To provide greater indirect heat exchange than conventional heat exchange tubes with or without fins in usage in a medium comprising a liquid phase, in which flowing gas bubbles or particulate solids and flowing gas bubbles are dispersed.

Ribbed heat exchange tubes in which an aspect ratio of ribs is less than 5 are used. The ribs, and typically also the longitudinal axis of the heat exchange tube, are aligned spirally along the gas bubble flow direction. This is useful to increase either of (i) the productivity of a slurry Fischer-Tropsch hydrocarbon synthesis reactor without any increase in the temperature in the reactor or (ii) the production quantity of higher molecular weight hydrocarbons by reducing the reaction temperature at a given material conversion rate.