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  • bit of fun (perpetual motion machines)

    Discussion in 'The main mechanical design forum' started by Bobamuntung, Mar 3, 2011.

    1. LinkedIn Gopher

      LinkedIn Gopher Little furry chap

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      Here's one for you then:
      A long tube with a semi-permeable membrane closing it's lower end is placed deep into the sea such that it's upper (open) end is just above the surface.
      The membrane allows fresh water to pass but not the salt.
      The tube soon fills with fresh water.
      Because salt water is denser than fresh water, when the pressure across the membrane has equalised the fresh water column will rise above the sea water level.
      If allowed to overflow, you have perpetual water circulation.
      Ha Ha....a gravity perpetual motion machine !!

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    3. LinkedIn Gopher

      LinkedIn Gopher Little furry chap

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      Actually, the semi-permeable membrane is not the issue. Such membranes do exist and are used all the time. Look up "visking tubing" for example. It occurs naturally in the natural world and can be manufactured. It's used everywhere from sausage skins to dialysis machines. Come on guys you'll have to try harder than that !!

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    4. Dana

      Dana Well-Known Member

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      The problem with your semi permeable membrane is that if there's any flow through it, there will be a pressure gradient across it, so the pressure of the fresh water inside will be lower than the pressure of the salt water outside it. Looked at another way, if the fresh water inside is at the same pressure as the salt water outside, then fresh water will pass through the membrane back into the seawater as fast as it comes in. Either way, the numbers will cancel and there will be no net flow; it will do no useful work.

      In the case of the bubble machine, even if you had a frictionless seal, it would take work to open the seal against the pressure of the water, cancelling out the buoyancy.

      Actually a perpetual motion machine is quite possible: A magnetically suspended (i.e. frictionless) flywheel in a vacuum will continue spinning indefinitely. Such devices have been proposed (and perhaps used, I don't know) for energy storage. However, you can't take any energy out of it without slowing it down; the energy you put into it to spin it up will be the same as you get back out of it.
       
    5. LinkedIn Gopher

      LinkedIn Gopher Little furry chap

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      Hi Dana, The pressure gradient across a filter corrolates to flow rate does it not? The pressure differential across the filter will be determined by the depth (or height of water column). Therefore why not find sufficient depth to achieve a pressure differential to overwhelm the flow related pressure drop across the filter for a predetermined flow rate.
      Ps Don't forget, this is just a bit of fun ( I know I'm messing with you ha ha )

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    6. MikahB

      MikahB Member

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      The issue is not pressure drop across the membrane, but pressure differential required to cause the reverse osmosis process to occur. According to wikipedia (yes, I know that's not a real source :D ) required delta P for saltwater is 40-82 bar.

      A-ha - found a nicely detailed explanation here. Perhaps a useful machine somewhere, just not on earth!
       
    7. maniacal_engineer

      maniacal_engineer Well-Known Member

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      Actually there was an article in Scientific American regarding thermohaline gradients. water heats and evaporates on the surface becoming saltier than the deeper colder water, a tube connecting the surface to the depths, acting as a heat exchanger, can allow the circulation of the two deeper cooler less salty water gets heated coming up the tube and become more buoyant, surface water gets chilled and, being saltier, sinks. There needs to be a way to separate intakes and exhausts for it to be self sustaining. but it does work.
      a similar thing also works in Cameroon where a pipe to the bottom of the lake brings water saturated with volcanic gasses (hydrogen sulfide for example) to the surface. as the water rises and the pressure decreases the gasses come out of solution (like a carbonated drink) making the water in the pipe more buoyant. Once it gets started it self perpetuates. Thia was done recently to prevent the lake from "turning over" as it did a few years ago and poisoning thousands of people and cattle

      http://scylinfo.files.wordpress.com/201 ... saster.pdf

      not actually perpetual motion - but it looks like it
       

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