Steel

[ISeeDeadBTUs]

Ok, so I don't know much about steel. The side skin of the GreenWood is 1/16", plain ole' steel (not galvanized, not stainless). The entire front panel is 1/4 steel. The bottom of the rear panel is likewise 1/4". The top is 1/16" skin material. So . .

I wanna go stainless. I been snoopin' around

http://www.cut2sizemetals.com/stainless-steel/flat-sheet/ssh/

TYPE Corrosion Resistance Max thick form/weld? Price(per [\]) Side (2 per unit) Top Back
(47 3/4 X 47 3/4) (50 X 36) (32 X 32)
304 Average 7ga (3/16) 36.25 $600 (per side) $625 $410

309 Excellent 16ga (1/16) 23.25 $390(per side) $410 $270

316 Better 11ga (2/16) 39.43 $650(per side) $680 $440
7ga (3/16) 61.58 $1,005(per side) $1,050 $680

430 Good 14ga (5/64) 10.68 $195 $200 $135

What am I missing, other than the casholla?? Looks to me like even the least expensive - $725 for +1/16 430 - would be slightly thicker than OE AND increased corrosion resistance.

Looks like $3,740 would get you an awesome boat anchor.

Looks like the 309 has the qualities suited to skins (prolly NOT internal 1,500>2,000 °F operating environs) and would set a person back 1,460 for a full set of skins. AND they would be the same thickness as the original.

Any ideas? Would you opt for thickness over corrosion resistence? Vice versa?

Jimbo

 

[pybyr]

I don't know enough to advise on the alloys. One place that I have sourced stainless at favorable prices has been www.onlinemetals.com

 

[in hot water]

The ability to weld it, hold pressure and handle thermal shock is a consideration

Aquatherm did some expensive testing of stainless alloys, including 316 and 409, years ago with that Omega. I don't know that it ever worked out.

I think a heavier gauge of plain steel males more sense.

hr

 

[jebatty]

Seems to me that corrosion resistance is not the first question. Rather, and first, why is corrosion present? My short answer mighty be O2 and/or acids with water. Second, where is the corrosion occurring and why is it occurring there? So many wood boilers are made of regular steel, have lives of 20 and more years, and corrosion is not the question that is being asked. So, why is it your question?

With my boiler, the system water is and remains crystal clear. I suppose, but I doubt, that there is any corrosion occurring on the system side. The other place for corrosion would be inside the firebox and/or through the path that the hot exhaust gases pass on their way to the atmosphere. If the fire is burned hot and not allowed to smolder, and condensation is not allowed to occur, corrosion here should be very minimal and a well made boiler with quality steel, welds, and design, should last a very long time. My best experience-based example is my wood stove, only a firebox, no water. That stove has been in 24-7 business during the winter heating season (often from September to June in MN) since 1990. I'm not aware of any corrosion of a type that might mean a failure lies in the near future. I don't know why a water boiler should be any different.

I would try to isolate the nature of the problem and why before asking what steel should be used. Perhaps you have a design problem? or an operating problem? Both will need to be addressed.

 

[ewdudley]

[I believe it's the sheet metal jacket that needs replacing.]

 

[dirttracker]

I know a little about stainless, so I'll put in my 2c. Take it for what it's worth as I am not a metallurgist, I primarily specify the use of stainless in my day job for fasteners, connector bodies, and other small parts. I am assuming the jacket you refer to is an outside cover that is probably subject to condensation, but is not in direct contact with the water in your boiler. I am also assuming the operating temperatures and stresses are low enough to not be a materials issue.

AFIK, all of the 300 (I also believe this alloy series is also referred to as 18-8 CRES) series stainless are good for corrosion resistance. I have never seen anyone separate them like the listing you have in your post. Specifications I have seen allow the use of multiple 300 series alloys (302, 302HQ, 304, 316, etc) for the same parts. I suspect any of the 300 series alloys would be acceptable (and probably overkill) for your application. 300 series is a work hardenable material - material strength increases as the material is deformed. For this reason it can be hard to work with. I know from experience that drilling holes in 300 can be a real PITA, you need a sharp bit and some pressure to get through. If you start with a somewhat sharp bit you will end up with a very dull bit and no hole (unless you melt though). Welding this material can also be a pain. I have never done it myself but I know you can get stress cracks around the welds if you are not careful. I also know 300 series also loses it's strength considerably with temperature.

