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Liquid to liquid heat exchanger idea

Discussion in 'Home Power and Microgeneration' started by Chris W, Sep 26, 2005.

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  1. Chris W

    Chris W Guest

    In order to heat my water and still use the existing hot water heater, I
    was thinking about building my own liquid to liquid heat exchanger.
    Since there would be antifreeze in the solar collector, I need some way
    to make sure it will never contaminate the water in the hot water tank.
    So I came up with this design, tell me what you think.

    Chris W

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  2. Ecnerwal

    Ecnerwal Guest

    Seems like it would work, though I doubt it would work as well as the
    commercial double-wall heat exchangers - simply difficult to
    home-fabricate that sort of thing with similar efficiency, and lost
    efficiency makes your system work less effectively. In any case, use of
    non-toxic antifreeze is a good idea, though some jurisdictions require
    both non-toxic antifreeze and a double-wall heat exchanger.

  3. For efficient transfer of heat, you need surface area. While your idea would
    work, the dead air space acts as an insulator.
  4. Tom Miller

    Tom Miller Guest

    I doubt if it would be practical as the air gap between the heat exchange
    surfaces will have so low a conductivity as to be useless.
    I suggest that you make a conventional tube-in -tube heat exchanger and use
    propylene glycol as an anti-freeze (not ethylene glycol, its poisonous). Any
    leak will show up as sweet taste in the heated water. Simply marking the
    level in the header tank will show if there is a leak anywhere in the
  5. Chris W

    Chris W Guest

    Did you not see the copper "fingers" that give an easy path for the heat
    to conduct from the center to inner tube in the cross section view?

    Chris W

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  6. YouGoFirst

    YouGoFirst Guest

    In order to heat my water and still use the existing hot water heater, I
    You may also want to consider looking into a cross flow, countercurrent heat
    exchanger. I once built a heat exchanger using some copper pipe and a
    bucket. For my purposes I was trying to cool a water mixture down. The
    system used a 5 gallon bucket for its shell and copper pipe inside. The
    system worked quite well. If you use a single piece of pipe to go through
    your system, and grossly overside the pipe you will help prevent leaks due
    to pressure buildup. Also, if you use a large pipe and can reduce the flow
    rate through the heat exchanger, that will also help improve the efficiency.
  7. SQLit

    SQLit Guest


    The fingers are a joke and would be hugely labor intensive to install.
    Simple is better.
    Have you considered mineral oil? Anything that uses water will eventually
    need maintenance.
    Our flat plate exchangers need to be torn down once a year to keep them at
    optimal condition. They are aluminum alloy and are water to water medium. We
    spend a million a month on water treatment. Starting out with distilled
  8. Guest

    Higher velocities INcrease heat transfer...

  9. News

    News Guest

    Low viscosity car engine oil? Cheap enough and everywhere.
  10. Michael

    Michael Guest

    Low viscosity car engine oil? Cheap enough and everywhere.On the other hand, it fails the "non-toxic" criteria rather spectacularly
  11. Guest

    Yes, always.

  12. wmbjk

    wmbjk Guest

    Caution: extreme thread drift - I am suddenly reminded of claims by
    some that if you drive fast enough on washboard roads, you'll only hit
    the high spots. :) Oddly enough, the same people claim that driving
    fast worsens the washboard, when by their own logic it should be
    wearing down the high spots...

  13. Michael

    Michael Guest

    To some degree you're both correct, but....

    It's important to distinguish between heat transfer RATE (which generally
    increases with flow speed) and the total amount of heat transfer (which is a
    function of both rate and residence time), which does not necessarily
    increase with increased flow speeds.
  14. Guest

    Raising a fluid velocity ALWAYS increases heat transfer.

  15. daestrom

    daestrom Guest

    This is true. BUT, increasing the flow rate will usually *reduce* the
    outlet temperature of the liquid you're heating. These two statements are
    not contradictory once you consider that heat transfer rate is equal to mass
    flow rate times specific heat capacity times the temperature rise.

    Q = mdot*Cp*(Tout - Tin)

    Increasing the mdot, while reducing the (Tout - Tin) can still result in a
    rise in Q.

    If the fluid is being recirculated to a holding tank, then then only concern
    with increasing flow rates is the increase in pumping power needed and
    possibly flow-induced vibration.

    But if its a once-through, or open-loop system, increasing the flow rate
    will reduce the outlet temperature for the 'once-through' fluid and may drop
    it below the desired range.

    So, it depends on the exact application, and whether the heat-exchanger's
    outlet temperature is paramount or maximizing the BTU/hr rate to a
    recirculating fluid is the main concern.

  16. daestrom

    daestrom Guest

    And there seems to be certain speeds where the wheel's seem to 'resonate'
    and act more like they're 'floating' than really following the ups/downs in
    the roads.

    (or maybe I just need some new shocks ;-)

  17. Interesting diversion.

    No question in my mind that washboard has a periodicity to it, and
    that, with some vehicles, you can null it out by driving at the
    "right" speed. I noticed that driving an old VW Beetle on gravel roads
    a lot. But I don't drive that fast anymore. :>)

    Washboard seems to show up first on hills, and around my home, where
    the gravel starts at the end of the pavement. My own theory is that
    the periodicity of the washboard is in fact caused by vehicle
    suspensions breaking traction, with a resulting periodic axle hop.
    This creates high and low spots in the road surface, which in turn
    induces more axle hop in succeeding vehicles. A positive feedback
    mechanism, if you will.

    Now, when I encounter washboard, I just slow down. Why beat the heck
    out my vehicle, its contents, and myself?

    Gordon Richmond
  18. Cosmopolite

    Cosmopolite Guest

    You probably noticed that rear engine, rear wheel drive is the
    smoothest ride on gravel.
  19. Rick Frazier

    Rick Frazier Guest


    This may seem too simple, but if you just make the concentric tubes, with
    the clean water on one side, pressurized at house pressure (say, 50psi) and
    the solar water (or antifreeze) at a much lower pressure (10psi or less,
    depending on the head that results from the difference of the collector
    height and the storage tank/exchanger height), you shouldn't have to ever
    worry about getting clean water contaminated with solar water... Of course,
    if you are a worry wart, you can use a non-poisonous anti-freeze and tint
    the solar water with food coloring (massive amounts, say red?) and you'd be
    able to detect a leak without any problems at all. Of course, if you have
    any significant leak of the clean system into the solar system, it's going
    to blow the collectors anyway, as most of them aren't rated for anything
    near house pressures.

    Simpler is typically better. Less maintenance, and often better heat

    Good Luck!
  20. wmbjk

    wmbjk Guest

    I agree. On a freshly graded level road, the washboard shows up first
    on either side of cattleguards where there's more braking and
    accelerating. I've also noticed that even on the 10% grade on my own
    personal road, I can go for months driving fast in a car without
    creating washboard, but if a dually pickup for instance makes a single
    trip up the hill, the washboard is started immediately wherever the
    material is loose enough.
    On some vehicles there just doesn't seem to be any speed that can
    improve the situation. I drove a moving van once 10 miles on the
    proverbial bad road. Even crawling speed didn't help. I tell people to
    drive at whatever speed they get the smoothest ride. The worse the
    ride, the more they're enhancing the washboard. Reducing tire pressure
    can help a lot. What I find is that the older people are, the more
    they enjoy blaming young whippersnappers for the problem, even if
    Grandpa's turtle speed on his own driveway earned him the worst
    washboard in the neighborhood. ;-)

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