This application claims benefit of priority under 35 U.S.C. §119 to Chinese Patent Application No. CN 201210426112.4, filed Oct. 30, 2012, the contents of which are also incorporated herein by references.
The present disclosure relates to a heat transfer tube which is especially suitable for a heating furnace. The present disclosure further relates to a cracking furnace using the heat transfer tube.
Cracking furnaces, the primary equipment in the petrochemical industry, are mainly used for heating hydrocarbon material so as to achieve cracking reaction which requires a large amount of heat. Fourier's theorem says,
wherein q is the heat transferred, A represents the heat transfer area, k stands for the heat transfer coefficient, and dt/dy is the temperature gradient. Taking a cracking furnace used in the petrochemical industry as an example, when the heat transfer area A (which is determined by the capacity of the cracking furnace) and the temperature gradient dt/dy are determined, the only way to improve the heat transferred per unit area q/A is to improve the value of the heat transfer coefficient k, which is subject to influences from thermal resistance of the main fluid, thermal resistance of the boundary layer, etc.
In accordance with Prandtl's boundary layer theory, when an actual fluid flows along a solid wall, an extremely thin layer of fluid close to the wall surface would be attached to the wall without slippage. That is to say, the speed of the fluid attached to the wall surface, which forms a boundary layer, is zero. Although this boundary layer is very thin, the heat resistance thereof is unusually large. When heat passes through the boundary layer, it can be rapidly transferred to the main fluid. Therefore, if the boundary layer can be somehow thinned, the heat transferred would be effectively increased.
In the prior art, the furnace pipe of a commonly used cracking furnace in the petrochemical industry is usually structured as follows. On the one hand, a rib is provided on the inner surface of one or more or all of the regions from the inlet end to the outlet end along the axial direction of the furnace coil in the cracking furnace, and extends spirally on the inner surface of the furnace coil along an axial direction thereof. Although the rib can achieve the purpose of agitating the fluid so as to minimize the thickness of the boundary layer, the coke formed on the inner surface thereof would continuously weaken the role of the rib as time lapses, so that the function of reducing the boundary layer thereof will become smaller. On the other hand, a plurality of fins spaced from one another are provided on the inner surface of the furnace pipe. These fins can also reduce the thickness of the boundary layer. However, as the coke on the inner surface of the furnace pipe is increased, these fins will similarly get less effective.
Therefore, it is important in this technical field to enhance heat transfer elements so as to further improve heat transfer effect of the furnace coil.
To solve the above technical problem in the prior at, the present disclosure provides a heat transfer tube, which possesses good transfer effects. The present disclosure further relates to a cracking furnace using the heat transfer tube.
According to a first aspect of the present disclosure, it discloses 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.
In the heat transfer tube according to the present disclosure, under the action of the twisted baffle, fluid flows along the twisted baffle and turns into a rotating flow. A tangential speed of the fluid destroys the boundary layer so as to achieve the purpose of enhancing heat transfer.
In one embodiment, the twisted baffle is provided with a plurality of holes. Both axial and radial flowing fluids can flow through the holes, i.e., these holes can alter the flow directions of the fluids, so as to enhance turbulence in the heat transfer tube, thus destroying the boundary layer and achieving the purpose of enhancing heat transfer. In addition, fluids from different directions can all conveniently pass through these holes and flow downstream, thereby further reducing resistance to flow of the fluids and reducing pressure loss. Coke pieces carried in the fluids can also pass through these holes to move downstream, which facilitates the discharge of the coke pieces.
In a preferred embodiment, the ratio of the sum area of the plurality of holes to the area of the twisted baffle is in a range from 0.05:1 to 0.95:1. When the ratio is of a small value in the above range, the heat transfer tube is of high capacity, but the pressure drop of the fluid is great. As the value of the ratio turns greater, the heat transfer tube would be of lower capacity, but the pressure drop of the fluid grows smaller accordingly. When the ratio ranges from 0.6:1 to 0.8:1, the capacity of the heat transfer tube and the pressure drop of the fluid both fall within a proper scope. The ratio of an axial distance between the centerlines of two adjacent holes to an axial length of the twisted baffle ranges from 0.2:1 to 0.8:1.
