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Second Thermal and Fluids Engineering  Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

AN EFFECTIVENESS−NTU RELATIONSHIP FOR EVAPORATORS WITH NON-NEGLIGIBLE BOILING POINT ELEVATION INCREASES

Gregory P. Thiel
Rohsenow Kendall Heat and Mass Transfer Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA

John H. Lienhard V
Rohsenow Kendall Heat and Mass Transfer Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA

Abstract

In some evaporators, such as those used in the desalination of high-salinity produced waters, the rise in temperature of the evaporating stream owing to boiling point elevation increase may be non-neglible. In such cases, neither classical effectiveness−NTU relationships, which are valid for constant capacity rates, nor traditional falling film evaporation analysis, which generally considers uniform vapor saturation temperature, are applicable. An effectiveness number of transfer units (ε−NTU) model is developed for use in evaporators where the evaporating stream: (1) comprises a volatile solvent and non-volatile solute(s); and (2) undergoes a significant, but linear change in boiling point elevation with increasing solute molality. The model is applicable to evaporators driven by an isothermal stream (e.g., steam-driven or refrigerant-driven) in parallel flow, counterflow, and crossflow configurations where the evaporating stream is mixed. The model reveals nonlinearities in the driving temperature difference for large changes in BPE, which can impact staging design and systemwide energy consumption. The broad applicability of the model is demonstrated by applications to high-salinity desalination systems and juice evaporators.

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