Abstract: A mobile cleaning device for large flat surfaces includes a housing which forms a spray chamber having an open bottom. A first pair of exhaust ducts is mounted on the front and rear walls of the housing, and a second pair of exhaust ducts is mounted on the top wall of the housing adjacent the housing side walls for removing the debris and spent cleaning fluid from within the spary chamber as the device is moved along a surface being cleaned. An improved fluid-powered rotor is mounted on the housing and rotatably mounts and drives a plurality of fluid spray nozzles by use of the back pressure exerted by the cleaning fluid as it is sprayed from the nozzles against the surface being cleaned. The rotor is provided with an effective seal for the high pressure fluid with respect to the stationary components adjacent the movable rotor.