Spiral baffle heat exchanger

       Many heat transfer problems are often involved in petroleum, chemical, power, metallurgy, energy and other industrial sectors. Shell and tube heat exchangers are the most widely used heat transfer equipment in current industrial production. Compared with other types of heat exchangers, its main advantages are large heat transfer area per unit volume and good heat transfer effect. In addition, simple structure, wide range of materials required for manufacturing, and high flexibility in operation, etc., have been widely used in the field of chemical engineering.
       In order to increase the velocity of the shell-side fluid and increase the degree of turbulence to improve the heat transfer film coefficient of the shell-side, baffles are usually installed on the shell-side of the shell-and-tube heat exchanger, the most common being a circular baffle. When the fluid flows meanderingly in the shell with the circular baffle, the direction and speed are constantly changing, especially at the edge of the baffle, the fluid separation is easy to occur. Due to the flow dead zone between the arcuate plate and the shell, the fluid repeatedly cross-flows in the baffle plate, which reduces the heat transfer driving force. To obtain higher heat transfer performance, the only way to reduce the spacing of the arcuate plates , which is bound to be accompanied by higher flow resistance at the expense of higher energy consumption. Therefore, it is urgent to change this traditional baffle form.

　　Helical baffle heat exchangers are ideal alternatives due to their unique advantages. The spiral baffle heat exchanger uses a continuous helical support plate to support the heat exchange tube, so that when the shell side medium enters from the shell side inlet, it advances obliquely along the spiral channel formed by the spiral plate, changing the traditional transverse baffle method. It is formed into a longitudinal helical baffle mode, which greatly enhances the heat transfer effect while reducing the shell side resistance. Its characteristics are:

(1) The medium flows continuously and smoothly in the shell, avoiding the serious pressure loss caused by the lateral baffle, so it has the characteristics of pressure drop.

(2) Compared with the bow-shaped baffle, under the same pressure drop, the flow velocity of the medium on the shell side can be greatly increased, thereby increasing the degree of turbulence and increasing the heat transfer capacity of the medium.

(3) The medium on the shell side advances spirally, thus generating a velocity gradient on the radial section, forming radial turbulence, and reducing the thickness of the bottom layer retained on the surface of the heat exchange tube, which is beneficial to improve the heat transfer coefficient of the membrane.

⑷The ratio of the lateral baffle mode has no dead zone. While improving the heat transfer coefficient, it reduces the deposition of dirt, and the thermal resistance is stable, so that the heat exchanger can always be in a high-efficiency operation state.

⑸The confinement of the swirl baffle to the heat exchange tube is stronger than that of the arcuate baffle, which reduces the vibration of the tube bundle and prolongs the operating life of the equipment.

⑹ When the shell side is used for condensing heat exchange, the spiral baffle can play the role of draining the condensed liquid, reducing the coverage of the condensed liquid on the lower pipe, thereby improving the heat exchange effect.
　　The research conducted by StehlikP et al. concluded that compared with the traditional arcuate baffle heat exchanger, the heat transfer coefficient of the helical baffle heat exchanger under the same conditions can be increased by 1.8 times, and the flow resistance can be reduced by 25%. Chen Shixing et al. found that for high viscosity oil, the shell-side convection heat transfer coefficient per unit pressure drop of the spiral baffle heat exchanger is about 1.5 times that of the ordinary arcuate baffle heat exchanger; for water, the spiral baffle heat exchanger The shell-side convection heat transfer coefficient per unit pressure drop of the heat exchanger is about 2.4 times that of the ordinary arcuate baffle heat exchanger. Song Xiaoping introduced the application of more than ten sets of spiral baffle heat exchangers in oil refineries. The results show that its various performance indicators are better than the original bow baffle heat exchangers. The use of spiral baffle heat exchangers The heat exchange efficiency after the device is greatly improved, and the heat exchange area and metal consumption are reduced, thereby reducing the equipment investment of the device.
However, it is difficult to process the helical surface, and it is difficult to realize the cooperation between the heat exchange tube and the baffle. Taking into account the convenience of processing, a series of quasi-sector flat plates (called helical baffles) are used to replace the curved surface connection, and an approximate helical surface is formed on the shell side, so that the fluid on the shell side of the heat exchanger is continuously generated. spiral flow, see Figure 2.

