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Why doesnt water evaporate from central heating?

Discussion in 'Plumbing and Central Heating' started by boolie, 23 Mar 2021.

  1. boolie

    boolie

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    Just curious, water in the central heating system gets heated up to 80 degrees Celsius - why doesn't it evaporate significantly?

    e.g. I can check my heating system pressure after a year and its still on 1.5 bar; i.e. no noticeable drop after a year of use.
     
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  3. bellairaphon

    bellairaphon

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    Can’t evaporate if there’s nowhere vapour or liquid can escape from.

    Nathan
     
  4. oldbuffer

    oldbuffer

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    Because its sealed and there is nowhere for the water vapour to go to. Or at least there shouldn't be!
     
  5. Nige F

    Nige F

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    Because the system is sealed - nowhere to evaporate to o_O
     
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  6. boolie

    boolie

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    interesting. makes sense. it does evaporate but can't really escape so just cools down back into the pipes.
     
  7. denso13

    denso13

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    No, the expansion vessel takes up water expansion but the water doesn't evaporate at all.
     
  8. boolie

    boolie

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    ok so the conditions inside a sealed system mean that the water cant evaporate - there is no air
     
  9. stem

    stem

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    There shouldn't be air in a heating system but the lack of air per se is not why the water doesn't escape.

    If you were to half fill a plastic bottle with water seal the top, then leave it out in the sun, the water will stay there permanently even though the bottle is also half full of air. The water remains, because as the previous posters have pointed out there is nowhere for the water (or water vapour) to escape from.
     
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  11. boolie

    boolie

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    interesting. so the water get's hot and expands into the EV. Now what I'm not sure about is does any evaporation occur inside the sealed system (evaporation without escape)?

    Can the water evaporate in the pipes? i.e. the water evaporates, the vapour remains in the pipe and then turns back to liquid form when it cools down

    @denso13 said no evaporation occurs at all. Theoretically lets assume the sealed system is entirely full of water and there is no air. So given @stem example; the bottle is full to the brim with water - does any evaporation occur?
     
  12. bellairaphon

    bellairaphon

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    Some evaporation occurs definitely some water molecules won’t need as much energy to become superheated.. and because of this the system will be saturated, a mixture of liquid and vapour.. which is latent heat and cannot be measured.
     
  13. denso13

    denso13

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    Not if they are full of water, as in a sealed system.
     
  14. vxxbcks

    vxxbcks

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    The pressure prevents the evaporation. Water can remain in liquid state far beyond 100°C in a sealed system, and actually this is the reason that sealed storage cylinders have lots of regulations around them. If there were a fault leading to the cyliinder being heated to say 130°C at 7 bar, then there was a fault leading to depressurisation, then the water boils off, expanding by hundreds of times in volume and hence creating the explosion risk.
     
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  15. Mottie

    Mottie

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    That’s why you never take the rad cap off of a car with a hot engine. It’s not the pressure that blows off, it’s water past it’s atmospheric boiling point instantly turning to steam. (y)
     
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  16. MJN

    MJN

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    It's not so much that the water/vapour can't escape (although this is of course true), it's more the case that once sufficient water has evaporated to saturate the air space (to 100% relative humidity i.e. the point at which it cannot hold any more water) then condensation starts to form. Evaporation still continues but it is balanced out by an equivalent rate of condensation hence the equilibrium gives the appearance that it's stopped (it really hasn't though - some molecules will be going liquid-to-gas and others gas-to-liquid).

    It does - indeed if you could see inside your heat exchanger you'd see small bubbles forming in localised hot spots (like you do in a kettle or pan) where sufficient energy has enabled water molecules to break free from their liquid suspension and become gaseous. However, as with the situation with the sealed bottle, as those molecules lose energy once they've left the heat source (and their temperature falls below the boiling point) they will condense back into liquid form again due to the evaporation/condensation equilibrium having been reached; it's an endless cycle. Note however that the majority of what's happening here is more actually described as *boiling* as opposed to *evaporation* - the latter is, strictly speaking, a phenomenon that only occurs at the liquid-gas interface which, inside a sealed system with all air removed, represents a very small surface area. With an open vented system it is much bigger by virtue of the open tank being exposed to atmospheric air hence resulting in far more evaporation.
     
    Last edited: 27 Mar 2021
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  17. D_Hailsham

    D_Hailsham

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    At Last! an accurate description of evaporation.

    Even with an open vented system the boiler, pump, pipes and radiators form a closed circuit. So the only place where evaporation can occur is at the surface of the F&E tank.
     
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