I don't know much about 400 series stainless alloys but they are less corrosion resistant than the 300 series. 400 series is more common as it is less expensive and easier to work, it is also a heat treatable alloy. 400 series is commonly used for automotive exhaust systems and other commercial items (read less expensive). For the price this may be the way to go as you indicate. You will get much better corrosion resistance than mild steel with something that is much less expensive and easier to work when compared to 300 series. If you weld this material you will need a stainless filler material, using mild steel filler will result in corrosion around the welds. I know form experience (on my old OWB) that 400 series is harder to produce a nice looking weld compared to mild steel - for what it's worth.

If it was my boiler I would make the skins out of 400 series and see how long they last. For the price you could be $$ ahead in the end. The disclaimer is; I don't know how long the original skins lated or what is causing the corrosion. I am looking at it this way, how often do you replace the exhaust system on newer cars using 400 SS? How often did you replace exhaust systems when they were made of mild steel? I know it's not a similar environment, but you can see how much benefit comes from using cheap SS.

 

[DaveBP]

If this "skin" is just the outside jacket that is commonly painted steel over the fiberglass insulation around the actual water jacket why is it corroding so fast?

If it's caused by humidity in the boiler room would a dehumidifier and the electricity to run it be cheaper and just as effective?

If there is a leak in the water jacket and the dampness is corroding it from inside wouldn't that need to fixed first?.

If you need to cosmetically cover it might good ol' galvanized sheet steel available at any local tin knocker be a reasonable compromise? Or painted metal rood panels if you fancy the OWB look

 

[Singed Eyebrows]

Ok, so I don't know much about steel. The side skin of the GreenWood is 1/16", plain ole' steel (not galvanized, not stainless). The entire front panel is 1/4 steel. The bottom of the rear panel is likewise 1/4". The top is 1/16" skin material. So . .

I wanna go stainless. I been snoopin' around

http://www.cut2sizemetals.com/stainless-steel/flat-sheet/ssh/

TYPE Corrosion Resistance Max thick form/weld? Price(per []) Side (2 per unit) Top Back
(47 3/4 X 47 3/4) (50 X 36) (32 X 32)
304 Average 7ga (3/16) 36.25 $600 (per side) $625 $410

309 Excellent 16ga (1/16) 23.25 $390(per side) $410 $270

316 Better 11ga (2/16) 39.43 $650(per side) $680 $440
7ga (3/16) 61.58 $1,005(per side) $1,050 $680

430 Good 14ga (5/64) 10.68 $195 $200 $135

What am I missing, other than the casholla?? Looks to me like even the least expensive - $725 for +1/16 430 - would be slightly thicker than OE AND increased corrosion resistance.

Looks like $3,740 would get you an awesome boat anchor.

Looks like the 309 has the qualities suited to skins (prolly NOT internal 1,500>2,000 °F operating environs) and would set a person back 1,460 for a full set of skins. AND they would be the same thickness as the original.

Any ideas? Would you opt for thickness over corrosion resistence? Vice versa?

Jimbo

My advise would be to call your local industrial steel suppliers & get quotes from them. They should deal with you as your order should meet minimums. I would not buy from a speciality steel co. You will probably need to buy full sheets & they should be able to shear them to your specs. I would consider A516/70 which is boiler plate(the best I know of) & designed for a tough environment. It seems you are just replacing the outside sheet metal & not the boiler steel itself. This is going to be much cheaper than stainless. If you must go stainless they can sell it to you & get you the most corrosion resistant alloy that your pocketbook can handle. $3740 is more than my boiler cost. New Econoburns were $4500.00, Randy

 

[leaddog]

