Vertical thermosyphon reboilers and evaporators are widely used in the process industries. However, increasing the thermal efficiency of these units is very difficult. They are commonly used for about 70% of all evaporation duties in chemical industries. The flow in these units depends on the amount of buoyancy created by vaporization. The flow rate is therefore related not only to heat transfer rate, but also to evaporation, friction, and static pressure loss. The hydrostatic heat present at the base of a vertical thermosyphon reboiler suppresses boiling, creating a sub-cooled region. At the base of the tube bundle, this region length sometimes approaches a significant percent of tube length. Because single-phase convective heat transfer is the dominant heat transfer mechanism in this region, tube inserts can be used to promote heat transfer without blocking flow. In this article, using a simulation model that has been validated against the result of HTFS software, the effect of using different types of tube inserts in a sub-cooled zone of a vertical thermosyphon reboiler on the thermal performance of this unit is discussed.