Design considerations of heat exchangers for vaporizing fluids

The problems encountered in determining the steady-state operating characteristics of certain types of vapor generating equipment are examined. The first type of equipment considered is the vertical tube reboiler with upward flow. A calculation method applicable to industrial mixtures is described, wherein the evolution of mixture composition along the tube is accounted for. Based on results obtained with an experimental unit of this type, an attempt has been made to develop correlations for the pressure drop and the heat transfer rate that would apply at relatively low pressures (a few bars). Also discussed are falling film, long tube vertical evaporators and a boiling systen often found in certain industrial situations. Methods and results are presented that permit predictions of the heat transfer coefficient and the pressure drop. For the case of horizontal, once-through vapour generators at high pressures, the influence of gravity leads to non-symmetrical flow and heat transfer patterns inside the tubes. First results with freon are presented to permit an analysis of this asymmetry, and in addition, to permit examination of the effect of elbows. Finally, the problems associated with boiling outside horizontal or vertical tubes are considered. This study is tied to the operation of low pressure boilers often encountered in sea water desalting units (subatmospheric pressures) and in the chemical industry