In a sense they do lower the pressure in a clothes dryer by forcing hot air through it. Bernoulli’s principle says that moving air will have a lower pressure than still air. More importantly, however, the incoming stream of air encourages evaporation by continuously replacing the saturated air in the dryer with dry air from outside. After all it’s not the air pressure, per se, that matters, but the partial pressure of the water vapor in the air.
With regard to your practicality questions:
The world is heading away from cheaper water. And what you really are asking for here is to use the energy stored in the water, rather than the water itself. There are cheaper ways to make energy than to use running water. If your invention really caught on market forces would quickly drive it to the cheapest energy supply. My guess is it wouldn’t be mains pressure.
Somewhere in your design there’d have to be a rotating seal. Good ones are expensive and cheap ones are no good. And this would have to be for a household appliance – it would have to be very reliable and last a long time under a lot of heavy usage. In the current designs nobody cares too much if their clothes dryer leaks a little hot air from the odd seam or two, but if you’re trying to maintain a vacuum even a little leak is a big problem.
And now, after having rained all over your parade, I want to congratulate you for coming up with a new idea. It got me thinking about different ways to do this, and even though I don’t think you got it right this time, don’t give up hope. “There’s a pony in there somewhere!!”
If man was meant to fly faster than the speed of sound he would have been born with 5000 pounds of thrust.