Whenever the productivity of an indirectly cooled, isoperibolic so semibatch reactor (SBR) must be enhanced by keeping the reaction temperature nearly constant, an additional heat transfer surface, with respect to that provided by the reactor jacket only, must be installed. In these cases, internal coils are normally adopted, which additional surface is, however, intrinsically limited and not fully available from the beginning of the supply period. However, a suitable fraction of the overall heat transfer surface can be installed on an external reactor recycle loop: this allows also for a simpler monitoring of the reaction system by comparing the measured temperature to a nearly constant target value. In this work, a kinetic-free sizing criterion has been developed for isoperibolic SBRs (both homogeneous and heterogeneous), through which the required external heat transfer surface can be easily estimated.