my diy cooling system

[xrunner1234]

Originally posted by AnnArborBuck
If you go and read the entire thread that I posted concerning cooling properties you will realize i was not talking apples to oranges. Thermodynamics is thermodynamics.

If the ground is at 70 degrees and your tank water is at 80, it will take a very long time to get to 70. The driving force of heat transfer is the temperature difference. as the hot and cold temperatures approach each other the amount of energy transfered from the hot to cold GREATLY decreases.

You may not believe what I am telling you, but trust me, I know what I am talking about. I have a masters degree in chemical engineering with way more heat transfer knowledge then I will ever need in my life.

When i was talking about increasing flow and decrease efficiency due to frictional losses what I mean is the heat caused by friction, not from lack of flow.

I promise you this, If I pump 1000 gallons of water through the coil and drop the temp .5 degrees that will remove heat then flowing 2 gallons that removes 10 degrees. Again, go back to the original thread and read about Q=U*A*Delta T. Q is the amount of heat removed per unit volumn. As Delta T approaches zero the amount of heat removed approaches 0.

If you have anymore questions feel free to let me know, but trust me on this on.

Well I’m not a thermodynamics expert, and it looks like you have a lot more experience. I’m not doubting your expertise, but I need to see the math, so please bare with me and don’t take offense.

Isn’t there an equation for heat changers that include flowrate (Q = m * Cp * DT,)? So for a give Qremoved from tank, when flow is increased (m), DT goes down? And the basic comment that I made was that if we increase water flow, the delta of tank temp and return temp from the heat exchanger would be reduced- so if we plot flow rate vs DT, there would be an exponential curve upward (and maybe it comes back down once friction comes into play).

[spazz]

annarborbuck

i will try to do some testing this weekend and give you some hard figures to work with that way you will better under stand what i have and how it works.

[AnnArborBuck]

Isn’t there an equation for heat changers that include flowrate (Q = m * Cp * DT,)? So for a give Qremoved from tank, when flow is increased (m), DT goes down?

That equation is not the equation used for heat echangers. That equation is used if you have a static body of water (or what ever substance you want). That equation is used if you were calculating the amount of energy required to heat 1 gallon of water 10 degrees, etc. The DT in that equation is actually different then the DT in the heat exchanger equation.

DT in the heat exchanger equation is the Log Mean Temperature which is a relationship between the cold and hot sides. The driving force for heat transfer in a heat exchanger is the difference between the cold and hot sides, or in this case the ground and the tank water.

Just imagine if the tank water was the same temp as the ground (Florida). Geothermal cooling won't work in this case. The ground has to be cooler then the water to cool the water. The colder the water is compared to the tank water the faster it can cool the tank water. So if you have slow flow, the tank water temp approaches the temp of the ground, and as it does the amount of energy transfered decreases. This is why you have to use the log mean temp for heat exhangers.

Make sense?

[xrunner1234]

I think we’re talking about 2 diff things.

Originally posted by AnnArborBuck
That equation is not the equation used for heat echangers. That equation is used if you have a static body of water (or what ever substance you want). That equation is used if you were calculating the amount of energy required to heat 1 gallon of water 10 degrees, etc. The DT in that equation is actually different then the DT in the heat exchanger equation.

Yes, but that equation is also used for heat transfer flowrate, where m is mass/unit time or flowrate.

[i]DT in the heat exchanger equation is the Log Mean Temperature which is a relationship between the cold and hot sides. The driving force for heat transfer in a heat exchanger is the difference between the cold and hot sides, or in this case the ground and the tank water. [/B]

Yes LMDT is that and that is the driving force.

[i] So if you have slow flow, the tank water temp approaches the temp of the ground, and as it does the amount of energy transfered decreases. This is why you have to use the log mean temp for heat exhangers.

Make sense? [/B]

This I disagree with. You are saying that if you have slow flow, the tank temp approaches the temp of the ground and energy transfer decreases. Energy transfer does decrease, but don’t forget we are putting in energy into the tank through pump, skimmer, etc. So if energy transfer decreases, then tank temp increases, not decreases.

And the DT that I am talking about is not the DT from coil to coil, or pipe to ground (which I think you are talking about) but rather pipe inlet to pipe exhaust.

Maybe we are talking about different things.

[lessthanlights]

spazz-

digging up the past a little here but you've got me interested. it's over 2 years later- are you still using this system? has it stood the test of time? being from minnesota too i've often thought about using a system like yours but outside to take advantage of winter.

