Standing Column Pumping Energy

I'd like to get some post-installation feedback on my standing column system. Some background: I live in New England where standing column designs are fairly common. In New Hampshire domestic wells can be used for dual purpose exchange/potable water. I have a 395 foot deep well with the static water table at 100 feet. With most designs I am aware of, pumping energy is relatively high, especially when the pump serves double duty for house pressure.

In order to reduce pumping energy I figured I needed to reduce the head - which I did by taking the unorthodox approach of working with the natural suction created in the return line. By installing a second pressure transducer on the VFD with a different range, the geo system operates at 20" mercury. When the house usage exceeds what is stored in the pressure tank, the system switches to house pressure to refill the tank, then switches back to low pressure operation. At the end of the cycle, the system pressurizes again to reduce mineral precipitation in the HX.

The upshot is, I'm pumping 7 gallons per minute through a 3 ton unit from 100 feet underground using only 1.5 amps. My installation cost was a fraction of what new bore holes would cost me and the house usage effectively serves as the bleed on the well. All the concerns about air infiltrating the system have yet to materialize. What tiny amount of air does get in there is not in contact with the water for more than a minute or so before bubbling back up to the well surface. In another unorthodox move, all domestic water is routed through the unit's HX so the water is never stagnant - it is always fresh water drawn from the bottom of the well.

There's more to it than just this, like trying to correctly size a pump for this application, but this gives a pretty good idea of what I did. I should also note that the pros I worked with fought this design tooth and nail (as they should) but in the end had to admit that it was a novel solution that may or may not work for others. So, "others", what do you think?

 

Thank you for the info and the write up! That was definately a clever work around that yielded some good results. I had thought about installing two transducers several times at happy hour but the application never presented itsself to me. Stick around we need more clever ideas here to help others.
Eric

 

Bravo. I am genuinely impressed with that pumping efficiency and simplicity of open loop design with just one well. Enjoy the cheap btu's.

 

I imagine the system is almost siphoning. How low is the pressure in psi when there's no domestic water demand just low pressure- high flow for the geo unit? What size pipes are used to and from the well to minimize headloss to the house and to maximize return suction? What was the most difficult detail to engineer?

 

Well, in PSIG (Gauge) the pressure is negative, meaning it operates under vacuum. In PSIA (Absolute) I measure the "pressure" in two locations, upstream and downstream of the constant flow valve. Since that valve is downstream of the HX, I'm not measuring peak system pressure, so add a couple of PSI for HX loss. Upstream of the constant flow valve is 5 PSIA or about 20" hg. The constant flow valve requires 3 PSI drop to function properly and downstream of the valve measures 2 PSIA or about 26" mercury. The pressure transducer is also located upstream of the flow valve and is calibrated to operate at 20" hg.

Understand that this level of vacuum would not normally be achieved by conventional pump due to cavitation. But there are no such restrictions when gravity is the motivating force. I would bet big money that nearly every standing column system has some part operating at high levels of vacuum - it's just the physics of moving water downhill. This is one reason I felt confident that I wasn't overstepping best practices too far. This system simply operates both the return and supply at vacuum pressures, rather than just the return.

Also note that the pump is still overcoming a not insignificant head. It's still lifting the water about 70 feet and then there's pipe friction, HX loss and valving. Because the pipe is not an integral part of the ground exchange, I don't need to worry about turbulent vs laminar flow so there's no reason not to minimize pipe friction. I used 1 1/4 " throughout, though the 130 ft drop pipe is actually 1" to help fit everything down the casing. At 7 GPM, pipe friction is minimal.

 

Most difficult to engineer. Hmm, besides keeping my marriage intact with a newborn in the house during this endless project?

As with any new idea (or at least new to me) convincing myself it would work was not easy. Then convincing others that it would work was much harder still. Actually getting it to work at times seemed impossible! The first time I ran a test with the manifold I realized I had no provision for starting the syphon, so I had to engineer a priming circuit. The first prime switch I purchased had a dead band the size of Kansas. Took me a week of calling around to find one that was sensitive enough, and so on and so on....

Every conceivable thing was much harder than I anticipated. I went through 3 different transducers trying to calibrate the system. Even then, I had huge oscillations pressure as the VFD tried to zero in on a design pressure that was well out of specifications for the unit. There just isn't much elasticity in the system at these reduced pressures. It took a trial and error combination of bladder tanks (acting as vacuum accumulators) and the right transducer to finally get it stabilized.

Fitting the pump curve was also interesting. I had to figure a pump curve that was the MOST efficient at the low pressure/volume required from running and yet still provide enough pressure and flow for house use when sped up. In the end, it was educated guesswork, and I have a feeling I got it pretty close to be at 1.5 amps. BTW, the pump runs at 3.5 amps when I take a shower in the morning with my 1.6 GPM shower head!

I also scoped an additional challenge for myself in that I wanted the house to operate at constant pressure when the unit wasn't operating, rather than cycling with the storage tank pressure. That took some mental gymnastics, since my storage tank was grossly oversized for parameters of the VFD drive.

The control circuitry turned out quite elegant, I must say, but getting everything to sequence and start/stop on cue without resorting to a bank full of relays was not easy and took several attempts before it was completely reliable.

 

Hi - I understand that because I am a new member, my posts are moderated. How long is the delay, usually? I have had a couple of posts pending for a few days now.

 

Blake, thanks for the reply posts. Because there was a long delay due to moderator's schedule not yours, I had the idea that you wanted to keep the engineering Jeanie in a bottle. My state prohibits reinjection so it's great engineering that I can't personally use, but I found it very interesting food for thought nonetheless.

