What's the longest lasting tubing for use in low pressure air systems? I'm looking at building a greenhouse with double or triple layer ETFE panels and the tubing would be run through the metal pipe structure, so it would experience exposure to air an either steel or aluminum with low (a couple PSI or so at most) air pressures. It would be a much bigger pain to replace the tubing than the ETFE panels so I'd like something which goes significantly beyond the 30-40 lifespan of the ETFE panels.
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$\begingroup$ Will any of the tubing be exposed to sunlight (UV radiation)? $\endgroup$– Jonathan R SwiftCommented Mar 26, 2018 at 0:22
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$\begingroup$ @JonathanRSwift Nope, it would be inside the frame of the structure (e.g. welded or otherwise joined metal pipes.) $\endgroup$– CoryGCommented Mar 26, 2018 at 5:15
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1$\begingroup$ And any other requirements for e.g. flexibility? Minimum bend radius? etc. - I'm no tubing expert, but I figure these sort of details will help other users to answer your question more effectively :) $\endgroup$– Jonathan R SwiftCommented Mar 26, 2018 at 8:28
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$\begingroup$ So far as I know, PEX radiant heat tubing (designed for hot water) has a lifetime warranted for 25-30 years; I'd expect the lifetime if it carries room temp air to be double that. MIght be an expensive way to go. $\endgroup$– Carl WitthoftCommented Mar 26, 2018 at 17:45
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1$\begingroup$ @JonathanRSwift good point: PTFE tubing like that used for waste water exit in houses will last about 1000000 years $\endgroup$– Carl WitthoftCommented Mar 26, 2018 at 17:46
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1 Answer
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In general, for air life-time, no tubing manutcurer will recommend a useful lifetime beyond 20 years, no matter the service. This isn't really due to shoddy craftmanship, but due to creep and test data.
All plastics experience creep - a phenomenon where the material will continue to deform so long as it is being stressed. No matter how low the pressure of the air, the tubing will be stressed, and the material will deform, and at some point it will rupture because it cannot deform anymore.
Some plastics resolve this phenomenon with reinforcement - either steel reinforced on the outside, or in the case of fiber-reinforced plastic, fibers of a material that will not creep. In this case, 100% of the stress can be diverted to the fibers and the material can last forever.
However, there is a catch to this reinforcement phenomenon - the testing for such materials requires strict reproduction to ensure it's final life - and this rarely happens. As such, most manufacturers will simply stick to the plastic life - 20 years, as they cannot provide a better number for the fiber reinforced version.
In this scenario, most polyethylene tubing would work wonderfully, with a double-containment pipe around the tube consisting of either steel or PVC. However, I won't even be able to promise you it would last longer than the panels.
