A very serious problem of instability that arises in the vertical thermosyphon reboilers can be solved by a newly developed equipment called internal thermosyphon reboiler. Experiments were carried out in both quartz tube and copper tube with the same geometrical dimensions. The former allowed better visual observation and the latter gave more quantitative informations. Water, sulfuric acid, triethanol-amine, heavy oil, ethanol-water, methanol-ethanol, ethanol-propanol and some other mixtures were used as working media. Particles of polypropene with density very close to water were used as the tracer in water system for investigating the flow characteristics. Rate of circulation, flow patterns, rate of heat transfer and fouling in this equipment were studied. A flow model and a design method were proposed. The results calculated could predict the characteristics of two-phase flow and heat transfer inside this boiling equipment. A very serious problem of instability that arises in the vertical thermosyphon reboilers can be solved by a newly developed equipment called internal thermosyphon reboiler. the latter gave more quantitative informations. Water, sulfuric acid, triethanol-amine, heavy oil, ethanol-water, methanol-ethanol, ethanol-propanol and some other mixtures were used as working media. Particles of polypropene with density very close to water were used as the tracer in water system for investigating the flow characteristics. Rate of circulation, flow rate, rate of heat transfer and fouling in this equipment were studied. two-phase flow and heat transfer inside this boiling equipment.