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Thread: Sizing pond loop for 20-ton chiller

  1. #1
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    Sizing pond loop for 20-ton chiller

    My brother and I recently bought 60 acres so we could move our families and our business out of the city. We're going to have 3 houses plus our shop/office and I managed to get my hands on a 20-ton Carrier 30MPW water cooled chiller we're going to use for cooling. Rather than a cooling tower, I would like to sink a closed-loop system in the lake on the property, its about 16' deep and has about 9 acre-feet of water. I figure it would be similar to a water-source heat pump that way. Can anyone point me to some resources to help me size the loop system?

    Thanks.

  2. #2
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    The most common system like this is the spray pond. Spray ponds are not very efficient and at times waste a lot of water. The submerged pipe I assume you intend to use is even less efficient. It would take a lot of pipe (money) to make a heat exchanger that would work because the water in the pond isn't moving and you would have to size it for the worst condition. The type of heat transfer you are looking for is called free convection. The approximate u factor for free convection ranges from 25 to 60. The forced convection u factor is much better ranging from 150 to 300. Your final design will almost certainly be based on experiments under operating conditions because it is doubtful that you will find a u factor anywhere to allow anything better than ballpark design. One of the first things you need to know is what the highest expected temperature is of the pond water. You should consider a dry cooler if you don't want a cooling tower. A plate and frame heat exchanger with an approach of 1 or 2 degrees is another option. There are problems to solve either way you decide to go. I would go with the dry cooler.
    Last edited by WAYNE3298; 03-01-2016 at 04:46 AM. Reason: post not complete

  3. #3
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    Im not sure what acre feet of what is, so are you saying its an average of 9' deep and its roughly an acre, that is not enough for a 20 ton system. We typically look for a 1/2 acre pond with 7-9' of average depth per 4 tons, then we size around 300' of 3/4 pipe per ton. So at 20 tons you would need roughly 6,000 feet of pipe laid out in the pond.
    However have you actually done a load calc? 20 tons could be way oversized or way undersize and cause other issues. When you say "managed to get my hands out" it makes me think you are getting in over your head unless you have left out some information here.
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  4. #4
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    Thread Starter
    9 acre-feet is approximately 3,000,000 gallons. The pond is actually 16-25 feet deep and has multiple fountains as well. The "managed to get my hands on" is referring to the fact that I used to work for Carrier and got it much cheaper than retail. The load calc was done with Carrier's Hourly Analysis Program and shouldn't be too far off. And the reason I was asking was because I came up with 6000' as the length needed for burying the pipe in a horizontal slinky pattern in the ground. I also read that if its sunk in a lake/pond, the water to water exchange would allow for a shorter run.

  5. #5
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    Quote Originally Posted by agibbstx View Post
    9 acre-feet is approximately 3,000,000 gallons. The pond is actually 16-25 feet deep and has multiple fountains as well. The "managed to get my hands on" is referring to the fact that I used to work for Carrier and got it much cheaper than retail. The load calc was done with Carrier's Hourly Analysis Program and shouldn't be too far off. And the reason I was asking was because I came up with 6000' as the length needed for burying the pipe in a horizontal slinky pattern in the ground. I also read that if its sunk in a lake/pond, the water to water exchange would allow for a shorter run.
    You came up with 6,000 feet of pipe in the ground for a 20 ton system... Your reading that wrong, thats probably 6,000' of TRENCH not pipe. Then you have around 600' of pipe per 125' trench that is 3' wide.

    Oh boy.

    We just finished up a 11 ton pool heating system on a 5 acre pond with slight flowing water and between 5 and 20' deep and had 11 loops with 300' of pipe uncoiled and separated for better water flow. So thats 3,300' of pipe for a POND system

    On the other hand we do 4 ton systems all the time with in ground pipe and we use 3,600 of pipe in a racetrack as I am not a fan of slinkies, they are the worst possible loop type IMO. Well if a 4 ton system takes 3,600 of pipe in a 5' deep trench that is 5' wide that would mean that for 20 tons you would need 18,000 feet of pipe by rough extrapolation.
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  6. #6
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    Just do an open system instead of closed loop. but you'll probably be better off using individual warer to air units. 20ton chiller plus pump plus a 3" loop will cost you big time even though you have the chiller already, you'll prob need to get 480V 3ph brought in 7.5-10HP in pump. I just see lots and lots of money instead of standard water to air units

  7. #7
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    This is a 3ph chiller?

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    The most common system like this is the spray pond. Spray ponds are not very efficient and at times waste a lot of water. The submerged pipe I assume you intend to use is even less efficient. It would take a lot of pipe (money) to make a heat exchanger that would work because the water in the pond isn't moving and you would have to size it for the worst condition.

  9. #9
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    How often will the piping need to get the barnacles off?
    I know my fish pond has stuff settling on my pump etc. I only ran across one pond HP but
    didn't know how large the loop was.
    What kind of pump head is needed for that much pipe?
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  10. #10
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    Use open loop, but install a suitable pleated filter on the side going into the chiller and figure on checking/changing it based on pressure drop across the filter...

    Curious, are you providing cooling for all three homes with this one unit? If so, you might want to install a buffer tank on the chilled water side so that the compressor has a decent run time and does not short cycle.
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