Abstract: An undulating prismoid module made of relatively rigid material. The prismoids may have bases of any number of sides, and preferably a plane passing through the centers of the prismoid bases in any row of prismoid bases is nonparallel to the sides of the module. The modules may be stacked and fluid inlets and outlets provided in the void volume between modules so that the modules may be used as a heat exchanger, an extraction tower, absorption towers, distillation towers, an adsorption tower, chemical or bilogical reactors, and water cooling towers. The modules are disposed in repeating patterns of four in the stack, each module in the stack being rotated 180.degree. about a first or second horizontal axis with respect to the previous model.