Heat exchanger for air conditioning fresh water loop

Discussion in 'Boat Design' started by Narcosis, Sep 11, 2013.

  1. Narcosis
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    Narcosis Junior Member

  2. philSweet
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    philSweet Senior Member

    hi Narcosis

    The pool heat exchanger is supposed to work for a wood-burning stove. I think the temperature difference between the two units would make it hard to compare. There may be a 40 degree delta assumed in one unit and a 10 degree delta in the other.

    I think a small forced exchanger will work wonders if you direct the wastewater away from the hull. There is a lot of transfer from the heated water back into the boat in stagnant conditions. You can improve the capacity of your units substantially and reduce the heat migration back into the boat by putting the exchanger on the hot side of the keel cooler.

    For what it's worth, I don't much like keel cooling for ac. I think way too much heat comes back in. Suppose 40% comes back in - you would need to size the ac at 170% of the load! That's a 70% reduction in efficiency. If only 20% returns - 125% of load. You have to actually account for the heat transfer out the hull and the way that it changes with keel coolers. You need to get that hot water away from the hull. You may not need to resize the ac's if you go to pumped raw water. Just make sure it is directed as far away from the hull as possible. Preferably out one end. Make sure your diesel fuel temp stays above the outside dew point always and that the top of the tank doesn't get cooled. Assuming the desired inside temp is not more than about 5 degrees cooler the the outside and you are just fighting off solar radiation uptake, every degree you gain in feedwater will translate to a nearly a degree in cabin temp. Reducing the heat migration back inside could make this more than a one to one proposition.
     
  3. FAST FRED
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    FAST FRED Senior Member

    < I think way too much heat comes back in. Suppose 40% comes back in ->

    One would assume the top of the tank has insulation so very little would heat the ER and little of that be passed into the living spaces.

    I dont think the 2 or 3 percent improvement ( IF THAT) in a lower cooling water temp is going to solve the posters problem.

    It could be that service is required on all the units but usually if the freon is a liquid at the TX valve its temperature is meaningless.

    Perhaps the mid west sizing of the installed air cond is just too small for the tropics, not the size of the circ water transfer area.?

    DO all the AC units run together 100% of daylight?
     
  4. Bglad
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    Bglad Senior Member

    I think Phil hits the nail on the head in his second paragraph. You need additional capacity for a specific circumstance. Add in the heat exchanger and when you don't need it secure the seawater valves, flush it clean, and install antifreeze so it is treated and ready for its next use.

    Pros - It only needs to be sized to cure an incremental shortage (smaller costs) and you retain your onboard treated water system (if you were actually treating it)

    Cons - Additional pump and piping located inboard

     
  5. Bglad
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    Bglad Senior Member

    One other thing. Possibly the inlet for the raw water pump might induce some waterflow under the hull instead of relying solely on convection to move the water around the keel cooler.
     
  6. jonr
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    jonr Senior Member

    A related point for anyone doing AC: seal up air leaks. Not only is incoming air warm, it also carries moisture. And removing moisture (latent load) is quite energy intensive (can be 1/2 of the total load).

    Another point is that when it comes to preventing mold, a few hours/day of drying works. Ie, on an unoccupied boat, you should find that running a dehumidifier 4 hours/day is enough.

    The only real concern I have with a water to water heat exchanger is how fast it will foul and how hard it is to clean. Acid? A plate heat exchanger that can be disassembled?
     
  7. jonr
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    jonr Senior Member

  8. Yobarnacle
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    Yobarnacle Senior Member

    http://weaponxmotorsports.myshopify.com/products/amsoil-radiator-and-heat-exchanger-addative

    " Maximize your engine and/or super charger's efficiency with Redline Water Wetter's radiator/heat exchanger additive for a cooler intake charge and more HP! Add this to your distilled water system for maximum cooling.


    WaterWetter is a unique wetting agent for cooling systems which reduces coolant temperatures by as much as 30 degrees Farenheit"

    I have also been told, distilled water cools beter than tap water.

    So maybe changing your coolant is all that is necessary. Cheap to test if it does.

    We use coolant additives in keel coolers on ships diesel engines.
     
  9. Narcosis
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    Narcosis Junior Member

    Excellent replies guys, thanks.

    I'm still leaning toward this solution. In the cooler months the systems are adequate and the exchanger could easily be decommissioned. Additionally, I could add a thermostat activated switch that turns on the sea water pump when the fresh water loop reaches 90 degrees. No reason running a pump if its not needed.

    I emailed Flagship Marine with my issue. They are an aircon manufacturer. There response was this:
    I can assume that their heat exchanger solution, properly sized, would keep the system very close to seawater temp (80 degrees). I'm wondering if the existing geothermal units have oversized condenser coils? They never throw a high pressure error and the cooling delta is always greater than 15 degrees regardless of the keel cooler temp being 80 degrees or 105 degrees. Confusing.

    I attached a picture of one of the keel cooler (really a skin cooler) channels.
     

    Attached Files:

  10. jonr
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    jonr Senior Member

    10F above the seawater temp would be a achievable value. Very close (say 1 or 2F) makes for a ridiculously large system.

    Done right, you could reduce the size/energy use of your over-sized circulation pump. For example, plumb your additional heat exchanger in parallel, not series.

    Note that most heat exchanger ratings are scams - for example, I just looked at one rated for 150K btu. But in the fine print, you see that this is with a 108F delta-T. So it would do about 14K btu in your application.
     

  11. sdowney717
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    sdowney717 Senior Member

    Why not get an engine heat exchanger?
    Surely that could lower the water temp and likely cheaper than what you talking about.
    For the sea water flow perhaps a large centrifugal style pump which is more reliable and will use less power than an impeller pump.
     
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