Heat transfer tube and cracking furnace using the same

Abstract

A heat transfer tube includes a twisted baffle arranged in an inner wall of the tube. The twisted baffle extends spirally along an axial direction of the heat transfer tube. The twisted baffle is provided with a non-through gap extending along an axial direction of the heat transfer tube from an end to the other end of the twisted baffle. A cracking furnace uses the heat transfer tube. The heat transfer tube and cracking furnace have good heat transfer effects and small pressure loss.

Claims

1. A heat transfer tube comprising a twisted baffle arranged on an inner wall of the tube, said twisted baffle extending spirally along an axial direction of the heat transfer tube and being provided with a non-through gap extending from one end to the other end of the twisted baffle along an axial direction of the heat transfer tube without penetrating the twisted baffle in the axial direction; wherein the non-through gap has a contour line of a smooth curve, the smooth curve comprises two identical curve segments, which are centrosymmetric with respect to an axial centerline of the heat transfer tube; wherein the contour line is unclosed U-shaped and the non-through gap is not enclosed on all sides by material nor connected with an ear.

2. The heat transfer tube according to claim 1, characterized in that there are two gaps, which extend from different ends of the twisted baffle towards each other along the axial direction of the heat transfer tube without intersection.

3. The heat transfer tube according to claim 2, characterized in that the area ratio of an upstream gap to a downstream gap is in a range from 20:1 to 0.05:1.

4. The heat transfer tube according to claim 1, characterized in that the twisted baffle is further provided with a plurality of holes.

5. The heat transfer tube according to claim 4, characterized in that the ratio of an axial distance between centerlines of two adjacent holes to an axial length of the twisted baffle ranges from 0.2:1 to 0.8:1.

6. The heat transfer tube according to claim 3, characterized in that the area ratio of an upstream gap to a downstream gap is in a range from 2:1 to 0.5:1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) In the following, the present disclosure will be described in detail in view of specific embodiments and with reference to the drawings, wherein,

(2) FIG. 1 schematically shows a side view of a heat transfer tube with a twisted baffle according to the present disclosure;

(3) FIGS. 2 and 3 schematically show perspective views of a first embodiment of the twisted baffle according to the present disclosure;

(4) FIGS. 4 to 6 schematically show cross-section views of A-A, B-B and C-C of FIG. 1 using the twisted baffle of FIG. 2:

(5) FIGS. 7 and 8 schematically show a perspective view of a second embodiment of the twisted baffle according to the present disclosure;

(6) FIG. 9 schematically shows a perspective view of a third embodiment of the twisted baffle according to the present disclosure;

(7) FIG. 10 schematically shows a perspective view of a prior art twisted baffle; and

(8) FIG. 11 schematically shows a radiant coil of a cracking furnace using the heat transfer tube according to the present disclosure.

(9) In the drawings, the same component is referred to with the same reference sign. The drawings are not drawn in accordance with an actual scale.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) The present disclosure will be further illustrated in the following in view of the drawings.

(11) FIG. 1 schematically shows a side view of a heat transfer tube 10 according to the present disclosure. The heat transfer tube 10 is provided with a twisted baffle 11 introducing a fluid to flow rotatably. The twisted baffle 11 extends spirally along an axial direction of the heat transfer tube 10. The structure of the twisted baffle 11 is schematically shown in FIGS. 2, 3, 7, 8 and 9 and will be explained in the following.

(12) FIGS. 2 and 3 schematically show perspective views of a first embodiment of the twisted baffle 11 according to the present disclosure. The twisted baffle 11 has a twist angle between 90° and 1080°. The ratio of the axial length of the twisted baffle to an inner diameter of the heat transfer tube falls in a range from 1:1 to 10:1. The twisted baffle 11 is arranged with a gap 12, which extends along an axial direction of the heat transfer tube 10 from an upstream end to a downstream end of the twisted baffle 11 without completely penetrating the twisted baffle 11. Generally, the gap 12 can be understood as having a U shape. Under this condition, the area ratio of the gap 12 to the twisted baffle 11 ranges from 0.05:1 to 0.95:1.

