Consider a nozzle connected to a vacuum source. As the pressure ratio gets to about 2, the flow through the nozzle will approach the local speed of sound, and the flow becomes choked flow. When the absolute pressure of the vacuum is decreased further, the flow speed will not increase. This is the Kantrowitz Limit, which limits the mass flow because the velocity is limited to the speed of sound, and the area, inlet pressure and density are all fixed. Aircraft jet engines are very much affected by this limit, once the inlet flow speed gets to Mach 1 the mass flow rate is limited, regardless of how much suction the engine creates.