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Old 08-30-2011, 05:57 PM
GpsFrontier GpsFrontier is offline
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Join Date: Mar 2009
Location: Lake Havasu AZ.
Posts: 1,855

OK well I had hoped to have more pictures to post by now, but there isn't that much more to look at quite yet. I do have the space for the greenhouse staked out and marked. But because it took almost two weeks for all the city utility's (water, sewer, electric, phone, cable, and traffic signals) to respond to the request to mark their utility's, I wasn't able to mark out where the subterranean system's will be going quite yet. Even though they are supposed to respond within 2 working days, water and sewer needed a reminder (second request) to come out, and traffic held me up until yesterday and only finally responded after 4 requests.

But the good news is the business credit card I needed got approved, and for over $6,000, witch is more than I'll need. So now I'm just waiting on the card with my name on it to arrive to begin getting supply's. The bad news is because of scheduling issues, I wont be able to begin digging until the weekend of September 17 at the earliest. But I just wanted to share my calculations on building the subterranean system for anyone interested in the details of the build.

First I needed to calculate the volume of the 4 inch perforated irrigation tubing. So I found a online calculator to do that. It takes 11 (11.454) feet of 4 inch tubing to hold 1 cubic foot of air volume. And 300 feet of 4 inch tubing holds 26.16 cubic feet of air volume. The recommended amount of tubing for a sub "T" system is 1.5 feet (in length) for every square foot of greenhouse floor. So the recommend amount of tubing needed for a 200 square foot greenhouse comes out to 300 feet of tubing.

Also because the air speed through the tubing is critical to how well it functions, I needed to do some more calculations. If the air speed going through the tubes is too fast, there wont be sufficient contact time for the warm water vapor to condense, and that's what makes the sub "T" system work. So the air speed is very critical, and a bigger fan wont make it work better, in fact quite the opposite. The maximum air speed through each tube shouldn't be more than 10 cfm. So here are the numbers I came to.

With each level of the sub "T" system (4 foot deep, 3 foot deep, and 2 foot deep) having 8 tubes for each level, that's a total of 24 equal sized tubes. 300 feet of tubing divided by 24 makes each tube basically 12 feet long.

Now it's also critical to the system working well that the system be able to circulate the entire air volume of the greenhouse underground 5 times per hour. So I needed to know the total air volume of the greenhouse. With 200 square feet of floor space, and 10 foot tall walls (it will probably only actually have 9 foot walls), 200x10= 2,000 cubic feet. But the ceiling will be a upside down "V" shape and hold air volume as well. So even though I don't know exactly how much more air volume that will hold, I just added another 1,000 cubic feet of air (50% more), and feel that should be more than enough extra air volume to add. So with a figure of 3,000 cubic feet of air I was ready to calculate the fan cfm.

3,000 cubic feet of air need to be circulated underground 5 times every hour. Fans are rated in cfm (cubic feet per minute), so 60 minutes divided by 5= 12. So 3,000 divided by 12= 250.

With each tubes air speed being 10 cfm, I'll need about 25 tubes to circulate 250 cfm of air 5 times per hour. Remember I'm building two systems, not one. So it's OK if each system circulates slightly less air volume than needed for the entire greenhouse on its own.

24 tubes total (4 inch wide, 12 foot long), 24x10 cfm= 240
3,000 divided by 12= 250
2,000 divided by 12= 166

So each system will be capable of moving 240 cfm of air at 10 cfm in each tube. And total air volume being between 2,000 and 3,000. So I will need a inline duct fan rated between 166 and 250 cfm like this one: 6-In 110VAC 250CFM In-Line Duct Fan - DB200 - Smarthome for each of the two sub "T" systems. This particular fan will cost me 9 cents a day to run 24/7, and $2.70 a month. With 2 systems/fans in the greenhouse, the electrical cost to run two of the exact fans in the above link 24/7 all month long would be about $5.40 a month. I will also be using a variable switch (like a dimmer switch) for each fan to control the fan speed. That way I can reduce the fan speed and increase contact time with the underground tubes when I want (thus increase condensation).

Total cfm between both systems at 10 cfm in each tube, 480 cfm. That's enough cfm exchange to circulate the entire 3000 cubic feet of air in the greenhouse underground every 6.25 minutes, with no more than 10 cfm in each tube.
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Last edited by GpsFrontier; 08-30-2011 at 11:42 PM.
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