Patent Publication Number: US-2021190442-A1

Title: Heat transfer enhancement pipe as well as cracking furnace and atmospheric and vacuum heating furnace including the same

Description:
TECHNICAL FIELD 
     The invention relates to the field of fluid heat transfer technology, in particular to a heat transfer enhancement pipe as well as a cracking furnace and an atmospheric and vacuum heating furnace including the same. 
     BACKGROUND 
     The heat transfer enhancement pipe refers to a heat transfer element capable of enhancing fluid heat transfer between the interior and the outside of the pipe, that is, enabling unit heat transfer area to transfer as much heat as possible per unit time. The heat transfer enhancement pipes are used in many industries, such as thermal power generation, petrochemical, food, pharmaceutical, light industry, metallurgy, navel architecture, etc. The cracking furnace is an important equipment in petrochemical industry, therefore the heat transfer enhancement pipe has been widely used in the cracking furnace. 
     For a heat transfer enhancement pipe, there is a flow boundary layer between the fluid flow body and the pipe wall surface, and the heat transfer resistance is large. At the same time, due to the extremely low flow velocity in the boundary layer, coke is gradually deposited and adhered to the inner surface of the furnace pipe during the cracking process to form a dense coke layer, which coke layer is extremely large in heat transfer resistance. Therefore, the maximum resistance of the heat transfer pipe in the radiation section of the cracking furnace is in the boundary layer region of the inner wall of the pipe. 
     U.S. Pat. No. 5,605,400A discloses to enhance heat transfer by providing a fin on the internal wall of the heat transfer enhancement pipe. The fin not only increases surface area of the heat transfer enhancement pipe but also increases turbulent kinetic energy inside the pipe. The fin is in the form of a distorted blade. The fin is usually arranged in the interior of the heat transfer enhancement pipe to thin the boundary layer of the fluid via rotation of the fluid itself, thereby achieving the purpose of heat transfer enhancement. Although the heat transfer enhancement pipe with fin has a relatively good heat transfer enhancement effect, cracks can often occur between the fin and the pipe wall of the heat transfer enhancement pipe due to high stress at the welding site during operation, since the fin is connected with the pipe wall of the heat transfer enhancement pipe by welding. Especially in long-term operation combined with ultra-high temperature environment, it is more likely for cracks to occur between the fin and the pipe wall of the heat transfer enhancement pipe, thereby shortening service life of the heat transfer enhancement pipe. 
     Therefore, it is necessary to reduce thermal stress of the heat transfer enhancement pipe to increase service life of the heat transfer enhancement pipe, while ensuring heat transfer effect of the heat transfer enhancement pipe. 
     SUMMARY OF THE INVENTION 
     Objects of the present invention are to overcome issues of short service life of the heat transfer enhancement pipe existing in the prior art and to provide a heat transfer enhancement pipe capable of reducing its own thermal stress and thereby increasing service life of the heat transfer enhancement pipe. 
     In order to achieve the above objects, one aspect of the present invention provides a heat transfer enhancement pipe including a pipe body of tubular shape with an inlet for entering of a fluid and an outlet for said fluid to flow out, internal wall of the pipe body is provided with a fin protruding toward the interior of the pipe body, wherein the fin has one or more fin sections extending spirally in the axial direction of the pipe body, and each fin section has a first end surface facing the inlet and a second end surface facing the outlet, at least one of the first end surface and the second end surface of at least one of the rib sections is formed as a transition surface along spirally extending direction. 
     On the other aspect, the present invention provides a cracking furnace or an atmospheric and vacuum heating furnace comprising a radiation chamber, in which at least one furnace pipe assembly is installed; the furnace pipe assembly comprises a plurality of furnace pipes arranged in sequence and heat transfer enhancement pipe communicating adjacent furnace pipes, the heat transfer enhancement pipe is heat transfer enhancement pipe as described as above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective schematic view of the heat transfer enhancement pipe according to a preferred embodiment of the present invention, wherein the fin has a rectangular cross section; the transition angle is 30°. 
         FIG. 2  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 1 . 
         FIG. 3  is a perspective schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein the fin has a trapezoidal cross section. 
         FIG. 4  is a left-side structural schematic view of the heat transfer enhancement pipe shown in  FIG. 3 . 
         FIG. 5  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 3 . 
         FIG. 6  is a perspective schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein the fin has a trapezoidal cross section, the number of intervals arranged at the fin is 1; the transition angle is 35°. 
         FIG. 7  is a side perspective schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein the cross-section of the fin is triangular-shaped viewed from aside. 
         FIG. 8  is a perspective schematic view of the heat transfer enhancement pipe according to another embodiment of the present invention, wherein the fin has a trapezoidal cross section, the transition angle is 38°, and the height of the fin gradually increases from one end. 
         FIG. 9  is a cross-sectional structural schematic view of the heat transfer enhancement pipe according to another embodiment of the present invention. 
         FIG. 10  is a stress distribution diagram of the heat transfer enhancement pipe of the present invention vs a prior art heat transfer pipe. 
         FIG. 11  is a cross-sectional structural schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein the fin has a trapezoidal cross section, the number of intervals arranged at the fin is 2; the transition angle is 38°. 
         FIG. 12  is a perspective schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein the fin has a trapezoidal cross section, the transition angle is 35°, and the top surface of the fin facing the central axis of the pipe body is formed as the third transition surface of concave shape. 
         FIG. 13  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 12 . 
         FIG. 14  is a structural schematic view of a furnace pipe assembly in the cracking furnace according to a preferred embodiment of the present invention. 
         FIG. 15  is a perspective schematic view of the heat transfer enhancement pipe according to a preferred embodiment of the present invention, wherein a heat insulator is provided at the outside of the pipe body, the fin has a trapezoidal cross section, the transition angle is 30°. 
         FIG. 16  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 15 . 
         FIG. 17  is a perspective schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein a heat insulator is provided at the outside of the pipe body, the fin has a trapezoidal cross section, the transition angle is 35°. 
         FIG. 18  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 17 . 
         FIG. 19  is a perspective schematic view of a heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein a heat insulator is provided at the outside of the pipe body, the fin has a trapezoidal cross section, the transition angle is 40°. 
         FIG. 20  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 19 . 
         FIG. 21  is a perspective schematic view of a heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein the connecting part supported between the pipe body and the heat insulator is the second connecting part. 
         FIG. 22  is a perspective schematic view from another angle of the heat transfer enhancement pipe shown in  FIG. 21 . 
         FIG. 23  is a perspective schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein a heat insulator is provided at the outside of the pipe body, the fin has a trapezoidal cross section, the number of intervals arranged at the fin is 1, the transition angle is 35°. 
         FIG. 24  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 23 . 
         FIG. 25  is a perspective schematic view of the heat transfer enhancement pipe according to another preferred embodiment of the present invention, wherein a heat insulator is provided at the outside of the pipe body, the fin has a trapezoidal cross section, the transition angle is 35°, and the top surface of the fin facing the central axis of the pipe body is formed as the third transition surface of concave shape. 
         FIG. 26  is a cross-sectional structural schematic view of the heat transfer enhancement pipe shown in  FIG. 25 . 
         FIG. 27  is a cross-sectional structural schematic view of the heat transfer enhancement pipe according to a preferred embodiment of the present invention, wherein a heat insulating layer is provided on the external surface of the pipe body, the fin has a trapezoidal cross section, the number of intervals arranged at the fin is 1, the transition angle is 35°. 
         FIG. 28  is a local structural schematic view of the heat transfer enhancement pipe shown in  FIG. 27 , wherein a heat insulating layer is provided on the external surface of the pipe body, which includes a metal alloy layer, an oxide layer, and a ceramic layer sequentially stacked at the external surface of the pipe body. 
     
