Abstract: A fluid flow diverter or reverser comprises a housing having a transverse separator wall dividing it into first and second axially adjacent chambers. The first chamber has a diametrally extending separator wall dividing it into subchambers. Each subchamber has an inlet or outlet, respectively, to which a conduit, that may serve as the feed or return of a process loop circuit or the feed or return of a fluid supply circuit, is connected. The transverse wall has a pair of openings which communicate the respective subchambers with the second chamber. The second chamber has inlet and outlet openings, respectively, which can connect to a fluid supply and return circuit or a process loop circuit. A diverter blade in the second chamber is reversibly rotatable for changing direction of fluid flow between the inlets and outlets and, hence, in the loop circuit.