Abstract: A jet pump apparatus for mixing a higher pressure gas from a higher pressure gas supply and a lower pressure gas from a lower pressure gas supply, includes an inlet section, an outlet section, a diffuser section and a mixing section, the inlet section including a higher pressure gas inlet and a lower pressure gas inlet, the mixing section including a plurality of separated mixing chambers, and the diffuser section including diffusers for receiving mixed gases from each of the mixing chambers, and the outlet section receiving mixed gases from the diffuser section and conveying the mixed gases to a mixed gas outlet, and the inlet section including a higher pressure gas inlet duct which receives higher pressure gas from the higher pressure gas inlet, and a lower pressure gas inlet duct which receives lower pressure gas from the lower pressure gas inlet, and for each of the plurality of mixing chambers, there being a primary nozzle for introducing primarily higher pressure gas from the higher pressure gas inlet d