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Getting cold water into pressurized hot water container

  1. Mar 12, 2015 #1
    How can this be done?

    So there is a container with hot water, and it's highly pressurized. It also has an output potential with a pressure reducing valve. I'm trying to workout how to feed the pressurized container with colder water (to refill).

    My first thought was that maybe when pressure is released through the pressure reducing valve, will it be possible that as it's releasing water, it can also be fed as the pressure going out can also bring water in? -Although I'm wondering if the uniform pressure still contained inside will prevent inlet.

    What do you guys think? -Hope this isn't too vague.
     
  2. jcsd
  3. Mar 12, 2015 #2

    Bystander

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  4. Mar 12, 2015 #3
  5. Mar 12, 2015 #4

    Bystander

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    What pressure?
     
  6. Mar 12, 2015 #5
    I'm not sure yet, depends on a lot of unconsidered factors (fresh design). Think of it as a 3 litre pressure cooker you want to refill without wasting heat. Hope that helps.
     
  7. Mar 12, 2015 #6

    CWatters

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    The cold source will have to be at a higher pressure than the hot water tank or some sort of pump is required. To make life easier for any pump look at adding some sort of expansion vessel between the pump and the hot tank. Bladder tanks are used on heating systems. Unfortunately these live longer if they are attached to a cold/cooler part of the system - otherwise I'd suggest making the hot cylinder out of one.

    http://en.wikipedia.org/wiki/Expansion_tank

    You don't need one this big but just to illustrate..
    http://www.screwfix.com/c/heating-plumbing/expansion-vessels/cat830984
     
  8. Mar 12, 2015 #7
    Started realising the colder would need to be at a higher pressure. Can't there be some sort of vacuum effect. Maybe this principle:
     
  9. Mar 13, 2015 #8

    CWatters

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    That works because the bottle is filled with hot water vapour not air. When the hot water vapour is condensed back into water it's volume, and hence the pressure in the bottle reduces dramatically. The pressure outside the bottle (eg in the cold water) is therefore higher than the pressure inside the bottle and cold water is pushed into the bottle.

    The very first steam engines used the same principle. Instead of high pressure steam to push on a piston the early steam engines used condensing steam to "suck" and atmospheric pressure to "push" on a piston in exactly the same way shown in your vid.

    We can't know if that method can be used in your case as you haven't really explained what you are trying to do.
     
  10. Mar 13, 2015 #9
    What if there is 3 litre chamber with steam (I imagine all the water has vapourized due to high temp) and then then a serperate chamber where you refill the water manually, and whilst doing this the hot chamber is sealed, but once the refill chamber is closed, there is an opening in a small pipe from this refill tank to the hot chamber (from the bottom), will that hot steamed area then be able to refill itself with this vacuum effect? This steam would combine with the room temp water and suck the water in the tank right?, as long as the steam volume didn't consume the volume of the hot chamber?

    Like I mentioned above, imagine you have a pressure cooker full of steam, then some of that steam gets released with pressure reducing valve, but now you want to refill the pressure cooker without opening it (loss of energy).
     
  11. Mar 13, 2015 #10

    CWatters

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    If you cool the hot chamber then the pressure in the hot chamber will reduce. If it reduces below that of the cold/refill chamber then yes some water will be transferred from the cold to what was the hot chamber until the pressure is equalised. You could then isolate the two and reheat the hot vessel.

    You have opened it....to let the cold water in.

    It's best not to think of it as a "vacuum effect". What pushes the cold water into the bottle is always the excess pressure in the cold bottle compared to the hot.
     
  12. Mar 13, 2015 #11

    : ) OK, well imagine that you can open one part to let water in, which doesn't affect the hot chamber, but by the flick of a switch the cold water is exposed to the hot chamber by a small tube.
     
  13. Mar 13, 2015 #12

    CWatters

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    I'm not sure what you want me to say. The laws of thermodynamics will tell you what happens when two vessels, one cold, one hot are connected together. If you know the starting conditions in each vessel (pressure, temperature, volume etc) you can work out exactly what the conditions will be after they are connected together by a small tube.

    The important thing to understand is there is no way to end up with more energy in the system than you started with. To do that you have to supply it with more energy by using a pump (post #2) or some more complicated scheme. So it will always require energy to raise the pressure and temperature of water.
     
  14. Mar 13, 2015 #13
    I'm still pondering this.. Is it not the same as what occurs in that video? Hot vapour chamber meeting with cold, draws in cold in to hot chamber? ..I'm going mad with this. I've a great idea for a design, but this is may last problem.
     
  15. Mar 13, 2015 #14

    billy_joule

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    No. Energy is removed from the hot chamber, the vapour condenses and the pressure drops. It is no longer a hot chamber.
    The pressure in the chamber is now below atmospheric. Atmospheric pressure forces the water into the cold, low pressure chamber.
    Is you want water to flow in to a high pressure chamber you need the water to be at a higher pressure than the chamber, there are no two ways about it.
     
  16. Mar 13, 2015 #15
    It's just hard for me to imagine what your saying, when I think of the video. Vapour in glass, in cold water, sucks in the cold water. Does it not work in my case simply because of the added pressure built up in the hot chamber? Before seeing that video I just imagine the higher pressure chamber would force it's release into the lower pressure chamber to lower its energy.
     
  17. Mar 13, 2015 #16

    billy_joule

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    What do you need to cook for so long that you need to top up the water? Why can't you simply start with more water in the vessel?
    At what rate are you losing steam out of your vessel?
     
  18. Mar 13, 2015 #17
    Think I solved it. Pressure builds in hot chamber,
    Haha not cooking. Don't worry, not building a bomb either.

    Wondering if I could accomplish it by having the hot chamber trigger a switch which allows the cold water to rush in. The switch could be triggered by low pressure in the heat chamber..

    There will be a definite amoutn of cold water which will go in to a refill chamber, then safely sealed.
     
  19. Mar 13, 2015 #18
    hmm or I could have it where high pressure force in the hot chamber exerts a force on a button which keeps the two chambers seperate, as water is let out from the hot chamber as steam (outside the unit) then pressure drops, allowing cold water in for heating. Although I would need to have it so the door temporarily opens until hot pressure can keep it shut again..
     
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