Just guessing, but probably pressure? PEX has much lower pressure ratings than metal. I plan to use PEX for a water purification system, but it will be sub 100 PSI. Most PEX-A are rated to 200 PSI at room temp and 120-150 PSI at 180 degrees F. They will rupture at 400 to 800 PSI of water depending on the brand. Metal pipes can handle thousands of PSI, required for pushing water through membranes.
If that is the case it still seems like you would just need metal for a section that reduces the size of the pipe down to something smaller to give you the pressure. A larger diameter pex should be able carry the water around with a reducer only where the pressure is needed.
Don't you only get differences in pressure due to diameter changes proportional to flow rate? If a membrane requires 400psi to get meaningful flow through it, it doesn't seem like you'd be able to sustain flow with much less than 400 psi of static pressure.
I don't understand what the problem would be. If there is a pressure limit with PEX, use larger diameter PEX, then reduce it where you need the higher pressure. This is the same concept of cleaning something by putting your thumb on the front of a garden hose.
Going from a larger diameter to a smaller diameter only results in a change in pressure if there's flow. In fact, the pressure ends up lower in the smaller diameter section according to wikipedia. Putting your thumb on the front of a garden hose increases pressure inside the entire hose because it reduces the flow rate, not just where your thumb is.
> Going from a larger diameter to a smaller diameter only results in a change in pressure if there's flow
There isn't a question of how much water can flow, the question was only if you need metal to have the high pressure needed for filtering.
> In fact, the pressure ends up lower in the smaller diameter section according to wikipedia.
You are probably seeing the pressure applied to surrounding pipe itself without taking into account the increase in velocity from the conservation of kinetic energy.
> Putting your thumb on the front of a garden hose increases pressure inside the entire hose because it reduces the flow rate, not just where your thumb is.
I think you are missing the point, if you want more focused you can reduce the flow rate since you still have the same kinetic energy of the water.
This is all irrelevant to the main topic though, apparently industrial PEX can go over 1400psi
Is Pex used in industry? I’ve only seen it in newish residential construction. Is there such a thing as a 6” diameter pex pipe? And fittings? 12”? 24”?
Where corrosion is a problem i’ve Seen metal pipes coated with epoxies.
I’ve seen big HDPE pipe used for low pressure sections to save cost over metal.
What do you mean by 'in industry'. In what industry? I don't know about anything larger than 3", but the general point was more than it doesn't seem right that corrosion of metal pipes would be a huge barrier to desalination since there are options for non metal pipes. There is also PVC which can be huge in diameter and CPVC which is used in some situations that need pipe that is more inert, but it is very brittle.
“Industry” generally refers to factories or plants as opposed to a commercial or residential setting. Quite often different codes, standards, and construction methods are used in industry than you see elsewhere. But large pex pipes isn’t something that I have seen.
