PE vs PE with FEP liner

[Serrand]

Hello everybody,
I am designing a heat exchanger which involves corrosive materials, which excludes any metal as possibilities as materials. I seem to have narrowed it down to PE by comparing polymers with their respective thermal conductivities.
Now I am faced with the choice between High Density Polyethylene or Polyethylene with a FEP liner. The second has slightly thinner wall thickness, but I am not sure of it's thermal properties.
Is there anybody out there that might be able to tell me if the FEP liner is likely to improve PE's properties or the opposite.
Thank you
Domenic

 

[Pud]

I assume this is a low temperature heat exchanger e.g. less than 50/60C, and you will be using off the shelf tubing?

What corrosive materials/temps? I would have thought Titanium tubing would be about the same cost as FEP - much better conductivity, but not as good as copper (or steel for that matter).

Some domestic boilers have secondary heat exchangers made from PPS. (UK)

A bit more info would be helpful.


Cheers


Harry

 

[patprimmer]

PE and FEP are both very poor conductors. I expect any difference in thickness would offset any difference in conductivity.

You can add fillers to improve conductivity, but you will lose some corrosion resistance.

Can you use a thin PE coating on copper.

Regards

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[Demon3]

When I worked at Electrolux (Frigidaire to the US folks) they made a heat exchanger from polymer (PP). The idea was to replace an aluminium one in the prooduct at lower cost. It seemed doomed to failure due to the poor thermal conductivity of polymers. Amazingly it was found that the polymer part actually performed better than the metal one. It seems that if you take advantage of superior design afforded by the moldability of plastics you can make a really effective part.

Don't forget that PE or PP will need exceptionally good stabilization using a specialty stabilizer package when used in a heat exchanger.

1. High temperature degrades the plastic (the FEP should be way better than PE and PP)
2. The flow of liquid and / or gas extracts the stabilizer
3. The thin walls mean there is no "reservior" of stabilizer to replace that caused by extraction

Does gold plating the metal work? That would be a nice solution.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[Serrand]

Thank you very much for your input, I should have mentioned that the fluid to be cooled/heated is salt water and the temperature range is between 2 and 40 degrees. I have had a look and I don't think titanium is anywhere near the price of FEP, this will be a one off heat exchanger so all parts will be ordered from Mc Master or similar suppliers. From what I gather between the two PE the difference of conduction coefficient is less important than the difference in wall thickness?
Thanks again
Domenic

 

[Demon3]

Similar to what Pat suggested, you could buy some polymer tape and stick it to metal. That's a great way to test a possible solution. These guys make / sell really good tapes that should work well.

http://www.cshyde.com/tapecat.htm

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

If it's salt water, what's wrong with stainless steel.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Serrand]

Stainless steel is only stainless as along as it has an oxygen source to replenish its protective layer, however, this system will be submerged for extended periods of time. The experience my company has made with 316 and saltwater has usually ended in rust (sooner or later). So I think polymers is still the way to go, however, I haven't excluded the tapes on metal yet.
Thank you once again for everybody's help

 

[Pud]

Modern car brake pipes are usually coated with PVDF these days, although if fittings have to be immersed not good.


Cheers

Harry

 

[rodneybennet]

Probably polyethylene with fillers is an appropriate solution. Indeed chemical resistance may go down if fillers are applied (however this depends on the mixture involved and the type of fillers used).

In the conditions stated, polyethylene on top of copper is less practical since an adhesive is expected to be required to bind the materials. Nowadays (with current technologies), it is questionable whether the appropriate interface is created for long term resistance to water/acid accumulation and subsequent bulge formation.

Regards,
Rodney

For more integrated knowledge on diffusion and corrosion, also take a look at:

http://www.composite-agency.com