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

    My boat has 7 stand alone AC units on a fresh water loop that is keel cooled (aluminum hull skin cooler). Works great while at anchor in a current and I love the fact that I don't have sea water piped throughout my boat or a sea strainer to constantly clear, but its not so great at our home marina where there is zero current and the water direclty under the hull heats up making the loop temperature reach 100 degrees and air conditioners highly inefficient.

    So... this got me thinking: http://www.flagshipmarine.com/heatexchanger.html They have a 180,000BTU fresh water to sea water heat exchanger for about $3,000. Why can't I use a titanium pool heat exchanger instead to cool my fresh water air conditioning loop like this? http://www.amazon.com/Titanium-Water-Exchanger-Outdoor-Boiler/dp/B00B1GU8WW

    My plan is to add a properly sized heat exchanger and raw water pump inline with the fresh water keel cooled loop. This will will be more efficient at a marina and also allow me more BTUs of heat exchange to add larger sized air conditioning units needed in some areas of the boat.

    So... what am I missing here? $3,000 cupro-nickel purpose-built vs. $600 titanium mass produced. Seems that BTUs of heat exchange is BTUs of heat exchange right?
     
  2. FAST FRED
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    FAST FRED Senior Member

    but its not so great at our home marina where there is zero current and the water direclty under the hull heats up making the loop temperature reach 100 degrees and air conditioners highly inefficient.

    100F is hardly a problem for most cooling systems , not sure you could measure the -inefficiency-.

    Probably the required circulation pump would eat more juice than you could save.
     
  3. Narcosis
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    Narcosis Junior Member

    I'm no refridgeration expert, thats good to know. Quite a frew refer guys were amazed the units were able to function with such high entering water temp.

    I suppose I can replace three of these 12,000BTU units with three 18,000BTU (the salon gets too hot in the afternoon) and add the heat exchanger if they are throwing high pressure errors. I live aboard full time and the goal is comfort. Efficiency is the secondary directive.
     
  4. jonr
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    jonr Senior Member

    Perhaps easier:

    http://www.de-icer.com/mounting-options.php

    But if you can get the hull to 110F, it should kill the barnacles.

    Heating sea water causes fouling in a heat exchanger and it takes very little fouling to kill heat transfer efficiency.
     
  5. Narcosis
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    Narcosis Junior Member

    I thought of something like that, but didn't realize there was a commercialy designed version (although for ice).

    I'm not convinced my skin cooler is properly sized to begin with and my plans are increasing my air conditioning capacity from 84,000BTU to 112,000BTU.

    I'm sitting in 80 degree water on a 90 degree day and the circulation water temp is about 105 degrees on the cool side and 120 degrees on the hot side. I have a 1.5HP 220V circulation pump circulating approximately 3,500GPH through four 20 foot long by 12 inch wide skin cooler channels (so 80 square feet of surface area). Layers of epoxy and anti-fouling paint covering the aluminum hull including skin cooler seems like it would make any thermal calculations irrelevant.
     
  6. jonr
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    jonr Senior Member

    At those temperatures, you are better off using air source heat pumps. Ie, add an inverter driven mini-split (or multi-split).

    Regarding water flow, 3,500 gph at a 15 degree rise comes to ~400,000 btu/hr. One of your numbers isn't right.
     
  7. FAST FRED
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    FAST FRED Senior Member

    First thing I would check is the amount of antifreeze in the cooling system.

    Flush the system with a cleaner that is OK for your aluminum tank, and refrigeration goodies.

    Then use no more than 35% antifreez for corrosion protection.

    Water is a better conductor than antifreez which is only 5/8 as good.

    Perhaps simply better coolant will lower the coolant temperature?

    Another simple method while dockside could be a submersible pump as designed to clear ice from boat hulls in freezing areas.

    The extra water flow on you not moving vessels cooling section could be worth the attempt.

    Does the existing system work when underway with more water flow?
     
  8. Narcosis
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    Narcosis Junior Member

    Since we never get freezing temperatures here there is no glycol/antifreeze in the system, just water. While underway and/or anchored where there is current the system works adequately.

    If the keel cooler is properly sized, shouldn't it keep the loop water very close to the sea water temperature? Example: 80 degree seawater, 80 degree keel cooler loop.

    As a comparison, would it be possible to run a keel-cooled dry-stack engine at WOT indefinately in stagnant tropical water with NO water moving over the keel? Or wouldn't water movement over the hull be required?

    I'm starting to believe keel cooling is only effective when there is water moving across the hull. Otherwise, the seawater under the hull just warms up and slowly reduces the efficiency of the keel cooler.

    The A/C in the salon is undersized to begin with, although the boat is very well insulated. Ultimately I'm going to be replacing the three salon air conditioners with larger units and I'm concerened that it will just amplify the problem.

    How would pumping raw water through a heat exchanger that is in-line with the keel cooler loop be any different from an efficiency standpoint than running a submersible ice breaking pump under the hull?

