Sorry if this has been asked before but...
Why are all domestic hot water cylinders made of copper/stainless steel? Don't we have any plastic (e.g. PEX) ones?
Also, are they any shape other than cylindrical?
I'm researching an essay for college and any insight will be appreciated! Thanks, James.
Hot water cylinders are pressure vessels, albeit at a relatively low pressure. Take my house (which admittedly is not typical). Water tanks in loft 40 foot in the air, hot water cylinder on ground floor, so head of water is 40 foot or about 1.2 bar or 17 psi. so if the tank is
4 foot high by 18" diameter, it has an area of about 3250 sq inches which means that there is a force of 55,250 lbs pressing outwards ie about 25 metric tons !The strongest form of pressure vessel is a sphere, but a cylinder with a convex domed top and concave domed base comes fairly close. The weakest form is a vessel with flat sides.
So you need a vessel made from a material strong enough to resist such a force at temperatures varying from close to freezing up to close to boiling point reliably over many years. There may be exotic plastics able to do this, but can they be jointed by normal plumbing methods?
AWEM
If a tank holds 200 litres, the force on all tank walls will be just over 200kg (bit of water in pipe & header)
But thats still a lot of force.
Why plastic? Same as everything, a copper container is cheaper than a plastic one that can withstand that heat & pressure. We know that simply because everyone uses copper.
NT
The others have answered the basic question.
But I can conceive of a system with a loft mounted plastic tank, lagged with 100mm of polyurethane foam, containing a copper coil fed from the boiler with hot water via lagged pipes, and feeding the hot taps by gravity. Such a tank would not need to take much pressure, but it had better not deform when hot! It could be done, but it probably isn't economic (in fact we can be certain it isn't economic, or someone would be offering it - either that or I'm a genius :-)
R.
copper/stainless
inches
plastics
methods?
No ! The WEIGHT of the tank contents would be 200Kgs, not the force acting on its walls. The 200Kgs would act directly down on the base as a dead load in addition to the force acting from water pressure.
AWEM
Agreed but the pressure it is required to withstand is still only 1.2 bar. Since PEX pipework can be used anywhere in the system, it must be able to withstand at least this pressure (and associated temperature) so why are there no PEX cylinders?
I would argue that cylinders are only round simply because they *are* made from thin copper sheets which would deform otherwise - this shouldn't be an issue with PEX.
It is not environmentally friendly to keep digging huge holes in the ground to extract the ever-reducing reserves of copper so should we not be looking at replacing them with something else?
James.
I reckon the force on the tank walls depends almost entirely on the height of water in the header pipe. If the header pipe was a quarter of an inch in diameter and 100 feet high, the tank would burst.
Wouldn't copper, which is almost indefinately recyclable, be better than an oil based plastic?
Owain
Agreed! When my cylinder was first installed 22 years ago it leaked at the seams, but when the label was examined it turned out it was only rated for a 30 foot head of water, which would be ok in most houses. We had to have a high pressure one installed to cope with the extra head (40 foot or so) which is why I am aware of this issue !
AWEM
Hello,
Cylinders *are* available in shapes other than cylindrical. I've recently had one custom-made to my spec by IFS, who can make them square, rectangular etc to suit the space available:
Alan.
I dont think we are reducing copper reserves, just moving them around. Old cu tanks dont vanish, theyre recycled.
NT
Yep. My old copper tank and some large copper pipe attracted a bloke and his van to take all my scrap away including a large steel hot air boiler that I would have had to hire a van to remove to the tip. He was happy, I was happy. Fair trade.
Except it's running out - we're building more new houses (here and in China and elsewhere) than are being recycled. Check out "Peak Copper" if you're interested in finding out more.
I agree oil-based plastic is not the ideal solution - my question should have been can tanks be made in *anything* other than copper and stainless steel! ;-)
James.
|!Sorry if this has been asked before but... |! |!Why are all domestic hot water cylinders made of copper/stainless steel? |!Don't we have any plastic (e.g. PEX) ones? |! |!Also, are they any shape other than cylindrical? |! |!I'm researching an essay for college and any insight will be |!appreciated!
Research the strength of plastics at hot water temperature, say 90 deg C, as compared with copper.
Research the strength of various shapes when under internal pressure.
'Fraid not - the static pressure of a column of water has nothing to do with its volume. It is related solely to the density of the liquid and the head (height).
This is why to increase the pressure of your non-mains water supply you increase the head of the water - you don't increase/decrease the size of the pipe (this affects the flow *rate* and not the static pressure).
Every 10m of head approx. = 1 bar. Most UK cylinders are B.S. Grade 3 which is rated to 10m max head. (Grade 2 are 15m and Grade 1 are 25m. Grade 4 are now illegal).
James
Interesting - where do you have the 2nd coil? Is it still below the primary coil (i.e. pre-heating)? Did you consider an external mantle for the solar heat exchanger?
James.
Cross-linked polyethylene (PEX) is on the limits at 90C but there are plenty of other (and more rigid) plastics that work at temps higher than this.
I still contend that the shape is only cylindrical because *copper* is so soft and deformable!
Thanks James
If you read more carefully, I am not talking at all about the volume of water, but the height. Obviously we are talking about water, so the density is irrelevant. I said "almost" because the height of the cylinder has to be taken into account.
"James" wrote
No! Whatever the material you use, the optimum shape for resisting pressure (both internal and external) is round. Spherical first although rarely practical (the obvious DIY exception being the pressure vessel for a sealed heating system) and cylindrical with domed ends being a more practical second choice. In a circular vessel, internal pressure acting on the vessel wall gives rise to tensile force in the material which is unlikely to result in deformation. Pressure acting on a flat surface results in bending stresses and likely deformation hence as has already been hinted, containing pressure with flat sided vessel leads to lots of stiffening being required.
Phil
Fair enough. Your mention of the diameter of pipe (which is irrelevant) threw me and implied you were talking about volume.
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