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Heating and cooling: Stratification – is it destroyed by coil heat exchangers for DHW?

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  1.  
    • CommentAuthorcookie
    • CommentTimeJan 24th 2009
     
    The commercial examples of thermal stores pump hot water from the top of the tank through a plate heat exchanger (heating the DHW) and deposit the colder water at the bottom of the tank.

    I would like to use an internal coil type heat exchanger for my DHW but I’m worried the stratification effect will be damaged and all I’ll be doing is turning my thermal store into a conventional hot water tank. Will the convection effect (cooler water now at the top of the tank will sink) make stratification pointless?

    The coil will only be in the top of the tank.

    Has anyone seen a plate heat exchanger mechanically driven by the pressure of the DHW (I think its against the water by-laws or something)? If not someone get your patent pad out!

    Does anyone know how to calculate the temperature at different heights of a tank as it cools or is heated up? Tank height say 2 metres?

    Why internal coil….
    I’m making it myself (I’m from Yorkshire, enough said)
    I don’t want an additional pump because I’ll be off grid electric

    Cheers Cookie
    • CommentAuthoradwindrum
    • CommentTimeJan 27th 2009
     
    If not too late/complicated why not have two coils for the DHW, one near the bottom for preheating and then another nearer the top for topping up thus helping to preserve the stratifiaction.
    • CommentAuthorcookie
    • CommentTimeJan 27th 2009
     
    I never thought of that, so basically a large section of coil at the bottom then the pipe feeds up to a second section of the coil at the top.... hmmm... might just wait.

    Although that would then take me back to one of my other posts that if water at the bottom of the tank is only 30C and it sits all week till I return it could develop legionella, hmm, it wasn't a massive problem in the tank, but in the DHW could be a serious problem.

    I really don't want to have to run / buy a pump and plate heat exchanger if I can get away with it, plus it would mean I'd need power for hot water, which is a problem if you haven't got power periodically :os

    Like your thinking though!

    Cookie
  4.  
    Use an over-large plate heat exchanger. It is also cheaper too as a plain cylinder is used with no expensive coils. The plate can be screwed off for cleaning or replacement too. Legionella is not such a problem with a plate as little water is stored in it. When the tap is turned on, after leaving the store for a number of days/weeks, leaving it a minute or two will ensure fresh water.

    Use a Danfoss RAVK valve on the return from the plate and pump. I posted some details of this. This will restrict the water back into the cylinder and preserve stratification. Also have a spreader pipe on the return to the cylinder. Drill out the pipe stop on a brass compression fitting. Slide in a copper pipe with holes facing downwards and an internal stop end in the pipe. BES supply the stops and parts http://www.bes.co.uk

    A boiler flow spreader pipe kicked upwards towards the tapping to the DHW plate heat exchanger. This also ensures boiler heated water is taken directly to the plate.

    Look at this thread. A DIYer was guided through making his own stainless steel heat bank by a professional:
    http://www.diynot.com/forums/viewtopic.php?t=137289
    • CommentAuthorcookie
    • CommentTimeJan 27th 2009
     
    Thanks for the input I'll have a good read of that.

    My cylinder is a large concrete tank lined with something, not sure yet, but all pipe work will have to enter from the top and drop down to the correct height I think, ensuring a leak free tank, not sure how I'll deal with expansion and contraction of the water as I don't know if I can construct it so its completely water tight with a header tank as yet :os

    Is it possible to have an open tank design as long as the water level in the tank is higher then the highest water pipe in the house? It’s going to be a bungalow. The tank would have an insulated lid and be fairly tight fitting, just not water tight enough to be under pressure?

    Cookie
  6.  
    First. Why concrete? :bigsmile:

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