heat exchanger rising film to natural circulation

[gdpatelp]

We are using shell and tube evaporator as an economizer. Shell side has flue organic gases and tube side has the same organic fluid (as solvent) which is to be evaporated. The evaporator is working as rising film but we are not getting required results. We would like to know if the same is converted into natural circulation (thermosyphon) through external downcomer, would there be any increase in efficiency of heat transfer so as to recover more condensate from the shell side and vapourize more fluid on the tube side