Here's one to ponder... SE

Steve Ebels

I'm working with a greenhouse owner on a new system for a 90' x 210' greenhouse presently heated with 1.2 MBTU worth of LP fired unit heaters. He's in the process of changing what he grows in there and wants to have the root zone for plants growing in the floor beds to be a certain temp. That part is no problem to figure out.

Here's the one I don't know how to put a handle on. He's going to have 22, 190' long rows of tomato plants up off the floor on decks. These decks will have 1" blueboard foam on them. The tomato plants will grown in plastic bags (3 per) that will sit on the foamboard deck. He wants to keep the roots, and therefore the water in the bags at 75-78* by running pex under the bags and circulating warm water through it. My question is not how to pipe it to keep the water temp even over the 190' but instead how to calculate the heat lost through the pex in order to determine flow rates and water temp.

Got any thoughts???

Mike T., Swampeast MO

r-value of "Aluma PEX"

http://www.weil-mclain.com/FTP/Alumipex_Manuals/QualPex_Tube_Tech_Info.pdf

I've asked others before about the r-value of PEX but never got an answer. This is the FIRST I've seen of such, but as you see it's a specific product. If other mfgrs have such data I can't find it...

Hydroponic tomatoes with the "root zone" kept at 75°-80°; the soil temp they "like", right?

I'm guessing that the air temp surrounding those bags of water is going to be variable?

If we're thinking along the same lines your method of keeping such a long run at the same temp involves an even number of tubes with supply/return ends flip-flopped?

Still might be difficult maintaining those bags at a relatively constant temp when the air temp is likely to fluctuate quite a bit--plus the loss of the bags is going to vary with how much water they contain--don't know how variable that will be either.

Any chance that the rows of bags themselves can be put into a container of fluid? That might help to provide one of the things I've always heard that tomatoes LOVE--warm, muggy nights.

Paul Pollets

I'd use

Wirsbo Multicor or Pex/Al/Pex. Still need to provide either plates or an overpour. The emmision rates are in the new Wirsbo CDAM manual. Or call their design dept. for particulars.
Regards, Paul

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Alan(CaliforniaRadiant)Forbes

Based on

70° surrounding air temperature, ½" galvanized pipe will give off 60 BTU/[], ½" tarnished copper 30 BTU/[]

½" PEX? My guess would be around 20 BTU/[]. But that is surrounded by air. If you are going to put this in sand, especially moist sand, it's going to be a lot more.

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Mike T., Swampeast MO

I think the tube is going to be in contact with the water-containing bags but sitting on foam insulation--not embedded in anything...

Steve Ebels

Yep

The tube will just lay on top of the foam and the root bags will lay on the tube. For piping I will use an arrangement similar to a two pipe primary secondary with the crossover piping underneath the bags. Each crossover will be short, probably less than 15' in length to ensure uniform water temp. The whole loop will be reverse returned back to the heatsource which is an interesting thing in itself. It'll consist of 4, 300K corn fired "boilers". The math works out that burning corn will save him about 60% vs the LP he's using right now. A bushel of 15% moisture corn can be had for slightly above $2. This will have a gross btu output of around 450-500,000 btu's. That my friends, is cheap heat.

I love off the wall stuff like this.

hot_rod

What about

some of the thin inexpensive dual tube transfer plates. Seems like the increased transfer surface, and root bag contact would do you well. Noise shouldn't be an issue:)

Crop King has a similar system I installed years ago for hydroponic tomato greenhouses. They used a thin layer of gravel over the tube for better heat spread, I imagine. Protected the tube from UV also. They used a twin rubber tube similar to solar roll but larger diameter, made in California as I remember.

hot rod

Steve Ebels

Plates

Sound like an excellent idea. Have to make sure there are no sharps on them to tear the bags though. Thanks Hot Rod!

hydronicsmike

WOW

You guys blow me away. Incredible stuffe here!

What about another option?
If the turf, waterbags or whatever it may be at the end, has to be maintained at a constant temperature, I would consider piping the runs in a way that assures equal heat transfer. Maybe like reverse return.

Then incorporate some sort of mixing (Variable Speed or Floating) with the ability to average a couple of sensors that are spread out throughout the soil or waterbags. I dont want to spam you with any particular product, but the ones I am familiar with are tekmar.

I am thinking maybe a Mixing Control like a 362 or 361. You may have 4 remote sensors wired to the control for temperature averaging. I would look into using 4 of them and spread those out throughout the loops to gain feedback as far as temperature goes in the turf or bag. This would allow you to gain continuous circulation, which will decrease the temperature swings as you are sensing the temperature right where it counts and the supply water temperature would change accordingly based on the load at any given time. A fixed water temperature may not be enough as the load can change all of the time. Its a greenhouse. Those are very hard to control. With all the glass on the outside, clouds, sun or wind changes the situation massively. If you have equal flow through all loops by utilizing the reverse return piping method, technically, the heat transfer should be the same and no more than 4 sensors would be required to average.

I hope this makes sense. Almost lost myself when I read over it again....:)

Regards,

Mike

[Deleted User]

HOw about solar panels

How about using solar panels? Little tubes, lots of spread on the heat, reverse return on piping. FAFCO.com has some really good ones.

Steve Ebels

Wintertime in Michigan

Usually does not bless us with a lot of solar gain. Clouds about constant due to cold air blowing across the big lakes which are warmer than the air. At least until they freeze over like last winter.

hydronicsmike

Steve...

...I would be very interested to find out how you ended up doing this job. What was your strategy? Please advise, if you don't mind. Sounds like a heck of a system to get it all worked out.

Thanks,

Mike