Tayana 42 Refrigeration: Keel - Air - Both?

[lostatsea]

What a puzzle- I have heard opposing opinions which appear well balanced- unfortunately! This leaves me no better off in search of a decision. I am leaning now towards a friend's advice (he's a marine refrigeration installer) of doing a combo water and air: when in cold-enough-water disable, via a toggle switch (or simply unplug) the fan on the air-cooled part of the circuit. I'm living aboard on SF Bay, so the water is plenty cold! Then, when hauled out or in warmer water, enable it or plug it in. I am pretty set anyway on the keel-cooled option, so the decision is: add an air-cooled circuit at the same time, before making all the pre-charged compression connections? I am hauling out the boat for other btw (below the waterline) work so want to install the cooler then. The rest of the system I can do at the dock.
Question who, besides Frigoboat (friggin' boat?) and Isotherm, makes these units?

My next question is for anyone with the same boat or familiarity with it: (1984 Tayana 42, port galley, centerline sink w/two top-loading doors and one front-loading door on the icebox, oriented athwart-ships at the bulkhead aft of the main salon against the master head shower... sorry for the description- they built these things so many different ways)
Reason for asking: I cannot ascertain the quantity nor extent of foam insulation- I did notice during my re-power, (YEAH!) when I was cleaning the bilge areas (oh yeah, I know how to have fun!) that the foam sprayed into the contained area around the bottom of the icebox space had squirted out of the holes made for the icebox drain-hoses. I inferred from this that the insulation was well installed (injected, sprayed) with a minimum of voids. Anybody out there with enough experience in this boat's icebox installations to confirm or deny this for me?
Obviously, I am trying to get away with as much as possible with as little as possible. The prospect of removing and rebuilding what appears to be a well built, well insulated, double icebox is fairly daunting at this moment as I am not sure I could come up with the necessary cabinet-making skills to do it well. I am hoping to install a "kit" type system with a spill-over vent/hole/fan from the freezer side thereby hopefully avoiding the cost of hiring "experts"... of course, I have been frequenting restaurants for almost all my meals so I probably could have deep frozen the whole boat with what I've spent delaying this install! Thanks in advance- Johnston

[bstreep]

Deferring to your friend who is a marine fridge installer...

But, GENERALLY, water cooled refrigeration is much more efficient than air cooled. It's just that water transfers heat so much better than air.

In warmer climates, the water is still likely to be as cool as the air, most of the time. A keel cooler eliminates almost all of the downsides to water cooled refrigeration (marine growth, corrosion, etc.).

As well, if you air cool your fridge, AND that exchange is inside your boat, you will then have to deal with removing that heat. In the tropics.

[lostatsea]

Thanks! I agree but had heard stories about the water temp getting so warm in the tropics. The best reason to have an air-cooled "auxiliary" circuit that I can see is the time spent out of water on the hard. If I could go 120v a.c., on that side it solves all the problems. I heard about a cold plate that has dual circuits for different compressors- this might be the way to go. How the little K.I.S.S. bell rings in my head when I think about double anything!

[amarinesurveyor]

I agree with bstreep. Keel cooling is more eficient; how often is the water warmer than the air? I think almost never, and you're not going to be out of the water that often. I would avoid the extra expense of adding the air cooling option. Keep it simple stu...
Brian

[lostatsea]

Ok thanks all... Now for the second and third parts of the inquiry... Who makes these units and what about the amount and quality of insulation found in these boats?

[GordMay]

I'm not any more qualified than the previous contributers to explan the thermodynamics, but I believe that some of the assumptions expressed here are erroneous.

If our assumptions are wronge, so might be our conclusions.

Ie: The Sea Surface Water Temperature is generally warmer, on average, than the Atmospheric Air Temperature immediately above it (tho’ perhaps not in SF Bay).
Of course, water also has a higher specific heat capacity (SHC) than air.

See Richard Kollmann’s expert opinion on

WATER COOLING SMALL CONDENSING UNITS

Question: There are many questions on one forum thread regarding water cooling small Danfoss compressors and which method is best; pumped seawater, Keel coolers or using water from fresh water tank?

Answer: I have followed this thread with great interest having seen these different methods of refrigerant condenser cooling mediums in operations, but no one has mentioned all of the adverse effects of using water as a cooling medium. Most boaters understand the problems with circulating water with a pump, restricted water flow and poor reliability but what about the damaging effect to aluminum tanks, or the boat’s aluminum hull or total destruction to the refrigerant system caused by electrical discharge?

It is a fact that water cooled units do experience material loss do to electrolysis. It is also a fact that Danfoss BD compressors do discharge small amounts of current into the seawater causing failures at the point of voltage leakage. On circulating water systems it’s the seawater condenser that fails, on keel coolers it is the line or block that fails, on aluminum tanks it’s the tank or hull that loses metal. A few of the responsible refrigerator manufactures address the electrolysis problem by adding Zincs to the heat transfer units but most ignore the problem because the rate of metal loss is not predictable on all boats do to their use.

I have three questions for boaters with water cooled condensing units; After how many refrigeration operating hours do you replace the zinc on your water cooled unit? How many hours is the refrigerator run while connected to shore-power, between zinc replacements? Why do you believe that water cooling of a small 12 volt unit is more efficient than air cooling? ”

Goto ➥ kollmann-marine.com :: View topic - Book's Unanswered Questions Revised 1-23-07

Richard also discusses several other aspects of water vs air cooling elsewhere on his website.

His website and books are a treasure-trove of valuable information:

➥ KollmannMarine Boat Refrigeration Specialist

[mesquaukee]

Originally Posted by GordMay

Ie: The Sea Surface Water Temperature is generally warmer, on average, than the Atmospheric Air Temperature immediately above it (tho’ perhaps not in SF Bay).
Of course, water also has a higher specific heat capacity (SHC) than air.

Why do you believe that water cooling of a small 12 volt unit is more efficient than air cooling? ”

I have played with a number of configurations and have found that it is better to go with an oversized air cooled condenser than have the problems associated with water cooled condensers. It also uses less ampHrs/day.
If you use a air cooled condenser 4-6 times the size of the ones that typically come with a BD35 or BD50 compressor the condenser temperature (and pressure) is as low or lower than water cooled in tropical waters. If you turn the fan off it warms up a bit but is cooler than as configured for a do it yourself fan cooled unit. You can separate the compressor and condenser thereby splitting the heat produced, this once again reduce condenser pressure (temperature). The compressor and the control module on it also runs cooler. A lot of failures occur due to this module overheating. It is possible to use a very tall condenser which needs no fan whatsoever. It relies on air flow due to a thermal difference (look at the back of a household fridge).
Dropping the condenser temperature 10F gives an 12% energy savings (AmpHr/day), 20F saves 24%.
There are a number of other things that can be done to reduce energy use.
Considering that the fridge/freezer is the largest use of energy it does make a large difference on a boats energy budget.