question for the engineers

Discussion in 'Geothermal Loops' started by waterpirate, Feb 28, 2017.

  1. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    So this is the set up. Open loop system on the dune line, pumping salt water. It has failed after a short number of years in operation due to mineral fouling of the return well, and then the supply wells production diminished.
    The owner wants to switch to closed loop, no biggie. The load is 16 tons. The two failed water wells are each 300' deep. What would be the thermal penalty I would take from the schedule 40 pvc if I were to insert two loops per well and grout the whole mess shut?
    The lithology is wet sand and silts from -4 to 300'. Typically we use a 300' bore to cover 1.5 ton of load in this lithology. The plan to insert 2 loops into each casing and cover 3 tons of load per old well is what is giving me pause.
    Obviously the owner is keen on getting 6 tons of load out of the failed system and only having to construct 10 tons of loops from scratch.
    What say you lot? If your name and reputation was behind this, what would you feel comfortable with? I am prepared to drill 16 tons from scratch, but the possibility of recovering something from the failed system is a sirens song I can not ignore.
    Eric
     
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    What is your groundwater temp when at equilibrium?

    And 16 tons of cooling? Or heating?

    Back of the envelope, yeh I think you can get 6 tons, but depends a bunch on runtime of equipment.
     
  3. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I think if you are short 3 ton (13 instead of 16) it would not be the end of the world. Use larger pipes with larger surface area, and higher conductive grout. Consider the newer graphite grout to get that grout conductivity up. Why do you want to use PVC instead of HDPE? What size pipe? 16 tons of heating I assume?

    In terms of capacity, the biggest impact is the amount of BTUs rejected, or extracted.

    Adding 2 circuits to the borehole instead of one will get you more capacity, but not doubling it, but still increasing 25-30%. So with a few tricks you might get closer to your 6 tons goal.

    It will also depend on how you grout and install the other loops.

    If you have the other info such as spacing, borehole size etc I can run some numbers for you.
     
  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Doc,

    I assumed the well was sleeved already with PVC which is why WP brought it up.
     
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Aha! So schedule 40 instead of steel.

    Eric, do you have any feedback where your 200ft / ton boreholes usually end up in terms of minimum EWT?
     
  6. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Thanx for the replies guys. Deep earth temp here is 50 degrees and is what the designers use for a baseline. Using 3/4 inch pipe at 200' per ton of load we normally see 72 degrees in cooling and 45 degrees in heating. We are cooling dominated. I am not privy to how the load was calculated other to say that the hvac contractor that did the job initially, never has an issue with being short.

    The crux is that if I insert 2 3/4 inch loops at 300' in each cased well, those loops will have to stand alone from the others drilled at 200' due to flow imbalance. The way I could correct that issue is to only do standard 200' loops in the cased well and then split them up so they do not stand alone. I know this flies in the face of many designers here, but I still prefer to split my loads between several stand alone fields to prevent a total failure of the entire house.

    This would only reduce the tonnage by 2 tons in the cased wells, and play it more safely?

    Thoughts?
    Eric
     
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The EWTs indicates that you are having an well performing, almost oversized, loop. We are more aggressive with sizing, and usually shoot for 30 heating and 90 cooling.
    Do not worry about flow imbalance, the loops pressure drop will balance them out.
    While you might have flow imbalance due to 50% higher pressure drop in the 300" loops, it will direct more flow to the 200" pipe, with 0.75" pipe, just a small increase in flow will increase the pressure drop so much, that when the loop field is balance, you might see 10% more flow in the 200' circuits, not a big deal or worry at all. So they will heat up (or cool down) a bit more in the 300' circuits with lesser flow, but the total BTUs will not be significantly different between the circuits.

    Use the wells for their full length, the more you get out of them the better. Put 2 circuits in the 300' wells for a total of 16. The issue is more the total pressure drop, the more circuits you have the better.

    On a side note, 16 tons with nominal 3 gpm, 48 gpm total, 16 circuits, would be better with 1" pipe in terms of pressure drop.

    What is your antifreeze?

    Do not worry about safety margins here, you basically cannot have such thing as a total failure. Given your EWTs, even (10) 200'circuits total would work OK. Not super efficient like you have now, but not even close to a failure either....
     

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