Fridge cooled through diesel tank

[flemmingsveen]

I have been thinking of make my fridge more efficient with cooling down the return line from from the evaperator. I have a steelboat and one of my diesel tanks is in the keel and easu access near the compressor. Instead of using water I`ve been thinking of making a loop down in the diesel tank (300L) that would be a easy solution for me. But is this a solution with backfire? Or have anyone tried it before?

[markpj23]

There have been several similar discussions on this forum. The main point to remember is that the BTUs end up inside the boat - with you. Yes you can cool the system by running a coil into the diesel / water tanks, but ultimately you're transferring that heat to the interior of your boat.

Whether or not that's significant to your setup is an individual choice based upon the amount of heat you need to transfer and your particular boat. Transferring the heat overboard leaves no question...

People do this with water tanks all the time.... but diesel fuel is not water.

Diesel has much higher viscosity and much lower thermal conductivity than water, which means it will be much less efficient at sucking the heat away from the cooling loop. This means that you will need a much longer cooling loop than you would need in a water tank.

The other problem is that diesel has a heat capacity less than half that of water. In other words, the same amount of heat raises the temperature of diesel by twice as much as water.

If you raise the temperature of the diesel tank too high (above about 125 F for #2 diesel) it becomes flammable, just like gasoline.

Before I made a change outside of "normal practice" like this I would contact my insurance company about it. You don't want to give them an excuse for denying a claim.

Bill

[gosstyla]

Diesel in your tank may not be that cool. I just recently had to have fuel coolers installed on my return lines after on a long passage the diesel got so hot it melted my Racors!

[philip van praag]

Originally Posted by gosstyla

Diesel in your tank may not be that cool. I just recently had to have fuel coolers installed on my return lines after on a long passage the diesel got so hot it melted my Racors!

this highlights the fact that the engins return unburnt fuel and this has risen in temprature so your plan may backfire.
if you use the water tanks this will rase the temp of your domestic water and may cause bacteria problems and you wont have any cold water on board.

why cant you use a hull cooling plate?

[flemmingsveen]

Thanks for quick reply

Its the efficient of the diesel versus water that is my only worry.
The diesel in the keel is always pretty cold, since it get cooled by the surrounding sea.
On top of this tank is the second diesel tank(Boat bottom) with 800L and 900L water on top water is always cool too.
To heat up the keel tank would take very much energy how I see it.
And the solution wouldnt cost me much money....
My other alternativ is to some how use a water intake and a inline cooler like a oil cooler etc.
I dont want to make moore holes in my steel hull either.

Greatketch is coming up with good points here...
Insurance its not a issue in my case.

hmm not such a genius idea after all. Maybe I have to test how easy it is to raise the temperature on diesel first.

[Weyalan]

Wont work. Deisiel in your tanks is usually warmer than ambient (if you have run your engine for any significant time).

Originally Posted by Weyalan

Wont work. Deisiel in your tanks is usually warmer than ambient (if you have run your engine for any significant time).

Actually, that is not quite true. Liquids are much more efficient at heat transfer than air is, so even at the same or higher temperatures you get more efficient heat transfer with liquids. There IS a limit to this of course :P

But it does point out an additional problem. The tank that is being used to feed the engine does get warm, but of course if you are using the fuel in this tank you can't consume it.... If you DO decide to try this, remember that you have to keep this tank more or less full. You have essentially reduced your usable fuel capacity by 300 L.

Bill

[Gene :^)]

Couldn't you lay a cooler on the inside of the hull (beneath the WL), then weld up a box over it with a fill cap. Fill it with coolant or mineral oil or some non corrosive fluid? Sort of an inside keel cooler or how about a regular outside keel cooler?

[David M]

I like the last idea, a keel cooler. It gets the heat off the boat and they are reliable.

[flemmingsveen]

I have been thinking of making a box on the inside but never got around to do it, now i just sandblasted and primed the boat so I dont want to start welding on the hull again. Another thing I have been looking in to is to coil up some line against the hull and use a material i havent yet found that transfer heat effecient on top to isolate from the air temperature.
maybe just welding a stainless steel box with a big contact area and put it against the hull with thermo transfer grease between? And isolate the box so air doesnt heat it up.
Im just guessing around here.