    
    
     DESCRIPTION OF THE REFERENCE CHARACTERS 
       1 —heat transfer enhancement pipe;  10 —pipe body;  100 —inlet;  101 —outlet;  11 —fin;  110 —first end surface;  111 —top surface;  112 —side wall face;  113 —smooth transition fillet;  115 —second end surface;  12 —interval;  120 —side wall;  13 —hole;  14 —heat insulator;  140 —straight pipe section;  141 —first tapered pipe section;  142 —second tapered pipe section;  15 —gap;  160 —first connecting piece;  161 —second connecting piece;  162 —connecting rod;  17 —heat insulating layer;  170 —metal alloy layer;  171 —ceramic layer;  172 —oxide layer;  2 —furnace pipe. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the present invention, without indicated on the contrary, words such as “up”, “down”, “left”, and “right” used herein to define orientations generally refer to and are understood as orientations in association with the drawings and orientations in actual application; “interior” and “external” is relative to the axis of the heat transfer enhancement pipe. In addition, the height of the fin refers to the height or distance between the top surface of the fin facing the central axis of the pipe body and the internal wall of the pipe body. The axial length of the fin refers to the length or distance of the fin along the central axis in the side view. 
     The present invention proposes to provide a heat transfer enhancement pipe in a furnace pipe assembly, to enhance heat transfer, thereby reducing or preventing formation of coke layer. As shown in  FIG. 14 , a plurality of furnace pipe assembly are provided in a radiation chamber of a cracking furnace. In each furnace pipe assembly, each furnace pipe assembly is provided with heat transfer enhancement pipes  1 , two heat transfer enhancement pipes  1  disposed at intervals along the axial direction of the furnace pipe  2 . Each heat transfer enhancement pipe  1  has an internal diameter of 65 mm. In each furnace pipe assembly, the axial length of the furnace pipe  2  between two adjacent heat transfer enhancement pipes  1  is 50 times the internal diameter of the heat transfer enhancement pipe  1 . It is to be understood that, the number and interval of the heat transfer enhancement pipes  1  may vary depending on particular applications, without departing from the scope of the present invention. In addition, the heat transfer enhancement pipe  1  of the present invention may also be used in other applications, such as a heating furnace. 
     As shown in  FIGS. 1-8 , the heat transfer enhancement pipe  1  includes a pipe body  10  of tubular shape having an inlet  100  for entering of a fluid and an outlet  101  for said fluid to flow out. The internal wall of the pipe body  10  is provided with fin  11  protruding towards the interior of the pipe body  10  and spirally extending in an axial direction of the pipe body  10 . The fins  11  may extend continuously or in sections. When the fins  11  extend in sections, the fins  11  include a plurality of the fin sections divided by intervals  12 . Similarly, when the fins  11  extend continuously, the fins  11  may be considered to include a single fin section. Therefore, the fins  11  have one or more fin sections extending spirally in the axial direction of the pipe body  10 . It is to be understood that the length of each fin section may be the same or different. In addition, each fin section includes a first end surface facing the inlet  100  and a second end surface facing the outlet  101 . At least one of the first end surface and the second end surface of at least one of the fin sections is formed as a transition surface along a spirally extending direction. In order to facilitate the distinction, in the present application, the first end surface  110  closest to the inlet  100  is referred to as the first transition surface; the second end surface  115  closest to the outlet  101  is referred to as the second transition surface; the first end surface and the second end surface defined by the side walls  120  of the intervals  12  are referred to as the fourth transition surface. When the first end surface and/or the second end surface of the plurality of the fin sections are transition surfaces, the transition surfaces formed by the first end surface and/or the second end surface of each fin section may be the same or different. 
     In addition, it should be noted that the transition surface may be a curved face or a flat face. The curved face may be convex or concave. Preferably, the curved face is concave to further improve the heat transfer effect of the heat transfer enhancement pipe and to further reduce the thermal stress of the heat transfer enhancement pipe. In addition, the transition surface can also reduce the impact force of the fluid on the fins. “Transition angle” refers to the angle between the transition surface or the tangent plane of the transition surface (when the transition surface is a curved face) and the tangent plane of the pipe wall at the connection position. The transition angle extends at an angle greater than or equal to 0° and less than 90°. 
     As shown in  FIGS. 1-5 , the first end surface  110  of fin  11  closest to the inlet  100  is formed as the first transition surface in a spirally extending direction. By providing on the internal wall of pipe body  10  with fin  11  protruding towards the interior of pipe body  10  and by forming the first end surface  110  of fin  11  closest to the inlet  100  as the first transition surface in a spirally extending direction, it thereby enables the heat transfer enhancement pipe to have a good heat transfer effect, while thermal stress of the heat transfer enhancement pipe  1  can be reduced and the ability to resist local over-temperature of the heat transfer enhancement pipe  1  is correspondingly improved, so as to increase service life of the heat transfer enhancement pipe; furthermore, the first end surface  110  forming as the first transition surface has a relatively strong turbulent effect on the fluid in pipe body  10  and reduces coking phenomenon.  FIG. 10  is a stress distribution diagram of the heat transfer enhancement pipe of the present invention vs a prior art heat transfer pipe. As can be seen from  FIG. 10 , in the prior art heat transfer pipe, there is a significant stress concentration at the connection between the fins and the pipe wall of the reinforced heat transfer tube (as shown in the upper half of  FIG. 10 ); as compared with the prior art heat transfer pipe, the thermal stress of the heat transfer enhancement pipe  1  of the present invention is significantly reduced (as shown in the lower half of  FIG. 10 ). 
