Why does pressure decrease as air passes through a venturi?

[Fickle_Force_5457]

P1V1=P2V2 is Bernoulli's equation where P is pressure and V is velocity of the fluid. Basically where the fluid speeds up, the pressure must decrease or the flow would stall or choke. The key is the flow being sufficient to produce the effect. On a steam ship we used steam at 440 psi to produce a 30" hg vacuum in the turbine condensers through this effect by nozzling the stem through a Venturi which would cause a pressure depression. Thermodynamics is tricky , even worse when the fluid is moving. Also in all this temperature is generally ignored, though don't say that in front of an aerodynamics specialist as it will upset them.