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[Greywynd]

Hey everyone,

Starting to gather up material for a OWB build. Wondering what sort of thickness the outer shell, and in particular, the firebox should be made of.

I'm thinking that thicker is better for longevity, but not so good for heat transfer. So where's the ideal medium between the two?

[Greywynd]

No thoughts?

I knew one friend that had 1/4" firebox, but the rear wall started to crack/leak after a while, he figures it was the wood hitting it when reloading, denting and distorting it. Eventually he welded in a piece of 3/4" plate across the back to reinforce it.

I'm thinking that 3/8" might be a good thickness, at least for the bottom and rear, and maybe 1/4" for the rest?

Of course this is assuming perfect material selection, and not just what comes along that I can make work....

[BoilerHouse]

For the firebox it's a bit of a balancing act I think.  Thicker lasts longer, thinner is better heat transfer.  I would think that 1/4 inch is probably a good compromise.  My firebox is 3/8 but it is also fire brick lined.  I try to get heat transfer through the thinner walled fire tubed heat exchanger.

[Fourced]

All the material in my build is 1/4 or really close to it.

[peacmar]

Thickness only effects heat transfer very little when you look at it from a scientific stand point. Heat transfer through materials is referee to as "latency of heat" or the ability to transfer heat in relationship to water and its ability to transfer heat. The heat holding capacity of a material is related to the latency of heat, but is entirely different. While one may think that thicker metal will transfer heat less efficiently, most grades of steel have a latency of heat of .91 or roughly translated as %91 efficient as water. Most grades of steel have a higher holding capacity than water. It takes longer to dissipate the heat. Therefore heat will be transfered to the water equally as fast whether it is 1/8" thick or 1" thick. The difference is that the 1" thick plate will take longer to dissipate all the heat it has stored to the water than 1/8" thick. This has many benefits, more thermal storage capacity, may be negligible but more nonetheless. And a higher surface temperature of the burn chamber which will result in less heat being stolen from the flames, more complete combustion, and more efficiency with less smoke. The deciding factor generally is cost effectiveness of the creation. Mobility for installation second. If you have the money and the means to move it around when completed, make the firebox walls as thick as possible. Water jacket thickness should be determined by the whole of the mass of the water contained within. We don't want it bursting now do we.....?

[weasel]

I built my wood boiler in 1994 and still use it today. My fire box is 7ga carbon steel.  I stick welded all the seams both outside and inside. The bottom and 3 sides are fire brick lined.  I'm surprised it has lasted this long. The first 3 years I had anti freeze in it but since then it has been all water and no boiler treatment.  My water jacket is a 275 gal ob- round oil tank. The kind every one used for their house heating oil.  I stood it on end cut a square hole, slid in the 36" long fire box and 2 horz flues, added secondary burn, forced heated draft, and it works better than all the units that were on the market back in 94.  I had a leak the third year because the screw in plugs in the oil tank were made out of lead, and lead and anti freeze dont like each other. Since then I have rotted out the chimney and replaced it with stainless steel.  The sheet metal roof under the chimney cap has holes rotted through it.  Creosote and rain water will take the stripes off a zebra if ya know what i mean.  Personally I wouldn't go any thicker than a 1/4". There is a trade off between strength and heat transfer efficiency. One of these years I'll build a new wood boiler and retire this one. So I happened on this site snooping around looking for some good ideas for my next boiler.

Weasel

[MrDan27611]

There have been some posts on here debating material thicknesses before. I'm sure you could find them with a bit of searching and see all the points and counterpoints.

Peacmar, I'm always glad to have a scientific basis for a conversation vs. "my buddy says..." however I think one point may not have been clear in your post. A thicker piece of metal, while eventually transferring heat at the same efficiency, doesn't do so in the time frame. If we took your recommendation and took it to extreme ends and put in 5" thick walls, the heat we generate would eventually make it's way into the water even through 5" of metal however the recovery time would go down as it takes longer to simply heat that thermal mass meaning we'd continue to drop water temperature for some time after the aquastat called for heat. Additionally, once we did start gaining heat, we'd have a good bit more heat in the metal after the aquastat called for the heat to end meaning we'd continue to heat our water after the fire had been reduced, possibly even leading to a boil over, again I'm speaking in extreme circumstances. When we are talking about any realistic thickness that somebody would use, it probably doesn't make much difference. 1/4" seems to be a good compromise thickness. My install with two water tanks (one on the boiler, one on a holding tank) has been installed for 30 years. I've had two stress cracks in the boiler, and one crack in the weld on the holding tank in 30 years. 3/8 wouldn't have helped much in either case.

