Publisher Summary This chapter discusses the ejectors and mechanical vacuum systems. “Vacuum” refers to the degree of emptiness of a process system. A perfect vacuum represents an absolute zero of pressure, which is technically not maintainable. A vacuum system indicates a system that can be a matter of the degree to which the system approaches absolute zero pressure. To create a vacuum in a fixed system, it is necessary to draw out or pump out the air in the volume. Ejectors may be single or multi-stage and also multi-jet inside a single housing or stage. The extra stages, with or without the interstage condensing of steam, allow the system to operate at lower absolute pressures than a single stage unit. The capacity of an ejector is expressed as pounds per hour total of noncondensable plus condensables to the inlet flange of the unit. For multistage units, the total capacity must be separated into pounds per hour of condensables and noncondensables. The final stages are only required to handle the noncondensable portion of the load plus the saturation moisture leaving the intercondensers.