Thermosyphon reboilers require pumping of the column bottoms liquid into the reboiler. Natural circulation is obtained by using the density difference between the reboiler inlet column bottoms liquid and the reboiler outlet liquid-vapor mixture to provide sufficient liquid head to deliver the tower bottoms into the reboiler. Thermosyphon reboilers are more complex than kettle reboilers and require more attention from the plant operators. There are many types of thermosyphon reboilers including vertical, horizontal, once-through or recirculating.

Thermosyphon reboilers play a wide role in the chemical industry, which provides a simple, low maintenance design for distillation tower reboiler system. The thermosyphon reboiler contains the two endearing qualities of the evaporator, namely mechanical simplicity and operation in the nucleate boiling regime with its attractive high fluxes. These reboilers require rational design procedure as several flow patterns manifests during the heat transfer to a flowing two phase boiling mixture, which in-turn depends on upon the flow rates, physical properties of the components, pipe diameter and orientation. The circulation rate, heat-transfer rate and pressure drop all are interrelated, and hence, iterative design procedures must be used.