(13) The axial length of the twisted baffle 11 can be called as a “pitch”, and the ratio of the “pitch” to the inner diameter of the heat transfer tube can be called a “twist ratio”. The twist angle and twist ratio would both influence the rotation degree of the fluid in the heat transfer tube 10. When the twist ratio is determined, the larger the twist angle is, the higher the tangential speed of the fluid will be, but the pressure drop of the fluid would also be correspondingly higher. The twisted baffle 11 is selected as with a twist ratio and twist angle which can enable the fluid in the heat transfer tube 10 to possess a sufficiently high tangential speed to destroy the boundary layer, so that a good heat transfer effect can be achieved. In this case, a smaller tendency for coke to be formed on the inner wall of the heat transfer tube can be resulted and the pressure drop of the fluid can be controlled as within an acceptable scope. By arranging the gap 12 on the twisted baffle 11, the contact area of the fluid with the twisted baffle 11 is significantly reduced, thus reducing the resistance of the fluid in the heat transfer tube 10 and the pressure drop of the fluid. In addition, the gap 12 is non-through, i.e., the twisted baffle is actually an integral piece with two side edges thereof both connecting to the heat transfer tube 10, which improves stability of the twisted baffle 11 in the heat transfer tube 10.

(14) FIGS. 2 and 3 show a contour line of the gap 12 of the twisted baffle 11 as a smooth curve, which can reduce the resistance of the fluid, thus reducing the pressure drop of the fluid. The smooth curve can be understood as comprising two identical curve segments 13 and 13′, which are centrosymmetric with respect to a centerline of the heat transfer tube 10. With this understanding, the gap 12 possesses the following technical features. The ratio of the width of an starting end of the gap 12 to the inner diameter of the heat transfer tube 10 is in a range from 0.05:1 to 0.95:1 with the curve segment 13 (which is taken as an example for the explanation) extending from a starting end 14 towards a tail end 15 of the gap 12. The ratio of the x-axis component of the curvature radius change rate of the curve segment to the inner diameter of the heat transfer tube ranges from 0.05:1 to 0.95:1; the ratio of the y-axis component of the curvature radius change rate of the curve segment to the inner diameter of the heat transfer tube ranges from 0.05:1 to 0.95:1; and the ratio of the z-axis component of the curvature radius change rate of the curve segment to the inner diameter of the heat transfer tube ranges from 1:1 to 10:1. In the present disclosure, the terms “x-axis”, “y-axis” and “z-axis” respectively refer to a diameter direction of the heat transfer tube 10, the direction perpendicular to the drawing sheet and the axial direction of the heat transfer tube 10. The gap 12 in this form possesses the best hydrodynamic effect, i.e., the gap 12 of this form generates the smallest fluid pressure drop and the highest resistance to impact of the twisted baffle 11.

(15) As a matter of fact, the twisted baffle 11 indicated in FIG. 2 or 3 can be understood as a trajectory surface which is achieved through rotating one diameter line of the heat transfer tube 10 around a midpoint thereof and at the same time translating it along the axial direction of the heat transfer tube 10 upwardly or downwardly followed by intersecting a spheroid or the like with the trajectory surface and removing the intersected portion. In this way, the twisted baffle 11 comprises a top edge and a bottom edge parallel to each other, a pair of twisted side edges which always contact with the inner wall of the heat transfer tube 10 and the contour line of the gap. FIGS. 4 to 6 schematically show different cross-sections of the heat transfer tube 10 at different positions, from which the twisting manner of the twisted baffle 11 can be seen. The cross section of the gap 12 as indicated in FIG. 4 is larger than that indicated in FIG. 5, because the cross-section A-A is closer to a minor axis of the spheroid which forms the gap 12. The twisted baffle as indicated in FIG. 6 possesses no gaps because the cross-section C-C is arranged at a portion of the twisted baffle 11 not being penetrated by the gap 12.

(16) Although FIG. 2 indicates that the gap 12 of the twisted baffle 11 is arranged as with an opening facing upstream and a top end facing downstream, the gap 12 can actually also be arranged as with the top end facing upstream and the opening facing downstream. Under this condition, the impact force from the fluid to the twisted baffle 11 would be significantly reduced, so that the resistance to impact of the twisted baffle 11 would be improved.

(17) FIGS. 7 and 8 schematically show a second embodiment of the twisted baffle 11. This embodiment is similar with the twisted baffle 11 as indicated in FIGS. 2 and 3. The difference therebetween lies only in that the twisted baffle 11 is provided with two gaps 12 and 12′, which extend respectively from an upstream end and a downstream end of the twisted baffle 11 towards each other, but still spaced from each other. The downstream gap 12′ can further reduce the resistance of the fluid so as to reduce pressure drop thereof. In addition, the arrangement of the upstream and downstream gaps is beneficial for lowering the weight of the twisted baffle 11, facilitating arrangement and use of the heat transfer tube 10. Preferably, the area ratio of the upstream gap 12 to the downstream gap 12′ ranges from 2:1 to 0.5:1. In this case, the ratio of the sum area of the gaps 12 and 12′ to the area of the twisted baffle 11 falls within a range from 0.05:1 to 0.95:1.

