A vacuum exhaust system for evacuating a plurality of vacuum chambers , said vacuum exhaust system comprising: a plurality of low pressure vacuum pumps configured to operate in the molecular flow region of the gas and configured for evacuating said plurality of vacuum chambers ; a plurality of chamber valves (V3) for isolating or connecting said plurality of low pressure vacuum pumps with said plurality of vacuum chambers ; a plurality of branch channels each connected to a corresponding exhaust of said plurality of low pressure vacuum pumps ; a main channel formed from a confluence of said branch channels and configured to provide a fluid communication path between said plurality of branch channels and an intermediate pressure vacuum pump , said intermediate pressure vacuum pump being configured to evacuate said main channel and to operate in a viscous flow region of said gas; and a higher pressure vacuum pump configured to operate in a higher pressure viscous flow region of said gas than said intermediate pressure vacuum pump , said higher pressure vacuum pump being connected to an exhaust of said intermediate pressure vacuum pump ; a plurality of bypass channels for providing a fluid communication path between at least some of said plurality of vacuum chambers and a higher pressure vacuum pump ; wherein said plurality of bypass channels each comprise a valve (V1) configured to open or close said bypass channel ; and characterised in that said higher pressure vacuum pump connected to said exhaust of said intermediate pressure vacuum pump and said higher pressure vacuum pump in fluid communication with said plurality of bypass channels is a same higher pressure vacuum pump . a plurality of low pressure vacuum pumps configured to operate in the molecular flow region of the gas and configured for evacuating said plurality of vacuum chambers ; a plurality of chamber valves (V3) for isolating or connecting said plurality of low pressure vacuum pumps with said plurality of vacuum chambers ; a plurality of branch channels each connected to a corresponding exhaust of said plurality of low pressure vacuum pumps ; a main channel formed from a confluence of said branch channels and configured to provide a fluid communication path between said plurality of branch channels and an intermediate pressure vacuum pump , said intermediate pressure vacuum pump being configured to evacuate said main channel and to operate in a viscous flow region of said gas; and a higher pressure vacuum pump configured to operate in a higher pressure viscous flow region of said gas than said intermediate pressure vacuum pump , said higher pressure vacuum pump being connected to an exhaust of said intermediate pressure vacuum pump ; a plurality of bypass channels for providing a fluid communication path between at least some of said plurality of vacuum chambers and a higher pressure vacuum pump ; wherein said plurality of bypass channels each comprise a valve (V1) configured to open or close said bypass channel ; and characterised in that said higher pressure vacuum pump connected to said exhaust of said intermediate pressure vacuum pump and said higher pressure vacuum pump in fluid communication with said plurality of bypass channels is a same higher pressure vacuum pump . <EOS>