Abstract: A vacuum exhaust system includes a plurality of low pressure vacuum pumps that operate in a molecular flow region and evacuate a plurality of vacuum chambers. A plurality of chamber valves are positioned between the low pressure vacuum pumps and the plurality of vacuum chambers. A plurality of branch channels are each connected to a corresponding exhaust of the plurality of low pressure vacuum pumps and a main channel is formed from a confluence of the branch channels. An intermediate vacuum pump is connected to the main channel and operates in a viscous flow region. A higher pressure vacuum pump operates in a higher pressure viscous flow region and is connected to an exhaust of the intermediate pressure vacuum pump. A plurality of bypass channels, each having a valve, provide a fluid communication path between at least some of the plurality of vacuum chambers and a higher pressure vacuum pump.