(18) FIG. 9 schematically indicates a third embodiment of the twisted baffle 11. In this embodiment, the twisted baffle 11 is provided with a hole 41, so that the fluid can pass through the hole 41 and smoothly flow downstream, thus further reducing the pressure loss of the fluid. In one specific embodiment, the ratio of an axial distance between two adjacent centerlines to an axial length of the twisted baffle 11 ranges from 0.2:1 to 0.8:1.

(19) The present disclosure further relates to a cracking furnace (not shown in the drawings) using the heat transfer tube 10 as mentioned above. A cracking furnace is well known to one skilled in the art and therefore will not be discussed here. A radiant coil 50 of the cracking furnace is provided with at least one heat transfer tube 10 as described above. FIG. 11 schematically indicates three heat transfer tubes 10. Preferably, these heat transfer tubes 10 are provided along the axial direction in the radiant coil in a manner of being spaced from each other. For example, the ratio of an axial distance of two adjacent heat transfer tubes 10 to the inner diameter of the heat transfer tube 10 is in a range from 15:1 to 75:1, preferably from 25:1 to 50:1, so that the fluid in the radiant coil would continuously turn from a piston flow to a rotating flow, thus improving the heat transfer efficiency. It should be noted that when there are a plurality of heat transfer tubes, the twisted baffle of each of these heat transfer tubes 10 can be arranged in a manner as shown in any one of FIGS. 2, 7 and 9.

(20) In the following, specific example will be used to explain the heat transfer efficiency and pressure drop of the radiant coil 50 of the cracking furnace when the heat transfer tube 10 according to the present disclosure is used.

EXAMPLE 1

(21) The radiant coil of the cracking furnace is arranged with 6 heat transfer tubes 10 with twisted baffles as indicated in FIG. 2. The inner diameter of each of the heat transfer tubes 10 is 51 mm. The ratio of the x-axis component of the curvature radius change rate of the curve segment to the inner diameter of the heat transfer tube is 0.6:1; the ratio of the y-axis component of the curvature radius change rate of the curve segment to the inner diameter of the heat transfer tube is 0.6:1; and the ratio of the z-axis component of curvature radius change rate of the curve segment to the inner diameter of the heat transfer tube is 2:1. The twisted baffles 11 and 11′ respectively have a twist angle of 180° and a twist ratio of 2.5. The distance between two adjacent heat transfer tubes 10 is 50 times as large as the inner diameter of the heat transfer tube. Experiments have found that the heat transfer load of the radiant coil is 1,278.75 KW and the pressure drop is 70,916.4 Pa.

COMPARATIVE EXAMPLE 1

(22) The radiant coil of the cracking furnace is mounted with 6 prior art heat transfer tubes 50′. The heat transfer tube 50′ is structured as being provided with a twisted baffle 51′ in a casing of the heat transfer tube 50′, the twisted baffle 51′ dividing the heat transfer tube 50′ into two material passages non-communicating with each other as indicated in FIG. 10. The inner diameter of the heat transfer tube 50′ is 51 mm. The twisted baffle 51′ has a twist angle of 180° and a twist ratio of 2.5. The distance between two adjacent heat transfer tubes 50′ is 50 times as large as the inner diameter of the heat transfer tube 50′. Experiments have found that the heat transfer load of the radiant coil is 1,264.08 KW and the pressure drop is 71,140 Pa.

(23) In view of the above example and comparative example, it can be derived that compared with the heat transfer efficiency of the radiant coil in the cracking furnace using the prior art heat transfer tube, the heat transfer efficiency of the radiant coil in the cracking furnace using the heat transfer tube according to the present disclosure is significantly improved, and the pressure drop is also decreased. The above features are very beneficial for hydrocarbon cracking reaction.

(24) Although this disclosure has been discussed with reference to preferable examples, it extends beyond the specifically disclosed examples to other alternative examples and/or use of the disclosure and obvious modifications and equivalents thereof. Particularly, as long as there are no structural conflicts, the technical features disclosed in each and every example of the present disclosure can be combined with one another in any way. The scope of the present disclosure herein disclosed should not be limited by the particular disclosed examples as described above, but encompasses any and all technical solutions following within the scope of the following claims.