Compression of water vapour as R718 is disclosed with and without addition of additives as an aqueous solution in rotational displacement machines, i.e. refrigeration, air-conditioning and heat pump technology. To largely avoid over or under-compression, it is proposed for the easiest possible adaptation of the currently effective internal volume ratio as so-called iV value in the displacer that the compressor housing starting from the outlet side with rotor profile length LR over a length LiV comprises planar, i.e. flat iV disks (3j) with the index j for 1≤j≤n; n is the number of disks; n≥1 with a width bj per iV disk having planar surfaces PF preferably perpendicular to the neutral axis AN. The iV disks are displaced in a targeted manner individually by movement control devices (5j) per iV disk in each case by a distance si where 0