Reynolds number importance with variable speed pumps

Discussion in 'General Discussions' started by ddipert, Dec 18, 2019.

  1. ddipert

    ddipert New Member

    Started to think about Reynolds numbers of variable speed pump. How are the systems working when the pump is on low? Are the Reynolds numbers low when the pump is set on low?

    Anyone have any good data an install with low Reynolds numbers?
    Can more pipe in the ground make up for a low Reynolds number by spending more time in the trench transferring heat.
    How are variable speed pumps able to have good performance with their low Reynolds numbers (or do you design a loop field differently)?
    I searched but could not find anyone talking about this subject.

    Recently installed a 3 ton CM TE30 with a horizontal loop field. (6) trenches 4' wide trenches 100' with 800' of slinky 3/4" per trench 8' deep.

    I know my Reynolds number are very low (have valves to close loops if needed). Added the extra capacity for another Geo pump in the future.
  2. arkie6

    arkie6 Active Member Forum Leader

    If my memory is correct, the ClimateMaster TE30 variable speed loop pump controls flow based on differential temperature at the loop inlet/outlet to the unit. If the sensed differential temperature (dt) is too high due to insufficient heat transfer in the loop, then the pump will automatically increase flow as needed to maintain desired dt. This increase in flow will also improve your Reynolds number which will further enhance heat transfer. During periods when there is low demand on the unit and hence low flow from the loop pump, the fact that your Reynolds number may be low is of little consequence.
  3. wing

    wing Member

    You have 1600 feet of 3/4 inch horizontal hdpe pipe per ton of heat pump capacity. Accordingly, your loop field is way over designed. BTW, I am a big fan of over designing the loop field.

    A low Reynolds number will negatively impact heat transfer, but since you have such an overdesigned loop field, it won’t matter.

    Having a variable speed pump is another issue. You only need a pump with 9 gallons per minute capacity through your loop. The fluid velocity and head are both low, so using a variable speed pump versus fixed may save some money on pumping the loop field versus fixed speed.
  4. ddipert

    ddipert New Member

    My DT's do not seem to change much with pump speed.
    If I run on DT mode 5* DT with anywhere from 5-7.5GPM.
    Manual mode pump 50% and still have DT of 5.5* around 3-3.5GPM
    Water temp is 60*
    This has me thinking why won't my DT's change with pump speed.

    The loop field was over designed for expansion (another 3 ton unit) in the future. With the valves I can make it more conventional size. I'm trying to figure out which way would be better. Keep the loop as is and turn the pump to manual mode (seems like a waste of an expensive pump) but might be cheaper to run at the slower speed.

    I read about the horror stories loop being to small. I would rather have more loop in the ground and turn a loop, two, or three off than have to dig it back up to add later.
  5. wing

    wing Member

    I agree 100%. Over sizing the loop field is cheap insurance. A minimal amount of excavator time and cheap loops of HDPE pipe.

    Heat of extraction is a function of the pumped fluid volume and delta temperature across the heat pump. If you pump at a lower rate and keep the same delta T you are lowering your heat of extraction.

    When you go to dual 3 ton units, it appears your loop field will still be very well sized at 800 feet of buried 3/4 tubing per ton.
  6. ddipert

    ddipert New Member

    "Heat of extraction is a function of the pumped fluid volume and delta temperature across the heat pump. If you pump at a lower rate and keep the same delta T you are lowering your heat of extraction."

    Shouldn't the the delta T change with pump speed? If the fluid moves slower it should be able to transfer more temperature (more contact time in the heat exchanger).
    Last edited: Dec 20, 2019
  7. SShaw

    SShaw Member

    If you want to figure out which way is better to run the pump, then you should calculate your heating capacity and COP to see what works best. You should find variable speed is best.

    Heat of Extraction (HE): GPM x DELTA-WATER-TEMP x 485
    Heating Capacity (HC): HE + (TOTAL WATTS x 3.413)
    Efficiency (COP): HC / (TOTAL WATTS x 3.413)

    Heating capacity is proportional to GPM. Based on your comments above, it looks like constant mode uses 100 less Watts but outputs far less BTUS due to the lower GPM.

    From the numbers you posted it looks like variable is better: let's say with constant mode your flow dropped 1.5 GPM at 5-degree delta T and you saved 100 Watts pumping power. You lost (1.5 x 5 x 485 + 100 x 3.413) = 3,979 BTU heating capacity, and you saved only 341 BTU (100 Watts) input power. That means you are worse off, unless your heat pump is runs with a COP better than 11.6 (3979 /341.3), which it doesn't.

    By the way, all of the above is related to heat transfer inside your heat pump, so I don't believe it has anything to do with the Reynolds number out in your loop field, which is related to heat exchange with the earth.
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    FIRST you only need a good reynolds number when you are trying to extract(transfer) a lot of heat from the ground so only when the heat pump is running at higher speed. Single or dual stage is different since all that heat transfer has to happen when the pump is running, and since it will cycle much more, this might happen a lot. Antifreeze also affects your reynold number significantly, methanol is much more favorable in terms of Reynolds numbers than glycol.

    Heat capacity of a geo system has very little to do with GPM. The heat capacity is determined by the refrigerant circuit, mainly by the compressor size, and to a lesser degree the operating conditions and heat exchangers. The formulas you posted are meant for heat extraction or rejection in fluids with antifreeze.

    You lower your gpm, and your delta T goes up, but your heat extraction stays the same. If reduce your flow, your heat pump does not know. All it does is reduce your flow inside your heat exchanger, so your heat pump has more time to take more heat out of a given volume of water. As a result your avenger fluid temperature inside your source heat exchanger gets slightly reduced.

    You can fine tune your system and reduce your pumping power by 100 watts, and it might slightly increase your average water temp , which might cost you about 20 watts. So you gain 80 watts, and can optimize your system.

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