In one embodiment, the twisted baffle has a twist angle of between 90° to 1080°. When the twist angle is relatively small, the pressure of the fluid and the tangential speed of the rotating fluid are both small. Therefore, the heat transfer tube is of poor effect. As the twist angle turns larger, the tangential speed of the rotating flow would increase, so that the effect of the heat transfer tube would be improved, but the pressure drop of the fluid will be increased. When the twist angle ranges from 120°-360°, the capacity of the heat transfer tube and the pressure drop of the fluid both fall within a proper range. One single region of the heat transfer tube can be provided with a plurality of twisted baffles parallel to one another, which define an enclosed circle viewed from one end of the heat transfer tube. In a preferred embodiment, the diameter ratio of the circle to the heat transfer tube falls within a range from 0.05:1 to 0.95:1. When this ratio is relatively small, the heat transfer tube is of high capacity but the pressure drop of the fluid is great. As the value of the ratio gradually increases, the capacity of the heat transfer tube would be decreased, but the pressure drop of the fluid would accordingly turn small. When this ratio ranges from 0.6:1 to 0.8:1, both the capacity of the heat transfer tube and the pressure drop of the fluid would fall within respective proper scopes. This arrangement renders that only the portion closed to the heat transfer tube wall is provided with a twisted baffle while the central portion of the heat transfer tube actually forms a channel. In this way, when the fluid flows through the heat transfer tube, part of the fluid can directly flows out of the tube through the channel, so that not only a better heat transfer effect can be achieved but the pressure loss is also small. Moreover, the channel also enables the coke pieces to be quickly discharged therefrom.
In a preferred embodiment, the ratio of the axial length of the twisted baffle to an inner diameter of the heat transfer tube is a range from 1:1 to 10:1. When this ratio is relatively small, the tangential speed of the rotating flow is relatively great, so that the heat transfer tube is of high capacity but the pressure drop of the fluid is relatively great. As the value of the ratio gradually increases, the tangential speed of the rotating flow would turn smaller, and thus the capacity of the heat transfer tube would be decreased, but the pressure drop of the fluid would turn smaller. When this ratio ranges from 2:1 to 4:1, both the capacity of the heat transfer tube and the pressure drop of the fluid would fall within respective proper scopes. The twisted baffle of such size further enables the fluid in the heat transfer tube with a tangential speed sufficient enough to destroy the boundary layer, so that a better heat transfer effect can be achieved and there would be a smaller tendency for coke to be formed on the heat transfer wall.
In one embodiment, along the trajectory of the circle a casing is arranged and fixedly connected to a radial inner end of the twisted baffle. With the arrangement of the casing, the rotating flow of the fluid would not be affected by the flow inside the casing, which further improves the tangential speed of the fluid, enhances the heat transfer and reduces coke on the heat transfer all. Furthermore, the casing also improves the strength of the twisted baffle. For example, the casing can effectively support the twisted baffle, thus enhancing the stability and impact resistance thereof.
According to a second aspect of the present disclosure, it discloses a cracking furnace, a radiant coil of which comprises at least one, preferably 2 to 10 heat transfer tubes according to the first aspect of the present disclosure.
In one embodiment, the plurality of heat transfer tubes are arranged in the radiant coil along an axial direction thereof in a manner of being spaced from each other. The ratio of the spacing distance to the diameter of the heat transfer tube is in a range from 15:1 to 75:1, preferably from 25:1 to 50:1. The plurality of heat transfer tubes spaced from each other can continuously change the fluid in the radiant coil from piston flow into rotating flow, thus improving the heat transfer efficiency.
In the context of the present disclosure, the term “piston flow” ideally means that fluids mix with each other in the flow direction but by no means in the radial direction. Practically however, only approximate piston flow rather than absolute piston flow can be achieved.
