SCW length requirements with bleed

Discussion in 'Standing Column Well (SCW)' started by Brian, Jul 13, 2010.

  1. Brian

    Brian New Member

    I have an unusual situation regarding a SCW I'm working on. Originally it was designed to be a closed loop system, but we had to switch due to a very high water table (less than 10 feet down). We now have an extra well, which lead to the idea using a large amount of bleed and putting it into the extra well. From what we have tested so far, either well can handle a maximum of about eight gallons per minute of additional water without overflow. Since we intend to use a system with a flowrate of about 14 gallons per minutes, that would lead to a bleed rate of well over 50% (which isn't practical).

    I, however, am worried that our second well isn't deep enough to handle the system we plan to put in. I've tried looking around for information on specific requirements, but have found little. The closest I've found is that a SCW needs to be about 50 to 55 feet deep per ton with a 10% bleed. Due to bleed effect on the well length being a curve, that doesn't let me calculate a required length for greater than 10% bleed.

    Does anyone have any information? Links to information would be great.
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I can't claim direct experience with SCWs but I might be able to help you work through some of the thermodynamics involved. I have researched SCWs off and on.

    What you propose seems to me to be a hybrid system, combining elements of SCW with open loop / reinjection.

    You wrote that you contemplate a system with flow rate of 14 GPM. What does that translate to in tonnage?

    Closed loop systems are typically designed for 2.5 - 3 GPM per ton but there is often considerable flexibility in that guideline. Keeping to that flow per ton results in a temperature difference across the unit of 6-8 degrees, but again there is nothing sacred about that Delta-T

    You are in CT. A groundwater temp map commonly cited / used for design purposes suggests that your deep ground temp runs around 50 degrees. You could reasonably plan to extract water at 50 and return it at 36-40 degrees, allowing for substantially less than 3 GPM per ton. Depending on your tonnage you might be able to meet the load with 8 GPM and still stay safe from freezing. You could withdraw 8 GPM fromone well and reinject it to the other. That's essentially an open loop system and those often operate with 1.5 - 2 GPM per ton.

    My understanding of the application of bleed in a SCW is that it is event-driven rather than continuous. Bleed is activated when the entering or leaving water temperature reaches low or high limits. The greater the bleed percentage, the faster the water temperature will return to within bounds. In other words, during periods of light load no bleed may be necessary. If leaving water temp gets low enough that freezing is a possibility, then bleed (in your case reinjection of several GPM into the other well) is activated until leaving water temperature rises out of freezing range. The same principle applies during cooling season - if loop water gets too hot, bleed some off until the temperature drops a bit.

    It is hard to predict thermal performance of a particular well used as a SCW owing to the large number of variables, chief among which are convection and advection. If the formation is loose enough to allow water in the well to naturally move away from the well as it warms / cools, that has a good effect. If the entire aquifer is on the move laterally (advection) that helps as well.

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