need some design help

Well guys all my free trials are expired or disallowed. I need a sanity check for a off center project due to a polluted aquifer. Typically in my area we install 400' of active loop per ton of load. This job has a vertical stop depth of 80' =ing only 160' of active loop.
My driller design math says we need to drill 2.5 loops to equal 1 normal loop to cover the load. The load is 5 tons. my concern is the boreholes being that short they will not perform the same as the 200' bores. The 80' bores are coarse sand 100% wet starting at 15'. Am I worrying about nothing? Would be curios what someones software has to say about the depth and layout of the loops.
Thanx in advance!
Eric

 

Caveat - rules of thumb follows. And I don't want the lay person thinking this is how to design a system.

One could say the top 20' is going to be more representative of a horizontal system (that usually has 2x the pipe). So that would be 10% of your normal borehole length and now 25% of your new borehole length.

I would look at 360' being "vertical quality" in vs. now 120' being "vertical quality". So 3x is somewhat the safety factor you need.

But if you threw me some loads I could run it through the actual software.

 

Thanks Chris!
Just wanted a gut check, and your suggestion makes sense to me.
Caveat- no lay person should follow here.
Eric

 

How do you keep your bore hole from collapsing? Temporary casing?

Saturated sand is a good thermal conductor. What are you going to use to backfill the hole? Can you use the cuttings, i.e. sand in this case, to backfill the hole and then cap it with a bentonite plug? Is that allowed in this case since you are drilling in unconsolidated material?

Climatemaster has some free software you can download to determine length of pipe in the ground needed based on load, actual soil conditions, and type of grout.

It is called GeoDesigner Software and can be found here: https://www.climatemaster.com/geothermal-dealer/home/sales--system-sizing-tools

 

The "cheap" software will not show the difference in this type of drilling unless you specify different soil types. As an example, you'll get one 1000' borehole being the equivalent to 100x10' boreholes. And they are not equivalent.

I don't know about GeoDesigner specifically, but I have noticed this about others such as WF's GeoLink.

 

I know with the GeoDesigner software, one of the inputs is average depth of the loop. It has been a while since I used it, but if I recall correctly, changing from an 100' deep hole with an average loop depth of 50' to a 200' deep hole with an average loop depth of 100' did change the total amount of pipe needed.

 

While you pick up more "true" geo component with a deeper borehole (about 1F every 70F), and you exclude the seasonal effect of outdoor temps on the loop (for the first 15F or so ft), overall your loop should not differ more then 5 % in thermal performance by spreading the borehole out 2.5 times.

Would need details in terms of grout, expected design loads, pipe diameter, bore hole diameter, assumed formation conductivity, borehole diameter etc. More or equally important is pressure drop and that you still have somewhat of turbulent flow during peak load, so overall design matters a bit here, since you are now spreading the flow between more circuits, and the circuits are slower, both are now creating lesser pressure drop and lesser velocity.

Not sure if it is worth to follow the illusion of precision, all factors being equal your minimum winter EWT might differ by 0.5 F degrees. You might start making ice earlier, having the water close to the borehole going through the phase change earlier, which will stabilize the borehole temp anyway. At what temp do you loops usually run at the end of the heating season, and what pipe size do you use.

I would not worry.....with a single family house I assume.

 

Wow! lots of questions and thoughts.
First the drilling:
we do mud rotary, the hydraulic pressure of the fluid in the borehole holds it open until the loop is inserted. then the entire bore is pumped full of 20% solids bentonite from the bottom to the top.

typical loops here are 3/4 x 200 inserted to a TD of 200'. using 1 borehole per to, ewt at the end of the winter is in the 40 degree range.
Eric

 

Doc's got a good point as usual about the necessary flow through the loops. Say you install 12 loops at of 3/4" pipe 160' each (80' down and back). That would give you 1920' of pipe in the hole for 5 tons or ~384' of loop/ton which is pretty close to your normal 400'/ton. You may want to connect them with 3 loops in series (3 x 160' = 480' of pipe) and then connect those 4 groups of 3 series loops in parallel. Assuming 3 gpm/ton x 5 tons = 15 gpm total / 4 parallel loops = 3.75 gpm per 480' of 3/4" pipe. I don't have the Climatemaster pressure drop calculator software on this computer, but that is a little higher than typical flow rate for that length of loop and I suspect the pressure drop would be higher than recommended for optimum efficiency. It might be worth investigating going with 1" pipe for the loops if configured in this 3 series x 4 parallel configuration to reduce overall pressure drop of the loop field. The larger 1" pipe would also improve heat transfer slightly over 3/4" pipe. I found a Turbulent Flow Rate Calculator online and it showed 1" pipe with 20% glycol at 40F water temperature with 3.75 gpm flow to have a Reynolds number of ~4500 which is in the turbulent region (Reynolds number >2500) and where you want to be for good heat transfer.

 

I get with 4 x 480ft 3/4" pipe and 20% glycol a Reynolds around 2155 (3.8 gpm flow), but you are already at 20ft/hd just for the loop field. Glycol is always pretty bad, either your reynolds number stinks, or you have to get the flow up so high that your pressure drop is too high (much pumping power). With 20% methanol, your reynolds number would be around 3800.
With Methanol, you could do 6 circuits @ 320ft, going down to 2.5 gpm per circuit, and still have a reynolds # above 2500, and 5.9 ft/hd loopfield pressure drop.
Now, if you use 4 x 320 with 1" pipe and glycol, your Reynolds drops to 1720.

If you can't use Methanol (New Jersey?), could you use higher conductive grout and go with lesser pipe length? Right now your grout is not more than 0.40 conductivity, if I recall the 20% solids betonide correctly.

