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Flat plate HX question.

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by maple1, Oct 12, 2013.

  1. maple1

    maple1 Minister of Fire

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    Kind of general, but basically - what is the point of diminishing return when heating just DHW with a flat plate, as far as how big of a hx? (Number of plates)

    Looking at 5x12, wondering if 10 plates will do, or if I should bump up to 20. Looking to get a good close approach, and send the return back to storage as cool as I can. I've got an Alpha on the supply side, and a B&G Ecocirc ready to plumb in on the DHW side - so I can vary my flow on both sides fairly good.

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  2. ewdudley

    ewdudley Minister of Fire

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    Here's some ball-park/sanity-check numbers.

    An electric household DHW supplies 4500 watt or about 15000 btu / hour.

    Below is part of a table for a 5x12-10 HX I found:
    Code:
    OKO Brazed 5x12-10
    
    hot side 180 degF => 160 degF
    cold side 40 degF => 140 degF
    
    btu/h  hot cold
           gpm  gpm
    25000  2.6  0.5
    40000  4.1  0.8
    50000  5.2  1.0
    
    Back when I got my thermodynamics merit badge I might have been able to calculate what the flows and deltaTs would be, but all I can offer today is guesswork. Looks like for a 5x12-10 at 15000 btu / hour you might be able to get 180 => 140 on the hot side and 55 => 140 on the cold side, with a couple gpm on the hot side and less than a gpm on the cold side.

    [Edit:] This all assumes you'd be using the HX to charge a DHW storage tank or conventional electric DHW tank. On demand is a whole different problem.
    Last edited: Oct 13, 2013
  3. Bob Rohr

    Bob Rohr Minister of Fire

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  4. Bob Rohr

    Bob Rohr Minister of Fire

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    How many GPM do you want to transfer. I started with a 5X10- 30 on my solar storage/ DHW HX. With well water at 55F I could provide a endless shower, about 1.5 gpm with the solar tank as low as 120F.

    I now have a 3X8-40 and when the solar tank drops to 130 I have to flow back the shower gpm to keep the shower temperature warm enough.

    If budget allows more plates = more performance, keeping in mind domestic water conditions will tend to foul the plates and limit transfer as you use the system.

    Some good HX sizing info starting on page 34 of this journal. The example shows a 5X12 -30 With 130F on the "A" side, 120F "b" side 50F incoming supply, 5 gpm flow rate.

    http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_11_us.pdf
  5. maple1

    maple1 Minister of Fire

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    I'll d/l that file & read through it later when I have more time - thanks very much.

    Asking as I want to be able to preserve stratification & draw down my storage as much as possible, when recharging an 80 gallon electric heater from storage only. GPMs are pretty loose - I can dial each side to some extent & demand doesn't really factor in with a tank that big & intermittent water usage.

    I had pretty well decided to go with a 20 over a 10, but had no info to base that on, just a hunch. But then also got to wondering if it would gain me much by going even bigger.
  6. jebatty

    jebatty Minister of Fire

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    Look at the tables. Relevant are Side A in and out and Side B in and out, gpm, pressure drop Sides A and B. Then pick circulators that will move the desired gpm at the specified pressure drop. Remember that 1 psi = 2.34 feet of pump head. For dhw, issues also will include minerals in the water and scale build-up in the hx. Provide for shut-offs and unions on the hx to so that it can be removed and flushed to clean scale as needed. And likely a mixing valve on the side feeding the house to keep water temp at about 125F maximum, although dishwashers need or can use hotter water.
  7. jebatty

    jebatty Minister of Fire

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