       FIG. 4  clearly shows the first transition surface forming in the spirally extending direction, wherein the first end surface  110  is sloped in the spirally extending direction. The aforementioned heat transfer enhancement pipe  1  is suitable for heating furnaces and is also suitable for cracking furnaces. Additionally, it should be noted that the fluid in the heat transfer enhancement pipe  1  is not specifically limited and can be selected according to actual application environment of the heat transfer enhancement pipe  1 . 
     In addition, the first transition surface can be formed as a first curved face. The first curved face can be either convex or concave shape; preferably, the first curved face is of concave shape so as to further improve heat transfer effect of the heat transfer enhancement pipe  1  and further reduce thermal stress of the heat transfer enhancement pipe  1 . Specifically, the first curved face can be a partial paraboloid taken from a paraboloid. 
     In addition, the transition angle of the first transition surface can be greater than or equal to 0° and less than 90°, so as to further reduce thermal stress of the heat transfer enhancement pipe  1  and greatly increase service life of the heat transfer enhancement pipe  1 . The transition angle of the first transition surface can be 10°, 15°, 20°, 25°, 30°, 35°, 38°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, or 85°. 
     In order to further reduce thermal stress of the heat transfer enhancement pipe  1 , the second end surface of the fin  11  closest to the outlet  101  can be formed as the second transition surface in a spirally extending direction; wherein the second end surface  110  is sloped in the spirally extending direction, so as to correspondingly increase service life of the heat transfer enhancement pipe. In addition, the second transition surface can be formed as a second curved face. The second curved face can be either convex or concave shape; preferably, the second curved face can be of concave shape. In addition, the transition angle of the second transition surface can be greater than or equal to 0° and less than 90°, so as to further reduce thermal stress of the heat transfer enhancement pipe  1  and greatly increase service life of the heat transfer enhancement pipe  1 . The transition angle of the second transition surface can be 10°, 15°, 20°, 25°, 30°, 35°, 38°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, or 85°. 
     As shown in  FIG. 12 , the top surface  111  of the fin  11  facing the central axis of pipe body  10  can be formed as the third transition surface, so as to reduce thermal stress of the heat transfer enhancement pipe  1  without affecting heat transfer effect of the heat transfer enhancement pipe  1 . It is further preferred for the third transition surface to be concave. Specifically, the third transition surface takes form of a paraboloid. 
     Preferably, two opposite side wall faces  112  of the fin  11  gradually approach to each other in a direction from the internal wall of pipe body  10  to the center of pipe body  10 ; that is to say, each of the side wall faces  112  can be inclined, so as to enable fin  11  to enhance disturbance to the fluid entering into pipe body  10  and improve heat transfer effect, while further reducing thermal stress of the heat transfer enhancement pipe  1 . It is also understood that the cross section of the fin  11 , which is the cross section taken from a plane parallel to a radial direction of pipe body  10 , can substantially be trapezoidal or trapezoidal-like. Of course, the cross section of the fin  11  can substantially be rectangular. 
     In order to reduce thermal stress of the heat transfer enhancement pipe  1 , a smooth transition fillet  113  can be formed at the connection of at least one of two opposite side wall faces  112  of the fin  11  with the internal wall of pipe body  10 . Further, the radius of smooth transition fillet  113  is greater than 0 and less than or equal to 10 mm. Setting the radius of smooth transition fillet  113  within the above range can further reduce thermal stress of the heat transfer enhancement pipe  1  and increase service life of the heat transfer enhancement pipe  1 . Specifically, the radius of smooth transition fillet  113  can be 5 mm, 6 mm, or 10 mm. 
     In addition, the angle formed by each of the side wall faces  112  and the internal wall of pipe body  10  at the connection with each other can be 5° to 90°; that is to say, the angle between the tangential planes of each of the side wall faces  112  and the internal wall of pipe body  10  at the connection with each other can be 5° to 90°; setting the angle within the above range can further reduce thermal stress of the heat transfer enhancement pipe  1  and increase service life of the heat transfer enhancement pipe  1 . The angle formed by each of the side wall faces  112  and the internal wall of pipe body  10  at the connection with each other can be 20°, 30°, 40°, 45°, 50°, 60°, 70°, or 80°. 
     In order to reduce thermal stress of the heat transfer enhancement pipe  1 , the height of the fin  11  is preferably greater than 0 and less than or equal to 150 mm; for example, the height of the fin  11  can be 10 mm, 20 mm, 30 mm, 40 mm, 50 mm, 60 mm, 70 mm, 80 mm, 90 mm, 100 mm, 110 mm, 120 mm, 130 mm, or 140 mm. 
     As shown in connection with  FIG. 6-7 , intervals  12  can be arranged on fin  11  to separate fin  11  so that not only the heat transfer enhancement pipe  1  has a good heat transfer effect but also thermal stress of the heat transfer enhancement pipe  1  can be reduced, while the ability to resist local over-temperature can be improved. When the heat transfer enhancement pipe  1  provided with intervals  12  is applied to a heating furnace or a cracking furnace, operating cycle of the heating furnace or cracking furnace can also be increased. Wherein the number of intervals  12  is not specifically limited and can be selected according to actual needs. For example, it can be provided with one interval  12 , or two, three, four, or five intervals  12 . When provided with a plurality of intervals  12 , the plurality of intervals  12  are preferably arranged in the extending direction of fin  11 . 
     Preferably, at least one of two sidewalls  120  of intervals  12  is formed as the fourth transition surface. For example, as shown in  FIG. 6-7 , both of the sidewalls  120  of intervals  12  can be formed as transition surfaces, and the distance between two sidewalls  120  gradually increases in a direction from close to the internal wall of pipe body  10  to away from the internal wall of pipe body  10 . Wherein the distance between two sidewalls  120 , i.e. the width of intervals  12 , can be greater than 0 and less than or equal to 10000 mm; for example, the distance between two sidewalls  120  can be 1000 mm, 2000 mm, 3000 mm, 4000 mm, 5000 mm, 6000 mm, 7000 mm, 8000 mm, or 9000 mm. In addition, the fourth transition surface can be concave toward a direction facing away from the center of intervals  12 . 
