Researches And Application Of Direct-Sizing Method For Heat Exchangers

In the process of heat exchangers design, it is difficult to establish the subjection function of the optimal points of the objective functions due to the contradictions and indefinite relations among the design objectives. The main contradiction is when the heat transfer capability of heat exchangers is enhanced, more energy is consumed at the same time. The variation of fluid properties in heat exchangers is one of the main factors to cause the uncertainty. The conventional design methods have not thoroughly solved these problems. The further development of strengthening heat transport in heat exchangers urgently needs the breakthrough in the heat exchanger design method. Actually it may be achieved by synthetically considering the objective functions and the uncertain factors in the process of heat exchanger design.In the present work, based on the fundamentals of heat transfer,hydrodynamics and thermodynamics, the Direct-Sizing design method of the heat exchanger was investigated which is suitable for the heat exchangers with geometric similarity structures. By analyzing the fundamental concepts and summarizing the design steps of Direct-Sizing design method, it was pointed out that the key point is to determine the temperature profile as well as the resistance profiles. By analyzing the impact of the flowing state and temperature difference on the temperature profile, the impact of the heat exchanger structure on the resistance profile, the approach to determine the optimal design point was discussed.A new heat exchanger design method called segmented design method was also proposed in the present work by taking into account of the variation of fluid properties of the hot and cold fluids with respect to the temperature. In this method, the whole heat exchanger is firstly divided into several parts. Based on the energy conservation principle, the connection conditions between each part are established. Applying the initial condition and the connection conditions leads to the inlet and outlet temperatures of each part. The application of LMTD method gives rise to the heat transfer surfaces. Thus the whole design of the heat exchanger is completed. Since the segmented design method reflects the reality of heat transfer process, it should yield more accurate design result.As one example, the helical baffle shell-and-tube heat exchanger was designed by Direct-Sizing design method. By analyzing the shortages of discontinuous helical baffle plate, a novel helical baffle plate which can form integrated helical channel was designed. The structure characters of the helical baffle shell-and-tube heat exchanger and the properties of its local heat exchanger were discussed. The correlation of the thermal design and resistance design was established according to the characteristic dimension. A good result was achieved. The advantages of the Direct-Sizing method were demonstrated.Finally, the future development and improvement of the Direct-Sizing design method were discussed. An idea to propose a so-called segmented-direct-sizing design method was pointed out, which is the combination of the segmented method and Direct-Sizing method. This method would not only eliminate the limitation of the Direct-Sizing method and enable it to design the heat exchangers without the geometric similarity structures, but also improve the design accuracy.