How do Submersibles work?

Answer 1

While it's easy to push on water, it's hard to pull on water. When you drink soda through a straw, you may feel like you're pulling on the water, but you're not. What you are actually doing is removing some air from the space inside the straw and above the water, so that the air pressure in that space drops below atmospheric pressure. The water column near the bottom of the straw then experiences a pressure imbalance: the usual atmospheric pressure below it and less-than-atmospheric pressure above it. That imbalance provides a modest upward force on the water column and pushes it up into your mouth. So far, so good. But if you make that straw longer, you'll need to suck harder. That's because as the column of water gets taller, it gets heavier. It needs a more severe pressure imbalance to push it upward and support it. By the time the straw and water column get to be about 40 feet tall, you'll need to suck every bit of air out from inside the straw because the pressure imbalance needed to support a 40-foot column of water is approximately one atmosphere of pressure. If the straw is taller than 40 feet, you're simply out of luck. Even if you remove all the air from within the straw, the atmospheric pressure of the water below the straw won't be able to push the water up the straw higher than about 40 feet. To get the water to rise higher in the straw, you'll need to install a pump at the bottom, or a submersible pump. The pump increases the water pressure there to more than 1 atmosphere, so that there is a bigger pressure imbalance available and therefore the possibility of supporting a taller column of water. The submersible pump can boost the water pressure well above atmospheric and thereby push the water to the surface despite the great height and weight of the water column. Multiple stage submersible pumps are arranged in series so that the discharge from the first stage becomes the intake for the next stage with each successive stage adding its pressure to the previous one. Surface suction pumps are really only practical for water that's a few feet below the surface; after that, deep pressure pumps are a much better idea.