     Further, a plurality of fins  11 , for example, two, three, or four fins  11 , can be arranged on the internal wall of pipe body  10 . As viewed in the direction of inlet  100 , the plurality of fins  11  can be clockwise or counterclockwise spiral. Configuring the plurality of fins  11  with the above structure not only improves heat transfer effect of the heat transfer enhancement pipe  1 , but also reduces thermal stress of the heat transfer enhancement pipe  1 , improves the ability of the heat transfer enhancement pipe  1  to resist high temperature, and greatly extends service life of the heat transfer enhancement pipe  1 . 
     Preferably, as viewed in the direction of inlet  100 , the plurality of fins  11  can be enclosed at the center of pipe body  10  to form a hole  13  extending in the axial direction of pipe body  10  to facilitate the flow of the fluid into pipe body  10  and to reduce pressure drop. In order to reduce pressure drop to as low as possible, the ratio d:D between diameter d of hole  13  and internal diameter D of pipe body  10  can preferably be greater than 0 and less than 1; for example, the ratio d:D can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9. 
     In order to increase disturbance effect of fin  11  to the fluid, the rotational angle of fin  11  can preferably be 90-1080°; for example, the rotational angle of fin  11  can be 120°, 180°, 360°, 720°, or 1080°. 
     Generally, the ratio of the axial length of fin  11  rotated by 180° to internal diameter D of pipe body  10  is a distortion ratio that determines the length of each fin  11 ; while the rotational angle of fin  11  determines the degree of distortion and affects heat transfer efficiency. The distortion ratio of fin  11  can be 2.3 to 2.6; for example, the distortion ratio of fin  11  can be 2.35, 2.4, 2.5, 2.49, or 2.5. 
     In addition, the ratio L 1 :D of length L 1  of fin  11  in the axial direction of pipe body  10  to internal diameter D of pipe body  10  is 1-10:1; preferably, the ratio L 1 :D=1-6:1. 
     The present invention also provides a cracking furnace comprising a radiation chamber, in which at least one furnace pipe assembly is mounted, as shown in  FIG. 14 . The furnace pipe assembly comprises a plurality of furnace pipes  2  sequentially arranged, in which heat transfer enhancement pipes, i.e. the heat transfer enhancement pipes  1 , communicating adjacent furnace pipes  2  can be axially arranged in a spaced manner; the heat transfer enhancement pipes are the heat transfer enhancement pipes  1  provided by present invention. Specifically, the furnace pipe assembly can be provided with 2, 3, 4, 5, 6, 7, 8, 9, or 10 heat transfer enhancement pipes  1 . Preferably, the ratio L 2 :D of axial length L 2  of furnace pipe  2  to internal diameter D of pipe body  10  is 15-75, so that heat transfer effect and operating cycle of the cracking furnace can be further improved. It is further preferred that the ratio L 2 :D=25-50. 
     Effects of the present invention will be further illustrated through embodiments and comparative examples in the following. 
     Example 11 
     A plurality of the furnace pipe assemblies are arranged in a radiation chamber of a cracking furnace. The heat transfer enhancement pipes  1  are arranged in three of the furnace pipe assemblies. Two heat transfer enhancement pipes  1  are arranged in each furnace pipe assembly at intervals in axial direction of the furnace pipe  2 . Each heat transfer enhancement pipe  1  has an internal diameter of 65 mm. In each furnace pipe assembly, the axial length of the furnace pipe  2  between two adjacent heat transfer enhancement pipes  1  is 50 times the internal diameter of the heat transfer enhancement pipe  1 . Structure of each of the heat transfer enhancement pipes  1  is as follow: two fins  11  are arranged on the internal wall of pipe body  10  with their two ends respectively formed as the first transition surface and the second transition surface of concave shapes in a spirally extending direction as shown in  FIG. 4 ; the transition angle of the first transition surface is 30°; the transition angle of the second transition surface is 30°; the cross section of each fin  11 , i.e. the cross section taken from a surface in the radial direction parallel to pipe body  10 , is substantially rectangular; a smooth transition fillet is formed at connection of each side wall face  112  and the internal wall of pipe body  10 ; as viewed from the direction of inlet  100 , two fins  11  take shapes of clockwise spirals; two fins  11  enclose at the center of pipe body  10  to form hole  13  extending in the axial direction of pipe body  10 ; the ratio of the diameter of hole  13  to the internal diameter of pipe body  10  is 0.6; the rotation angle of each of the fins  11  is 180°; the distortion ratio of each of the fins  11  is 2.5, wherein the outlet temperature of the cracking furnace is 820-830°. 
     Example 12 
     Example 12 is the same as Example 11 except that: the transition angle of the first transition surface is 35°; the transition angle of the second transition surface is 35°; the cross section of each fin  11 , i.e. the cross section taken from a surface in the radial direction parallel to pipe body  10 , is substantially trapezoidal; the angle formed by each side wall face  112  and the internal wall of pipe body  10  at the connection with each other is 45°; and one interval is arranged on each of the fins  11 . Other conditions remain unchanged. 
     Example 13 
     Example 13 is the same as Example 11 except that: the transition angle of the first transition surface is 35°; the transition angle of the second transition surface is 35°; the cross section of each fin  11 , i.e. the cross section taken from a surface in the radial direction parallel to pipe body  10 , is substantially trapezoidal; the angle formed by each side wall face  112  and the internal wall of pipe body  10  at the connection with each other is 45°; and the top surface  111  of each fin  11  in the direction towards the central axis of pipe body  10  is a concave transition surface as shown in  FIG. 12 . Other conditions remain unchanged. 
     Comparative Example 11 
     The heat transfer enhancement pipe of the prior art is arranged, wherein in the pipe body is provided with only one fin that extends spirally in the axial direction of the pipe body and separates the interior of the pipe body into two mutually non-communicating chambers, with the remaining conditions unchanged. 
     Respective test results of the cracking furnaces in the examples and the comparative example after operating under same conditions are shown in Table 1 below. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Test items  
               
