heat transfer, pex vs. copper

[emesine]

Does anyone know the difference in heat transfer between oxygen barrier pex and copper?

Can you make a heat exchanger out of oxygen barrier pex instead of copper? It would be a heck of a lot cheaper if the heat transfer is similar to copper.

Andrew

 

[bpirger]

In a quick answer....no. Take a look here:

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

The thermal conductivity is copper is one of the highest.....it's great stuff. Compared to plastic (PVC) its, copper is 1000 times better. I can't imagine a thin layer of Al sandwiched between PEX layers is going to help much. A little, sure, but not much.

 

[tom in maine]

The heat transfer is only one part of the issue. Since the walls of both tubes are rather thin, the thermal conductivity is only one factor.
There is a bit of a time lag when transferring heat across PEX.
All that being said, you need 4-6x the amount of PEX to perform similarly to copper.

This sizing is critical for DHW. When mother is not getting hot water when filling the tub, you will hear about it.

A lot of the PEX DHW heat exchangers you read about on builditsolar.com are banking on the volume of water in the coils. If you run these hx steady state for a while, they will tend to fade in temperature if not sized properly.

When you build a hx to perform equally with copper, the costs become very close to the same. As I mentioned in another post earlier today, there are some potential leaching issues that are of some concern with immersed PEX hx.

NREL (the National Renewable Energy Labs) has done a lot of research on polymer heat exchangers. They are still trying to come up with a competitive design when objectively compared to copper. I suspect the most reasonable design is with coiled PEX similar to what we currently do with copper.

My concern (aside for leaching and cost), is the volume of tubing that has to go in the tank.

When using PEX as a wood boiler hx, you actually might have a little more forgiveness since the delta T can be bigger, but no one hollers about a lack of hot water.

 

[emesine]

Interesting.....

So if I need 200 feet of 1/2 inch copper tubing, I would need somewere around 800 feet of pex. Probably cheaper to just go with the copper.

According to the pex literature, the "thermal conductivity" is 2.2 btu-in/(something I don't understand). Can you translate this into something I can use in calculations?

Andrew

 

[tom in maine]

I think copper is something like 200 vs. the 2.2 for pex.
Fortunately, the wall thickness is thin enough that thermal conductivity is only a partial factor.

 

[emesine]

Copper is really pretty expensive. 1/2 inch copper is $1.50+ per foot on a quick google search. I could do a very large heat exchanger in pex. I'd rather work in pex, it is a lot easier to work with. I want to be able to transfer about 100K to 150K BTU at a 20 degree temperature differential with water flowing at 20 gpm.

One solution would be to run 4 parallel 3/4 inch, 250 foot pex lines in the heat storage tank. Space in the tank is not a problem. That is around 200 square feet of pipe surface area. I may just build it and add on a few more lines if necessary. Pex is so easy to work with it might be easier than building a more complicated copper heat exchanger.

Does anyone know how to calculate the heat transfer if such an array? I know it is not easy!

Andrew

 

[DaveBP]

Keep in mind that if you are just heating the storage tank with these PEX HXs letting them flop to the bottom will be an efficient way to install them (assuming they actually sink). But if you are also going to extract heat from the tank with the same HX, then you will get much better efficiency if they are arrayed evenly from top to bottom of the water column (vertically speaking). That way you can get a counter flow heat exchange both ways, pumping heat in and out with the reversal of flow direction. That means you'll need some kind of frame to hold all that PEX in place, especially if it is full of air at any time during the installation.

Remember, too, that PEX tubing wall thickness increases a lot with diameter. 3/4" tubing has a wall thickness damned near half again as thick as 1/2". I don't know how much that effects the heat exchange rate but it will be a bigger effect than copper. Lots of speculation about optimum heat exchanger tubing diameter in the past here.

Keep reporting in on the project. Lots of conjecture about PEX heat exhangers here. Be interesting to hear some more experiences.

 

[kabbott]

Copper is really pretty expensive. 1/2 inch copper is $1.50+ per foot on a quick google search. I could do a very large heat exchanger in pex. I'd rather work in pex, it is a lot easier to work with. I want to be able to transfer about 100K to 150K BTU at a 20 degree temperature differential with water flowing at 20 gpm.

One solution would be to run 4 parallel 3/4 inch, 250 foot pex lines in the heat storage tank. Space in the tank is not a problem. That is around 200 square feet of pipe surface area. I may just build it and add on a few more lines if necessary. Pex is so easy to work with it might be easier than building a more complicated copper heat exchanger.

Does anyone know how to calculate the heat transfer if such an array? I know it is not easy!

Andrew

I have about 780 feet of 1/2 inch pex in four parallel coils. It wont come close to handling 150 kBTU's. It won't handle my new boiler (about 112 kbtu ) I "borrowed" 100 feet
of 1 inch copper from dad (retired plumber) and it does slightly better than the pex. I am going to bite the bullet and add another 100 feet of copper. One problem with the
pex is the reduced diameter compared to copper. I am using an taco 00R pump. The 1 inch copper flows good bit more than 4x 1/2 pex.

If you have to go unpressurized for 150 kbtu I think I would stay away from pex, It,s gonna take ALOT. If I had to do it again I would go with a flat plate.
Best case.... Go with pressurized storage, I am working on collecting a couple propane tanks now.

3/4 inch K copper is ~3.60 a foot in 100 foot rolls. 1 inch ~4.73 a foot. 3/4 inch L ~3.25
I just put 300 + feet of 2 inch K in the ground, woulda made a hell of heat exchanger :coolmad:

If you go pex I'd say you will need about double what you said, 4 x 500 feet for 150 kbtu.

 

[tom in maine]

Of course, the simple solution would have been to use one of our hx (Sorry, I had to get that in somewhere!), but since this forum focuses on DIY approaches, I would suggest tying the PEX and 1" copper together. It might be slightly tricky to balance the flow between the two since they should be in parallel but since you already have all this in place, it should work well.

Another option would've been to use plate hx. They are not as elegant a solution as a well designed coil and require a second pump, but they do work well.

 

[Willman]

I had some leftover fin tube BB. I removed the aluminum fins and found that they fit fairly tight on 3/4 pex. I was wondering how much more efficient that would make the pex?
Next question to answer would be to find a supply of the fins prior to assembly at a good price. Maybe even come up with a heat transfer glue or paste for a better exchange of btus.

Will

 

[tom in maine]

Would be an interesting test. I suspect the differential movement due to varying temperatures could be an issue.
I am extremely skeptical of anything aluminum in any water system that had other metals in it. There have been too many fiascos.

Fins and liners can be an issue.
And some patient soul gets to install the fins. Even if it brought PEX and copper down to parity, thats a lot of fins!

 

[Willman]

Valid points Tom. Are unpressurized tanks charged as well as discharged via HX ? I do agree on the dissimilar metals being a huge issue I figured the water in the tanks was used to store btus. Or is it actually circulated thru heating systems ? The feet of pex needed to equal copper is known. Wonder if PAP would make any difference to justify extra cost. Much rigid-er to help hold the spiral. Fins would have to really cut down the length needed. Get the kids around the tubing and tell em its a game putting em on.

Will