Wang Fenglin Wang Fei Qu Yinglin Jiang Hongmei

(Beijing Guangsha Huaneng Technology Co., Ltd. 100083)


Abstract: The origin and development of the spiral baffle heat exchanger, the shell structure and flow principle are introduced. The research status of heat transfer and drag reduction is summarized. The calculation method of related dimensions in structural design is given. . At present, the research on spiral baffle heat exchanger is still in its infancy, solving the problem of baffle processing and tube bundle assembly, improving production efficiency, and speeding up the popularization and application of spiral baffle heat exchanger are urgent problems to be solved.

Keywords: spiral baffle heat exchanger size algorithm introduction

The appearance of shell-and-tube heat exchangers has been around for hundreds of years and is one of the most widely used unit equipment, accounting for more than 50% of the total heat exchanger output in China [1]. The common shell-and-tube heat exchanger adopts a bow-shaped baffle [2]. The baffle plate is arranged vertically with the heat exchange tube, and there is a flow dead zone, and the heat exchange efficiency is difficult to be improved; the fluid sweeps across the heat exchange tube laterally, and the tube bundle is prone to vibration. The connection between the heat exchange tube and the tube sheet is loose or the heat exchange tube is broken, resulting in failure of the heat exchanger, and the vibration problem in the large diameter heat exchanger is more serious. In the 1960s, some scholars proposed the structural type of spiral baffles, but due to the difficulty in manufacturing, industrial production has not been realized. In 1986, Czech scientists obtained patents for spiral baffles. In 1994, ABB realized industrial manufacturing [3] and was applied in Western countries such as Europe and the United States. In the late 1990s, some units in China obtained patented technology and obtained application in production [4].

1 Structure and analysis of spiral baffle heat exchanger

1. 1 structure of spiral baffle

The structural type of the spiral baffle heat exchanger is derived from the auger that transports the material. The original spiral baffle is a continuous spiral structure. Due to the difficulty in manufacturing, the current spiral baffle heat exchangers adopt an intermittent approximate spiral structure, that is, a sector shape with a plurality of quarter-shell cross-sections. The plates are assembled in a spiral shape, and each baffle is obliquely intersected with a heat exchange tube passing therethrough. In the spiral baffle heat exchanger, the medium is spirally propelled from the inlet of the shell to the outlet, and the centrifugal force generated thereby increases the turbulence of the fluid and avoids the pressure loss caused by large angle reentry. The currently used spiral baffles have a helix angle α and a back rake angle β, and the structure thereof is as shown in Fig. 1.



The flow of fluid in the shell side is not only related to the helix angle, but also to the specific dimensions of the spiral plate. The angle of the helix angle can be changed to adjust the flow area of ​​the shell side. A certain amount of overlap can be used to reduce the unsupported span of the heat exchange tube to increase the rigidity. The double helix or double shell structure can be used to adjust the fluid flow of the shell side. It is clear that the influence of the size of the spiral baffle on heat transfer and resistance performance is an important issue in the study of spiral baffle heat exchangers.

1. 2 Interaction between flow fields in a spiral baffle heat exchanger

1. 2. 1 Influence of spiral angle on flow field [5]

In general, the tangential velocity ut of the shell-side fluid of the spiral baffle heat exchanger is greater than the axial velocity uz, and the smaller the α, the larger the ut. The pulsation speed is very sensitive to α, and when α decreases, the pulsation speed increases. The decrease in α increases the resistance drop, but the drag drop is much smaller than that of the bow baffle heat exchanger.

1. 2. 2 Influence of flow on flow field [5]

When α is the same, the flow velocity tends to be uniform along the radial direction as the flow rate increases, and the pulsation velocity increases, which is favorable for heat transfer. This is because when the flow rate increases, the laminar boundary layer becomes a turbulent boundary layer. When the separation point is advanced, a large number of vortices are generated after the tube bundle, and the vortex motion can enhance the liquid radial mixing, so that the velocity tends to be uniform along the radial direction.

1. 3 Influence of spiral flow on heat transfer

(1) In the spiral flow, the tangential velocity produces a centrifugal force acting on the fluid. The pressure on the outside of the fluid rises and the pressure on the inside decreases. The fluid flows from the outside to the inside under the pressure difference, and the fluid in the center appears to flow back. Secondary flow [5]. The spiral flow and the secondary flow increase the turbulence greatly and the turbulence is uniformed in the radial direction to enhance heat transfer.

(2) The spiral flowing fluid obliquely washes the tube bundle, and under the dual action of tilting and rotating, the velocity boundary layer becomes very thin, and the heat transfer coefficient is greatly improved.