            
           
           
               
               
               
               
               
            
               
                   
                 Heat  
                   
                 Maximum  
                   
               
               
                   
                 transfer  
                 Pressure  
                 thermal  
                 Service  
               
               
                 No.  
                 load/W  
                 drop/MPa  
                 stress/MPA  
                 life/year 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 11  
                 94620  
                 0.108350  
                 40  
                 6-7  
               
               
                 Example 12  
                 94700  
                 0.10780  
                 35  
                 6-7  
               
               
                 Example 13  
                 94700  
                 0.10820  
                 40  
                 6-7  
               
               
                 Comparative  
                 88080  
                 0.120909  
                 110  
                 4-5  
               
               
                 example 11 
               
               
                   
               
            
           
         
       
     
     It can be known from the above that arranging the heat transfer enhancement pipe provided by the present invention in the cracking furnace increases heat transfer load maximally by 6620w, significantly increases heat transfer efficiency, and significantly reduces pressure drop, while increasing service life of the heat transfer enhancement pipe due to maximum thermal stress reduction of the heat transfer enhancement pipe being over 50%. 
     In addition, according to another example, a height of the fin  11  gradually increases from one end in at least a part extension of the fin. In the example shown in  FIG. 8 , the height of the fin  11  gradually increases in an extending direction from the inlet  100  to the outlet  101 ; however, it is to be understood that, the height of the fin  11  may also gradually increases in an extending direction from the outlet  101  to the inlet  100 . In addition, the height of the fin  11  may also gradually increases in a direction from both ends to the middle. By providing on the internal wall of pipe body  10  with fin  11  protruding towards the interior of pipe body  10  and by causing the height of the fin  11  to gradually increase in the extending direction from the inlet  100  to the outlet  101 , it thereby enables the heat transfer enhancement pipe to have a good heat transfer effect, while thermal stress of the heat transfer enhancement pipe  1  can be reduced and the ability to resist local over-temperature of the heat transfer enhancement pipe  1  is correspondingly improved, so as to increase service life of the heat transfer enhancement pipe; furthermore, the height of the fin  11  gradually increasing in the extending direction from the inlet  100  to the outlet  101  has a relatively strong turbulent effect on the fluid in pipe body  10  and reduces coking phenomenon. The aforementioned heat transfer enhancement pipe  1  is suitable for heating furnaces and is also suitable for cracking furnaces. Because the height of the fin  11  gradually increases in the extending direction from the inlet  100  to the outlet  101 , the thermal stress of the heat transfer enhancement pipe  1  is reduced and the service life of the heat transfer enhancement pipe  1  is increased. Additionally, it should be noted that the fluid in the heat transfer enhancement pipe  1  is not specifically limited and can be selected according to actual application environment of the heat transfer enhancement pipe  1 . 
     In order to further reduce thermal stress of the heat transfer enhancement pipe  1 , a ratio of the height of the highest part of the fin  11  to the height of the lowest part of the fin  11  is 1.1-1.6:1. For example, the ratio of the height of the highest part of the fin  11  to the height of the lowest part of the fin  11  is 1.2:1, 1.3:1, 1.4:1 or 1.5:1. 
     Effects of the present invention will be further illustrated through Examples and comparative Examples in the following. 
     Example 21 
     Example 21 is the same as Example 11, except that: the height of each fin  11  gradually increases in the extending direction from the inlet  100  to the outlet  101 , the ratio of the height of the highest part of the fin  11  and the height of the lowest part of the fin  11  is 1.4:1. The heat transfer enhancement pipes  1  are used in atmospheric and vacuum heating furnaces. The inner diameter of each heat transfer enhancement pipe  1  is 75 mm, the transition angle of the first transition surface is 60°, and the second transition of the second transition surface is 60°, and the outlet temperature of the heating furnace is 406°. 
     Comparative Example 21 
     Comparative Example 21 is the same as Example 21, except that: the structure of the enhanced heat transfer tube is changed, that is, the heat transfer enhancement pipe of the prior art is arranged, wherein in the pipe body is provided with only one fin that extends spirally in the axial direction of the pipe body and separates the interior of the pipe body into two mutually non-communicating chambers, with the remaining conditions unchanged. 
     Respective test results of the atmospheric and vacuum heating furnaces in the Example 21 and the comparative example 21 after operating under same conditions are shown in Table 2 below. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Test items  
               
            
           
           
               
               
               
            
               
                   
                 Outlet  
                 Maximum  
               
               
                   
                 temperature/  
                 thermal  
               
               
                 No.  
                 ° C.  
                 stress/MPA 
               
               
                   
               
               
                 Example 21  
                 406  
                 32  
               
               
                 Comparative example 21  
                 396  
                 60 
               
               
                   
               
            
           
         
       
     
