Michael D. Hagan and Victoria N. Kruglov of CCTI Write Article for CEP Magazine

November 2010

Understand Heat Flux Limitations on Reboiler Design: Avoid the transition to film boiling to obtain better control and lower operating costs.  

Most chemical engineers involved with heat exchanger design are familiar with the concepts of nucleate versus film boiling and critical heat flux. Although a sensible heating medium, such as hot oil, may be used for some applications, most facilities employ steam at different pressure levels. This article explains how to account for the critical heat flux to avoid film boiling, and discusses the impacts of various heating media on the reboiler design. 

Heat flux, the heat transferred per unit surface area (Q/A), is equal to the product of the heat-transfer coefficient and the temperature difference across the exchanger:

The heat exchanger acts as a series of resistances represented by the inside heat-transfer coefficient hi, the wall resistance rw, and the outside coefficient A0. The heat transferred must pass through every resistance and the flux is constant across each of them. Therefore, for a vertical thermosiphon reboiler with boiling in the tubes, the heat flux expression becomes...

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