A manufacturing system consists of operating units converting materials of different properties into further materials. In a design problem, we are to find a suitable network of operating units which produces the desired products from the given raw materials. If we consider this network design from structural point of view, then we obtain a combinatorial optimization problem called Process Network Synthesis or (PNS) problem. It is known that the PNS problem is NP-complete. In this work, we present such a subclass of PNS problems which is well-solvable.