[spazz]

this was a very good system but the tank was not drilled so i had issues there. the tank was taken down but i left the cooling grid in place for the next tank. it worked very well for being under the house. if i were to do it again i would dig up the yard and put it 3 ft down in the yard to use the cooler ground temps. there very effecient to use but can cost a little more then you would think to install. unless you have a new house going up and can burry the piping next to the foundation. then you have the added benifit of heating the outside the the foundation to prevent it form cracking.

[BrainBandAid]

Would you still use regular (sch 40?) PVC?

[kentrob11]

Sweet! Does it have a bubble diffuser or a needlewheel?

[xtrstangx]

Originally posted by BrainBandAid
Would you still use regular (sch 40?) PVC?

For the best cooling, you'd want the thinnest thickness possible. Schedule 40 is your best bet for PVC and then look for one that has a low PSI rating (they come from 120 PSI to 600 PSI at Lowes/HD... the difference is the thickness)

[reefez]

i didnt meen to start an argument in the dynamix of geothermal cooling. ann arbor buck is right about one thing and that is the closer the tank water going through the grid gets to the temp of the sand, the longer it takes for the water to cool down. thanks to living in minnesota the groung is very cool. the sand temp at ground level where i put my grid stays at an even 60 deg or so. that makes the water cool down to 70 deg comming out of the grid. by putting 79.8 deg water through the grid i get a 10 deg drop in water at about 100 gph

What about the 10-20 below winters? Will heaters be able to keep up? Nevermind I guess you would just turn it off on cold spells. Cool idea.

[BruiseAndy]

You could use a 3 way valve and pipe it into discharge line and tank or to go from discharge back to loop, or a mix of the two to control outlet temperature. Could also integrate a temperature controller as a low and high limit.

Worked on chillers and boilers for quite some time and AnnArbor seems to make the most sense as far as equations go.

Heat transfer coefficient for PVC is 0.16 W/m.K by the way.

Why not use PEX instead of PVC? Thermal conductivity is about 3x higher and would probably be easier to use.

[Gudwyn]

spazz, looks you you are running tank water thru the loop instead of using an exchanger.

How did that work for you? Any problems with rotten-egg smell?

I just buried 300' of 1/2" pex pipe in my yard. It's down in the new sprinkler system trenches dug into the clay under the top soil. With the rain/sprinklers, the clay is always soggy and wet down there. So it should transfer heat well.

And I have a 20rlzt to feed it. My preference is to just feed it tank water, but I'm a little worried about anaroebic conditions down there.

My chiller runs at least a couple minutes every single day, so the 3-4gal in the pex will never sit for more than 24hrs. Even earlier this week when we had record lows of 17 overnight and highs below 30 it still turned on. But still worried. Any tips appreciated.

[Sparkss]

do you have a spec sheet on pex ? We are considering using it for a heat exchange loop in our sump and wanted to ensure that it was completely inert, and didn't have any negative coatings or properties, like ABS pipe can have. We were going to be using it for heating purposes, but it could be used either way.

[Gudwyn]

I don't have the spec sheet. The stuff I got has an O2 barrier and was listed as primarily for in-floor heating but also acceptable for potable water lines.

[Sparkss]

that is similar to what we were looking at (the o2 barrier and the primary use for in floor heating, but also rated for potable water). But then again copper is rated for potable water but we wouldn't use it in our reefs (ok, extreme example). I just want to be sure that the PEX doesn't include any sort of mildew inhibitors on it's exterior as a standard (I know that manufacturer to manufacturer can be different). Again, just trying to do as much due diligence as possible before taking the plunge (and plunging the PEX into our sump -- pun intended ).

[spazz]

you might want ot consider using a heat exchanger to run between the cooling grid and the tank itself. this would prevent cross contamination and also prevent stagnent water from hurting the tank. you can build a titanium heat exchanger for alot less than you think. and they work great.

[Sparkss]

the heating grid was going to be the heat exchanger between the heater and the tank water. No tank water would be used in the heating grid (Pex).

I know that it seems to be counter to your original thread (heating vs cooling), but the concepts are the same, just the heat transfer would be in a different direction .

[Randall_James]

Originally posted by Gudwyn
And I have a 20rlzt to feed it. My preference is to just feed it tank water, but I'm a little worried about anaroebic conditions down there.

There is actually a thread about this particular subject and nitrate reduction. Coiled tube in a bucket (no light) works to reduce nitrates. I would imagine that your system will have the same effect. As there is no place for "dead spots" I doubt that you will have to deal with any sulphur issues.

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