Using water tanks for refrigeration loop

[Therapy]

I read it only once.

Use your water tank">fresh water tank instead of a keel cooler or other external cooling method.

Save a sea-cock?
Save maintenance?

It must not be that good or it would be used more often huh?

[w42]

Depending on how much water is in the tank and how hard your refrigerator is working, your water tank will eventually warm. the warmer it gets, the harder your refrigerator needs to work, less efficient.

With an external keel cooler, it will be a while before you warm the entire ocean. But this too will eventually be inefficient when global warming hits.

[Therapy]

I guess there might be a lot of math (something I can't do well) involved with low volume limit for said tank, water temp etc.

In the summer here in Florida the water temps are frequently 86F plus.

I am wondering.

[MidLandOne]

It introduces a fatal failure mode - When one runs out of fresh water there will be no more refrigeration so no more ice to go in the whiskey .

But in a more serious vein I think it is a very bad design decision to make one system dependant on the reliabilty of another independant system where other quite suitable and well proven alternative solutions exist. For example, in this case the refrigeration system is made to be dependant on the potable water system in that there must be a minimum quantity water in the tanks for the refrigeration to work.

It also indroduces the, admittedly remote, possibilty of fouling of the potable water.

These things may be ok maybe for a weekend sail boat (but I would never do it even then) but not for an independent crusing boat in my view.

[sailorboy1]

I think this is an excellent orginal question! Hope we get some responses by people who have used the potable water tank as the heat sink in their systems. I would do the math, but I have learned that the theory and reality don't always meet in this type of thing.

[Therapy]

Thanks DonL.

Many times there are things done "against the grain". It does not make them poor in function though.

I am just curious because I don't think I can do the BTU math.

I do know that water is much better at it than air and there are a lot of air cooled units out there (most?). Maybe because air is easier to handle. I know all the installations of air cooled units call for "adequate" ventilation. I wonder of the efficiency of those units in 94 degree summer air here in FL and elsewhere.

I am just curious.

[hellosailor]

Therapy-
Depending on who you ask, water conducts heat 14x-20x better than air. But I think the only math you are looking at is how many btus your water tank can absorb, and then how it will eventually transfer them out to someplace else.

I would think the typical plastic water tanks wouldn't radiate heat very well, so you'd wind up with a tank of rather warm water eventually--unless you had a huge tank, a lot of water weight to carry around.

Eight BTUs will raise about 1 gallon of water 1 degree Fahrenheit. 1200 BTUs will take one gallon from 60F to 212F, boiling.

Of course, the cooler your fresh water supply is, the less crud will be tempted to grow in it. I'd rather have the fresh water supply separated from any other systems and purposes.

[Therapy]

Originally Posted by hellosailor

Therapy-
Depending on who you ask, water conducts heat 14x-20x better than air. But I think the only math you are looking at is how many btus your water tank can absorb, and then how it will eventually transfer them out to someplace else.

I would think the typical plastic water tanks wouldn't radiate heat very well, so you'd wind up with a tank of rather warm water eventually--unless you had a huge tank, a lot of water weight to carry around.

Eight BTUs will raise about 1 gallon of water 1 degree Fahrenheit. 1200 BTUs will take one gallon from 60F to 212F, boiling.

Of course, the cooler your fresh water supply is, the less crud will be tempted to grow in it. I'd rather have the fresh water supply separated from any other systems and purposes.

So a 15 gal tank already at 80 would not do well for a refrig/freezer?

1200 BTU a day? Don't know what a "large" fridge and/or freezer uses as far as BTU in a day.

I don't think contanimation of the water would really be an issue. Warm shower water all the time? Warm beer too? Too much run time?

[S/V Illusion]

I'd question the practicailty of "doing the math". There are many variables which can't be calculated and over which you have no control, e.g., adjacent heat sinks, rate of radiation to proximate sources, efficacy of any ventilation, ambient temp variation, heat transfer coefficient of tank walls, stratification within the tank, etc....

I suspect your calc would be meaningless except under ideal conditions which most boats don't replicate well

[Therapy]

I suppose there could be reasons it is not done.

I was just curious if anyone had done it. I read one reference to it.

[hellosailor]

Therapy- http://www.defender.com/product.jsp?path=-1|406|10789|86418&id=86373A low-end Adler-Barbour reefer is rated at 650btu/hour capacity, you cna probably find the rating for whatever one you have in mind. 1200btu/hour would seem easily in the range of a larger unit designed for hotter climes...figure that would heat a full 50 gallon tank about three degrees during one hour?Of course if you superinsulated the tank and hooked it up to the espresso machine...[g]

[Therapy]

Originally Posted by hellosailor

Therapy- http://www.defender.com/product.jsp?path=-1|406|10789|86418&id=86373A low-end Adler-Barbour reefer is rated at 650btu/hour capacity, you cna probably find the rating for whatever one you have in mind. 1200btu/hour would seem easily in the range of a larger unit designed for hotter climes...figure that would heat a full 50 gallon tank about three degrees during one hour?Of course if you superinsulated the tank and hooked it up to the espresso machine...[g]

Thanks...........

[sailorboy1]

I would say there isn't really any doubt that cooling the refrig condenser with water instead of air will make it more efficient. But that doesn't mean that overall the whole think is more efficient as the heat has to go somewhere. As far as efficiency the keel cooler might be the most. But then again it depends on what you consider as the model; the refrig or the boat as a whole.

Yes a 1000 BTU load into a 50-gal tank should heat it about 2.6 degrees F. From this you would have to do the math to remove the heat into the air and ocean. This will take more guess work. Would need to determine the area of the tank exposed to each is, the ocean and air temp, and the insulation factor of the hull. I would be willing to say that the heat transfer out of the fresh water tank is close enough to almost just keep it at the same temp (as the fresh water heats up the transfer rate is going to increase till the heat in = heat out). Think about the air around a normal household refrig, how warm does this really seem to get as it is the same heat value as what was cooled.

I think the thing is to heard of others who did it as to how well overall it works to what they wanted to accomplish (I was thinbking this same starting question the other day as something to "figure" out later).

[George Elliott]

Hey Folks I think I started this thread when I wrote the internal keel cooler post. In it I explained that due to the distance of the compressor to the tank I used 15' of 3/16" cu and the line had cooled dramatically to the air. Anyway the thing is working great, the water tank certainly isn't hot and i'm sold on Danfoss bd50s. But I do have a question, right now the compressor is running at 3.2 a on minimum speed, I think i need it to shift a bit more heat and can speed by the compressor 500 rpm with a resistor or continue to add more 134a and make it work harder, that'll raise the amp draw. Does anyone have any experience with this problem. Is it better to allow the ocmpressor to operate at it's rated draw, 2.9 @ 2,000 rpm, 3.8 @ 2,500 or to overcharge it slightly and increase the rated draw. Thanks George

[Therapy]

Originally Posted by George Elliott

Hey Folks I think I started this thread when I wrote the internal keel cooler post. In it I explained that due to the distance of the compressor to the tank I used 15' of 3/16" cu and the line had cooled dramatically to the air. Anyway the thing is working great, the water tank certainly isn't hot and i'm sold on Danfoss bd50s. But I do have a question, right now the compressor is running at 3.2 a on minimum speed, I think i need it to shift a bit more heat and can speed by the compressor 500 rpm with a resistor or continue to add more 134a and make it work harder, that'll raise the amp draw. Does anyone have any experience with this problem. Is it better to allow the ocmpressor to operate at it's rated draw, 2.9 @ 2,000 rpm, 3.8 @ 2,500 or to overcharge it slightly and increase the rated draw. Thanks George

Where is that?