Metallurgy continued - AL294C Stainless Steel

SpeyFitter

This article: <a href="http://www.protechinfo.com/mc_images/category/8/1016_(10-00).pdf">http://www.protechinfo.com/mc_images/category/8/1016_(10-00).pdf</a>

talks about the benefits of the AL294C grade of stainless steel we see for various flue gas vents made out of stainles steel. IN the huge thread on metallurgy a few years ago talking about the question to build the perfect condensing boiler that would last a long time, the general consensus I got was if they made a heat exchanger out of AL294C, it would have the best chance at truely being a lifetime boiler comparable in longetivity to it's cast iron conventional counterparts. This article above talks about the benefit of AL294C not containing much nickel compared to 300 grades (e.g. the famed 316Ti consider the best alloy available in a mod-con to date) which makes it more economic. If it's more economic then some of the alloys that contain a higher percentage of nickel (e.g. 316 Ti or L), why haven't we seen a mod-con with an AL294C heat exchanger?

Constantin

What's the benefit?

The question in all these kinds of debates is probably one of cost versus benefit. There is no doubt that Al29-4C is tasty and performs admirably under a variety of conditions - but is it needed when combusting the usual variety of gas-based fuels?



Based on my very limited research, it seems like Al29-4C is of particular use when dealing with very low pH flue gases such as condensing oil (especially when the source fuel is up at or beyond the allowable 1700PPM sulfur limit - may result in a pH at 2 or lower). Under these conditions, crevice corrosion, etc. is a really nasty problem. But in a condensing gas boiler, the pH doesn't drop below 4 (IIRC) and hence the issue is not as pressing.



Most corrosive environments are at their worst when the corrosive medium is allowed to condense/dry on the spot instead of being washed away. In a condensing boiler, it's fairly likely that the condensate will clean the surface semi-continuously, especially in top-down designs like the Prestige and the IBC. In the flue, it's more likely for stuff to dry on the spot after the the boiler shuts down. See how the requirement for Al29-4C vs. 316Ti or 316L varies by fuel and chimney liner manufacturer.



My guess is that going slightly thicker on the HX matieral and/or upgrading to 316Ti is likely cheaper in the context of increasing a boiler HX longevity than switching to a very expensive metallurgy like Al29-4C. Plus, I'd wager that the majority of HX failures out there have something to do with Balance of Plant (BOP) failures, such as clogged sumps, fan disk failures, etc. Thus, it's the BOP where I'd pay the attention, with a special eye on detecting failure modes before they can ruin the HX.



Now, if I was running my boiler on a sour-gas well in my back yard...or a local landfill...  oh, I'm not, so I guess I won't worry about that. But if I were to run those kinds of fuel a super-resistant HX might become a requirement.

Glen

you could use this -

Centrotherm recently rec'd its 636 approval. By all descriptions - it looks like the "viessmann" type venting that was available. A quick "google will take you to the home site. It will not be as versatile as the "29" material due to limited sizes - but for most of the installs - may work nicely. Approved in BC - not sure  about other jurisdictions yet.

SpeyFitter

Glen...

Hey Glen, have you had a chance to work with this stuff?

I know IBC was recently experimenting with it lately and I could be wrong but I thought I heard they arranged to have it used on some big multi boiler job with common venting in Whistler or something along those lines. 

Thanks for the link, that's very interesting though and I'll have a look for sure.

Glen

the short answer is no -

my days on the tools are done; but still keep in touch with gas inspectors and other former colleagues throughout BC (which is why I still receive notices from BCSA). I would drop anything for a round of golf with Roy at IBC - he is a gentleman and leader in our industry; as I would for some others in the biz too. But I would be game in trying this product - high tech plastics have always appealed to me. Makes sense that IBC is considering this as an approved vent - its an innovative crew in Vancouver.