1923mack
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| 07/23/2009 2:40 PM |
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| 3/4" HDPE appears to be the standard pipe for horizontal closed loop systems. The perimiter of 3/4" pipe is about 2.4 inches. The perimiter of 1 1/2" pipe is about 4.7 inched. The area of the 3/4" pipe is 0.44 square inches. The area of 1 1/2 pipe is 1.77 square inches. A the same flow rate and same length of pipe, say 12 gmp for talking, would not the 1 1/2" pipe dissipate 2 or 3 times as much heat. Twice the contact area and 1/4 the speed would seem to put a large advantage on the larger pipe. So is the higher cost of the bigger pipe the reason smaller pipe is used or am I missing something? I have heard the term laminar flow mentioned with some loop systems. I would think slow speed would be best for heat transfer. |
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waterpirate
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| 07/23/2009 3:09 PM |
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Generally speaking the size of the piping is more a function of the pipe length than a doubling or trippling of surface area. It is more cost effective to circulate water a long distance through a larger pipe creating less velocity and turbulance.
Again generally speaking it is about footage, not diameter.
Eric Sackett
weberwelldrilling.com |
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Eric Sackett
WeberWellDrilling.com |
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jonr
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| 07/23/2009 5:26 PM |
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The bigger pipe also has thicker walls that insulate more. So from 1" down to 3/4" is only about 4% difference (at least in one sample I tried in software). Laminar vs turbulent is also a roughly 4% thing.
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1923mack
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| 07/23/2009 7:07 PM |
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| I agree, thicker pipe would impede heat transfer. Prior statements were ment to be based on same wall thickness HDPE. In reality same gauge pipe in larger sizes may have thicker walls. Thinner gauges are a posibility. |
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joe.ami
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| 07/25/2009 8:44 AM |
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Posted By waterpirate on 07/23/2009 3:09 PM
Again generally speaking it is about footage, not diameter.
Eric Sackett
weberwelldrilling.com
Exactly.
Design software directs us to the minimum pipe to achieve the design parameters.
3/4" is cheap and lets not forget easier to handle.....anyone wanna take a whack at building a 1 1/4" slinky?
My software automatically sizes pipe. So if I select a pipe that transfers heat more slowly, it will simply indicate more length.
joe |
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jonr
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| 07/25/2009 9:08 AM |
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> Twice the contact area
If you wanted to explore the theory on this, you could get the effect you are talking about by putting some thermally conductive material around the pipe to increase its soil contact area. If this material conducts heat better than soil, then you will get an increase in performance. Or add clip-on aluminum fins. Or you could use thinner pipe walls (PEX might allow this) or additives to HDPE to make it more conductive.
Is any of it practical or cost effective? I don't know.
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1923mack
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| 07/25/2009 10:46 AM |
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>conductive soil around the pipe.
I was playing with the idea of better soil around the pipe (similar to grouting in verticle) but quickly came up with the control point being the transfer point of the pipe to the thermally "improved" soil. You can put 3 feet of great soil around the pipe, but you still only have a few inches of contact area from the pipe to the soil. After a very few inches of the "good" soil, the control will be the ability of the native soil. That is why I was looking at increasing the contact area of the pipe. Am I missing something when I assume, (with the same pipe wall thickness), doubling the perimeter of the pipe will double the heat transfer.
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jonr
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| 07/25/2009 8:43 PM |
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If you put 3 feet of great soil around the pipe, you greatly increase the amount of contact area with the normal soil. So heat transfer goes up.
The same effect is seen in vertical loops using grout. If the grout is a better conductor than the soil, then the bigger the borehole the better. On the other hand, with insulating grout, the smaller the better.
Using "Ground Loop Designer 2009", I see a 16% shorter loop by doubling pipe diameter with the same wall thickness (not practical). Only 16% because you do have other limitations.
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joe.ami
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| 07/26/2009 8:14 AM |
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Posted By jonr on 07/25/2009 8:43 PM
If you put 3 feet of great soil around the pipe, you greatly increase the amount of contact area with the normal soil. So heat transfer goes up.
Using "Ground Loop Designer 2009", I see a 16% shorter loop by doubling pipe diameter. Only 16% because you do have other limitations.
You can increase the contact of pipe to ground, but still be limited by native soils ability to transfer btu's. This is why the gain is only 16%.
you can change an acre of soil and still not benefit greatly in cooling. Heating however could improve due to solar gain more quickly transfered to loop.
j
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jonr
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| 07/26/2009 9:39 AM |
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This figure involves more interpolation that I'd like, but 3 feet of great soil in all directions around an average soil loop should be in the 30% shorter range. A soaker hose would be far more practical and is what I would use in dry soils.
Another thing that is interesting is clay that was wet getting heated, drying out and pulling away from the pipe. At this point, you have an air gap and then dry clay around the pipe and performance can be terrible. I suspect (but can't prove) that this has happened to some people who reported AC problems in this forum. If this occurs, sand backfill or a soaker would be a requirement.
