It's all Plain to Sea

[justwait]

Hi John

I am a newbie to the world of solar greenhouses. My research, so far, has led me to consider a 22 ft dome for our a solar subterranean off the grid solar system.

I have read your FAQs and been overwhelmed by the amount of good information you are willing to share. I have had to read some of the questions two or three times to appreciate your knowledge. I have started to review your excellent SHCS Calculator Excel spreadsheet and I have a couple of questions.

You use a value of 3.75 as the “Effective Diameter” of a 4” ADS in your SHCS Calculator. Is that the actual diameter of the inside of a 4” ADS pipe?

I recently visited Lowes and measured “THEIR” 4” ADS pipe and found the inside diameter was a full 4.”

Are all ADS pipes inside diameter the same?

Can you give me your insight?

[mrhobbithhnet]

Good question my friend... it's based in not knowing what happens when you are trying to calculate how much air is moving down a tube. First of all, a portion of the space the air is moving down a tube in is indeed unobstructed free air moving. But, if you think of it, some of it isn't - the air closest to the sides is in fact going to be moving progressively more slowly as it approaches the surface of the inside of the tube. It just isn't possible for the air immediately next to the inside tubing wall to be moving the same speed as the rest of the tubing. That Boundary Zone or Layer, is a factor to consider, and in a SHCS of tubes it's significant because there are so many and the tubes themselves are relatively small compared to the entire flow, significant enough that it must be considered or the calcs will all be too far off to work properly.

I took a look at the formulas used to calculate the exact theoretical effect (requiring knowing the air pressure, the surface roughness, and the average lack of linearity to mention a few factors...) and immediately saw that most average souls were in no way interested or capable of skinning that onion... so I just did a few calcs, found an average effect with normal ranges you'd expect in a 4" tube with about 4 ft per sec of air movement and 0.1 inches of back pressure in a closed loop system. From what I could figure, the 4" tube performed about the same as a pure 3.75" open tube with NO surface interactions. It's not exactly a pure calculation, but good enough. And from my ad hoc measurements of existing builds and measured flows, it's closer to the truth than using 4" as the actual free opening size. It actually is closer to 3.5" in higher speed systems, but that little change wasn't too off to get too concerned.

So to answer your question, the 4" ADS is indeed a nominal 4" ID as you've measured, but it's performance as an air tube develops enough 'drag' on free air flow to be a equivalent *free* air flow of a 3.75" size opening with no sides.

I could have just subtracted an equivalent percentage at the end of the calcs... but for some reason or other, found it easier to do it this way. Now that the 5 exchanges an hour is the minimum starting point for optimal exchanges, I should actually be doing it a completely different way - the fact of the matter is that as the air speed goes up past a certain point, it is bound to increase beyond that point, has a very significant and non linear effect that really should be calculated in a manner that reveals a closer answer...

Hope that helps you out... keep thinking about things, it's the unknown explained that makes the trek all the more interesting.
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[justwait]

Thank you for your insight on the effective diameter of the tubes.

Would it make a difference if the duct fan exhausted the air, rather that push the air into the tubes?

[mrhobbithhnet]

Most fans are more efficiently designed to push air as far as I know. But if the greenhouse is closed up tight, it won't matter too much as the fan will be pushing onto itself anyway. I have a suspicion that to have the most efficient system whether closed up or open, it might be better to have two slightly smaller fans, one for suction and one for inlet. Of course, on small greenhouses the wee bit of difference anything makes and the total outlay being so small, one is free to experiment without risking too much... and then let us know what's up. Mind you on big commercial systems costing $1000 a year in power bills, it would be proper to consult with an electrical engineer in the HVAC arena to see what the proper way to go is.
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[justwait]

[mrhobbithhnet]

justwait,

It would seem to me that misting in the south half, about 1/3 of the way to the SHCS intake would be the place to start. (at least till you are full of plants - a full greenhouse shouldn't need much misting for cooling)

What kind of layout for a SHCS were you thinking of? Have you seen some of Hex's posts? He's doing a SHCS recirculating system in a dome.

Domes are tricky to get good use of the soil mass heat sink - the only straight forward way to get equal length tubing is to lay them out radially. But by doing so, you don't maximize the soil heat sink as well (the tubing in the middle is close together and the outside is too far apart)

I'd be interested in seeing what you had in mind...
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[justwait]

Yes, we have seen Hex's design. We are trying an different design. We have posted our design under a new topic - 4-H Depoe Bay, Oregon Dome Subterranean Heating Research Center. We welcome comments.

[rlynde045]

Well for those that are interested we are back online. We had a busy summer both at our jobs and getting one of our greenhouses operational. At this point we are repairing (considering some upgrades) on some older equiptment that came with the greenhouse. The back wall that opens for the cooler filter's has given us the biggest challange out of the whole project. At this point we are in contact with Chris at Micro-Grow out of Calif and are considering one of their environment control systems that will in short monitor and operate the SCHS system in conjunction with the cooling, heating, Humidity, air flow's, lights, watering (basically every aspect will be automatically controlled in kind with each other).

John: i'm still restricted by Corperate to get pictures on your site so if you have some spare time and dont mind downloading some picts for us we can send you some updated photos for you to share. Sorry it has been so long since we chatted. By the way just with the caveman method of operating our system manually it has so far impessed us both in cold weather and heat. We are excited to get everything buttoned up and give it a test run through this winter.