Exploration of process design issues for tubular heat exchangers

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Exploration of Process Design Issues for Tubular Heat Exchangers Abstract: Tubular heat exchangers exhibit good sealing performance, as well as the advantage of being easy to maintain and clean. In applications where the working pressure of the heat transfer medium in the heat exchanger is high and strict sealing requirements apply to the media in the tube side and shell side, U-tube structured heat exchangers are generally used for the design of the tube bundle in order to ensure proper sealing between these two sides. Keywords: heat exchanger, heat transfer coefficient, scaling, process design, heat transfer capacity. I. Heat transfer calculation for shell-and-tube heat exchangers In the design of shell-and-tube heat exchangers, it is necessary to initially determine the heat transfer coefficient based on the process conditions of the heat exchanger, so as to roughly determine its heat transfer area and structural dimensions. Based on the preliminary determination of the structural dimensions of the heat exchanger, heat transfer calculations are carried out for the heat transfer coefficients in the tube side and the shell side of the heat exchanger; finally, various evaluations are performed, including those of the pressure drops in the shell side and tube side, as well as the wall temperatures of the tube bundle. 1. Initially determine the heat transfer area and structural dimensions of the heat exchanger. The heat transfer area of a heat exchanger is determined by its heat load, heat transfer coefficient, and the average logarithmic temperature difference between the cold and hot fluids flowing through it. Once the heat transfer area is known, the structural dimensions of the heat exchanger can be determined initially. The formula for calculating the heat exchange area of a heat exchanger is as follows: Where A represents the heat transfer area ； Q is the heat load, W ； K is the heat transfer coefficient, ； For the average logarithmic temperature difference, K. 2. Calculation of the heat transfer coefficient in the tube side and the heat transfer coefficient in the shell side (1) Heat transfer coefficient in the tube side ( ). It can be calculated using the following formula: Where: is the heat transfer factor ； For the thermal conductivity at the average temperature of the heat transfer medium within the tube bundle, ； d is the inner diameter of the heat exchange tube, in meters ； c is the specific heat capacity of the heat transfer medium at the operating pressure within the tube bundle, ； Viscosity of the heat transfer medium at the average temperature inside the tube bundle, in mPa•s ； Coefficient for correcting the viscosity of the heat transfer medium inside the tube bundle. (2) Calculation of the heat transfer coefficient ( ) for the shell side of the heat exchanger. It can be calculated using the following formula: where b is a constant ； Equivalent diameter of the heat exchanger shell side, m ； It is the outer diameter of the heat exchange tube, in meters ； It is the thermal conductivity at the average temperature of the heat transfer medium in the shell side ； Reynolds number of the heat transfer fluid in the shell side ； It is the Prandtl number of the heat transfer medium in the shell side. By calculating the heat transfer coefficients in the tube side and shell side of the heat exchanger, if the value is between 1.1 and 1.2 h, it indicates that the preliminary structural design of the heat exchanger is reasonable. On this basis, the pressure drops in the tube side and shell side of the heat exchanger, as well as the wall temperature of the tubes, are calculated. If these values meet the design requirements, it indicates that the design of the heat exchanger satisfies the requirements of the process flow; otherwise, the structure of the heat exchanger needs to be adjusted again until the process flow requirements are met. 3. Calculation of the heat transfer coefficient (K) of the heat exchanger: The heat transfer coefficients for the tube side and shell side are both based on empirical data regarding the heat transfer coefficient of the heat exchanger. If these empirical values match those obtained through theoretical calculations, and if the main performance parameters of the heat exchanger (such as the pressure drops across the shell and tube sides, as well as the inlet and outlet temperatures of the heat transfer medium) meet the requirements of the process, it indicates that the overall design of the heat exchanger is reasonable. Otherwise, it is necessary to adjust the structural dimensions and carry out further design calculations until the design requirements are met. The heat transfer coefficient (K) of the heat exchanger can be calculated using the following formula: Where: is the thermal resistance of the outer wall of the heat exchanger tube bundle ； For the thermal resistance of the inner wall of the heat exchanger tube bundle, ； It represents the thermal resistance of the tube wall in the heat exchanger tube bundle. II. The influence of the structural design of shell-and-tube heat exchangers on their heat transfer capacity 1. Design of the inlet and outlet ports in the tube side (1) Design of the inlet and outlet