Using geothermal to cool pipes
Using geothermal to cool pipes
(OP)
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
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





RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
I jsut tossed some numbers into a spread sheet i have, and well alot fo thigns are different, but using a 2" carbon steel pipe with your given flow range and temperatures, and keeping in mind that CS will conduct heat away better than plastic will, and assuming you just lay the pipe out on the floor, i figure you will need about 1400 meters of pipe, so yeah go buy the cooler its much cheaper.
The thing is your working with a very small delta T here and water holds alot of heat that you want to dissipate. free convection tot he air from a pipe is a poor way to do this. and i can easily see any other methods quickly going well past 500 dollars.
Plus im not sure how constant your crawlspace temperature will really be i mean do you have a sampling form all year round?
RE: Using geothermal to cool pipes
Thanks a bunch for looking into this,
Steven
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
I am assuming you meant putting the pipe through a tank of water held at 40°F?
If thats the case you would need about 20m of pipe (my spread sheet is set up for 1" as the smallest so i went 1" sch 160 to approximate the flow volume of 1/2")
So this is a much better scenario to consider, but your water sink will warm up over time unless your constantly changing it, and if your going to that kind of trouble why not just bring in cold make upw ater, and build a little mixer? unless the water is treated with chemicals and such... Just a thought either way.
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
Ouch.... I plugged ina thermal resistance of .19 W/mK from http:
With steel at 54 W/mK i guess this is to be expected, sorry i should ahve dug itno the spread sheet a bit more first. you really will need to go to a metal pipe for this application, or just introduce colder make up water.
FYI i used 1" sch 10 for the thickness, of the PVC.
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
Perhaps I do not understand the basis for the question. It sounds like you are trying to circulating 6 gpm of water through the tank with a 12 degF temperature drop. How are you generating 36,000 BTU/hr in a 180 gallon tank? Have you considered providing some type of shade for the tank?
RE: Using geothermal to cool pipes
I only need to lower the temperature of the tank about 5 degrees. The system I was describing before wouldn't always be running; only when the temperature was getting too high and it would kick on.
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
That will give you the duty. Cp x m dot x delta T will give you the delta T you really need to keep up with this heat load.
Chances are, once you're more realistic about the duty, radiative loss alone out of a reasonable length of PEX tubing will give you the delta T you really need. Once we know the duty we can do the calc for you, or at least teach you how to do it yourself.
Be careful about the end connectors used with PEX tubing, though, as they're often brass (both Zn and Cu are phytotoxic if I remember correctly). Wirsbo do sell end connectors that are non-metallic, but only limited types etc. For a recirc situation like what you're talking about, non-metallic external compression fittings (Jaco etc.) will also do the job.
RE: Using geothermal to cool pipes
Being a structural guy I don't get into these kind of calcs much (read: ever). I would like to learn it, though.
Thanks!
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
I think I read that k for pex is around .26 Btu/(ft hr F). Can anyone confirm this? If so, my q would be about 80 Btu/hr resulting in only needing 25' of pipe to balance the heat gain due to the 600 watts. This seems low to me. Any ideas?
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
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.
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
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.
RE: Using geothermal to cool pipes
RE: Using geothermal to cool pipes
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.