions of the beads  12  are shifted in the circumferential direction by an angle corresponding to the central angle of the portion divided into six equal parts on the circumference.  
         [0041]     Since the beads  12 ,  13  are provided as described above, when the tube  10  is developed into a plane as shown in  FIG. 1C , the beads  12  and the beads  13  are alternately arranged in the longitudinal direction of the tube  10 , and the beads  12  and the beads  13 , which are adjacent to each other in the axial direction, are arranged being shifted in the circumferential direction by a half of the bead length in the circumferential direction.  
         [0042]     In general, in the case where two-dimensional protrusions are provided, a portion in which the flow becomes stagnant is generated right after the protrusions. In this portion, the heat transfer performance is deteriorated, and the pressure loss is increased when the pressure is increased. When a flow rate in the tube is reduced, the boundary layer is developed. Therefore, when the height of the protrusions is embedded in this boundary layer, the flow in the tube becomes the same as the flow in a smooth circular tube. In order to prevent the occurrence of this phenomenon, it is necessary to increase the height of the protrusions. However, when the beads are formed, the property of press forming is limited. Further, when the height of the beads is increased, the pressure loss is also increased.  
         [0043]     Therefore, as shown in  FIG. 1 , the two-dimensional protrusions are formed into the beads  12 ,  13  which are formed in such a manner that the circumference is divided in the circumferential direction, and portions of high pressure are generated after the beads  12 ,  13 , and portions of low pressure, in which the beads  12 ,  13  are not located, are generated. Accordingly, fluid flows from the portions of high pressure to the portions of low pressure. In the region of a low flow rate in which the flow velocity in the tube is low, a flow of liquid is generated along the beads  12 ,  13 , however, in the conventional case, in the region of a low flow rate in which the flow velocity in the tube is low, a flow of liquid is generated on the axial line in the tube. Therefore, the heat radiating performance can be enhanced, and the pressure loss in the tube can be reduced, that is, the effect of facilitating the generation of a turbulent flow can be provided in the region of a low flow rate.  
         [0044]     In the region of a high flow rate in which the flow velocity is high in the tube, when the beads  12 ,  13  are formed in such a manner that the circumference is divided into equal parts, a difference in pressure is generated on the downstream side of the beads, and liquid flows to a portion of low pressure. Therefore, the pressure loss in the tube can be reduced. Concerning the heat radiating performance, since the target of the two-dimensional protrusion itself is the facilitation of the generation of a turbulent flow, when the beads are arranged as described above, the heat radiating performance is seldom affected, that is, the heat radiating performance is seldom deteriorated.  
         [0045]     By virtue of the foregoing, since the beads, which are bodies to facilitate the generation of a turbulent flow, are formed and arranged in such a manner that the circumference is divided into equal parts and the phases of the beads  12 ,  13  adjoining in the axial direction are shifted from each other, even when the pressure loss is reduced in the tube, the effect of facilitating heat transfer is not deteriorated and the heat radiating effect is enhanced.  
       Second Embodiment  
       [0046]     The tube structure of the multitubular heat exchanger of the second embodiment is shown in  FIGS. 3 and 4 . The beads  14  protruding from the inner face of the tube at the same position in the axial direction are provided in such a manner that the circumference of the tube  10  is divided into four equal parts and the beads  14  are distributed to all the divided positions, which are not adjacent to each other on the same circumference, that is, the beads  14  are distributed to every other divided position. On the circumference at the intermediate position between these beads  14 ,  14  and the beads  14 ,  14 , which are adjacent to these beads  14 ,  14 , located at a different position in the axial direction, the beads  15 ,  15  are provided on the same circumference, which is divided into equal parts of an arbitrary number, being distributed to the divided positions not adjacent to each other, in such a manner that the beads  15 ,  15  are shifted from the beads  14 ,  14  by the circumferential length of the beads  14 ,  14  in the circumferential direction. By virtue of the foregoing, when an interval of the beads  14 ,  14 , which are adjacent to each other at the different positions in the axial direction, is one pitch, an interval between the bead  14  and the bead  15 , which is located at the intermediate position between the beads  14 ,  14 , is a half (½) pitch.  
