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Idea for indirect evaporative cooling

  1. Oct 12, 2009 #1


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    Could I get a hundred feet of 3" corrugated drain pipe for the air and run it through a 6" corrugated drain pipe for running cooled evaporated water which would be pumped out of one end and gravity fed through the other end. As for the air "tube", I would run one end into the home at one location and the other end at the opposite end of the home with a fan pulling the cooled air through the "tube". Being corrugated, they have twice the surface area as a smooth hose, and I would snake them for added turbulence. Let's say the water is cooled to 65 degrees, what could I expect on a 100 degree day by recirculating the air through the home with a 2000 cfm fan and 1500 square feet? What if I buried the pipe 3-4 feet underground where the temp is 50-55 degrees? A 3" tube at 100 feet holds 18.75 cubic feet of air. I just don't know how to calculate this in order to see if it would be worth doing.
  2. jcsd
  3. Oct 12, 2009 #2


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    What you are wanting to make is a http://en.wikipedia.org/wiki/Heat_pump" [Broken].

    Welcome to PF
    Last edited by a moderator: May 4, 2017
  4. Oct 13, 2009 #3


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    Thank you for the reply and the info regarding heat pumps.
    Last edited by a moderator: May 4, 2017
  5. Oct 13, 2009 #4
    Hi Ed,

    You are talking about making a horizontal geothermal system. A variation of what you are talking about that has been around for years is called an "earth tube" except that it just uses air instead of water.

    Though air is a very poor thermal transfer medium approximately 120 feet of tube is needed. Ambient earth temperatures below 6 feet are relatively stable. Earth tubes are fairly efficient because not much else is needed. The primary drawback is condensation so a slight grade would need to be used to gravity drain the tube.

    A lot of people use buried PVC pipe effectively though plastic is a very crappy heat transfer material.

    120 feet at 6 feet or so is no joke. You might want to investigate using a large water tank as your heat sink.

    We just bought 22 aluminum shipping containers at a government auction to make a variation of the earth tube. The containers hold about 1 cubic yard of material each and have sleeves underneath them so that a forklift can pick them up. Since the entire container is aluminum the heat transfer properties are excellent. I've attached a couple pictures of what I am talking about.

    Simple inserts will be used to bridge the gaps between the containers (kind of like gutter downspouts that are fitted into each other). Air will pass down one side, back up the middle, and down the opposite side and exit the system. that way we get 3 times the distance for each run.

    The containers will hold water and recycled clay bricks that used to be used in a cooling tower. They are 9x9x6 inches each and will allow vertical travel of the water through the stack.

    As the ducting under the container starts to warm up that heat will be transferred through the aluminum into the ground and into the water above the pallet fork sleeves. Warm water will rise away from the ducting and cooler water will settle. The bricks add additional mass to our "therml" battery.

    Since our humidity is usually 80-100% and our dew point during the summer is 70F, I expect a lot of condensation. We plan to use a gravity drain to capture that water. Simple solar pumps and shading techniques will be used to bring the water above ground that will be used in our U-pick section for vegetables. We don't want the pumps running all day so the shading will allow sun to strike the pump panel once or twice a day for a certain period of time. This means you don't have to worry about switches failing or corroding. No batteries are needed because we don't really care about pumping at night or energy storage.

    To push air through the system we are using 12v fans like found in RVs or boats. They move a lot of air and don't need much energy so you can get by with a smaller panel. At night we will just use grid power. It will only cost pennies to run the fans.

    We are pushing air through the system. This is important because when you push air into a system you create a higher pressure which will cause more condensation to form. If you pull air through the system you still get condensation, but not as much. We want the water for agriculture purposes so we will be pushing it into the system.

    The bottom line is that we could have used a gravity trap to drain away the water and had a very simple system (two fans and some solar panels). When you start getting into heat exchangers, etc. you are adding more energy usage and more possible points of failure.

    In our case we aren't actually using a solar driven pump. I've created a "pump" that will move quantities of water using air expansion that is heated by the sun. There are no moving parts so there is no maintenance. I can't easily explain it in this post so a solar driven pump was an easy stand-in to explain what we would be doing with the water.

    It's a simple system and if you use some of the key words we've provided (like "earth tubes") you can do a google search and come up with a lot of information.

    I hope this helps.


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  6. Oct 14, 2009 #5


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    Thanks OldHap,
    "120 feet at 6 feet or so is no joke. You might want to investigate using a large water tank as your heat sink." Sounds good.
    I can stick it under our mobile home and it won't even be seen.

    Thanks for your input,
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