tubes in the tube side. The inlet and outlet diameters of the tube side are determined through calculation, taking into account the allowable pressure drop on the tube side; the calculation formula is: < 3300 (where it represents the density of the medium in the tube side) ； For the inlet and outlet flow velocities of the pipeline medium, ). To ensure a uniform distribution of the fluid in the tube side and to fully utilize the heat exchange capabilities of the heat exchange tubes, the inlet and outlet of the tube side should be located at the bottom and top of the heat exchanger’s tube side. (2) Design of the shell side inlet and outlet. As the heat exchange medium in the shell side flows horizontally and washes against the tube bundle, it causes wear and vibration to the tube bundle, which significantly affects the service life of the heat exchanger, especially when the flow velocity of the heat exchange medium in the shell side is high or when the medium contains solid particles. To ensure the performance of the heat exchanger, an anti-scour plate can be installed at the inlet of the shell side to cushion the impact of the fluid and protect the tube bundle from damage. To prevent excessively high flow velocities at the shell side inlet, there is a certain limit on the flow rate of the fluid in that section: at a flow rate of 100, the fluid reaches turbulent conditions. Therefore, fluids with low flow rates or high viscosity are preferred as the heat transfer fluid in the shell side. Since cleaning the tube side is easier than cleaning the shell side, fluids that tend to form scale, contain precipitates, or have impurities should be used in the tube side. From an economic perspective, it is more reasonable to use fluids that are high-temperature, high-pressure, or highly corrosive as the heat transfer fluid in the tube side. For heat exchangers with rigid structures, when there is a large temperature difference between the cold and hot fluids, since the wall temperature is close to the temperature of the fluid with the higher heat transfer coefficient, it is more appropriate for this fluid to be used in the shell side in order to reduce the expansion difference between the tube bundle and the shell. When the temperature difference between the cold and hot fluids is small and the heat transfer coefficients of the two fluids differ significantly, it is more appropriate for the fluid with the higher heat transfer coefficient to be used in the tube side. III. Examples of applications in production: The 4113-E1 is a U-tube shell-and-tube heat exchanger; the efficiency of its heat exchange capability has a direct impact on the carbon black sedimentation issue in open-loop systems as well as on the water circulation system, and it also affects the equilibrium of the carbon monoxide conversion reaction. Therefore, it is necessary to ensure proper operation of 4113-E1 in production in order to improve its heat exchange efficiency. Since the addition of an open-circuit system and improvements to the coagulant, the heat exchange performance of 4113-E1 has improved significantly. During cleaning, it was found that although there was some scale blocking the pipe ends on the tube side, the situation was not severe; the pipes were easy to clean, and there was no scaling on the inner walls of the pipes either. After cleaning, it was reinstalled and put into use; the heat exchange performance was satisfactory. The flash volume of 13-V12 decreased significantly, and now the temperature of 13T3 can be maintained above 210°C. It shows that the open-circuit system and coagulant improved the viscosity of graywater, reduced scaling on the inner walls of the pipes, thereby enhancing the heat exchange efficiency of 4113-E1. Furthermore, attempting to gradually increase the amount of 13F30 yielded satisfactory results: as can be seen from Table 1, as the circulation water volume increased, the amount of carbon black in the tube side of 13E1 also increased, and its flow velocity within that tube side increased as well. As a result, the probability of carbon black settling in the tube side decreased, and the temperature difference between the media increased, indicating a significant improvement in the heat exchange efficiency. The relevant debugging data is as follows: Date: 13T1, 13T2, 13T3, 13T4. Temperature difference of carbon black water; Temperature difference of ash water: 13F30 °C, °C, °C, °C, °C, °C; Volume flow rate in m3/h: 2004-8-10: 224.5, 124.6, 183.6, 199.7, 24.8, 59.0, 114.8; 2004-8-14: 226.1, 125.7, 189.4, 196.2, 29.9, 63.7, 113.7; 2004-8-15: 226.3, 125.5, 188.8, 196.1, 30.3, 63.3, 112.0; 2004-8-16: 226.5, 125.7, 189.3, 196.2, 30.4, 63.6, 114.3; 2004-8-19: 225.4, 124.3, 189.9, 194.1, 31.4, 65.6, 115.2; 2004-8-20: 226.2, 123.8, 189.4, 193.7, 32.5, 65.6, 114.1; 2004-8-21: 225.8, 123.8, 189.3, 194.0, 31.8, 65.5, 114.4; 2004-8-24: 225.5, 119.2, 190.5, 188.5, 37.1, 71.3, 115.0. IV. Challenges in process design Although the principles and methods for the process design of shell-and-tube heat exchangers have become standardized and regulated, they are not yet perfect. Design work still faces or may face many issues that need to be resolved, and these represent current areas of research and development focus. 