bobf
New Member
Posts: 3
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Post by bobf on May 25, 2012 13:06:14 GMT -8
I am building a small (~10' x 32') metal pipe and polycarbonate greenhouse. I would like to install a RMH for additional winter heating. The GH will be 4' on the S side x 6' on the N with a sunken aisle ~30" deep. The entire floor area (ground level and aisle) will be covered in concrete (drawing ~18k at gophercreek.com/RMH/CrossSection.jpg). I am also installing about 800' of water tubing as deep as I can get it with a backhoe for winter heat exchange with the earth. The heat exchange tubing will be partly set into the bottom of the concrete floor. My main question is the buried heat tubes. Given my location in the Oregon Coast Range, generous rainfall and intermittent use of the RMH, would galvanized ducting still work without rusting out quickly? It will be below the greenhouse, but we certainly get lots of underground moisture in the winter. Is direct burial in the soil a guaranteed problem? I couldn't find any posts that gave any anecdotal answers to this. My soil appears to have adequate clay (~40k - gophercreek.com/RMH/GH_2.jpg), though I've not tested it. I'm hoping that it will be self supporting by the time the ducting rusts away. I had considered concrete pipes but was hesitant with the posts about contaminants. I could cover the GH floor with an epoxy to minimize contact problems in the GH but that would likely not work well in the heat tubes. Has anyone had any experience with direct burial longevity? Any other suggestions? Have a great day! Bob |
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Post by redmohawk on May 25, 2012 14:54:53 GMT -8
I'm New to the RMH game but have made many a furnace of similar issues. I'd steer clear of any metal less than stainless in that setup to be honast.
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bobf
New Member
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Post by bobf on May 25, 2012 16:50:48 GMT -8
Thanks for the response, though I'm unlikely to buy stainless duct to bury. What similar issues have you had?
bob
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Post by redmohawk on May 26, 2012 3:32:43 GMT -8
Heat transfer tubes run underground for 15 m 6 inch (150 mm) buried 600mm into clay with some sand packing 4 of to use as airconditioning using solar cells to power largish computer fans at both ends one suck other blow.
Initial trial test piece was heavy wall gal pipe same 6 inch outside dia 4 m long left under ground for 2 years same depth/soil type (Aus victoria in my area have slightly alk soils) about ph 7.8 at depth buried. After 2 years with condensation forming from warm moist air being draw into pipe during late spring summer early Autum months. Pipe was rusted with surface scale down to about 0.5 mm.
I'd imagine with cooling exahust gases from a rocket heater you'd get a similar mosture issue to some degree although prob more acid . Being underground as opposed to a heat bench the temp of the soil will be more dificult to maintain high enough with a long length of tube underground to avoid the exahust temp falling below 100 deg .
From what I have learnt of rocket heaters this is about the minimum temp you want your exahust temp at chimney. Not saying its not possable but it will take some work to balance right. Don't want to lay to much and have to dig it back up.
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Post by peterberg on May 26, 2012 6:09:25 GMT -8
Welcome to the boards, Bobf.
You are gonna use lots of concrete in the greenhouse. When you cover the ducting all around in concrete as well it can safely rust away. It's even possible to use the cheapest metal duct you'll be able to find, like aluminum.
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bobf
New Member
Posts: 3
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Post by bobf on May 26, 2012 6:49:06 GMT -8
Thanks for that info redmohawk. Our soils are more acidic (forest area) so I would assume I'd have more aggressive deterioration.
Thanks for the welcome peter. I've enjoyed perusing the forums!
That is probably what I will try initially ... ducting with some type of coating that will remain if the duct rusts away.
bob
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Post by cmdrron on Aug 21, 2012 13:34:12 GMT -8
I am building a small (~10' x 32') metal pipe and polycarbonate greenhouse. I would like to install a RMH for additional winter heating. The GH will be 4' on the S side x 6' on the N with a sunken aisle ~30" deep. The entire floor area (ground level and aisle) will be covered in concrete (drawing ~18k at gophercreek.com/RMH/CrossSection.jpg). I am also installing about 800' of water tubing as deep as I can get it with a backhoe for winter heat exchange with the earth. The heat exchange tubing will be partly set into the bottom of the concrete floor. My main question is the buried heat tubes. Given my location in the Oregon Coast Range, generous rainfall and intermittent use of the RMH, would galvanized ducting still work without rusting out quickly? It will be below the greenhouse, but we certainly get lots of underground moisture in the winter. Is direct burial in the soil a guaranteed problem? I couldn't find any posts that gave any anecdotal answers to this. My soil appears to have adequate clay (~40k - gophercreek.com/RMH/GH_2.jpg), though I've not tested it. I'm hoping that it will be self supporting by the time the ducting rusts away. I had considered concrete pipes but was hesitant with the posts about contaminants. I could cover the GH floor with an epoxy to minimize contact problems in the GH but that would likely not work well in the heat tubes. Has anyone had any experience with direct burial longevity? Any other suggestions? Have a great day! Bob |
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Post by cmdrron on Aug 21, 2012 14:24:06 GMT -8
@ bobf
Howdy, Bob
"Has anyone had any experience with direct burial longevity? Any other suggestions?"
I'm Cmdr.Ron - Newer to this forum than probably everyone else (about a week), and tickled to have some bit to contribute.
With about 13 years each of long distance telecommunications and commercial electrical, plus a ton of other design & construction work over the decades, I emphatically say dodge direct burying metal pipe. That is, unless you want it to rust, which would potentially clog & contaminate your heat transfer fluid.
Temperature differential means condensation.
In the acid sand of North Fla., most materials except concrete & plastic deteriorate severely & rapidly. Plastic pipe (ABS, PVC, etc.) transfers heat slower than metal, but has extreme longevity in soil, costs way less, and is far easier to work. No matter how you join the pipe, pressure test it with air before back-filling. It's a bit of bother, but nothing like digging to find and repair a leaky joint.
A design question for you, sir. I take it you plan to store heat in the ground. Would you share your plans to extract that heat for use?
Cmdr Ron
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