The present invention relates to a process for preparing polytetrahydrofuran, polytetrahydrofuran copolymer, diester or monoester by polymerizing tetrahydrofuran in the presence of at least one telogen andor comonomer and of an acidic heterogeneous catalyst based on activated sheet silicates or mixed metal oxides in a fluidized bed. The arrangement of the support slats eliminates the need for springs to support the mattress and creates a plurality of internal air chambers, thereby providing insulation within the apparatus. The return-flow line is coupled with at least one component return-flow line assigned to the electronic component. Each of the two inner flange holding lip has a connecting part which obliquely extends from the inner side wall toward an open mouth of the trim portion, and a main part which extends from an extending end of the connecting part obliquely toward the bottom wall of the trim portion, thereby defining a generally inverted v-shaped cross-section.