When one ejector discharges into the suction port of another ejector, this is referred to as 'staging'. Deeper vacuum requires more steam ejector stages. You can use this chart to get a rough idea of how many stages will be required to meet the vacuum level you need. A number of stages can be replaced with a Liquid Ring Vacuum Pump (LRVP). Systems incorporating both LRVP's and steam ejectors to produce high vacuum are referred to as 'hybrid' systems, which Fox Valve can, of course, supply.