The length of the tubes is defined by the designer. It is not advisable to use tube lengths higher than 6 m because the construction is more difficult. Cleaning operations are also more difficult with long tubes.
However, there are certain instances in which it is imperative to maintain the countercurrent configuration, and large heat transfer areas are necessary (e.g., in natural gas conditioning plants to exchange heat between the inlet and outlet gas). In such cases, tubes up to 20 m long can be used.