         [0047]     The beads  14 ,  15  are provided as described above. When the tube  10  is developed to a plane as shown in  FIG. 3C , the beads are arranged as follows. In the ranges of 0° to 90° and 180° to 270° on the circumference, the beads  14 ,  14  are formed being separate from each other by one pitch in the longitudinal direction of the tube  10 . In the ranges of 90° to 180° and 270° to 360° on the circumference, the beads  15  are provided being separate from the beads  14  by a half pitch in the axial direction of the tube  10 , and the beads  14  and the beads  15 , which are adjacent to each other in the axial direction, are arranged being shifted from each other in the circumferential direction by the length of the bead  14  in the circumferential direction, that is, the beads  14  and the beads  15 , which are adjacent to each other in the axial direction, are arranged being shifted from each other in the circumferential direction by the circumferential length of the bead  14 , that is, by one fourth of the circumference.  
         [0048]     By virtue of the foregoing, since the beads, which are bodies to facilitate the generation of a turbulent flow, are formed and arranged in such a manner that the beads  14 , which are adjacent at the different positions in the axial direction, and the beads  15 , which are provided on the circumference at the intermediate position, are formed being shifted from each other in the circumferential direction by the length in the circumferential direction of the bead  14  (one fourth of the circumference). Therefore, a distance between the adjoining beads  14 ,  14  can be extended, and the pressure loss can be effectively reduced, and the heat radiating performance can be enhanced without deteriorating the heat transfer facilitating effect.  
         [0049]     In the case where the circumference is divided into equal parts of an even number except four, the pressure loss can be effectively reduced, and the heat radiating performance can be enhanced without deteriorating the heat transfer facilitating effect.  
       Third Embodiment  
       [0050]     The tube structure of the multitubular heat exchanger of the third embodiment is shown in  FIGS. 5 and 6 . The beads  16  protruding from the inner face of the tube at the same position in the axial direction are provided at positions where the circumference of the tube  10  is divided into three or more equal parts (four equal parts in the drawing) in the circumferential direction, being inclined by an arbitrary angle (30° in the drawing) of 45° or less with respect to the circumferential direction.  
         [0051]     When the beads  16 , . . . ,  16  are provided as described above, the tube  10  is developed into a plane as shown in  FIG. 5C . The beads  16 , . . . ,  16  are arranged at positions equally divided in the circumferential direction. The respective beads  16 , . . . ,  16  are inclined by a predetermined angle with respect to the circumferential direction and formed into a line in the longitudinal direction of the tube  10 .  
         [0052]     By virtue of the above structure, the thus formed beads  16  are inclined with respect to the circumferential direction. Therefore, the beads, which are bodies to facilitate the generation of a turbulent flow of exhaust gas, maintain the heat transfer facilitating effect and reduce the resisting action. Therefore, the pressure loss can be effectively reduced and the heat radiating performance can be enhanced.  
       Fourth Embodiment  
       [0053]     The tube structure of the multitubular heat exchanger of the fourth embodiment is shown in  FIGS. 7 and 8 . The beads  17  protruding from the inner face of the tube at the same position in the axial direction are provided in such a manner that the circumference of the tube  10  is divided into four equal parts and the beads  17  are distributed to all the divided positions, which are not adjacent to each other on the same circumference, that is, the beads  17  are distributed to every other divided position. On the circumference at the intermediate position between these beads  17 ,  17  and the beads  17 ,  17 , which are adjacent to these beads  17 ,  17 , located at a different position in the axial direction, the beads  18 ,  18  are provided on the same circumference, which is divided into equal parts of an arbitrary number, being distributed to the divided positions not adjacent to each other, in such a manner that the beads  18 ,  18  are shifted from the beads  17 ,  17  by the circumferential length of the beads  17 ,  17  in the circumferential direction. All beads  17 ,  17 ,  18 ,  18  are inclined in the same direction by an arbitrary angle (15° in the drawing) of not more than 45° with respect to the circumferential direction. By virtue of the foregoing, when an interval of the beads  17 ,  17 , which are adjacent to each other at the different positions, is one pitch, an interval between the bead  17  and the bead  18 , which are located at the intermediate position between the beads  17 ,  17 , is a half (½) pitch.  
