This research note proposes a mathematical model for a closed-cross-flow-structured tubular reactor, which is a reactor that contains a structured packing (closed-cross-flow structure, CCFS) with very good heat-transfer performance. The structure is modeled by staged short adiabatic channels, represented by a PFR model, with intermediate cooling at the wall intersections. A simplification of representing several parallel channels by just one channel with averaged properties only leads to a minor loss of accuracy. For a 5-cm-thick tube, the loss in accuracy is <2%, while computation is accelerated by a factor of similar to 80. For wider tubes, even more numerical simulation time can be saved, but the simplification will result in larger losses in accuracy.