Transient Behaviour of Economizers for Multi-Stage Refrigeration Plant

When a refrigeration plant operates with a large difference between condensing and evaporating pressures, multistage operation is required to obtain an acceptable performance. In two-stage systems, vessels that combine flash-gas removal and intercooling are commonly used. The disadvantage of these vessels is the large amount of refrigerant required for proper operation. An alternative for these vessels is the use of "economizers", dry evaporator heat exchangers. The advantages of this solution are: smaller volume requirements, lower costs and much smaller refrigerant contents. This paper describes a dynamic simulation model for economizers of multi-stage refrigerating plants. A finite difference method has been used to solve the mass and combined energy and momentum equations. In the two-phase flow region, the local void fraction is used to combine the separate equations for the two-phases. Both the void fraction models of Levy and Hughmark have been implemented. The two-phase flow heat transfer relationship of Gungor and Winterton has been used to predict the heat transfer rate in the two-phase flow region. The two-phase flow pressure drop relationship of Lockhart and Martinelli has been used to predict two-phase flow pressure drop. The model has been implemented in Fortran. The main objective of the model was to predict the effect of design criteria on the (dynamic) performance and refrigerant contents of compact economizers. The model results have been compared with experimental data obtained with a shell-and-tube economizer. The model predicts the performance of the economizer within 10%. Hughmark's void fraction model leads to refrigerant contents which are twice as large as obtained with Levy's modeL