         [0054]     The beads  17 ,  18  are provided as described above. When the tube  10  is developed to a plane as shown in  FIG. 7C , the beads are arranged as follows. In the ranges of 0° to 90° and 180° to 270° on the circumference, the beads  17 ,  17  are formed being separate from each other by one pitch in the longitudinal direction of the tube  10 . In the ranges of 90° to 180° and 270° to 360° on the circumference, the beads  18  are provided being separate from the beads  17  by a half pitch in the axial direction of the tube  10 , and the beads  17  and the beads  18 , which are adjacent to each other in the axial direction, are arranged being shifted from each other in the circumferential direction by the length of the beads  17  in the circumferential direction, that is, the beads  17  and the beads  18 , which are adjacent to each other in the axial direction, are arranged being shifted from each other in the circumferential direction by one fourth of the circumference. Other points are the same as those of the second embodiment.  
         [0055]     By virtue of the above structure, the beads  17 ,  18  are effectively inclined with respect to the circumferential direction. Therefore, the beads, which are bodies to facilitate the generation of a turbulent flow of exhaust gas, maintain the heat transfer facilitating effect and reduce the resisting action. Therefore, the pressure loss can be effectively reduced and the heat radiating performance can be enhanced.  
       Fifth Embodiment  
       [0056]     In the tube structure of the multitubular heat exchanger of the fifth embodiment, inclinations of the beads, which are arranged being adjacent to each other in the axial direction, are opposite to each other. The tube structure of the multitubular heat exchanger of the fourth embodiment is shown in  FIGS. 9 and 10 . The beads  17  protruding from the inner face of the tube at the same position in the axial direction are provided in such a manner that the circumference of the tube  10  is divided into four equal parts and the beads  19  are distributed to the divided positions, which are not adjacent to each other on the same circumference, that is, the beads  19  are distributed to every other divided position. On the circumference at the intermediate position between these beads  19 ,  19  and the beads  19 ,  19 , which are adjacent to these beads  19 ,  19 , located at a different position in the axial direction, the beads  21 ,  21  are provided on the same circumference, which is divided into equal parts of an arbitrary number, being distributed to the divided positions not adjacent to each other, in such a manner that the beads  21 ,  21  are shifted from the beads  19 ,  19  by the circumferential length of the beads  19 ,  19 . The beads  19 ,  19 , are inclined in the same direction by an arbitrary angle (15° in the drawing) of not more than 45° with respect to the circumferential direction. Further, the beads  21 ,  21 , which are provided while the positions are being shifted, are inclined in the direction opposite to the inclination direction of the beads  19 ,  19  with respect to the circumferential direction by an arbitrary angle (−15° in the drawing) of 45° or less.  
         [0057]     The beads  19 ,  21  are provided as described above. When the tube  10  is developed to a plane as shown in  FIG. 9C , the beads are arranged as follows. In the ranges of 0° to 90° and 180° to 270° on the circumference, the beads  19 ,  19  are formed being separate from each other by one pitch in the longitudinal direction of the tube  10  being inclined in the same direction. In the ranges of 90° to 180° and 270° to 360° on the circumference, the beads  21  are provided being separate from the beads  19  by a half pitch in the axial direction of the tube  10 , and the beads  19  and the beads  21 , which are adjacent to each other in the axial direction, are arranged being shifted from each other in the circumferential direction by the length of the bead  19  in the circumferential direction, that is, the beads  19  and the beads  21 , which are adjacent to each other in the axial direction, are arranged being shifted from each other in the circumferential direction by one fourth of the circumference. Further, with respect to the circumferential direction, the beads  21 ,  21  are provided being inclined in the direction opposite to the inclining direction of the beads  19 ,  19 . Other points are the same as those of the second embodiment.  
