Patent ID: 7364415

Claim:
An apparatus for pumping or compressing a fluid, the apparatus comprising: a housing which has an annular bore, at least one inlet and at least one outlet communicating with the bore, a first piston, a second piston, each said piston adapted for movement relative to the bore, drive means to drive one said piston relative to the bore, a first releasable locking means to temporarily lock and release a said piston to the drive means, and a second releasable locking means to temporarily lock the other said piston against movement along the bore, wherein the first and second pistons move according to a repeating cycle in which the first piston, when locked to the drive means, moves in a unidirectional at least partial loop around the bore until movement of said first piston unlocks the second piston which is until then locked against movement relative to the bore, and the first piston replaces the second piston by becoming the piston locked against movement relative to the bore and the second piston becomes the piston locked to the drive means.