     It can be known from the above that applying the heat transfer enhancement pipe provided by the present invention in the atmospheric and vacuum heating furnace, makes the atmospheric and vacuum heating furnace to have better heat transfer effect, and makes the heat transfer enhancement pipe to have less thermal stress. 
     According to another example, the outside of the pipe body  10  is provided with a heat insulator  14  at least partially surrounding the external circumference of the pipe body  10 . By providing the outside of the pipe body  10  with heat insulator  14  at least partially surrounding the external circumference of the pipe body  10 , heat transfer between high-temperature gas and the external wall of the pipe body  10  is impeded to reduce temperature of the external wall of the pipe body  10 , thereby reducing temperature difference between the pipe body  10  and the fin  11 , so as to effectively reduce thermal stress of the heat transfer enhancement pipe  1 , extend service life of the heat transfer enhancement pipe  1 , and correspondingly increase the allowable temperature of the heat transfer enhancement pipe  1 . When applying the aforementioned heat transfer enhancement pipe  1  to a cracking furnace, long-term stable operation of the cracking furnace can be ensured. Since the fins  11  are arranged in the interior of the pipe body  10 , the fluid entering into pipe body  10  can turn into a swirling flow; due to its tangential velocity, the fluid can destroy the boundary layer and reduces the rate of coking. It is to be understood that the heat insulator  14  can completely surround the external circumference of the pipe body  10  at the circumference of the pipe body  10 , i.e. at 360° around the external circumference of the pipe body  10 ; the heat insulator  14  can also partially surround the external circumference of the pipe body  10  at the circumference of the pipe body  10 , e.g. at 90° around the external circumference of the pipe body  10 ; of course, the heat insulator  14  can surround the external circumference of the pipe body  10  with a suitable angle according to actual needs; it should be noted that, when applying the aforementioned heat transfer enhancement pipe  1  to a cracking furnace and providing the heat insulator  14  that partially surrounds the external circumference of the pipe body  10  at the outside of the pipe body  10 , it is preferable to provide the heat insulator  14  at a heated surface of the pipe body  10 . In addition, the heat insulator  14  can preferably be arranged at the outside of the pipe body  10  that is provided with the fins, so that the fins are not easily cracked away from pipe body  10 , and service life of the heat transfer enhancement pipe  1  can be increased. 
     As shown in  FIGS. 15-26 , heat insulator  14  can be tubular and is preferably sleeved on the outside of the pipe body  10 , so as to further reduce temperature of the pipe wall of the pipe body  10 , thereby further reducing heat stress of the heat transfer enhancement pipe  1 . As for the shape and structure of the heat insulator  14 , they are not specifically limited: as shown in  FIG. 15 , heat insulator  14  can be cylindrical; or as shown in  FIG. 17 , heat insulator  14  can be elliptical. 
     In addition, the manner in which the heat insulator  14  is disposed is also not specifically limited, as shown in  FIG. 19  and  FIG. 20 , the heat insulator  14  can abut on the external surface of the pipe body  10 ; as shown in  FIG. 22  and  FIG. 23 , heat insulator  14  can also be sleeved on the outside of the pipe body  10 ; and gap  15  can be left between heat insulator  14  and the external wall of the pipe body  10 . By leaving gap  15  between heat insulator  14  and the external wall of the pipe body  10 , temperature of the pipe wall of the pipe body  10  in use is further reduced, thereby further reducing thermal stress of the heat transfer enhancement pipe  1 . 
     In order to further improve structural stability of the heat transfer enhancement pipe  1 , a connector that connects heat insulator  14  and pipe body  10  can be arranged there-between, wherein the structural form of the connector is not specifically limited as long as it can connect heat insulator  14  with pipe body  10 . As shown in  FIG. 23 , the connector can include a first connecting piece  160  that can extend in an axial direction parallel to pipe body  10 ; as shown in  FIG. 21 , the connector can include a second connecting piece  161  that can extend spirally along the external wall of the pipe body  10 ; as shown in  FIG. 15  and  FIG. 17 , the connector can include a connecting rod  162  with both ends thereof connectable to the external wall of the pipe body  10  and the internal wall of the heat insulator  14 , respectively. It is also to be understood that any two or more of the connectors of the above three structures can be optionally arranged between heat insulator  14  and pipe body  10 . Preferably, the connector is prepared and obtained from hard materials such as 35Cr45Ni or from soft materials such as ceramic fiber. 
     As shown in  FIGS. 15, 16, and 18 , heat insulator  14  can include a straight pipe section  140 , and a first tapered pipe section  141  and a second tapered pipe section  142  that are connected to the first end and the second end of straight pipe section  140 , respectively, wherein the first tapered pipe section  141  is tapered in a direction from close to the first end to away from the first end; the second tapered pipe section  142  is tapered in a direction from close to the second end to away from the second end. Heat insulator  14  is arranged as the above structure, so that not only temperature of the pipe wall of the pipe body  10  is effectively decreased, but also temperature variation in the axial direction of the pipe body  10  is relatively uniform, while thermal stress of the heat transfer enhancement pipe  1  is also reduced. 
     Further, the angle formed between the horizontal surface and the external wall surface of the first tapered pipe section  141  is preferably 10-80°; specifically, the angle formed between the horizontal surface and the external wall surface of the first tapered pipe section  141  can be 20°, 30°, 40°, 50°, 60°, or 70°. The angle formed between the horizontal surface and the external wall surface of the second tapered pipe section  142  is preferably 10-80°; similarly, the angle formed between the horizontal surface and the external wall surface of the second tapered pipe section  142  can be 20°, 30°, 40°, 50°, 60°, or 70°. 
     Further, the extension length of the heat insulator  14  in the axial direction of the pipe body  10  is preferably 1-2 times the length of the pipe body  10 . Setting the axial length of the heat insulator  14  within the above range can further decrease temperature of the pipe wall of the pipe body  10  in use and further reduces thermal stress of the pipe body  10 . 
     Effects of the present invention will be further illustrated through examples and comparative Examples in the following. 
     Example 31 
     Example 31 is the same as Example 11, except that: a heat insulator  14  of cylindrical shape is arranged on the outside of the pipe body  10 ; heat insulator  14  completely surrounds the external circumference of the pipe body  10  and leaves gap  15  with the external wall of the pipe body; heat insulator  14  is connected with pipe body  10  through connecting rod  162 . 
     Example 32 
     Example 32 is the same as Example 31 except that: heat insulator  14  is elliptical; the transition angle of the first transition surface is 35°; the transition angle of the second transition surface is 35°. Other conditions remain unchanged. 
     Example 33 
     Example 33 is the same as Example 31 except that: heat insulator  14  is attached to the external wall of the pipe body  10 ; the transition angle of the first transition surface is 40°; the transition angle of the second transition surface is 40°. Other conditions remain unchanged. 
     Comparative Example 31 
     Comparative Example 31 is the same as Comparative Example 11, that is, a heat transfer enhancement pipe of the prior art is arranged, wherein the outside of the pipe body is not provided with a heat insulator; the interior of the pipe body is provided with only one fin  11  that extends spirally in the axial direction of the pipe body and separates the interior of the pipe body into two mutually non-communicating chambers, with the remaining conditions unchanged. 
     Respective test results of the cracking furnaces in the examples and the comparative Example after operating under same conditions are shown in Table 3 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                   
                 Test items  
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Heat  
                   
