Abstract: Disclosed is a first stator defining an internal cavity and a second stator within the cavity of the first stator. A cylindrical rotor is disposed within the cavity. The rotor defines a first chamber within the cavity between it and the inner walls of the first stator and a second chamber within the cavity between the rotor and the outer walls of the second stator. The rotor carries vanes radially slidable in slots spaced circumferentially about the rotor. The inner and outer walls of the first and second stators respectively are spaced one from the other such that the linear distance therebetween along any extended radius of the rotor is substantially equal to the radial length of vanes whereby the outer and inner edges of the vanes are in continuous engagement along the inner and outer walls of the respective first and second stators upon rotation of the rotor. An inlet and an outlet are provided each of the first and second chambers.