         [0058]     By virtue of the above structure, the beads  19 ,  21  are effectively inclined with respect to the circumferential direction. Therefore, the beads, which are bodies to facilitate the generation of a turbulent flow of exhaust gas, maintain the heat transfer facilitating effect and reduce the resisting action. Therefore, the pressure loss can be effectively reduced and the heat radiating performance can be enhanced.  
       Sixth Embodiment  
       [0059]     The tube structure of the multitubular heat exchanger of the sixth embodiment is shown in  FIG. 11 . As the size of each portion of the tube structure is shown in the drawing, when the inner diameter D of the tube  10  used for a heat transfer tube is 5 to 30 mm, the height e of the bead is set at e=0.05 D to 0.2 D with respect to the inner diameter D, and the bead pitch P is set at P=6e to 25e with respect to the height e of the bead. This dimensional relationship can be applied to all possible embodiments according to the invention.  
         [0060]     By virtue of the foregoing, the beads of the most appropriate dimensions for the use, in which a flow rate of the exhaust gas greatly fluctuates, can be formed. Accordingly, the pressure loss of exhaust gas passing in the tube can be reduced and the heat radiating performance can be enhanced.  
       Seventh Embodiment  
       [0061]     The tube structure of the multitubular heat exchanger of the seventh embodiment can be applied without making a change in the operational effect even when the bead shape is somewhat changed. For example, a variation of the bead shape of the second embodiment is shown as follows. In  FIG. 12 , non-bead portions are formed at the boundary positions when the cross-sectional shape is equally divided on the circumference, which is referred to as Type 1, hereinafter. In  FIG. 13 , the beads are overlapped with the boundary position equally divided on the circumference, which is referred to as Type 2, hereinafter. In  FIG. 14 , the cross-sectional shape of the bead is formed into not an arc but into a straight line, which is referred to as Type 3, hereinafter. These types can be applied to a case in which the circumference is divided into equal parts of an arbitrary even number except four.  
         [0062]     In Type 1, the length in the longitudinal direction is formed short so that the beads  14   a  can be provided at the equally divided positions not adjoining on the same circumference of the tube  10  which is divided into four equal parts and so that non-bead portions can be formed at the boundary positions equally divided on the circumference. The beads  15   a ,  15   a , which are provided on the circumference at the intermediate position between these beads  14   a ,  14   a  and the beads  14   a ,  14   a  adjoining these beads  14   a ,  14   a  at a different position in the axial direction, are formed short in the length of the longitudinal direction.  
         [0063]     In the case of Type 2, the circumference of the tube  10  is divided into four equal parts, and the beads  4   b  provided at the equally divided positions, which are not adjacent to each other, on the same circumference are formed long in the longitudinal direction so that the end portions of the beads  4   b  can be formed at the boundary positions which are equally divided on the circumference. The beads  15   b ,  15   b , which are provided on the circumference at the intermediate position between these beads  14 ,  14   b  and the beads  14   b ,  14   b  adjoining these beads  14   b ,  14   b  at a different position in the axial direction, are formed long in the longitudinal direction in the same manner so that the beads  14   b ,  15   b  can be formed being overlapped with each other.  
         [0064]     In the case of Type 3, the cross-sectional shape of the primary portion of the beads  14   c ,  15   c  to be formed is not an arc formed along the tube wall but a linear shape which is made by means of pressing.  
         [0065]     When the above bead type, in which the bead shape is changed, provides the same operational effect as that of the original type, it can be applied.  
         [0066]     Concerning the characteristics of various bead patterns of the second to the fifth embodiment, relative evaluations of the heat radiating performance and the pressure loss resistance index are shown in  FIG. 15  in the case where the heat radiating performance and the pressure loss resistance index of the conventional tube structure having the two-dimensional protrusions are set at  100 .  
