Earth contact and thermal conductivity

Discussion in 'Vertical and Horizontal Loops' started by Olas, May 11, 2017.

  1. Olas

    Olas New Member

    Hi, I have been reading through the forums and found some very useful information so thank you for all the discsussions. I hope that you can offer some advice for my installation. I am installing a 30kw closed horizontal loop system. I am doing the ground work and someone else is doing the connecting and commissioning.

    I am 3/4 of the way through and am worried about the contact with the ground for thermal
    Conductivity. The soils is heavy clay. The ground is very very wet all year and water has been coming up into the ditches in a few places while digging even though we have had an unusual dry spell. The land is slightly sloping and not a stone in the ground just wet clay. The trenches are 1.6 meters deep or deeper and pipes are 1.2 meters apart. not slinkies just straight pipes. 9 100m long trenches so 200 meters each loop equating to 1800meters of pipe in the ground. It's quite a big system.

    My question is this. We did not use sand and I am concerned that there will not be enough contact with the pipe due to air gaps. I tried to convince myself that sand was not a good idea due to the poor thermal conductivity and that being so wet and clay the ground would settle around the pipes and even during winter have water around them. The use of sand I understood was for protection of the pipe but I now understand it is for thermal conductivity too but debatable regarding its potential insulators properties. I guess my question is around soil consolidation compaction and how long it will all settle with the ground condictions I have. How long will it take to compact do you think? I'm going to use some fine clay material I have on site on the final 3 loops but I did Not have anough of this for the whole lot. The sand for this would have been very expensive. It would be interesting to see the performancs of each loop individually but that isn't possible I guess. please forgive my lack of knowledge. Hoping you are not going to tell me I have cocked this up. The proof with be in the pudding though I'm sure. I would appreciate any comments positive or negative. Many thanks,
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Dry sand and dry clay are the concerns. Wet sand and wet clay are not an issue as the voids will just be filled with water.

    The issue with clay is that it shrinks when dry and can pull away from the pipe. The air gaps then just become an insulation layer (bad).

    Sand has better thermoconductivity/thermodiffusivity, but if you know things are going to remain wet, I wouldn't worry about it.
  3. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi and welcome!
    Wet clay is certainly better than dry clay. Water is a great conductor and if your trenches are/were wet that is a plus. when backfilling you should have avoided the dumping in of clumps as opposed to sifting in the smashed up native soil. I see that Chris just posted above and offers good advice. If the soil is normaly damp to wet, it will sort itself out.
  4. Olas

    Olas New Member

    Many thanks for your comments. The ground will always be wet that is for sure. It is an incredibly wet area with bog plants and all sorts thriving in it. At the depth we have gone it would never be dry that is for sure. I made sure the last loops had full contact with the soil. I do wonder if after install I could potentially turn some of the taps off at the manifold and read the efficiency results then turn that off and run the loops that I did differently and compare the readings. To be honest I am not sure if that data will be available from the system. It would be interesting though.
    Many thanks for your comments.
  5. Ron342

    Ron342 New Member

    If you are combining the individual loops where they are accessable (basement/crawl) you can equalize them for individual flow rates (assuming same lengths, diameters etc or do a correction factor if not) and measure the temp difference on each loop - which should give you individual loop efficiency??
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Usually the wet clay is great, with time it will compact nicely around the pipes and gives you one of the best thermo conductivities of any horizontal loop. But it has to be wet, which it is in your case.
    Do not try to throttle them back and be obsessed with individual loop flow rates or temp differences between loops, this will only increase your pressure drop, which will increase your needed pumping power.

    In our area clay is performing better than sand, and we are trying to avoid the sand as much as possible. Not sure how you come up with a 30 kw system, what is the load it is serving? So roughly 10 tons of heating? Who designed this?

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