Dynamic Modeling of Shell-and-Tube Heat-Exchangers: Moving Boundary vs. Finite Volume

Modeling the dynamics of shell-and-tube heat-exchangers is an important step in developing dynamic system models of liquid chillers that are used for studying transient system performance. Existing literature on the subject is limited and much of what exists uses either a lumped parameter approach or a finite volume approach for the shell-and-tube heat-exchangers. The lumped parameter approach is simplistic and provides neither spatial detail nor sufficient accuracy in predicting exit conditions. The finite volume approach provides extensive spatial detail but at significant computational expense. A third alternative, known as the moving-boundary approach, has thus far only been used for refrigerant-in-tube coils. It has the potential for fast execution due to the reduced number of equations as compared to the finite-volume method, while retaining some spatial detail. This paper details the formulation of shell-and-tube evaporators and condensers using the moving-boundary approach and presents comparative results of model execution with a finite-volume approach. Both formulations are developed to capture start-up and loadchange transients. The moving-boundary formulation has the ability to handle the discontinuities associated with phase-boundaries exiting and entering the heat-exchanger during transient operation. A significant saving in execution time is shown over the finite-volume approach with comparable accuracy.