1. 4 characteristics of spiral baffle heat exchanger

Compared to conventional bow baffle heat exchangers, spiral baffle heat exchangers have the following advantages.

(1) The shell-side fluid flows in a spiral shape, the fluid turbulence is intensified, and the thickness of the laminar bottom layer is thinned, which is beneficial to increase the heat transfer coefficient. According to the foreign literature, the heat transfer coefficient of the shell-shell film under the unit pressure drop of the spiral baffle can be increased by 1.8 to 2.0 times, so that the heat exchanger can be reduced under the same heat load. size.

(2) The spiral flow of the shell-side medium reduces the resistance drop significantly. Compared with the single-bow plate, the resistance drop can be reduced by about 45% under the same flow conditions.

(3) There is no stagnation zone and dead zone in the shell process, no dirt deposition, which can extend the equipment maintenance period.

(4) It is more suitable for viscous media and media with severe fouling.

(5) Effectively prevent the occurrence of fluid-induced vibration, and is suitable for working conditions with large flow fluctuations and vapor-liquid two phases.

The spiral baffle heat exchanger also has certain disadvantages. The processing of the spiral plate tube hole and the distance tube is difficult, and a special tire tool is needed. The tube bundle assembly is difficult, and the cost is higher than the bow plate heat exchanger.

2 Research on spiral baffle heat exchanger

The research on spiral baffle heat exchangers mainly focuses on the fluid dynamics study on the shell side. Through the study of heat transfer and drag reduction on the shell side, the appropriate helix angle is selected to meet the engineering needs. Research institutions abroad mainly include the American Society for Heat Transfer Research (HTRI) and the UK Research Center for Heat Transfer and Fluid Flow.

Xi'an Jiaotong University is an early research unit and has obtained a number of patents. Deng Bin et al. used a porous medium and distributed resistance model step approximation technique to numerically simulate the laminar flow on the shell side of the heat exchanger [6], indicating that the shell side fluid spirally flows and is tested with the corresponding heat exchanger. A comparative study was conducted. Wang Qiuwang et al. studied the heat transfer and resistance performance of the heat exchanger [7], and found that the use of false tubes in the 4-tube heat exchanger reduces the heat transfer efficiency and increases the resistance; at the same Reynolds number, there is no center. The heat transfer efficiency of the tube is 30% higher than that of the central tube; at the same shell side flow rate, the shell side heat transfer coefficient decreases as the helix angle increases. Wang Liang et al. conducted heat exchange and resistance performance tests on heat exchangers with helix angles of 10° and 15° [8].

The Key Laboratory of Heat Transfer Enhancement and Process Energy Conservation of the Ministry of Education of South China University of Technology has done a lot of research work on intensifying heat transfer and new shell-side heat transfer enhancement technology, taking the lead in using low finned tubes in spiral baffle heat exchangers [9] ]. Zhang Shaowei of Nanjing University of Technology and other researches on the effect of baffle spacing on the performance of heat exchangers [10]. Researchers at Fushun Petroleum Institute used a plexiglass to make a model of a spiral baffle heat exchanger. The characteristics of the flow field were measured in detail by a laser speedometer. The influence of the swirl angle on the velocity distribution and the pulsation velocity and the coupling of the flow rate were studied. Relationships, it is found that different helix angles and arrangements will affect the velocity distribution of the fluid and also affect the heat transfer effect. Peng Jie et al. studied the amount of spiral baffle lap joints. The results show that the lap joint arrangement is beneficial to reduce the pressure drop, but it is not conducive to heat transfer.

3 Calculation of structural design related dimensions

The commonly used spiral baffles are baffles with a helix angle α and a back rake angle β, as shown in FIG. Usually, four spiral baffles are alternately overlapped to form a pitch. α is the angle between the plane of the baffle and the plane of the tube sheet, and β is the angle at which the baffle with the helix angle α is inclined backward in the direction of fluid flow (axial direction). The side length of the spiral baffle is overlapped over a part of the center line of the device, which can effectively reduce the leakage between the baffles, and generally overlaps the tubes of about 2 rows.



The baffle projection and main view are shown in Figure 3. The left side of Figure 3 is a projection of the baffle on the section of the tube sheet of the device, which is 1/4 circle; the right side is the front view of the baffle, which is approximately fan-shaped. The algorithm for the relevant dimensions is as follows.