Compared with the prior art, the present disclosure excels in the following aspects. To begin with, the arrangement of the twisted baffle in the heat transfer tube turns the fluid flowing along the twisted baffle into a rotating fluid, thus improving the tangential speed of the fluid, destroying the boundary layer and achieving the purpose of enhancing heat transfer. Next, the plurality of holes provided on the twisted baffle can change the flow direction of the fluid so as to strengthen the turbulence in the heat transfer tube and achieve the object of enhancing heat transfer. Besides, these holes further reduce the resistance in the flow of the fluid, so that pressure loss is further decreased. Moreover, coke pieces carried in the fluid can also move downstream through these holes, which promotes the discharge of the coke pieces. When one single region of the heat transfer tube is provided with a plurality of twisted baffles parallel to one another, which define an enclosed circle viewed from one end of the heat transfer tube, a central portion of the heat transfer tube actually forms a channel, which can lower pressure loss and is favorable for rapid discharge of the coke pieces. Furthermore, along the trajectory of the circle a casing is arranged. Therefore, the casing, twisted baffle and inner wall of the heat transfer tube form a spiral cavity together, wherein the fluid is turned into a complete rotating flow, which further improves the tangential speed of the fluid, thus further enhancing the heat transfer and reducing formation of coke on the wall of the heat transfer tube. In addition, the casing can support the twisted baffle, thereby improving the stability and impact resistance of the twisted baffle.
In the following, the present disclosure will be described in detail in view of specific embodiments and with reference to the drawings, wherein,
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.
The present disclosure will be further illustrated in the following in view of the drawings.
The twisted baffles not defining the vertical passage 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. In contrast, the twisted baffles defining the vertical passage can be formed through removing from a cylinder coaxial with the heat transfer tube 10 a central portion of the twisted baffles not defining the vertical passage, by means of which two identical parallel twisted baffles as shown in
An embodiment of the twisted baffle as indicated in
Since the twisted baffles 11 and 11′ extend spirally, the fluid would turn from a piston flow into a rotating flow under the guidance of the twisted baffles 11 and 11′. With a tangential speed, the fluid would destroy the boundary layer so as to enhance heat transfer. Moreover, there would be a smaller tendency for coke to be formed on the inner wall of the heat transfer tube 10 in view of the tangential speed of the fluid. Further, besides improving the heat transfer effect, the channel defined by the twisted baffles 11 and 11′ (i.e., the vertical passage as mentioned above or the circle 12 as indicated in
It should also be understood that although the twisted baffles 11 and 11′ in the embodiment as indicated in
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.
In the following, specific examples will be used to explain the heat transfer efficiency and pressure drop of the radiant coil of the cracking furnace when the heat transfer tube 10 according to the present disclosure is used.
The radiant coil of the cracking furnace is arranged with 6 heat transfer tubes 10 as indicated in
The radiant coil of the cracking furnace is arranged with 6 heat transfer tubes 10 as indicated in
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
In view of the above examples and comparative example, it can be derived that compared with the heat transfer efficiency of the radiant coil n 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. The heat transfer load of the radiant coil is improved to as high as 1,270.13 KW and the pressure drop is also well controlled to be as low as 6,573.8 Pa. The above features are very beneficial for hydrocarbon cracking reaction.
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.
Number | Date | Country | Kind |
---|---|---|---|
2012 1 0426112 | Oct 2012 | CN | national |
Number | Name | Date | Kind |
---|---|---|---|
1056373 | Segelken | Mar 1913 | A |
4455154 | Blasiole | Jun 1984 | A |
5605400 | Kojima | Feb 1997 | A |
20020007941 | Zhu et al. | Jan 2002 | A1 |
20100147672 | Wang et al. | Jun 2010 | A1 |
20110094720 | Wang et al. | Apr 2011 | A1 |
20120203049 | McCarthy et al. | Aug 2012 | A1 |
Number | Date | Country |
---|---|---|
2 101 210 | Apr 1992 | CN |
2101210 | Apr 1992 | CN |
100365368 | Jan 2006 | CN |
103061887 | Apr 2013 | CN |
2430584 | Jan 1976 | DE |
2 133 644 | Dec 2009 | EP |
8-68526 | Mar 1986 | JP |
62-268994 | Nov 1987 | JP |
1-318865 | Dec 1989 | JP |
6-34231 | Feb 1994 | JP |
2009-186063 | Aug 2009 | JP |
Entry |
---|
Search Report in United Kingdom Application No. GB1319549.0, date of search May 18, 2014. |
Search Report in Singapore Application No. 2013080528, dated Sep. 26, 2014. |
Search Report and Written Opinion from Belgian Patent Office, Application No. 201300735, issued May 20, 2015. |
Number | Date | Country | |
---|---|---|---|
20140127091 A1 | May 2014 | US |