If you have to stay with glycol, I would do the 4 x 480 (3 in series) with a 2155 reynolds, I am not the biggest believer that the 2500 Reynolds must be reached, I don't even recall where the 2500 number came from. A dual flowcenter would give you around 14.5 gpm. Or use only 1 pump flowcenter (26-99?) and let the flow drop to 11 gpm and the reynolds to 1700. Throwing all the rules out of the window, that would cut your pumping power in half, your delta T would be a couple degrees higher, but who would care if you have 40 F degree water to play with....but you have lesser material cost and half the pumping costs...

Spinning this further, using some 1.0 grout would probably get your borehole length to 150ft/ton, allowing it to drop down to 30F would decrease you borehole down to 95ft/ton.

So (3) 1" circuits with 1" pipe and 2 bore in series would get you an 1830 Reynolds with a single pump flowcenter, and still 12 gpm. 1.0 grout needed. Half the loop field, half the pumping power, more compressor power due to 10F colder extreme loop temps (30F), probably equals each other out, but now half the bore field.

Who would have thought this geo stuff can actually be fun....

Let us know where you are going here. But I would not worry about shallower loops, putting them in series should help keeping the reynolds somehow at "ok" levels, whatever they are.

 

I have a budget meeting with the client this morning, followed by a site visit. Thanx for all the help and will keep you all posted. We never use glycol unless specified in the contract. Methanol is our go to anti freeze.
Eric

 

I've run several online calculators for Reynolds Number and I'm getting a number of approximately 7000, which is well into the turbulent flow region (>4000) using the following inputs:

1" SDR 11 pipe (ID = 1.06")
3.75 gpm = 1.36 ft/sec
fluid density = 0.977 kg/l (water + methanol @ 15% by weight @ 5C / 41F, which is 20% methanol by volume and gives a freeze point of -12C / 10F)
dynamic fluid viscosity = 1.5 cP (I used water at 5C because I couldn't find values for water + methanol mixture and assuming a mixture doesn't change viscosity appreciably).
Reynolds number = 7269

https://www.engineeringclicks.com/reynolds-number-calculator/

The following graph I extracted from the internet shows you need ~2.75 gpm through a 1" SDR 11 pipe with 15% methanol @ 25F to achieve a minimum Reynolds number of 2500.

 

You cannot use water for the calculations. The mixture changes the viscosity which is the key input into the formula. Temperature changes the density and viscosity.

The addition of methanol to water lowers the reynolds number, so does lower temperature.

Example: If 20% Methanol + Water (15F) at a given setting has a Reynolds number of 2293, with 32F water Reynolds # is 4,476. With 50F water it is 6,101, and with 110F water , it is 12,879.

Temperature and antifreeze change things significantly.

Again, not sure where the weird number of 2,500 is coming from, you have turbulences at 2,200, 2,400, and some more at 2,600. Not too concerned of Reynolds of 2,000 or 1,800 either, since your loop field should not drop down to 15F LWT either.

@Eric

Using methanol makes things very easy.

I would argue, you can save significant loop length if your EWT is around 40F at the end of the heating season, without having much performance impact.

You cost per ft might go up due to higher grout cost, but so does you overall margin. Plus you might save a pump with a loop with lower pressure drop.

Lesser upfront, higher margin, share the savings with the customer, more jobs with the same overhead...worth thinking about.

We are at less than 100ft/ton in our area, 1.5" pipe, 1.6 grout, single borehole up to 5 ton system size.
Looks like you would put in (5) x 200 ft 3/4", which is twice the footage and 5 times the rig mobilization on site. Maybe worth thinking about it.

 

Doc,
Here on the sand bar we are spoiled. 6 wheel drill rigs, light weight high speed production drilling. No rock or hard pan. I will definitely seek your guidance on loop configuration once a layout has been established.
Eric

 

Hey guys, long time no talk!
I had a similar issue a few years back too. I had to restrict our loops in a contaminated zone as well. I didn’t not get a concrete ansewer when looking for advice either.
We simply cut our 1” loops down from 300’ to 150’ each. We then hooked 2 loops in series to form a 300’ loop before doing a typical reverse return install. I hope this helps you.
I do not have any data on loops since the property has since changed hands, but no complaints or call back in since installation.

 

Dan,
We have missed you! Hope everything is going great for you in your neck of the woods. Things here are booming and the geo is picking up a nice head of steam as well.
Eric

 

So the bit hit the ground today and we drilled 12 x 3/4 x 205' total loop length to cover 4 tons of load. I did two rows of six. Total depth of bores were 80' each. I ran the numbers on geo flo calculator and just looking for a gut check. Planning a close header, outside 50' of 1 1/4 from circulator, inside piping 3', 2 ' of 1" rubber hose from circulator to unit, no elbows. It spit out 21' of head at 12 gpm. Anyone concur or disagree?
Eric

 

So the bit hit the ground today and we drilled 12 x 3/4 x 205' total loop length to cover 4 tons of load. I did two rows of six. Total depth of bores were 80' each. I ran the numbers on geo flo calculator and just looking for a gut check. Planning a close header, outside 50' of 1 1/4 from circulator, inside piping 3', 2 ' of 1" rubber hose from circulator to unit, no elbows. It spit out 21' of head at 12 gpm. Anyone concur or disagree?
Eric

 

Do you run 2 boreholes in series? If you have 12 gpm, you have 1 gpm per borehole, unless you run 2 in series. Assuming methanol, your Reynolds # is less than 1000. If you now run 2 holes in series, your Reynolds is about 2300, which I think is fine. If you run them in series, yes, I get about 21.5 ft/hd total system pressure drop, depending on the heat pump.

 

Thanx Doc. So two bores in series for a close header with 6 connections instead of 12?
Eric