I've been following this skin problen some and thought I would throw this out there. It seems that the problem with these is that the flue flow is around the insulation and moiture is rusting out the skins.
If you would make a sandwich skin with sheet metal on the outside, (gal, painted, or thin stainless) hightemp insulating in the middle, and 7ga boilerplate painted with a ceramic coating I think it would solve the problem. The high temp flue gasses wouldn't degrade the inside because of the ceramic and the insulation wouldn't get wet from condensation and the outside would be free of moisture so it wouldn't rust.
The eko lower chamber has this ceramic coating over the steel. I don't think any of this material would need to be to thick as it looks like most of the heat is going up. That is one of the reasons the sides of the hx seems to coat up. If you could have the coated steel and insulation sandwich I think you would raise the side temp inside some and the hx would not coat as much.
These are just some thoughts I've had from looking at pic. and writings on here and I may be way off base so take it as such.
leaddog

 

[ISeeDeadBTUs]

I see that many non-RMND owners still don't understand the condensation problem we deal with. I think all boilers have weaknesses . . . condensation is our big one.

Ok, the steel I speak of is just a 'skin'. It is not exposed to combustion. The water we speak of is not the system water. It's the water that is released from the wood during combustion.

For those that think 'regular' sheet metal should stand up to the corrosive force of water/creosote . . . a, 1.25" IRON nipple was eaten completly through on my unit within about 2 years. While we all take measures to minimize condensate (dry wood, hot fires, storage, proper draft) it's a reality. And it DESTROYS regular 1/16" sheet within 3 years.

But thanks for the input on the steel. Welding shouldn't be an issue (not much to weld and I'd have a pro do it) and drilling shouldn't be much of a issue.

Any knowledge on this is appreciated.

Jimbo

 

[ISeeDeadBTUs]

cool idea Dawg. . . but our Rock Wool is like 4 " thick.

But 7ga boiler plate ceramic coated . . . hmmm, that sounds like the idea about a steel inner shell that someone else esposed.

Prolly all good ideas . . .just doubt I will be footing the bill for anything that radical especially with 35 °F in the 'cast

 

[tom in maine]

I have not had my hands on a Seton/Greenwood boiler, but have been digesting all the information that has been posted on them.
It seems there are several factors that are complicite in this problem:

1. Combustion by-products are able to leak out from the combustion chamber into the "fluffy" insulation.
2. The insulation is not air or water tight.
3. There are periods of incomplete combustion whenever the system "goes idle". These periods produce unburned materials that condense on
cooler surfaces--the jackets after passing through or around the insulation.
4. Painted steel is not corrosion resistant, especially when exposed to occasional hot, moist, acidic flue gas with a lot of complex unburned hydrocarbons.
5. Units with water jackets on all combustion zones seem to not have this issue.

Considering all this, I built a prototype wood boiler for use last winter. It was similar to the Seton/Greenwood, etc. boilers in that it did not have a
complete water jacket, but it was contained in a galvanized shell that was insulated with 4" of vermiculite cement and a layer of kaowool inside of that. The unit also had a combustion chamber that was completely different that these units.

We have abandoned this design in our prototype work since I was unhappy with the heat loss through the shell and was concerned about the potential of a fire breach at some time in the future. In hindsight, I would not use galvanized again, but it has stood up well.
What was surprising was the amount of material that was able to sneak through the kaowool. It was not acidic, wet flue material that leaks through these units but there was some soot that apparently leaked through during startup before the combustion chamber got hot enough and also at times when I destroyed the integrity of the combustion chamber as we worked through different designs.

I am not sure if it would help, but the vermiculite cement seems to be a better choice for insulation than rock wool when working with an enclosure for
a boiler that is not completely encased in a water jacket. It has not cracked nor bee breached by seepage. I would still use the kapwool.

My boiler is not generating the amount of acidic stuff that a Seton/Greenwood might during idle since it does not idle, so it might not be a great choice.
My decision to use it was driven by the fact that Dick Hill and I have used it many times in the past with great success. It is simple and cheap and can be
a DIY project. I am not sure how you would cover the top of the unit since this material does not have much tensile strength but thought it might be of help.