Abstract: A refrigerant or coolant compressor of the radial type with an impeller hng a plurality of vane elements can be used for compressing water vapor as a refrigerant or coolant under vacuum conditions. The impeller of the compressor is constructed to produce a high volume flow rate at the required compression ratio, in view of the low density of water vapor as the preferred flow medium. The impeller has sufficient strength to operate at the required high circumferential velocities. The impeller includes vane elements, disk elements, vane support elements and a hub. The vane elements are individually connected to the hub by the support elements. The support elements may be either ring-shaped elements connecting a rear surface of the disk elements to the hub, or may be pin-shaped insert members connecting the root of each vane element to the hub. The components of the impeller are made of a polymer composite material reinforced preferably with carbon fibers.