No. Please read this carefully, for the third time: the water films dominate the series thermal resistance. The thin pipe wall material makes little difference. Ferguson sells 1/2"x400' rolls of 125 psi ENDOT HDPE pipe for $48, and the outside water film conductance of
1/2" pipe is higher than 3/4" pipe, per square foot.Nick, again repeating obvious truths to people who will not listen.
I think that the biggest part of the difference between U of 30 BTU/hr-ft^2-F that Nick measured, and the value measured in the test of 125 BTU/hr-ft^2-F is that in Nick's test there was no flow either inside the tube or outside the tube (except for natural convection). On the test that I described there was forced flow trough the tube, which reduces the film coefficient on the inside the tube side. If I understand your application correctly you will have pumped flow in the tube, and it seems like a value something like 125 would be in the ball park for either kind of pipe. Since the pipe is cheap, it might be good to err on the too much pipe side.
I am working on a prototype of a horizontal pond batch hot water heater that has a coil of 3/4 inch PE pipe sitting in an insulated and glazed 6 inch deep bath of sun heated water. I think that the water bath is going to hit about 120F today, and if so, I will test how good a job the PE pipe does in extracting heat from the bath. I'll pass on whatever U value comes out of this.
Gary
Gary,
It sounds like you agree with Nick, that copper pipe is a waste of money for the pool HX because without pumped (presumably turbulent) flow on both sides, at least one water film is going to dominate the series resistance. I certainly agree that once you go with plastic tubing, it's worthwhile to err on the side of too much pipe. It would be very nice if the plastic tube HX got a U anywhere near 125 BUT/hr-ft^2-F, it'll warm the hot tub up in a hurry!
I don't have any sense of how big a deal this oxygen diffusion is. Supposedly the PEX-Al-PEX tubing is better than just PEX specifically because the Al acts as an oxygen barrier. I guess the dissolved oxygen is supposed to eat your metal bits over time. If I use PE tubing for the pool HX, I assume I'll be pumping oxygen into the cistern water.
The oxygen issue seems slightly different that the open vs closed system issue. Supposedly open systems that expose the water to air require more maintenance. My assumption is that open systems accumulate both oxygen but also organics and bacteria and whatnot, which then all has to be killed and strained from the system to prevent accumulation in the pipes and so forth.
For the DHW HX, it seems to me that the dominating issue is safety, and copper tubing seems safer than plastic tubing: Less likely to leak or crack, less likely to blow out (stronger), less likely to support some nasty thing growing. Probably more likely to leach poisonous heavy metals into the stagnating hot water, but that's why we don't make coffee from the hot water tap.
Radiant floor people only worry about cast iron pumps.
...which loses 12% of its pressure rating for each 10 F rise above 73 F.
ENDOT has a lifetime guarantee that includes labor for the first 25 years.
Nick
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