Thermal design of large plate-fin heat exchanger for cryogenic air separation unit based on multiple dynamic equilibriums

This paper presents a thermal design method of large plate-fin heat exchanger (PFHE) for cryogenic air separation unit (ASU) based on multiple dynamic equilibriums (MDE). The enlarging of PFHE brings cascading thermal design difficulties. Renault number of multi-streams in long passages fluctuates in wide range which generates various working conditions. The fin type of each passage and the Re for flowing streams are pre-set firstly. Using single-banking with top priority, the Recursive Prior Plug-In (RPP) method is proposed to complete permutation of hot streams and cold streams. The preferable passage arrangement is selected and determined according to the variance of the cumulative heat load. The initial temperature of each ordered stream in all the passages is then obtained. The temperature difference sequence between the one-sided adjacent streams is built. The large heat transfer area in plane perpendicular to the flowing direction is divided into MDE according to the temperature difference sequence. For each equilibrium, the heat balance equations between hot quantity and cold quantity are built by Fourier's law of heat conduction and heat convection. The final equilibrium temperature of each ordered stream in all the passages can be obtained by iterative solving the simultaneous differential equations for all MDE. Each long stream is also divided into MDE along flowing direction and modeled by energy conservation principle in enthalpy change to obtain the actual phase. The heat transfer process about Re is divided into MDE according to the dynamic equilibriums between heat transfer and frictional resistance to determine the optimal fins under different working conditions. The heat transfer experiment is completed to verify the proposed method. The MDE is applicable for various stacking patterns of numerous long hot streams and cold streams under variable working conditions. The proposed method is especially useful for thermal design of large energy-efficient PFHE for ASU.