    Again, I appreciate efficiency, but comfort and practicality is most important since we live aboard full time. The boat is a custom 70' aluminum motoryacht. The way I see it, I have three options:

    1.) Convert to an open system and just pump sea water through the entire boat.
    PRO - Most efficient for cooling equipment (constant 80 degree supply of water)
    CON - Even though it would be the least expensive initially (replumb supply to seachest and return to through-hull) the existing keel cooler should be decommissioned by grinding off and welding shut / replating areas of the hull. Not cheap in the long run.
    CON - Sea water belongs on the OUTSIDE of the boat or pumped contained in the engine room with waterproof doors.
    CON - Pumping sea water to the flybridge 14 feet above sea level results in increased head pressure which requires larger pumping capacity and more elect current. The efficiency of the constant 80 degree water above is negated by a larger pump.

    2.) Haul the boat and reengineer a new keel cooler.
    PRO - Less complexities, no sea strainer to contantly clean, and I just like the idea of it.
    CON - Is the idea of a fresh water loop keel cooled air conditioning system the best option in the subtropics/tropics? It seems current or vessel movement is required for optimal keel cooling.
    CON - Estimated cost for welder and yard is between $5,000 and $10,000. Not cheap.
    CON - Properly engineering a keel cooler is beyond the expertise of most local pros.

    3.) Install in-line heat exchanger or even box cooler with raw water pump in engine room.
    PRO - Simple bolt-in solution. I can simply buy my required BTUs and upgrade/increase my cooling capacity.
    CON - More plumbing and complication to existing systems
    CON - Cost between $1,000 and $3,000.
    - ???
     
  9. lucdekeyser
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    lucdekeyser Senior Member

    water as a heat sink

    Theoretically, this is hard to believe, unless the fauna and flora sticking to the hull would form an insulating pad. And even that is at least 80% water.
     
  10. jonr
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    jonr Senior Member

    Except that when the return water flows back down 14 feet, it cancels out the head pressure. Net effect - the height makes no difference.

    Moving water always improves heat exchanger efficiency.

    But I'd consider option#4 if corrosion can be avoided: add an air based A/C unit.

    Or construct a closed loop heat exchanger that gets lowered down into the water but isn't attached to the hull. http://www.awebgeo.com/What_is_it.html (or just use a coil of plastic pipe)
     
  11. Narcosis
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    Narcosis Junior Member

    Great point! I hadn't realized this, so its the same as a closed circuit and there is only frictional loss. Still... the seawater goes on the outside of the boat. And we're talking about a couple THOUSAND gallons per hour.

    This is a no-go. Especially in the tropics. Where does this hot air go? Cooling tower on the hard top? What does that sound like? Lots of houseboats around here have rooftop AC units and their flybridge is steaming hot from the AC exhaust. The sound is annoying. I couldn't imagine what a marina of 100 boats with roof top AC units or window AC units would sound like. I don't know much about thermal dynamics, but I do know you need to move a lot more volume of air through a heat exchanger compared to water.

    Its funny you should mention that. My A/C units are non-marine geothermal units, but I don't see the advantage between throwing a plastic pipe in the water vs. having aluminum "pipe" welded to my aluminum hull covered with epoxy and antifouling paint. Might work for a boat that never goes anywhere, but when we set the anchor the last thing I want to do is "throw out the plastic hose thingy".

    I appreciate the input though and like ideas that are outside the box. Theres always more than one way to skin a cat so to speak!
     
  12. Narcosis
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    Narcosis Junior Member

    I used to agree, but diving under my boat you can literally feel 2 or 3 feet of heat radiating from the keel cooler channels when I'm tied up at the marina. Diving under the boat at anchor with a small amount of current tand there is a few inches of hot water surrounding the hull. Again, maybe my perception is dead wrong? Isn't it a bit like a radiator with NO fan?
     
  13. FAST FRED
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    FAST FRED Senior Member

    If the keel cooler is properly sized, shouldn't it keep the loop water very close to the sea water temperature? Example: 80 degree seawater, 80 degree keel cooler loop.

    NOTHING is 100% efficient.

    Sounds like internal corrosion or external corrosion are slowing down the heat transfer.

    Otherwise the fellow that designed the hull tank didnt have a very sharp pencil , or the setup wasnt built properly.

    Most NA will go for a worst case of 100F water with a non moving boat.This allows for some fouling and paint buildup.

    The ice eater would still be the cheapest and simplest method , I would guess you could borrow one from a transiting norther cruiser at the marina , to see how it works.
     
  14. lucdekeyser
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    lucdekeyser Senior Member

    Water is 50 to 100 times more heat conductive than air. In marina's there are other sources of warmer fluids. Next dive, take a thermometer and a uristick with you ;-)
     

  15. Narcosis
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    Narcosis Junior Member

    Possible. We boroscoped the inside of the keel cooler and saw nothing but shiny and smooth clean aluminum.

    Hehehe Also possible, but I doubt it. My best guess, considering the history of the vessel, is she was built with the Great Lakes in mind, not the tropics. When the seawater temp drops from 80 degrees to 70 degrees the keel cooler temp hovers around 90 degrees. According to a geothermal forum, 90/95 degrees is a target entering water temperature during the cooling season.

    So, I know aluminum has a specific thermal conductivity. Not sure if the alloy matters, but its 5086. How can I figure out the thermal conductivity of 80 square feet of 3/16 inch aluminum? How many BTUs of heat exchange is possible with just the keel cooler channeling assuming no epoxy barrier or bottom paint? (I believe the surface area of the rest of the 70 foot hull the keel cooler is welded to negates a lot of the fouling since it is radiating heat as well)
     
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