         [0067]     The experiment was conducted on an EGR gas cooler, in which the heated gas (air) is passed through ten tubes and the tubes are cooled by water outside, under the following conditions; 
        Outer diameter of tube: φ12     Tube length: 200 mm     Bead height: 1 mm     Bead pitch: 10 mm     Outer diameter of shell: +54     Water flow rate: 10 L/min     Water inlet temperature: 80° C.     Gas inlet temperature: 500° C.        
 
         [0076]     As a result, the following can be confirmed. When the adjoining beads are shifted from each other in the circumferential direction or the beads are inclined with respect to the circumferential direction, the pressure loss can be reduced and the heat radiating performance can be enhanced without deteriorating the heat transfer facilitating effect.  
         [0077]     As described above, in the tube structure of the multitubular heat exchanger according to a first aspect of the invention of the present invention, the shape and the arranging method of the bead, which is a body for facilitating the generation of a turbulent flow, are determined in such a manner that the beads are divided into three or more parts in the circumferential direction and the adjoining beads in the axial direction are arranged so that the phases can be shifted from each other. Therefore, when a flow rate in the tube is low, the heat radiating performance can be enhanced by the effect of facilitating the generation of a turbulent flow while the pressure loss is being maintained to be the same as that of the conventional case in which the beads are uniformly formed on the circumference. As the flow rate in the tube is increased, the heat radiating performance is the same as or lower than that of the conventional tube in which the beads are uniformly formed on the circumference. However, concerning the pressure loss, since the beads are divided, a portion of high pressure generated on the downstream side of the bead is decreased when the beads are divided. Therefore, the pressure loss can be greatly reduced.  
         [0078]     In the tube structure of the multitubular heat exchanger of a second aspect of the invention, the dividing number becomes divisible. Therefore, the tube can be easily manufactured, that is, the tube can be manufactured at a low manufacturing cost although the number of beads is relatively large.  
         [0079]     In the tube structure of the multitubular heat exchanger of a third aspect of the invention, the beads formed divided into three or more equal parts in the circumferential direction are appropriately inclined with respect to the circumferential direction. Therefore, the flow passage resistance caused by the beads, which are bodies for facilitating the generation of a turbulent flow for the exhaust gas, can be reduced and the pressure loss in the tube can be effectively decreased.  
         [0080]     In the tube structure of the multitubular heat exchanger of a fourth aspect of the invention, the shape and the arranging method of the beads, which are bodies for facilitating the generation of a turbulent flow, are determined in such a manner that the beads are formed being shifted in the circumferential direction by the length of the bead in the circumferential direction between the beads, which are adjacent to each other at the different positions in the axial direction, and the beads which are provided on the circumference at the intermediate position of the beads. Therefore, a distance between the adjoining beads at different positions in the axial direction can be extended. Accordingly, the heat radiating performance can be enhanced in the case of a low flow rate, and the pressure loss can be effectively reduced in the case of a high flow rate.  
         [0081]     In the tube structure of the multitubular heat exchanger of a fifth aspect of the invention, the beads, which are divided into equal parts by an even number in the circumferential direction, are effectively inclined with respect to the circumferential direction. Therefore, the flow passage resistance caused by the beads, which are bodies for facilitating the generation of a turbulent flow of the exhaust gas, can be reduced and the pressure loss in the tube can be effectively decreased.  
         [0082]     In the tube structure of the multitubular heat exchanger of a sixth aspect of the invention, the heat transfer facilitating effect can be effectively enhanced without increasing the resisting action of the beads which are bodies for facilitating the generation of a turbulent flow of the exhaust gas.  
         [0083]     In the tube structure of the multitubular heat exchanger of a seventh aspect of the invention, the beads of the most appropriate dimensions for the condition of use, in which a flow rate of the exhaust gas greatly fluctuates, can be formed. Accordingly, the heat radiating performance can be enhanced in the case of a low flow rate of the exhaust gas passing in the tube, and the pressure loss can be effectively reduced in the case of a high flow rate.  
         [0084]     The present invention is not limited to the embodiments and the description thereof at all. If various changes which can be easily conceived by those skilled in the art are not departed from the description of the scope of claim, they may be contained in the present invention.