                 Maximum  
                   
               
               
                   
                   
                 transfer  
                 Pressure  
                 thermal  
                 Service  
               
               
                   
                 No.  
                 load/W  
                 drop/MPa  
                 stress/MPA  
                 life/year 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Example 31  
                 94620  
                 0.10835  
                 40  
                 6-7  
               
               
                   
                 Example 32  
                 94620  
                 0.10835  
                 30  
                 7-8  
               
               
                   
                 Example 33  
                 95650  
                 0.10835  
                 30  
                 7-8  
               
               
                   
                 Comparative  
                 89889  
                 0.12085  
                 110  
                 4-5 
               
               
                   
                 Example 31 
               
               
                   
                   
               
            
           
         
       
     
     It can be known from the above that providing the heat transfer enhancement pipe provided by the invention in the cracking furnace increases heat transfer load, significantly increases heat transfer efficiency, and significantly reduces pressure drop, while reducing maximum thermal stress of the heat transfer enhancement pipe and significantly increasing service life of the heat transfer enhancement pipe. 
     According to another example of the present invention, a heat insulating layer  17  is provided on the external surface of the pipe body  10 . By providing the heat insulating layer  17  on the external surface of the pipe body  10 , heat transfer between high-temperature gas and the pipe wall of the pipe body  10  is impeded to reduce temperature of the pipe wall of the pipe body  10 , thereby reducing temperature difference between the pipe body  10  and the fin  11 , so as to effectively reduce thermal stress of the heat transfer enhancement pipe  1 , extend service life of the heat transfer enhancement pipe  1 , and also improve high temperature resistance performance, thermal shock performance, and high-temperature corrosion resistance performance of the heat transfer enhancement pipe  1  because of the arrangement of the heat insulating layer  17 . When applying the aforementioned heat transfer enhancement pipe  1  to a cracking furnace, long-term stable operation of the cracking furnace can be ensured. In addition, heat insulating layer  17  can preferably be arranged at the outside of the pipe body  10  that is provided with the fins, so that the fins are not easily cracked away from pipe body  10 , and thermal stress of the heat transfer enhancement pipe  1  can be reduced. 
     Preferably, heat insulating layer  17  can include a metal alloy layer  170  arranged on the external surface of the pipe body  10  and a ceramic layer  171  arranged on the metal alloy layer  170 . Through providing metal alloy layer  170  on the external surface of the pipe body  10  and ceramic layer  171  on the metal alloy layer  170 , the heat insulating effect of the heat insulating layer  17  can be improved to further decrease thermal stress of the heat transfer enhancement pipe  1 . 
     It is to be understood that metal alloy layer  170  can be prepared and formed by metal alloy materials including M, Cr, Al, and Y, wherein M is selected from one or more of Fe, Ni, Co, and Al; when M is selected from two or more metals therein, such as Ni and Co, metal alloy layer  170  can be prepared and formed by metal alloy materials including Ni, Co, Cr, Al, and Y; when metal alloy layer  170  contains Ni and Co, heat insulating ability of the heat insulating layer  17  can be further improved, and oxidation resistance and hot corrosion resistance of the heat insulating layer  17  are improved. As for the content of each metal in the metal alloy materials, it can be configured according to actual needs with no particular requirement. For example, the weight fraction of Al can be 5-12%, and the weight fraction of Y can be 0.5-0.8%, so that the robustness of the heat insulating layer  17  can be improved, while reducing oxidation rate of metal alloy layer  170 ; the weight fraction of Cr can be 25-35%. In addition, it should also be noted that the metal alloy materials can be sprayed on the external surface of the pipe body  10  to form metal alloy layer  170  by employing low pressure plasma, atmospheric plasma, or electron-beam physical vapor deposition. Thickness of metal alloy layer  170  can be 50 to 100 μm; specifically, thickness of metal alloy layer  170  can be 60 μm, 70 μm, 80 μm, or 90 μm. 
     In order to further improve oxidation resistance of the heat insulating layer  17  and extend service life of the heat insulating layer  17 , additive materials can be added to the metal alloy materials for preparing metal alloy layer  170 , that is, metal alloy layer  170  can be prepared and formed after mixing the metal alloy materials with the additive materials, wherein the metal alloy materials include M, Cr, Al, and Y, wherein M is selected from one or more of Fe, Ni, Co, and Al; the additive materials are selected from Si, Ti, Co, or Al 2 O 3 ; as for the amount of addition of the additive materials, it can be added according to actual needs with no particular limitations, wherein the metal alloy materials have already been described in the above, and will not be described in details herein again. 
     In addition, ceramic layer  171  can be prepared and formed by one or more materials from yttria-stabilized zirconia, magnesia-stabilized zirconia, calcia-stabilized zirconia, and ceria-stabilized zirconia. When ceramic layer  171  is formed by two or more materials from the above, any two or more of the above materials can be mixed and then form into ceramic layer  171  after mixing. Specifically, when selecting yttria-stabilized zirconia as the material for ceramic layer  171 , ceramic layer  171  can have a relatively high thermal expansion system, for example, it can reach up to 11×10 −6  K −1 ; ceramic layer  171  can also have a relatively low thermal conductivity coefficient of 2.0-2.1Wm −1 K −1 ; while ceramic layer  171  also has good thermal shock resistance. It should also be noted that when selecting yttria-stabilized zirconia as ceramic layer  171 , the weight fraction of yttrium oxide is 6-8%. In order to further improve heat insulating performance of the heat insulating layer  17 , cerium oxide can also be added to the above materials forming ceramic layer  171 ; specifically, the amount of addition of cerium oxide can be 20-30% of the total weight of yttria-stabilized zirconia; further, the amount of addition of cerium oxide can be 25% of the total weight of yttria-stabilized zirconia. Similarly, one or more materials of yttria-stabilized zirconia, magnesia-stabilized zirconia, calcia-stabilized zirconia, and ceria-stabilized zirconia can be sprayed onto the external surface of metal alloy surface  170  to form ceramic layer  171  by employing methods of low pressure plasma, atmospheric plasma, or electron-beam physical vapor deposition. In addition, the thickness of ceramic layer  171  can be 200-300 μm; for example, the thickness of ceramic layer  171  can be 210 μm, 220 μm, 230 μm, 240 μm, 250 μm, 260 μm, 270 μm, 280 μm, or 290 μm. It should be noted that when the heat transfer enhancement pipe  1  is in use, the Al in metal alloy layer  170  reacts with the oxygen in ceramic layer  171  to form a thin and dense aluminum-oxide protective film, thereby protecting pipe body  10 . 
     In order to improve peeling resistance of the heat insulating layer  17 , an oxide layer  172  can be arranged between metal alloy layer  170  and ceramic layer  171 , wherein oxide layer  172  is preferably prepared and formed by alumina, silica, titania, or a mixture of any two or more materials from alumina, silica, and titania. Preferably, alumina is selected for preparing and forming oxide layer  172  to improve heat insulating performance of the heat insulating layer  17 . Similarly, the above oxide materials can be sprayed onto the surface of metal alloy layer  170  to form oxide layer  172  by employing methods of low pressure plasma, atmospheric plasma, or electron-beam physical vapor deposition. In addition, the thickness of oxide layer  172  can be 3-5 μm; for example, the thickness of oxide layer  172  can be 4 μm. 
     Additionally, the porosity of the heat insulating layer  17  can be 8 to 15%. 
     In order to effectively reduce temperature of the pipe wall of the pipe body  10  and to make temperature variation in the axial direction of the pipe body  10  relatively uniform while also to reduce thermal stress of the heat transfer enhancement pipe  1 , heat insulation layer  17  can include a straight section, and a first tapered section and a second tapered section that are connected to the first end and the second end of the straight section, respectively, wherein the first tapered section is tapered in a direction from close to the first end to away from the first end; the second tapered section is tapered in a direction from close to the second end to away from the second end. It is to be understood that the thickness of the heat insulating layer  17  is thinner near the ends; the thickness of the heat insulating layer  17  can gradually decrease by a value of 5-10%. In order to further reduce thermal stress of the heat transfer enhancement pipe  1 , heat insulating layer  17  is thicker at positions corresponding to the fins. 
     Effects of the present invention will be further illustrated through Examples and comparative Examples in the following. 
     Example 41 
     Example 41 is the same as Example 11, except that: the heat insulating layer  17  is disposed on the external surface of the pipe body  10 , the heat insulating layer  17  includes a 70 μm thick metal alloy layer  170 , a 4 μm thick oxide layer  172 , and a 240 μm thick ceramic layer  171  sequentially arranged at the external surface of the pipe body  10 ; wherein the metal alloy layer  170  is spray-formed from metal alloy materials having weight fraction of 64.5% Ni, 30% Cr, 5% Al, and 0.5% Y via atmospheric plasma spray method; the oxide layer  172  is formed by spraying aluminum oxide to the surface of metal alloy layer  170  by a selected method of low pressure plasma spray; the ceramic layer  171  is formed by spraying yttria-stabilized zirconia mixed with cerium oxide of 25% weight fraction of the yttria-stabilized zirconia; in the yttria-stabilized zirconia, the weight fraction of cerium oxide is 6%, the transition angle of the first transition surface is 35°; the transition angle of the second transition surface is 35°; the cross section of each fin  11 , i.e. the cross section taken from a surface in the radial direction parallel to pipe body  10 , is substantially trapezoidal; the angle formed by each side wall face  112  and the internal wall of the pipe body  10  is 45°. 
     Example 42 
     Example 42 is the same as Example 41, except that: in heat insulating layer  17 , metal alloy layer  170  is prepared and formed by metal alloy materials having weight fraction of 64.2% Ni, 30% Cr, 5% Al, and 0.8% Y, respectively; ceramic layer  171  is formed by yttria-stabilized zirconia; in the yttria-stabilized zirconia, the weight fraction of yttrium oxide is 8%. Other conditions remain unchanged. 
     Comparative Example 41 
     Comparative Example 41 is the same as Comparative Example 11, i.e.: the heat transfer enhancement pipe of the prior art is arranged (the external surface of the pipe body is not provided with heat insulating layer), wherein the outside of the pipe body is not provided with heat insulating layer; the interior of the pipe body is provided with only one fin that extends spirally in the axial direction of the pipe body and separates the interior of the pipe body into two mutually non-communicating chambers, with the remaining conditions unchanged. 
     Respective test results of the cracking furnaces in the Examples and the comparative Example after operating under same conditions are shown in Table 4 below. 
     
