alternative material for heat exchanger

[stvsxm]

good morning,
I need to make a simple tubular air to water heat exchanger... i.e. a grid of tubing that circulate cold water thru a hot air environment thereby cooling the air and heating the water. on paper, it seems to me that the best two materials for this are either aluminum or copper tubing. aluminum is out of the question because the " water " is salt water and corrosion would be unacceptable and copper is cost prohibitive because of the quantity necessary. are there any other materials like typical pvc or polyethylene tubing that would have reasonable heat transfer properties and be cost effective ? thank you for your input.

steven

 

[MJCronin]

Some suggestions:

- Consider a HX with extended fins instead of bare tubes. Exchanging heat from a liquid to an air stream can be made more effective.

- Consider marine materials (cupro nickle and brass/bronze)to resist the corrosion of the salt water. You might even find a used Titanium HX..!! What is the percentage of salt in your liquid ?

- Consider a used HX.... Look for one with multiple rows of cooling coils

- Be prepared to spend a few bucks ...

http://www.marineheatexchanger.org/

MJCronin
Sr. Process Engineer

 

[stvsxm]

hello and thank you. ill investigate those.

 

[rickfischer51]

You do not say how hot, and temperature limits the polymers that could be used. You also dont give flow rates and pressure drops. Polymers are not good conductors of heat, but make sense it the conduction resistance is small compared to the convective resistance, such as a laminar flow air to air recuperator. But plastics creep, and the higher the temp and the higher pressure, the more they creep. To deal with that, you need thick sections, but then the conduction resistance goes up and thermal performance suffers. There are a few cases where plastics make sense, but they are very specialized. You probably need to stick with metals.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Compositepro]

Is there a reason you cannot allow direct air to water contact, as in a cooling tower or swamp cooler? If the water temperature is low enough the air can be dehumidified by contact with water.

 

[stvsxm]

thank you gentlemen, here is the deal.... I have a solar array of about 100 panels. i am in the carribean so they get VERY hot. the efficiency and production drops off the hotter they get. right now i cool them by direct airflow underneath the panels and that works fine. i also have a fairly large pool that is really nice 1/2 the year and a lot less nice in the winter months. so... my plan is to create a tubular heat exchanger under each panel i.e an accordion tube deal of 1" copper tube being each fed by a common inlet manifold and feeding a common outlet manifold .... seal the underside of the panels , feel the 75 degree water into these air spaces that get about 200 degrees f and take the hot water out back into the pool. there is no room for a finned system. i figure about 75 feet of 1 " tubing under each panel... flow rate is completely adjustable and there will be no or negligible pressure. now... I'm 70 years old and graduated as a mechanical engineer 45 years ago. my career was spent designing and manufacturing race cars so to suggest that i am a bit out of practice in my thermodynamics calculations is giving me far more credit than i deserve. i can see the heat transfer characteristics of the two materials that make the most sense... copper and aluminum. the al won't work because of the corrosion and the copper , on the face of it was just too expensive. i was simply hoping that you guys , far far more current than i and dealing with ultra modern materials on a daily basis might have some magic answer for me. it is a lot to hope for. i can see the heat transfer characteristics of the typical pvc and cpvc and i just can't see those doing the job...

thanks for having a think.

steve

 

[EdStainless]

Would PEX stand the heat?
You won't find a metal that costs less than Cu and will have any chance of resisting the corrosion.

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P.E. Metallurgy, Plymouth Tube

 

[stvsxm]

they use pex down here all the time for hot and cold water applications...

 

[stvsxm]

allow me to rephrase this question with a different concept.... what do you think would give me the greater delta T of the water... running 75 deg water thru 1000 feet of 1" copper tubing at some given flow rate ) thru a steady state air environment of 200 degrees f for between 6 to 8 hours or simply laying that same 1000 feet of copper tubing in the blazing carribean sun for for the same 6 and 8 hours depending on the time of year ?

 

[EdStainless]

These units will usually use a themal actuated valve to control the flow, if the outlet water gets over 180F then it allows the flow to increase. As the water cools the flow slows.
If your Cu out in the sun has a nice dull dark finish you might get more heat, but you will be at the mercy of the wind also.
At least if you boxed in the panels for this you would have a controlled environment.

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P.E. Metallurgy, Plymouth Tube