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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorrevor
    • CommentTimeJul 19th 2019
    Have installed a proprietary solar thermal system. The instruction for the orientation of the pressure vessel has the vessel positioned with the pressure relief fluid entering the bottom of the vessel and the gas side up.

    Being at odds with my understanding of this I queried it and was assured that the instruction was correct and there is even a installation video confirming this. I have trawled through illustrations of systems on the WWW and all examples I can find the vessel is orientated with fluid side up gas side down. I recently bought the very excellent handbook The Solar Heating Design & Installation Guide from CIBSE and it quite clearly states that the orientation of the vessel is fluid side up gas side down.

    Any views?

    (As an aside with no liquid in the system the solar collector temperature got up to 155 deg C I knew it would get hot but not that hot)
    I would expect that the pressure vessel has a membrane in it to separate the gas (air) from the liquid. That being the case it doesn't matter in what orientation the tank is placed.

    I have 5 such pressure vessels, 3 with the fluid coming in at the bottom and 2 with the fluid coming in at the top. (FYI 1 vessel on a DHW system the other providing pressure for cold water from wells). Commercial pressure vessels here have the fluid entering at the bottom with an air valve at the top for topping up/adjusting the pressure probably for ease of access to the valve, however you can also get pressure vessels that are mounted horizontally where the fluid enters at one end and the air valve is at the other, both in the middle of the tank.

    If you get a failure of the membrane with the fluid entering the bottom the gas will slowly be absorbed until there is none left and with the fluid at the top then gas will enter the system - and then get absorbed.

    Bottom line -I don't think it matters, they work in any orientation (if they have a membrane)
    • CommentAuthorrevor
    • CommentTimeJul 19th 2019
    Thank you GR8. That has confirmed a message I got just now from a technical author of the manual I mentioned. The vessel is as you describe with a membrane. It has a valve at the bottom which I have just been informed may be a form of auto isolating valve it also incorporates a drain valve.
    Your mention of them being mounted horizontally reminds me of having such a vessel on an irrigation pump fitted with on off pressure switch which I have for watering our vegetables on our smallholding.
    Posted By: revorThank you GR8.

    GR8 ??
    • CommentAuthortony
    • CommentTimeJul 19th 2019
    Damn text speak ... Great = G R eight
    • CommentTimeJul 19th 2019
    In a solar system I believe it is sensible to have the expansion tank at the bottom of a descending spur with the connection at the top, so that very hot liquid doesn't get to the membrane in the event of stagnation. It also means any dissolved air coming out of solution will find it's way up and out. Behaviour with a damaged membrane doesn't really matter, since the expansion tank needs replacing at that point.
    • CommentAuthorCWatters
    • CommentTimeJul 19th 2019
    My understanding is that on potable water (eg where the water eventually comes out of a tap or shower). The entry should be at the bottom. The idea is not to form a stagnant volume of water in the expansion vessel.

    On sealed systems I believe it should enter from the top. Elsewhere it says this is to minimise heat loss from the uninsulated expansion vessel. eg having a hot pipe above a colder uninsulated expansion vessel minimises convection. For same reason it should be on the colder return to the solar panels not the hotter flow.
    • CommentAuthorCWatters
    • CommentTimeJul 19th 2019
    Posted By: djhIn a solar system I believe it is sensible to have the expansion tank at the bottom of a descending spur with the connection at the top, so that very hot liquid doesn't get to the membrane in the event of stagnation.

    Google found a comment to that effect..

    "Continuous temperatures are in excess of rated values for some system components e.g. Circulation Pump and the membrane of the Expansion Vessel"

    Not sure if this link will work..

    • CommentAuthorphiledge
    • CommentTimeJul 19th 2019
    If you have the entry at the bottom and the pressurised gas at the top then you wont be able to vent the air out of the accumulator. If you design a system to have a slug of air in it then there isnt much point in having an accumulator as the slug of air will absorb any fluctuations in pressure.

    Unfortunately having residual air in the system isnt good from a corrosion point of veiw, nor from a pump cavitation point of veiw, nor from a heat transfer point of veiw, so mount the accumulator with the inlet uppermost on the return leg from the tank to the collector to keep the temp low and allow the air to be vented.
    • CommentAuthorrevor
    • CommentTimeJul 21st 2019
    GR8 Great!!
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