       
         
           
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 Test items  
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Temperature  
                   
                   
               
               
                   
                   
                   
                 difference  
                   
                   
               
               
                   
                   
                   
                 between the  
                   
                   
               
               
                   
                   
                   
                 fin and the  
                 Maximum  
                   
               
               
                   
                 Heat  
                 Pressure  
                 pipe wall  
                 thermal  
                   
               
               
                   
                 transfer  
                 drop/  
                 of the pipe  
                 stress/  
                 Service  
               
               
                 No.  
                 load/W  
                 MPa  
                 body/° C.  
                 MPA  
                 life/year 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 41  
                 94700  
                 0.10780  
                 20-25  
                 40  
                 6-7  
               
               
                 Example 42  
                 94620  
                 0.10820  
                 20-25  
                 40  
                 6-7  
               
               
                 Comparative  
                 88080  
                 0.12090  
                 35-40  
                 110  
                 4-5 
               
               
                 Example 41 
               
               
                   
               
            
           
         
       
     
     It can be known from the above that providing the heat transfer enhancement pipe provided by the invention in the cracking furnace increases heat transfer load, significantly increases heat transfer efficiency, and significantly reduces pressure drop, while reducing maximum thermal stress of the heat transfer enhancement pipe and significantly increasing service life of the heat transfer enhancement pipe. 
     Preferred embodiments of the present invention have been described in detail above in association with the drawings; however, the present invention is not limited thereto. Various simple alterations of the technology of the present invention including combinations of each specific technological feature in any suitable ways can be made in the scope of the technology contemplated in the present invention. To avoid unnecessary repetitions, the present invention will not illustrate further on various possible combinations. However, these simple alterations and combinations should be regarded as contents disclosed by the present invention and fall into the scope protected by the present invention.