|
Post by patamos on Dec 30, 2017 10:18:55 GMT -8
Hi Folks, I started this thread as a carryover from discussion of bells in 'non-testo results' I have lately been in discussions with MHA members, ASTM advisors and fire protection engineers about the pros and cons of 'single skin' construction of heat exchange channels. So the topic has been on my mind as i dig into past research here and there. While i do not build bells or flue runs as stand alone 'single-skin' per se (with clay brick on flat, and clay-sand mortar), i do build some areas of the heater body with said lay-up covered in 3 stages of cob plasters: 1 is a scratch (sub-dermis) coat to level things out and plug gaps. 2 is a clay-sand browncoat (dermis) with fine straw or fiberglass fibre. 3 is fine finish plaster (epidermis). This layering of clay-based plasters can be considered a 'double skin' approach in some respects, in that it does offer enhanced structural stability, and protection from gas leaks. This is especially important in seismic zones where these 'like bonded to like' materials make a resilient monolith that fares better than 'double skin' walls with separate refractory core and outer facing of dissimilar materials. And of course, this kind of multi-layered, or reinforced 'single skin' build up is simpler/faster to build and offers better heat transfer than double skin (with an air gap). But it does not guarantee as much resistance to cracking from thermal stresses as a double skin with air gap (that is built properly)
Double skin without air gap between core and facing improves heat transfer, but one has to know when and where this can be done without risk of cracking And the same goes for the technique i have described. So, when we look at the traditional 'stand alone' single skin designs (without an outer layer or coating of any kind), my sense is that there are a few design aspects worth adhering to. Mainly this involves dissipating the more intense heat before it reaches the single skin areas. This can be done via rapid radiation through cook tops or into doubles skin areas of the heater such as black ovens as in Alex Chernov's example: www.mha-net.org/docs/v8n2/wildac12g.htm
Also at a near by link (edited in): mha-net.org/docs/v8n2/wildac08f.htmwe can also see Alex and Igor Kutzenov building a double bell heater, in which case the first (hotter) bell is reinforced in the heart of the heater. (edit: or perhaps more importantly, it is small enough that the hot flue gasses move on to the second bell sooner than later…) And then there is the approach using long bench flue runs (see Lars Helbro's web page), where the heat is directed away from the core for effective dissipation of thermal stresses. Much in the way RMH with cob monolith benches work. All this to say, there is an art and science to building a sturdy and durable heater. I am still very much learning about the many factors involved, and hope i always will be. So I look forward to any and all discussion on this topic...
|
|
|
Post by mheat2 on Dec 31, 2017 14:39:02 GMT -8
It is a broad topic, and it depends on which aspects you are most interested in discussing.
Assuming you are most interested in code approval for single skin and/or rocket firebox designs, I can relate some observations from having participated in writing ASTM E-1602 (masonry heaters).
Bottom line here is that it is a safety standard. Building code does not care about emissions, efficiency, etc.
It is also useful to distinguish North American from European attitudes, where masonry heaters have a history and often the local certified chimney sweep is the final authority on the safety of an installation. In North America, particularly the United States, it is a legalistic attitude, perhaps related to the large numbers of lawyers per capita in the U.S. compared to Europe.
The bottom line becomes "liability" and "worst case scenario". In a UL safety test for a stove, you keep adding fuel every 7 minutes until the stove surface no longer rises in temperature. That is the limit you have to design your clearances for. This obviously causes a problem with heat storing appliances.
For masonry heaters, we defaulted to existing fireplace code, by assuming a minimum 7.5" thick wall at the firebox. Then, we doubled the masonry fireplace clearance to combustible framing from 2" to 4". There was no test data to base this on, and it was "best professional opinion". One of our task group members was a 7th generation German stove mason. Subsequently, we did some quick and dirty testing at an MHA meeting, and were able to prove pretty convincingly, that with an 8" brick wall (but not soapstone) it was impossible to exceed the UL limit no matter how much you over-fired the heater.
This may help to focus your discussion on just exactly how you might go about specifying rules for single skin heaters, establishing minimum construction requirements, and establishing clearances to combustibles. ...............Norbert
|
|
|
Post by patamos on Dec 31, 2017 16:28:43 GMT -8
Thanks Norbert, And welcome to these boards I appreciate your perspective and the depth of experience you bring to this forum. Your comments about worst case over-firing... echo concerns expressed by Mike (fire protection engineer). With earthen mass being semi-conductive we cannot rely on rapid dissipation of heat via radiation. Hence the question: What happens when the mass has reached peak accumulation? To your knowledge, has anyone applied the UL safety test to a double bell design such as this: mha-net.org/docs/v8n2/wildac08f.htm ? Where structural integrity of the heater's wall facing is the issue, would it be accurate to say that rate and distribution of thermal ramp up are the main contributing factors?
|
|
|
Post by patamos on Dec 31, 2017 19:45:25 GMT -8
Further thoughts, In my case, i am interested in cob-reinforced-single-skin (as described above, not sure what else to call it...) heat exchange channels/flue runs/bells that are sufficiently downstream from the fire box such that even under the heaviest of over firing they will maintain their structural integrity. These wall are of minimum 5" thickness (3.5" to 4" wide brick, with 1" to 1.5" of cob layers added) and always ascribe to existing ASTM criteria for clearance from combustibles. So for me the discussion focuses around what constitutes 'sufficiently downstream' (or distal) from the core. One rule of thumb i have seen applied in grundofen designs with initial flue run through the bench is 1' per Kw of burn cycle. But with thin radiating surfaces (metal or glass stove tops, or the classic RMH metal contraflow barrel) and/or black oven intake ports immediately downstream of the firebox... this ratio can be shortened. Then there are a myriad of factors such as how often it will be fired (cooking 3x a day?), reloaded... etc, and the worst case scenario of constant over-firing... I'm guessing much can be learned from the older tried and true designs, and any 7th generation builders we can find
|
|
serg247
Junior Member
The mountain can not be conquered, it can allow it to ascend...
