Experimental Investigation on Natural Circulation in a Thermosyphon Reboiler

Thermosyphon reboilers are widely used in refineries, petrochemical industries and other chemical processes. The liquid product stream coming from the bottom of the vapor-liquid separator is heated in an evaporator consisting of a vertical tube or tube bundle. When the evaporation occurs, the specific volume of the two-phase fluid increases. The upward buoyancy force caused by the density difference between the evaporator and down-flow pipe drives the fluid flowing through the evaporator in to the separator and forms a natural circulation. The experiments were conducted in a pilot scale thermosyphon system in which the evaporator consists of 7 steel tubes (outside diameter 30 mm, wall thickness 2 mm, length 4 m). A mathematical model was set up to simulate the heat transfer and pressure drop, in which empirical equations from literature were used. With the help of the simulation, the flow boiling heat transfer coefficients inside the tubes can be evaluated from a few measured local wall temperatures.