1. Multiphase flow and heat transfer: Compared to the design methods for systems without phase changes, the design of systems involving phase changes such as condensation and boiling is much more complex. This is especially true for the problems related to two-phase or multiphase flow and heat transfer that occur in process industries, such as boiling and condensation in multi-component systems; condensation of vapors containing non-condensable gases; boiling and condensation within tube bundles, etc. Since these processes involve complex gas-liquid or multiphase flows, as well as non-equilibrium phase change heat and mass transfer, it is not yet possible to carry out quantitative design for them. 2. Heat transfer enhancement: For shell-and-tube heat exchangers, the methods for enhancing heat transfer can be divided into active and passive techniques based on whether external power is required; the former consumes external energy, while the latter does not. The latter mainly aims to thin the temperature boundary layer on the heat transfer wall surface or to alter the fluid near the heat transfer wall surface. There are mainly two approaches for implementation: ① Appropriate modifications and adjustments to the structure and shape of the heat transfer surface; ② Installing turbulence promoters on the heat transfer surface or in the heat transfer flow path, or adding additives to the fluid, particularly appropriate solid particles, which not only enhance heat transfer but also help prevent and remove scaling. In active technologies, using various field strengths such as electric and magnetic fields and their synergistic effects to enhance heat transfer has been a research area of interest in recent years. 3. Fluid vibration: Due to the highly complex flow path on the shell side of shell-and-tube heat exchangers, various fluid vortices, flutter, elastic excitation, and acoustic resonance can occur; these oscillations combined result in intense vibrations. As heat exchangers evolve toward larger sizes, higher temperatures, higher pressures, higher flow rates, and greater loads, vibrations may become more intense. In severe cases, this can not only cause the tubes to break but also damage the entire heat exchanger. Therefore, it is necessary to study the mechanisms of vibration as well as measures for its prevention and control. Over the years, although some mechanisms of fluid-induced excitation and vibration prediction methods have been proposed theoretically, due to the complexity of fluid flow, our understanding of its patterns remains superficial, making effective control and prevention difficult. In terms of engineering applications, some vibration-resistant structures have also been developed, but their performance is not satisfactory. It should be noted that by appropriately controlling the vibration frequency, amplitude, and location of occurrence, heat transfer can be enhanced and scaling can be prevented. 4. Dirt: Dirt can be broadly categorized into crystallization, particle deposition, chemical displacement, polymerization, coking, the growth of organisms, and surface corrosion. From the perspective of heat exchanger design and operation, fouling has a significant impact on heat transfer and flow parameters; therefore, the issue of fouling is given considerable attention, and progress has been made both domestically and internationally in terms of designing heat exchangers to resist fouling as well as in methods for removing fouling. However, due to the complexity of the problem, a conservative oversized design approach is still used in heat exchanger design to address fouling issues. Therefore, further research is needed to find more reasonable design methods that take dirt into account. 5. Turbulence: Turbulence problems are very complex. Although the three-dimensional unsteady flow equations can be used to describe turbulent states, and analytical methods can be employed to solve these equations, it remains extremely difficult from an academic perspective. It is predicted that, with the advances in computer science, computational fluid dynamics, nonlinear science, experimental science, etc., turbulence problems may be solved in the 21st century, thereby laying the foundation for the numerical simulation of flow and heat transfer in shell-and-tube heat exchangers. V. Conclusion: Heat exchangers are important thermal equipment in the petroleum and chemical industries. Conducting scientific calculations for heat exchangers and designing their structures in a rational manner are essential to ensuring their optimal performance. The thermal calculation of heat exchangers is the foundation of heat exchanger design and a prerequisite for the structural design of heat exchangers. Therefore, in the design of heat exchangers, it is only through continuous adjustment of the structural parameters and repeated calculations that higher performance and more rational design can be achieved. References: Zhu Pingguan. Principles and Calculations of Heat Exchangers [M]. Beijing: Tsinghua University Press, 1987. Lanzhou Petroleum Machinery Research Institute. Heat Exchangers. Beijing: Hydrocarbon Processing Press, 1986, p. 20. Cheng Lixin, Chen Tingkuan. Techniques and Methods for Enhancing Boiling Heat Transfer. Chemical Engineering Equipment Technology, 1999, 20(1): 30–33. Deng Songjiu. 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