ZHANG Renping, ZHENG Ruiqiang, WANG Qi, XIN Jiajun

(School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended abstract:[Background and purposes] The ceramic industry often consumes a large amount of energy and emits waste heat during the production process. In response to the national dual carbon policy, it is necessary to promote technological progress in the ceramic industry. The current common energy-saving method is to add a heat exchanger at the tail of the kiln to collect the waste heat from the flue gases and circulate it to the preheating stage. However, currently the ceramic industry has relatively scarce technology in optimizing heat exchangers, while optimizing the structure of heat exchangers to improve the efficiency of waste heat treatment at the rear of kilns is still a problem.[Methods] Response surface methodology is a statistical method used to optimize experimental results. It can be used to analyze the impact of multiple factors on the results by establishing mathematical models and quickly find the optimal combination of parameters in a small number of experiments. It is mainly used in fields such as food and chemical engineering. At present, the research on kiln heat exchangers is mainly based on single factor analysis, while the impact of multiple factors on response is neglected. Based on the current situation, this article is aimed to introduce the use of response surface methodology to establish a multi factor experiment, explore the significance of response under multiple factors, realize the structural optimization of furnace heat exchangers in complex environments, established a physical model of the heat exchanger absorbing waste heat from flue gas and systematically analyze the influence of four factors, namely fin spacing, fin thickness, tube lateral spacing and tube longitudinal spacing, on the comprehensive heat transfer performance indicators of the heat exchanger.[Results] In the single factor experiment, when the fin thickness is increased from 0.8 mm to 1.2 mm, the end temperature of the heat exchanger is increased by 2% and the end pressure is increased by 22.1%. When the fin spacing is increased from 5 mm to 9 mm, the end temperature of the heat exchanger is decreased by 20.1% and the end pressure is decreased by 36.8%. When the lateral spacing of the tubes is increased from 9 mm to 12 mm, the end temperature of the heat exchanger is increased by 12.4% and the end pressure is decreased by 5.4%. When the longitudinal distance between the tubes is increased from 12 mm to 16 mm, the temperature at the end of the heat exchanger is decreased by 14% and the pressure at the end is decreased by 44.6%. In the response surface methodology experiment, the fin spacing is increased by 80%, the fin thickness is increased by 50%, the tube transverse spacing is reduced by 14.3%, and the tube longitudinal spacing is reduced by 16.7%. With the change in the structure of the heat exchanger, the temperature difference between the inlet and outlet is decreased from 109.8628 ℃ to 93.7935 ℃, with a decrease of 14.6%. The import and export pressure difference is increased from 86.656 to 92.762, with an increase of 7.05%. The response of the heat exchanger was improved, with an increase of 10.1% in heat transfer factor, a decrease of 23.3% in resistance factor, and an increase of 20.3% in overall heat transfer performance. The heat transfer efficiency of the heat exchanger was effectively improved and the internal flow resistance of the heat exchanger was reduced.[Conclusions] The heat transfer coefficient is increased with increasing fin spacing, fin thickness and tube longitudinal spacing, while it is increased with decreasing tube transverse spacing. The resistance coefficient is increased with increasing fin thickness and longitudinal spacing of the tube, as well as transverse spacing along the tube. Within a certain range, the drag coefficient is decreased first and then increased with increasing fin spacing. The comprehensive performance index is increased with increasing fin spacing, fin thickness and tube lateral spacing, while it is increased with decreasing tube longitudinal spacing. By observing the density of contour lines in the response surface contour map, the significance of factors on the response can be analyzed. Among them, the fin spacing and tube longitudinal spacing have the most significant effect on the heat transfer factor, the tube longitudinal spacing and tube transverse spacing have the most significant effect on the resistance factor, and the fin spacing and tube longitudinal spacing have the most significant effect on the comprehensive performance index. Based on the mutual influence of various factors on the response, the optimal combination of fin spacing of 9 mm, fin thickness of 1.2 mm, tube lateral spacing of 9 mm, and tube longitudinal spacing of 12 mm, was finally obtained, by using the response surface method. As compared with the original method, the new strategy exhibited improvement in comprehensive performance indicators by 16.89%.

Key words: finned-tube heat exchanger; response surface methodology; heat transfer factor; resistance factor; comprehensive heat transfer