[peacmar]

Thank you mrdan for elaborating on that subject, I see I only mentioned the ability of metal to hold onto the heat briefly in one sentence. Your very correct in that there can be such a thing as too thick a wall for practicality. I kind of took for granted and assumed that no one on here would attempt to use anything much over .5" thick materials as most home workshops generally don't have the capacity to work with anything that large all to well. I myself feel that .25" thick material should be the absolute bare minimum for a firebox surface, there are many many factors that come into play, but that should suffice. If one where to calculate expansion and contraction rates, material strengths, ability to resist stress, and all those other fun metallurgical engineering calculations, with a little black magic I'm sure one could come up with an absolute perfect thickness. But joint designs, overall shape, and heat movement within also play important roles. Surface erosion is another huge factor. Maybe one of these days ill crunch the numbers for the heck of it and see what I come up with using common shapes. For now though, I'm with the majority.

[Greywynd]

Some interesting facts and observations being discussed here.

If, in my ideal world, I could use what I wanted, I think I would opt for 3/8" bottom and rear, 1/4" on the sides and top. I'm also thinking to taper the bottom in, so that the wood automatically rolls into the middle as it burns down.

It's still way back on a back burner for me, so still have lots of time to scrounge up materials for it.

[beeman]

mine was built from a old 1,000 gal propane tank for the fire chamber front and back is 1/2 inch plat the outside water  chamber was 1/4 inch tank hot chick dont look pretty but she keeps me warm stil hope to do more to mine to bring out her iner buity gary

[peacmar]

I just want to throw this up for the sake of discussion here....


My firebox is lined with 2.5" thick fire brick, all sides, my only direct contact heat transfer is a 3" tall by 36" wide secondary flue pass. About 40" long. Now, when I do my efficiency calculations by weighing the wood I put in and recording the temperature increase in the water over a given amount of time,I actually have a higher rate of heat transfer with the brick than without. Which supports a theory that a firebox made with 1" plate should be more efficient than one of 1/4" plate. The reason here is that even though the heat travels to the water more slowly, it is still being absorbed and eventually transferred through. But, the greatest advantage lies in the fact that I can make a much smaller, hotter, more efficient burn, with less wood, and still make as much or more heat. Because the surface temperatures in the burn chamber are much closer to actual combustion temperature. Now, if it takes an hour for the heat to travel through the bricks, through the metal, to the water, big deal. It takes four hours of burning to store enough heat for twelve hours of use. So, after the 400 gallons of water I have has started to cool, which is probably around the 4-6 hour mark, those bricks are still adding heat. Holding the temperature at a reasonable level for quite some time thereafter. Consider comparing two 9 volt batteries paired as opposed to a deep cycle automotive battery, the two 9v make more voltage combined but the auto battery will run your lil lightbulb for days. Mass storage is key to any regenerative system, be it heat or electricity or whatever. The more storage you have the less of a change there is from start to finish of a cycle and the longer the system lasts as there is less direct load on it.  The simplest laws of chemistry and physics are at work here. If, say I build a burner with a firebox that has 8 inch thick walls, the heat will still travel through to the metal at almost the same rate as water, which with steel, is almost instantaneously, the steel will continue to heat until it has reached the phase change temperature of the water, 210 degrees, at with point there will be a gradient established. The steel will continue to heat hotter than the water, potentially all the way to the melting point. But the surface of the metal that is in direct contact with the water will never achieve higher than 210 degrees because the water absorbs the heat. As heat is taken from the water, it is then taken from the metal, the metal slowly cools and continues to do so long after the fire has gone out. Because it was able to absorb more heat than the water, heat that would have been wasted if the wall was only 1/4 thick. But the cool part is that the water temperature could potential remain at exactly 210 degrees for many hours long after there is no fire.


Just a small blurb of late night physics to ponder on. I'm not sure how clear it came across as its far past my bed time.