1 rotor 1 may be a cylindrical member mounted on a drive shaft 8 as shown in the embodiment of fig3 or may be of one piece construction having an integral drive shaft the drive shaft 8 is mounted at its ends for rotation by bearings 17 as shown in fig3 as will later be more fully described in the case of a compressor drive shaft 8 may be driven by any suitable means such as an electric motor for example a longitudinal slot 4 is provided on the inner surface of casing 2 of a width and depth sufficient to releasably retain one roller 3 when engaged thereby in a stationary position with respect to its circular path of travel through annulus 20 intake port 11 and exhaust port 12 are provided through the casing 2 on opposite sides and in close proximity to slot 4 the exhaust port being closed by a suitable valve such as the spring urged ball check valve 13 the spring being adjustable so that the valve can be set to open at any desired pressure in this instance the intake has no valve but remains open and is on the right side of slot 4 since the rotor is driven.