Designing Shell and Tube Heat Exchangers: Consider Two-Phase Flow
By G.T.Polley, E.E. Vazquez –Ramirez, and M. Riesco Avila; University of Guanajuato, Mexico |
In the chemical process industries (CPI) it is relatively common to use a homogeneous two-phase flow model to predict the pressure drop through the shell-side of horizontal heat exchangers that handle two-phase mixtures. Recent experimental studies reported by HTFS [1, 2], however, have shown that this approach leads to overprediction of pressure drop and to overprediction of thermal performance. As a result, many designs are flawed and perform below expectations.
Homogeneous model vs. data
Doo and others [1, 2] studied the evaporation of R134A in a TEMA AEW type heat exchanger fitted with 97 tubes of 1,240 mm length. Three different baffle arrangements were studied. In the first series of tests the unit was fitted with six vertically cut baffles (providing a horizontal side-to-side flow). The baffle pitch was 156 mm. In the second series of tests the orientation of the baffles was changed such that the flow was vertical. In the final set of tests the unit had only four baffles (with a baffle pitch of 260 mm) and flow was again side-to-side.
The first step taken in the analysis was a comparison between the predictions of a “homogeneous” model for two-phase pressure-drop with the experimental measurements. For all…