 　　The arrangement of the helical baffles should stabilize the helical flow field of the medium in the shell side, which requires a consistent interval between the helical baffles, called the baffle spacing F), and the same installation angle α. The helical baffle should be arranged below the upper inlet and outlet axis or above the lower inlet and outlet axis, see Figure 3.

　　Since 1997, when Fushun No. 2 Petroleum Plant applied spiral baffle heat exchangers for the first time in China, spiral baffle heat exchangers have been rapidly promoted in chemical refining units, and dozens of units of more than 20 companies have successively applied them. Thousands of spiral baffle heat exchangers. The application results show that the use of helical baffles on the shell side does have the effect of reducing the pressure drop of the fluid on the shell side than the use of vertical arcuate baffles, but the improvement of the heat transfer efficiency on the shell side is not very obvious. After the large-diameter heat exchanger adopts the spiral baffle, its heat exchange efficiency is even inferior to that of the arcuate baffle. After many simulation experiments on different diameter shells and different angles of helical baffle heat exchangers, it is shown that the main reason for this is the limitation of the level of mechanical processing, because the processing to achieve complete continuous helical baffles very difficult. The traditional helical baffles are overlapped by 2 or 4 flat plates, arranged in an approximate helical plane. Each of the fan-shaped baffles with a projection of 360°/x is placed end to end at a certain angle to the shell side axis. The straight sides of the two adjacent baffles are staggered and buttted at the top, and the triangular space formed between the two adjacent baffles (see Figure 4) is likely to cause the medium flowing along the baffles to form short-circuit leakage and deviate from the spiral flow state.

　　The short-circuit leakage reduces the flow of the ideal channel, especially in the large-diameter helical baffle shell-and-tube heat exchanger, because a large amount of medium flows through the short-circuit along the triangular space and the gap formed between the two adjacent baffles, so that the The helical flow of the main channel is reduced, which reduces the flow rate of the medium and seriously affects the heat exchange efficiency. New type of anti-short-circuit spiral baffle shell-and-tube heat exchanger In order to ensure the improvement of the heat exchange efficiency of the shell side, Dalian Haite Company has developed a new type of anti-short-circuit spiral baffle plate, which makes the medium in the shell side approximate the ideal spiral. The flow pattern is through the housing.

Fluid Flow Analysis
　　On the basis of the original fan-shaped baffle, the new anti-short-circuit spiral baffle widens the straight sides of both sides by 1 or 2 rows of pipe spacing at the same time. Flow simulation experiments were carried out with 1 or 2 rows of heat exchange tubes running through. The experimental results show that the short-circuit phenomenon still exists when the fan-shaped baffle is widened by 5~10mm on both sides of the straight sides at the same time, and the medium in the shell side is approximately ideal until it is widened to the width of 1 row to 2 rows of tube spacing. The spiral flow pattern passes through the shell, and the short circuit phenomenon is almost eliminated at this time. The structural arrangement of the anti-short-circuit spiral baffle is shown in Figure 5.                                                    

　　This cross-overlapping connection mode plays a good guiding role for the medium flowing through the tube bundle, which not only reduces the short-circuit phenomenon that the straight sides of two adjacent fan-shaped baffles cross to form a triangular space, ensures the improvement of heat exchange efficiency, and also Since the same row or two rows of heat exchange tubes pass through two adjacent fan-shaped plates, the rigidity of the tube bundle is strengthened and the tendency of separation between quadrants is avoided. The helical baffle structure of the overlapping part has a good anti-vibration effect.
　　To sum up, the spiral baffle heat exchanger shows its superiority in terms of comprehensive performance. As long as the design is appropriate, better comprehensive performance can be obtained. However, compared with the traditional bow baffle, further work is required. There are still many places for research. For example, so far, most of the research on shell and tube heat exchangers has focused on the arcuate baffles. The TEMA standard is also aimed at the arcuate baffles. Into the standardized design, it is necessary to carry out a detailed analysis and research on its flow and heat transfer mechanism, and the factors affecting its flow and heat transfer mechanism are not only the factors affecting the traditional bow baffle, but also geometric factors (arrangement type, spiral angle, pitch), phase transition, physical properties of different media, etc. It is reasonable to believe that with the continuous development of computer technology and in-depth research on the mechanism of helical baffle heat exchangers, the application of this heat exchanger will be more extensive.