question re MAX safe flow velocities

[pybyr]

a question for the pros and for the experienced and thoughtful amateurs:

this will affect my choice of pipe and tube and circ sizing and efforts to keep the wire to water side of the efficiency equations

I get it that a basic part of the recipe of hydronics is that for residential applications, you normally never want water moving at less than 2 feet per second (to avoid entrapped bubbles) or more than 4 feet per second (to avoid noise from flows).

There's also apparently a higher upper FPS limit for commercial and industrial applications, that allows for and ignores worries noise but which still sets an upper FPS limit that you have to stay under in order to avoid erosion of pipes and fittings.

that second "noise be darned" FPS figure, that's the threshold that one has to stay under to avoid erosion, seems shrouded in mystery.

what is that upper FPS limit to avoid fitting erosion?

thanks!

 

[Willman]

Hi Trevor,
Not a pro at all, but I have my trusty copy of Modern Hydronic Heating open to the page. Siegenthaler states copper tubing will erode when flow velocities extend into the 5-8 FPS range. This is known as "erosion corrosion" It will tend to occur at tight turns (els & T's). He also sates that it is not known to be a widespread problem except in extreme cases.
Not quite certain about pex though. Prolly will take less FPS. I have seen pics of PAP fittings that failed due to erosion.
Also not certain why you would want to exceed flow velocity beyond your design.

Will

 

[pybyr]

Hi Trevor,
Not a pro at all, but I have my trusty copy of Modern Hydronic Heating open to the page. Siegenthaler states copper tubing will erode when flow velocities extend into the 5-8 FPS range. This is known as "erosion corrosion" It will tend to occur at tight turns (els & T's). He also sates that it is not known to be a widespread problem except in extreme cases.
Not quite certain about pex though. Prolly will take less FPS. I have seen pics of PAP fittings that failed due to erosion.
Also not certain why you would want to exceed flow velocity beyond your design.

Will

thanks Will-

here's what got me wondering- a friend of mine in the HVAC supply trade gave me Grundfos' newest brochure for the "Magna" ECMs- big commercial circulators (really interesting "smart" efficient ECMS)- and they (pushing some absolutely huge flows compared to residential circulators) have only inch and a half outlets. Also, the brochure lists adapting flanges for things all the way down to 3/4 inch pipe thread. This makes me think that velocity alone isn't as big a limitation (if you set noise concerns aside) as we all think from residential norms.

why am I trying to maybe push the envelope on velocities?

partly because I am interested in learning, partly because I'm already hemmoraging $$$ left and right on the various parts of this project, and so if I can go with piping & fittings that's a bit smaller than the residential norms, and without any adverse effects/ risks other than a whooshing sound, then it will be less expensive than the really huge diameters, so I want to consider that.

and I'll still look forward, hopefully, from some input from the rest of the crew, especially Joe Brown, Hot Rod, jebatty, NoFo, and the rest of the Boiler Room's hive mind brain trust

thanks

 

[slowzuki]

How many hours of pumping are you doing a day? High pressure loss x high flows equals lots of power. Power that is paid for on your electrical bill. Steel pipe and fittings are pretty cheap. I'd be careful.

About 0.10$ a kw/hr here. A 1/2 hp pump (375 watt) fully loaded costs about 28$ a month to run. Add that to any other circs or blowers on your boiler and you get a mounting power bill. With properly sized pipe a 1/20 hp pump can do boiler circ duty, at about 2.80$ a month.

 

[Willman]

Thinking back to a shop I used to work in that had a huge radiant set up that would definitely qualify for "commercial" the piping was all steel. Prolly steel has a higher erosion corrosion resistance. The noise as well as electric usage in most commercial apps (barring a library install) would be the least of the design parameters. Just my opinion. Maybe one of the pros (experienced DIYers) will show up.
Will

 

[slowzuki]

The noise comes from a history of rads in the hydronic systems in the US. Nobody likes noisy heating systems!