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These questions are in context of a DIY solar water heater I am planning (another separate question). The copper pipes in question, are supposed to carry around hot water using thermo-siphon mechanism.

Are there some specific reasons why people recommend bending long copper pipes, instead of cutting them into sections and joining them using U-joints, elbow-joints etc. ?

Also, can such joints be made reliable (as in leak-proof, while withstanding water pressure) without brazing them ?

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Just a side note: Questions about Braising belong on cooking.stackexchange.com. Questions about brazing belong here. (I fixed it) –  Chris Cudmore May 15 '13 at 14:36
    
Thanks Chris. Looks like this is a common mistake. Even wikipedia has clarifications to this effect. –  icarus74 May 15 '13 at 18:29
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3 Answers 3

Fewer points of failure

Bending results in fewer high risk failure points.

A continuously bent pipe has 2 high risk failure points. Whereas a similar length of pipe made with a bunch of fittings, has 2 high risk failure points per fitting.

Quieter

Bending a radius will provide less resistance within the pipe, so water will flow smoother. In a pressurized system, this means water flow is quieter.
Thanks @kavisiegel

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This, along with the fact that bending a radius will provide less resistance within the pipe. Water will flow smoother. In a pressurized system, this means water flow is quieter as well. –  kavisiegel May 15 '13 at 13:07
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Are there some specific reasons why people recommend bending long copper pipes, instead of cutting them into sections and joining them using U-joints, elbow-joints etc. ?

Pipe fittings add resistance to the flow of the fluid in the pipe. This is usually expressed in terms of an equivalent pipe length for the different fittings (eg, elbows, tees, etc). For example an elbow bend is approximately equivalent to an additional 300mm of straight pipe. The total equivalent legnth of pipe in a system can then be used to determine the size of pump required, as pipe of a given diameter will have a known pressure loss (due to friction) per unit length.

A 90 degree swept bend in a pipe will have lower resistance than an elbow fitting (albeit with a much larger radius). It will be a little higher resistance than the same length of straight pipe, but not significantly so when compared to the approximately 300mm of additional equivalent length the elbow would introduce.

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Bending copper has to be done with annealed copper. Rigid copper is too brittle can be done with either drawn or annealed copper. This table from copper.org does suggest slightly larger minimum radii for drawn pipe. thanks to @UNECS for prompting further research.

You can make rigid copper easier to bend successfully by heating the area of the bend with a torch and letting the pipe air cool slowly, thus annealing that area.

mandrel bender

The mandrel bender will make very uniform bends.

Spring bender

The spring bender will make more freeform bends.

Once bent, compression connection joints can be made with ferrules or flared ends. Note, you cannot mix ferrules with flares without a intervening adapter.

Ferrule and compression fitting

Note the tubing extending out past the joint. Make sure the copper tube bottoms in the fitting THEN tighten the overcap firmly.

enter image description here enter image description here

Flare connections mate with flare nuts (be sure to put flare nut on BEFORE making flare. enter image description here

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Hard drawn copper can and is quite regularly bent without annealing (as I do it nearly every day) –  UNECS May 16 '13 at 9:03
    
In which case @UNECS, would annealing help bend copper tubing more easily, and with more rudimentary tools ? –  icarus74 May 16 '13 at 13:23
    
@UNECS based on your coments, I did more research and I agree concerning drawn copper. I've updated my answer to reflect this. –  HerrBag May 16 '13 at 16:26
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