Vapor flow nozzle exit loss

This suggests that the pressure in a water drain can get so low, that air could be sucked out of the bathroom and down the drain. Of course, we all see this happen several times a day—typically when we flush a toilet. So much air is drawn into the water drainage piping, that we install vents on our roofs, to release this air. The only requirement, then, for vapors to be drawn into a flowing nozzle is for the nozzle exit loss to be larger than the static head of liquid above the nozzle. 

Figure 16.4 Nozzle exit loss for vapor flow.

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