3. 1 The calculation of the side length, angle and inclination of the spiral baffle is shown in Figure 2. OO' is the equipment axis, AA′′ is on the equipment axis, and the thick solid part of BA′D′ is the actual space of the baffle. position. It can be seen from Fig. 2 that the two sides of the baffles are not equal when the α and β are not equal, which will bring great difficulty to the blanking in the production, so in the actual design, the two angles are made equal, below The calculations are based on two angles equal.

3. 1. 1 side length calculation

In Figure 3, OD is the centerline of the baffle projection on the cross section of the tubesheet, and OD' is the centerline of the main view of the baffle. From Figure 2, two baffles of equal length and equivalent to a single bow device can be seen. radius.

Combined with Figure 2 and Figure 3:

R=AB=AD

From Figure 2 it can be concluded that:

R'=BA'=A'D"=R /cosα

A spiral baffle with a helix angle and a back rake angle, when the two angles are equal, the actual shape (Fig. 3) is an approximate fan shape with two side lengths R', a center line length R, and an angle θ. Symmetric about the centerline.

3. 1. 2 angle θ calculation





The distance tube is a rotating body whose end surface is to be fitted to the end surface of the baffle, so the inclination angle of the end surface of the distance tube is equal to the inclination angle γ of the baffle, as shown in Fig. 4.



3. 4 distance tube length calculation

The baffle layout is shown in Figure 5. The distance from the inside of the center point of the first baffle on the axis of the equipment (excluding the wall thickness) from the inside of the tube sheet should be given in the construction drawing.



The arrangement of the tie rods is shown in Figure 3. When the number of drawbars of each baffle is even, it should be symmetrically arranged with respect to the axis of rotation of the baffle; when the number of drawbars is odd, one puller should be placed on the rotating shaft, and the rest should be symmetrically arranged with respect to the rotating axis. Reduce the type of distance tube. First determine the position of the tie rod on the tube drawing of the tube sheet, and give the coordinate values ​​of the center of all the rods with the center point of the inner side of the tube plate as the coordinate origin.

3. 4. 1 Length of the distance pipe (axis length) between two adjacent baffles

The two end faces of the spacer tube between two adjacent spiral baffles are inclined, and the length of the axis is calculated as follows:



3. 4. 2 Length of the distance pipe from the front 4 baffles to the inner side of the tube sheet (axis length)

The distance between the four baffles constituting the first spiral and the tube plate is a plane near the tube plate side, and the side of the baffle plate is a slope, and the inclination angle is γ.

The projection of the drawbar on the tubesheet is shown in Figure 6. Let the projection of the first baffle on the tube plate be the first quadrant in Fig. 6, then the distance tube at L11 is the shortest, the length of the distance tube in the clockwise direction increases sequentially, and the distance tube at L44 is the longest. . The coordinates of the center point of the four tie rods in the first quadrant are L11 (x1, y1), L12 (x2, y2), L13 (x3, y3) and L14 (x4, y4), respectively.

The order of the four distance pipes on the first baffle is from small to large as follows:



4 spacers on the 2nd baffle and on the 1st block



In the stage, the model size of the test is too small, and the research results are not up to the requirements of industrial practice. It is necessary to take the route of combining production, research and research, obtain actual operational data on the equipment put into operation, and establish mathematics for heat transfer and flow. Model, develop common heat transfer calculation software, and achieve standardized design as soon as possible. At present, companies have recognized the superior performance of spiral baffle heat exchangers and are willing to adopt them in the installation, but it is difficult to mass produce them due to mechanical manufacturing techniques. It is an urgent problem to be solved by adopting advanced manufacturing technology and transforming CNC machine tools to solve the problem of baffle processing and tube bundle assembly, improve production efficiency, and speed up the promotion and application of spiral baffle heat exchangers.

references

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[2] State Bureau of Quality and Technical Supervision. GB151—1999 Shell and Tube Heat Exchanger. 1999.

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[6] Deng Bin, Wu Yang, Tao Wenzhao. Digital Simulation of Shell Side Flow of Helical Baffle Heat Exchanger [G] //Chinese Society of Engineering Thermophysics. Proceedings of the 2003 Conference. 2003.

[7] Wang Qiuwang, Luo Laiqin, Zeng Min. Experimental study on heat transfer and resistance performance of staggered baffle heat exchangers [G] //Chinese Society of Engineering Thermophysics. 2003 Academic Conference Proceedings. 2003.

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[9] Zhu Dongsheng, Jiang Xiang, Lu Yingsheng. Application of spiral fin baffle low finned tube oil heat exchanger [G]. 2004 National Chemical and Petrochemical Equipment Localization Technology Exchange Conference Collection. 2004.

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