Apparatus comprising: a chamber having a chamber inlet and a chamber outlet; a first inlet for receiving a first gas stream; a second inlet for receiving a second gas stream; a first outlet for outputting the first gas stream to a first vacuum pump; a second outlet for outputting the second gas stream to a second vacuum pump; a conduit network for connecting the first inlet to the first outlet and for connecting the second inlet to the second outlet such that, for each gas stream, first and second flow paths are defined between its respective inlet and outlet, the first flow path passing through the chamber inlet and the chamber outlet, and the second flow path by-passing the chamber, and means for routing the gas streams through the network such that when one gas stream flows along its first flow path, the other gas stream flows along its second flow path.