heat exchanger minimums

I am planning on buying a tarm solo plus 40 with roughly 1000 gallons of home-built storage. I am in the process of researching the heat exchanger and leaning towards using a long coil of copper (or maybe pex). I have a few questions:
- What are the minimum inside diameter and lengths needed?
- What type of copper should I use? I can get 10 ft of 1/2 hard copper tube at HD for 10.50. 100 ft would come to $105 which is not bad. Is it possible to get hard straight pipes and bend them into circles then solder a bunch of them together to form a large coil?
- How about soft copper coil? I am sure it would be easier to work with but costs a lot more - 20 feet of 5/8 OD is $57.82, so 100 feet would be more than twice as much as the hard tube.
- I noticed sparke is using pex and there was some discussion about heat conductivity. I looked up the numbers and polyethylene (not cross-linked , but that shouldn't matter to much) is about 0.5 W/mK. Copper is about 400 W/mK or about 800 times more than pex. This doesn't seem to match what sparke has observed (his system seems to work) so I am wondering if the material property doesn't matter so much as the water to material interface. When doing heat transfer analysis the interfaces are often more significant than the materials themselves. Anyway, what is the minimum amount of pex needed based on people's experience? Sparke, if you are there, you mentioned the performance was good getting up to 120 F. Were you able to get up to 180 as easily?

ABGWD4U said:

Gimme,

Where did you get the numbers for your pex heat conduction? I haven't ever heard anyone else come up with numbers to that extreme for pex. I agree there isn't a match. so we know that it falls somewhere between 400X and 3X. We are nailing it down.

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I just did a google search for "common materials thermal conductivity."

By the way, I just ran some conversion on the 400W/mK... Assuming you have 100 feet of 1/2 inch copper, which seems to be typical, using the 400W/mK as the thermal conductivity means that you would be able to transfer 907,000 BTU / h*F. In other words, for every degree of difference between the boiler water and the storage water you could transfer almost a million BTUs / hour! If the temp difference is 10 degrees (i.e. almost up to max temp) then you would be able to transfer almost 10 million BTUs / hour. This sounds a bit ridiculous to me so it probably is not real. I think the problem is that we can't use the simple heat conductivity because the interface from the water to the copper and then back from the copper to the water is MUCH more significant than the thermal conductivity of the copper. So, while it is true that copper, as a pure material, has a conductivity that is 800 times higher than PEX, that is only a small part of the story. It's kind of like using a nice big fat wire to conduct electricity but then adding a couple large resistors to each end. In that case we wouldn't care too much about how good the wire was conducting. Instead we would worry about the resistors. I think sparkes' 3x number is probably a lot closer than 800x.

jebatty said:

I've asked this question before, that is, why not use a plate hx? Cost is very reasonable, efficiency is very high, compact, etc. Only downside mentioned so far is an extra pump is needed. I'm heating 1000 gal, now pressurized, Tarm Solo Plus 40, with a 5" x 12" x 30 plate hx.

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How much for a decent flat plate hx? Where did you gets yours?

nhjohn said:

I too am doing the same thing - Tarm 40 with a homemade 1000 tank. I am also looking into heat exchanger options. I'll let you know if I come up with anything. When are you buying your Tarm?

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I am also going to get mine as soon as I get the flyer in the mail announcing the April specials ;^)

NHFarmer said:

I am planning on buying mine within the next month,I here they will be on sale and they expect a price increase after that.I am going to pick mine up and save on the trucking

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Nice call on the pick up. I may have to do that too, although the NH store is about 2 hours from here :^(