 

I've used this design in combination with a standing column, but the principles could be applied to surface discharge or injection well. Any open or semi open system could lower pumping energy with lower pressures (to a point, anyway). Since many open systems also feed domestic supplies, a dual pressure system has many potential applications. Even if one wants to make an argument against operating under vacuum, running at even 5 psi will use a lot less than running at 60 psi.

The pros I worked with insist on running their open loop systems at a minimum of 20 psi. One install we discussed the static water table was at 15 feet. Pumping at 20 psi vs 5 psi in this case uses 230% more pumping energy. Much arguing ensued, but basically what it came down to was that the lowest setting on their VFD's was 20 psi. A pressure transducer with a different range can extend the working "pressures" of a VFD well beyond its rating. The VFD can only read the signal its sent, not the actual pressure in the pipe.

 

Sometimes we pros are limited to the tools available. Sometimes it doesn't make sense.

Pioneering offers more liability than profit to the average contractor for hire.

joe

 

Very true - sorry, I do sound like I'm unfairly bashing the hard working and knowledgable professionals out there. (And I just edited the post, taking out the "!!!") You're working in an industry that already has an uphill road to gain acceptance with the general public without risking your reputation and/or business on an experiment. With that said, I was a guy who was willing to PAY for the experiment and I still met resistance. My expectation was that since I was essentially funding their R&D they would be a little more open to a new idea.

I should also say that the project manager was far more enthusiastic (read, not angry and a little curious and mostly supportive) whereas the owner was screaming at me in my basement in front of my 14 month old.

So, maybe there can be a happy medium between coming unhinged and a fool's folly.

 

Wow !!! sorry that you had to deal with that.

 

I don't think I would continue to employ someone who "screamed" at me.

I meant to add we are watching your project with interest.
I didn't feel you were attacking us, just wanted to remind that licenses and liability (whether warranty or damage) can be fetters that don't permit us to do everything we might like.

 

The contract I signed would have made it difficult not to be on the losing end of "firing" the engineering firm, instead I made it very clear that I would only deal with the project manager. Essentially the owner was "explaining" in much louder terms exactly how he felt about licenses, liability, warranty, and damages.... I get it, but a hold harmless clause probably would have sufficed.

I'm happy to see the response here to this design approach has been overall positive. I really expected to be challenged with some tough questions and was only hoping I'd done enough homework and engineering to satisfy the knowledgeable critics on this site.

The system has been operating for a full year without any real hiccups. Not quite bulletproof, but I've learned enough with this prototype that I think the next one could be close. Qualitatively, my electric bill in February last year topped out at $150.51 for a 2,100 square foot house in New Hampshire @ 70ºF. Am I happy with THAT result? What do you think

I'm happy to answer any additional questions about the design, though since it's already installed and operating, I really won't have any updates to offer - unless something goes wrong - in which case hopefully I'd be reporting how I fixed it and not "oops, what was I thinking."

 

Does anyone know of any geothermal/well companies in Raleigh that could help do this kind of project? I'm using 11amps on my pump for my SCW situation and I'd love to find a way to minimize that but I have yet to find anyone who is qualified to help me with these types of details. They all seem to just want to do the minimum to keep the thing running without regard for cost savings. I want the system to be as efficient as possible.

Thanks!

 

Since geochallenged resurrected this thread, I'll post an update. The system has now operated for 3 heating seasons without drama or mishap. This January had some mighty cold days (down to 5 below a couple of nights) and my electric bill for the month was $121.37. Also, interestingly enough, after 3 years of circulating water in the well column, well yield is up slightly. I say slightly, but it was near-dead before, producing less than 0.5 GPM on a daily average. So, I'll take slightly. The increased production has raised the water table in the well column by a couple of dozen feet. I'm now pumping 7 GPM with 0.75 Amps.

While I'd love for everyone doing standing column to take the steps I did to reduce pumping energy, I've had no takers to date. To my knowledge, no one else has attempted to install a system pumping under vacuum. I'm planning a small 6 house subdivision that will use this approach, so perhaps I'll get a little more play. In the meantime, I really feel badly for those stuck with inefficient pumping systems. In New Hampshire, I'm meeting many people with standing column systems that are paying way more than they expected to heat their house with geo.

 

Blake - what kind of filtration are you using on your SCW? Also what size mesh do you recommend to protect the heat exchanger? Grit/sand is a big issue in my SCW unfortunately due to a strong water vein that is washing the walls of the well.

I have a 5 ton and a 2 ton running off my well with household water. Thus far I have only found people who know residential wells, and people who know how to install the HVAC equipment but NO ONE who knows how to deal with a SCW that is pumping a boatload of water every day and has sand issues, nor anyone who knows how to optimize the system so it runs efficiently by doing the proper measurements. All they do is put a pressure gauge on the equipment and say its fine. I'd like them to actually measure what it is costing me to run these things.

It's a steep learning curve for me (I bought this house and had no idea what I was getting into) and I could sure use some help from someone who understands the overall system aspects of a SCW with reinjection and a geothermal HVAC.

 

Around these parts, New England, standing column systems are generally bored in solid granit. I have no useful experience for solving a sand problem. Wells here don't typically even have screens. Unfortunately, it sounds like standing column probably shouldn't have been specified given the water quality issues.

The only thing that comes to mind is setting the pump higher in the well (near the top of the water table) and extending a poter shroud to the bottom. The might let some of the sand settle out before it gets to the pump. I have no way of predicting if this would work.

 

Geochallenged, Did you ever find someone in the Raleigh area who could do this? I live in the same area.