1. Pinch theory and its application in heat exchange network: Guizhou University of Technology, Guiyang 550000, Guizhou)

Abstract: At present, there are three main methods for optimization and synthesis of heat exchanger networks: trial method, pinch technology, and mathematical programming method. However, in practical application, pinch technology has proved to be effective and has achieved significant economic benefits. The solutions of several key problems in pinch technology and how to find pinch points in heat exchanger network accurately and quickly are given. The application of these methods can effectively carry out the heat exchanger network Optimize the design and achieve. In view of the special situation encountered in production and practice, the solution is given, and a specific example is used to illustrate the heat transfer effect after the transformation. Based on the analysis of the theoretical basis and thermodynamic principle of pinch technology, the theoretical limitations of pinch technology, its related reasons and solutions are proposed.

Based on strict thermodynamic and mathematical rules, pinch technology has a complete theoretical basis, simple and reliable calculation, flexible and practical methods, and is easy for engineering technicians to master. It can also give play to designers' years of experience in engineering design and production practice to better engage in design work. Therefore, pinch technology represents a new and powerful design method.

Design of heat exchanger network using pinch technology

Ma Lianqiang, Zheng Kaixue, He Xinping, Gao Jianhong, Hualu Engineering Technology Co., Ltd., Xi'an 710054

Abstract: This paper introduces the basic concept of pinch technology and the principle of using pinch technology to design heat exchange network, lists examples of using pinch technology to design heat exchange network, and briefly introduces the basic knowledge of heat exchange network optimization.

Pinch Point Technology was developed by the system integration team of I. C. I led by Linnhoff. The team had redesigned and calculated 18 engineering designs for the technical transformation of old plants and the construction of new plants from 1977 to 1981, and found that the new principle design can save 30% of energy on average. Some projects can not only save energy, but also save investment after rearrangement. In 1982, United Carbon asked Linnhoff for guidance and calculated 9 engineering examples in one year. The results show that this method can save 50% of energy on average, and the investment in equipment used for technical transformation of old plants can generally be recovered within 2 to 12 months. Therefore, this technology is considered mature and can be widely used in industry. Experience has proved that this method can save energy and equipment investment in new design, and recover as much energy as possible with less equipment investment in technical transformation of old plants.

If a heat exchange network is composed of two hot streams and two cold streams, the relevant parameters are shown in Table 1

3.1 Determine the pinch point temperature and the consumption of cold and hot utilities, and complete the cold and hot combination curves respectively Δ Tmin is 10 ℃, and the pinch point temperature and the consumption of cold and hot utilities are determined, as shown in Figure 5.

4 Optimization of heat exchange network

The optimization of heat exchanger network must first determine the scientific objective function, which is generally energy cost and investment cost The optimal combination is the total cost Small. Heat exchanger network optimization is based on the redistribution of heat exchanger load. Some heat exchangers may become larger, some may become smaller, and some heat exchangers may be completely removed from the design. The optimization of heat exchange network generally starts from the heat load loop and utility path. The so-called heat load loop refers to the sequence of closed heat exchangers and streams that start from a heat exchanger or stream, walk down the heat exchanger and path without repetition, and can return to the starting point again. The utility path refers to the sequence of heat exchangers and streams that start from one utility stream and go along the heat exchanger and path without repetition to reach another utility stream. As shown in Figure 9, heat exchanger 2, stream 4, heat exchanger 4 and stream 1 form a heat load loop, and cooler C, stream 4, heat exchanger 4, stream 1 and heater H form a utility path. The number of heat exchangers can be reduced by redistributing the heat load in the heat load circuit, which does not change the utility consumption. As shown in Figure 9, we can transfer the 30kW heat transfer load of heat exchanger 4 to heat exchanger 2 and save heat exchanger 4, although this is contrary Small heat transfer temperature difference Δ The Tmin principle may increase the heat transfer area, but omit a heat exchanger. The feasibility of this optimization can be determined by the objective function. The number of heat exchangers can also be reduced by redistributing the heat load in the utility path, but this adjustment will change the utility consumption. Similarly, as shown in Figure 9, the heat load of cooler C and heater H can be increased by 30kW at the same time while the heat exchanger 4 can be saved, which can also reduce one heat exchanger, thus increasing the utility consumption. The feasibility of optimization also needs to be determined by the objective function.

5 Conclusion

This paper briefly describes the basic principle and process of pinch technology design of heat exchange network. The actual design of heat exchange network is very complex, for example, sometimes there is a reasonable heat transfer temperature difference between two streams, but because the leakage will cause serious consequences, it can not be matched in a heat exchanger; Sometimes the distance between the two streams is too far, and it is not economical to lay too long pipes. In addition, problems such as threshold problem, multi pinch problem, stream diversion, thermal power integration, etc. may also be encountered in the design process, which need to be specifically analyzed in the design process.