Using geothermal to cool pipes

[cookyb]

I wanted to get some help on setting up a cooling system for a saltwater aquarium. The tank water is 82F and needs to be held constant by the use of heaters and some type of cooling system. They make chillers for this use but they are upwards of $500 and I figured I could make something for cheaper than that.

My proposal is to pump the water from the tank through a pipe that is X' long and use the crawlspace under the house to cool it, either by just laying the pipe in the crawlspace or burying it. The crawlspace temp ranges from 60F-65F and I'll need a flow rate of approx 6 gallons per minute of 70F water to cool the tank. My question is, how long (or how do I calculate) the length of pipe I will need to cool the 82F water to 70F before it returns to the tank?

I was initially hoping to use copper tubing but I've heard stories of it leaching copper into the water which can kill the fish so I may have to use PEX, which I know is much less efficient at transferring heat through the walls.

Any thoughts or suggestions would be helpful. I think the time it will take for the water to cool may keep this idea from working but I thought I'd check into it first.

Thanks,
Steven

 

[cookyb]

Zapster: How do I take into consideration the film coefficients?

Danberry: there is a decent chance that an aluminum radiator will undergo galvanic corrosion when exposed to saltwater. Not good for the fishies. I have thought about it, though.

I'm leaning toward a small fridge and running the pipe through it (think: giant rat's nest of flex pipe in small fridge) to aid in the cooling. This should be able to get the "outside temp" down to 35-40 degrees and greatly help in the cooling process. Then I can just have the pump come on as needed. I would like to try it without the fridge first though.

 

[Zapster]

There are many facets to your problem and I suspect that some of them have not been addressed on this forum. Evaporative heat loss from the air/water surface and conductive heat loss from the tank’s sides and bottom come to mind. I would recommend that you find a good technical book if you are interested in further analysis. One text book that might be helpful is “Heat & Mass Transfer” by White. The heat transfer analysis should be applied to both the net heat gain of the tank (remember in addition to the gains there are also losses from the tank if it is in an environment cooler than the water), and then to the design of the cooling system.

 

[moltenmetal]

My back of the envelope calc has you dissipating only about 90 W/m2 at best via free convection and radiation (each doing about half the job) from a piece of tube, with an inlet/outlet water temp of 82 F and a surroundings temp of 65 F in your crawlspace. Others may ground-truth that for you and find me in error, as the result seems low to me, but that's the number I came up with.

Forget about metallic exchangers- it's warm seawater and you can't afford titanium, and your fishies and other critters won't like brass or copper.

5/8" OD PEX tubing is only 0.05 m2/m of length. 20 m of tubing is only going to lose you 90 W...that ain't much. Your circulator pump might put 1/2 of that back in...

You're going to be losing a fair bit of heat via evaporation and radiation from your aquarium, so maybe you only have to do half of your 600 W peak load. That's still a mightly long coil, or probably string of parallel coils.

I thought of telling you to enhance evaporation with a fan or with more aeration, but 600 W is what- nearly a litre of water evaporated per hour. Doubt your house or your fish would like that much, even if you made up with fresh.

The coil of PEX in the fridge might be the best bet- if you can get the coil into the fridge- the bend radius isn't all that good on that stuff.

 

[blarge]

if you decide to go with the coil in a fridge you may want to look at the U S Plastics web site, or a similar provider, as they have coils of tubing. their may be a material that is ok for the fish and comes pre-coiled in a size that will fit neatly in the fridge.

 

[cookyb]

Good to know. Thanks blarge.

I also upgraded some of the equipment in the tank with newer more efficient (much less wattage)stuff and it has made a big difference. With the ambient air temp being at 79F the tank has been hovering right around 81.5F for the last couple of days. Before the room temp would need to stay around 74F to keep the tank at 83F.

Thanks again to everyone else that responded. If I end up having to go through with making a chiller I'll update this post with the results.