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joe.ami
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| 07/26/2009 10:43 AM |
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Posted By jonr on 07/26/2009 9:39 AM
This figure involves more interpolation that I'd like, but 3 feet of great soil in all directions around an average soil loop should be in the 30% shorter range. A soaker hose would be far more practical and is what I would use in dry soils.
The most practical thing is properly sized loops for the soil. That's what I use.
A soaker hose is designed to assisst in compaction. Further use as a "fix" for short loops is something I'll leave to the less competent.
j |
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geodean
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| 07/26/2009 4:41 PM |
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Posted By joe.ami on 07/25/2009 8:44 AM
Exactly.
Design software directs us to the minimum pipe to achieve the design parameters.
3/4" is cheap and lets not forget easier to handle.....anyone wanna take a whack at building a 1 1/4" slinky?
My software automatically sizes pipe. So if I select a pipe that transfers heat more slowly, it will simply indicate more length.
joe We installed 120,00' of 1 1/4" slinky on a school. My crew had arms like Arnold after trying all of those loops.
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Dewayne Dean
PalaceGeothermal.com
Why settle for 90% when you can have 400%
We heat and cool with dirt!
visit- http://welserver.com/WEL0114/- to see my system |
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1923mack
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| 07/27/2009 9:14 AM |
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| So the control for pipe design is the ability of the soil mass to receive the heat, not the contact area of the pipe to the soil. I can see that if the soil is a poor receiver of the heat, then it could be a stumbling point. Only 19% increase seems very conservative. Dried clay can shrink and crack as mentioned. Overly wet clay has poor thermal properties. So I guess back to 3/4 inch HDPE with native backfill and custom soaker hose above. The soaker would be to enhanse performance, would not use it in the loop design calculations. |
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joe.ami
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| 07/27/2009 9:31 AM |
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Posted By 1923mack on 07/27/2009 9:14 AM
1)So the control for pipe design is the ability of the soil mass to receive the heat, not the contact area of the pipe to the soil.
2)So I guess back to 3/4 inch HDPE with native backfill and custom soaker hose above. The soaker would be to enhanse performance, would not use it in the loop design calculations.
1) Obviously both have an impact, the larger point is it's hard to make something that's not there. More surface contact with a poor conductor is still limited by the poor conductor.
2) I've no objection to a soaker hose employed to aid in compaction and occasionally assisst in extreme operating conditions (say lower EWT's on a hot day to improve EER) as long as it is not counted on for system operation.
You can also employ design software to determine how much extra pipe it would take to lower EWT's (raise EER) or raise EWT's (raise COP) all the time.
Good luck
Joe |
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1923mack
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| 07/27/2009 2:34 PM |
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| There is enough land (2 acres, light vegitation) to do as much pipe as necessary. With 84 degree soil temperature and I guess EWT's of 89 degree or so is any 5 ton system going to perform well? |
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joe.ami
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| 07/28/2009 8:41 AM |
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Posted By 1923mack on 07/27/2009 2:34 PM
There is enough land (2 acres, light vegitation) to do as much pipe as necessary. With 84 degree soil temperature and I guess EWT's of 89 degree or so is any 5 ton system going to perform well? Default EWT on my design software is 95*. So of themselves those numbers aren't a deal breaker.
Nature of soil/BTU's extracted can of course rapidly change that EWT. That's where local experience is most preferred. The ability of your acreage to give you what you need (or not) can be calculated, but in the age old engineer vs installer barbs I'd prefer experience to math.
One thought is to find a geo ME willing to design and guarantee your system and it's performance. As long as he has the right insurance, that might be the best bet for the few extra bucks you are willing to spend. Might even be a business oppurtunity for you or a local HVAC guy if no one is doing horizontals round there.
J
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1923mack
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| 07/29/2009 10:08 AM |
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Just a Mechanic:
While I am not really looking to start a new business, am thinking a nice demo project to show the feasibility of horizontal loops would be nice. There any many 'Greenies" here in California with money in their pockets, so a workable system that uses the earth and saves electricity should take off here in crazy California. My install could be a good demo project. |
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joe.ami
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| 07/29/2009 11:25 PM |
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mack,
my fear for you is that horizontals are ineffective in your area.
but if you can find a local hvac guy and you are willing to fund experiments, you might get some duct work for your trouble and only risk verticals later.
Joe |
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Geothermal; Savings Underfoot |
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jonr
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| 07/30/2009 9:34 AM |
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It's going to come down to "is the earth much warmer/cooler than the air". It is in climates with large seasonal temperature swings.
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engineer
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| 08/01/2009 8:40 PM |
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| I don't think 84 deg soil will yield 89 deg EWT for long unless the field is very large. |
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Without data, you only have an opinion. |
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