This might explain better As air is ejected from nozzle A, it mixes with the air in the tube at B Imagine that the air from the nozzle mixes with four times as much air. Momentum is conserved, so this cloud of air, now five times as much as came out of the nozzle, is now moving at one fifth the speed of the jet from the nozzle. All that air moving to the right requires replacement air to be pulled in from the left. So we now have suction at C. Moving five times as much air may seem efficient, but the kinetic energy of the moving air is a function of velocity squared. So we may have five times as much air moving, but the kinetic energy per volume of air is now 1/25th of what it was in the jet, and the total kinetic energy is one fifth of what we started with. Venturi pumps are not efficient. regards Mike

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