Cooling with ground water

[TheEquineFencer]

The following is a copy of some of my post/responces from another chat room that is a bit over my head as far as the math and such. Please bear with me and give me an idea of what I need to know; how much 3/4 Pex pipe do I need submerged in 60* water  to get 3-5 Tons of cooling.

How can I figure out how to calculate the heat/cooling exchange rate of water inside pex tubing? I want to build a ground source cooling system simular to a chiller system but I want a closed loop system. My game plan is to pump ground water into a holding tank that is constantly flowing ground water into the tank and dumping back out. Inside I'd like to use pex tubing with the cooling water going to my shop inside pex tubing in a closed loop. I have a brand new air handler rated at 12 Ton for the building end. It's a bit overkill I know, but for $250 it's what I have to work with. I'm wondering how much pex tubing I need to use for my loops inside my holding tank to get at least 3-5 tons of cooling. My ground water is 60-62* F. My air temp inside the shop will be 80*F or better.This is my first post, so I'm hoping someone can lead me in the right direction without getting tossed.

PEX tubing thermal conductivity: 2.43 BTU in/hr-ft^2-F per DIN52612
3/4 Pex 0.875 +/- .04, wall thickness .097 +/- .01, ID 0.677

The 3/4 Pex is going to be submerged in a tank of water. The ground source water is going to be3 constantly turned over and dumped. The water inside the 3/4 pex is going to be circulated through a cooling coil in my air handler and retured back through the 3/4 Pex to be cooled again in a closed loop. The water inside the cooling coil in the air handler will take the heat out of the air and transfer it to water inside the coil to return to the Pex in the holding tank. Then the heated water inside the 3/4 Pex inside the holding tank will transfer to the holding tank water and be lost through "new" cooled water that is turned over. That way I can control the Ph of the water so it does not eat the coiling coil with untreated ground water.

With T0 = 20F, x = 50m (165') I calculate you get from 5 to 10 tons of cooling as the flow rate rises from 2 m/s to 10 m/s.

What is 2m/s in relation to GPM?

I hope someone in hear can explain what the "geeks" ; no offence intended, from the other room are trying to convey to me, they are way over my head.

[joestue]

aside from the obvious question, which is why not pump the ground water through a junkyard variety truck radiator, how much heat you can get through the pex into the tank of water is mostly a function of how fast the water is flowing over the pex.

what i would suggest you do if you insist on using pex is to coil the pex up into a cylinder, with about 3/4th inch spacing between turns.
then using a cylinder on the inside and outside of the coil structure to force the water to flow across the length of the coil.


one ton of cooling is 3500 watts, but at what temperature do you need the 5 tons of cooling? 20F difference means you need
about 6.2 GPM to get 5 tons of cooling. theoretical minimum...

lets say the water goes into the cooler at 70F and exits at 80F you need 12 gpm.

now for the heat exchanger to cool that down to 70F you need 12 gpm as well. water goes in at 60F and comes out at 70F.

i'm not sure you could get much better than that.


note that 12 gpm doesn't require but a couple psi to push that much water through a medium sized radiator, so you'd have in practicallity a few days to find the leaks when they start to show up.
how bad is your ground water?
you might be able to find some pretty good sized aluminum radiators these days, i can't see corrosion being a problem.

getting back to the 2 meters per second required.. that is simply a matter of calculating the minimum gallons per minute required and finding the right sized tubing.
12 gpm through 3/4th inch pex is 2.6 meters per second, that does sound like it is turbulent flow.. 32-40 psi required to make that happen according to Hazen-Williams.

you can't parallel the pipes because that reduces the velocity and that reduces the heat flow..
32 psi times 12 gpm is theoretically 153 watts so that means you need at minimum a half horsepower pump for each side of this 165 foot heat exchanger.

in practicality to get 2.6 m/s flow rates across the outside of the pipe will require a lot more power.

[TheEquineFencer]

If using a "truck" radiator doesn't work any better for cooling than it does for heating, it's waste of wood. The Chiller air handler I have is rated at 12 tons. It's unused and still capped from shipping, cost me $250. I can thank the USA government for that auction item. It has a rather large copper coil for hot water and an even larger aluminum coil for cooling.
The water going into the tank holding the water will be at 60*F - 62*F. In a perfect world I'd be able to cool the water inside the Pex down to 60*F - 62*F and then pump the water to the chiller air handler. The reason for the closed loop is so I do not eat up the cooling coil inside the AH from the PH, Iron and other crud in the water.