So, I spent most of the day refreshing myself on Thermodynamics and Fluid Flow from my school days, to then do some ground loop calculations. I've done a couple whole-house load calcs, and I'm coming up with 22,000 to 26,000 BTUs heat loss for our county's 26F winter design temp. So, I've been sizing my ground loop for 3 tons to account for DHW that's not accounted for in the load calcs. With that my ground loop design: Soil type: Sand down to hardpan at 5 feet, where my loops will be. Soil Thermal Conductivity: 0.6 Btu/hr ft F (Annual rainfall = 70 in. almost all in the winter, and I'm putting infiltration trenches in-between the geo trenchs, so it could be greater than 1.0 at times) Loops: Four 800' loops of 3/4" HDPE, 8 pitch slinky in Four 100' long 3' wide trenches Design Ground Temp: 52F 20% propylene glycol mix Design Loop Flow: 3.2 gal/min Pump Flow: 12.8 gal/min Loop Head Loss: 24.8 ft Now, I need to find a pump. What's the typical head loss in the manifolds and heat pump? I don't want the pump to be undersized because I'm getting pretty close to laminar flow in the loops.
Climatemaster has a pretty good loop pressure drop and pump sizing calculator that you can download here: http://www.climatemaster.com/geothermal-dealer/geothermal-sales-tools/pressure-drop-calculator/ For a 3 ton geothermal unit with closed loop, you should only need around 9 gpm (3 gpm/ton) at full capacity. That changes your loop flows to 2.25 gpm/loop which will reduce your loop head loss. The pressure drop across the unit heat exchanger would typically be around 8-10 ft head loss. The program above shows that 10 fittings = 2 ft head loss. This would include the fittings exiting the unit and in your loop manifold. I count every short 90 degree turn as a fitting, i.e. a 180 degree U-bend at the end of a vertical loop = 2 fittings. A long sweep in the return pipe to make a turn = actual ft of pipe. You need to count the fittings on both the supply and return side.
Grundfoss 26-99 should do the job. With 9 gpm, you are indeed in laminar flow territory. Especially with glycol. Using 20% Methanol would change that. I assume you meant 18" pitch, not 8".
By pitch I mean to say 8 feet of pipe per foot of trench. About 16" on-center spacing of the loops. Laminar flow in the 3/4" pipes is estimated at about 3.1 gal/min with 20% PG mix.
Rather than 4 loops of 3/4" x 800', have you considered 3 loops of 1" x 1000'? Both have ~3000' of pipe in the ground, but the 1"x 1000' loops have less head loss. Also, rather than 20% PG antifreeze, have you considered 15% Methanol? Both protect down to ~18F freeze point, but the Methanol results in less head loss and higher Reynolds Number. See attached sample run using a Climatemaster TE_038 with internal Magna 25-140 pump that I did with 4 loops of 3/4" x 800' pipe with 20% PG vs. 3 loops of 1" x 1000' pipe with 15% Methanol:
You are not getting into turbulent flow territory with 3.1 gal/min per circuit and 20%PG with 3/4" pipe. Roughly a Reynolds number of 1800.