Posts: 111
|
Post by serg247 on Jan 1, 2018 1:10:57 GMT -8
|
|
|
Post by Jura on Jan 1, 2018 8:22:43 GMT -8
Serg! Have you seen such contraption in operation or it it is just idea yet to be built?
|
|
serg247
Junior Member
The mountain can not be conquered, it can allow it to ascend...
Posts: 111
|
Post by serg247 on Jan 2, 2018 5:23:51 GMT -8
A concept from individually tested nodes. Modified. Professional stove workers say they will work.
|
|
|
Post by pinhead on Jan 3, 2018 9:27:39 GMT -8
My available materials being what they are (and the cheapskate that I am), I only use brick for the core of the firebox. My bells have all been built with either pure cob or, lately, limestone posts with clay/sand mortar.
For those of you who aren't from the middle of Kansas, a limestone post is a big chunk of limestone generally around 8 to 10 inches square and between 6 and 8 feet long. The posts I use, though, have been broken and are too short for fence posts so they're anywhere from 1 to 4 feet long.
With an eight-inch-thick bell wall, building a double-skin bell is redundant, IMO, since thermal expansion in the large mass is very low and even, and the mass is high. The ceiling of the bell has been a monolithic slab of clay/sand/straw.
Admittedly, I haven't built a "full bell" above the heat riser; I have a steel barrel that provides quick radiant heat before feeding into the bell. The warmest I've seen the surface of the roof of the bell is 270°F. The hottest I've seen the outermost surface of the limestone is 140°F.
With this discussion's focus being on making efficient and effective single skin bells, wouldn't something such as thick limestone suffice? Structurally, using these large pieces of limestone would be hard to beat - especially when compared to a double skinned brick bell with an air gap.
|
|
|
Post by patamos on Jan 3, 2018 12:06:50 GMT -8
Ya, makes sense. Making use of what is readily available. It is good to hear that the limestone is working well for you. That 'low and even' expansion of the thermal mass is surely a big factor in the overall stability. I think low heat duty firebrick, soft fired red brick and unfired cob also offer minimal thermal expansion. Unfired clay bricks are poor conductors, but cob with 3-1 sand clay ratio and short fine fibres likely conducts much better. As the clay is little more than the glue that holds the aggregate together and the fibers are sparse and fine enough that they will not create air pockets. The most important element for me is working with clay. Such an amazing material with so many health benefits. We have a little pond we dug in the back 40 with a good 4' deep seam of pure brown clay that i can always draw from. My next door neighbour stables horses (fine manure fibre), another neighbour imports compressed bags of dust free finely chopped straw from Manitoba, and there is sand n' gravel pit down the road... I build adobe houses, cabins, outdoor kitchens... all kinds of structures with these materials. I've built a few RMHs and masonry heaters entirely with cob, but notice there is a maximal temperature after which it starts to abrade. So it makes sense to transition to fired brick or refractory materials or w.h.y. materials. Or at least boost the mix with alumina or sodium silicate etc...
I wonder if Norbert's MHA test of running the system full blast til the surface no longer rises in temperature ought to be the test also for structural integrity of the heater body?
|
|
|
Post by patamos on Jan 5, 2018 20:32:14 GMT -8
Thanks Josephcrawley for digging this up:
mha-net.org/docs/temp/080428oehme.htm
short version being, the hotter the firebox, the more we had best isolate from the facing.
|
|
ekw
New Member
Posts: 14
|
Post by ekw on Jan 9, 2018 17:12:29 GMT -8
We have started isolating the J-style fireboxes almost all the time now. The manifold/plinth area under the barrel does this pretty well at the heat riser end. In the feed area, we either use a refractory insulation (perlite-clay, ceramic wool/felt, or something like that with some 'give' to it), or we do a quick barrier/gap holder like cardboard and hope for the best. In the past, many builders felt that straw-rich cob or earthen plasters at 4" thickness was enough to resist the thermal expansion strain around firebox and barrel, but I find with the firebox proportions that seem to work best, it's hard to get enough thickness (about 4") around all parts of the feed.
When it comes to benches/thermal mass away from the firebox, our double-skin approach does not use expansion joints. We have found that the rounded steel pipe, bedded in cob/fireclay mortar, does not get hot enough to cause expansion cracking (especially considering it's round without edges/corners to concentrate the force). It's possible that some of the thermal expansion 'give' is coming from slippage, or that we're gaining the space we need with hot-wet curing in. A lot of these systems do have the pipes screwed together, and I've never seen expansion cracking downstream of the manifold area. It's likely the temps are just not hot enough for it to be that big an issue.
do I understand correctly that for kachelofens, they use tile and refractory cement in a way that is considered a "double skin"? Or are they also doing a second firebrick channel inside the outer skin of brick-laid tile, mortar, and firebrick?
... I would welcome an opportunity to put one of the J-style rockets through the "until the surface temperature stops rising" tests in front of qualified witnesses. Us backwoods builders have done a lot of attempted destruction-testing with encouraging results, and some metered emissions and burn efficiency tests, but these don't seem to carry as much weight as the German/Austrian or EPA labs, or a 7th generation mason for that matter.
The one place we don't have double-skin of some kind is the radiant metal barrel, or contraflow container, or bell, or whatever you want to call it. It can be made with thicker steel and gives a wonderful immediate gratification for folks who like that quick radiant heat-up in the evenings.
I understand that most masonry heaters have some cleanout access points in their smoke channels, are these allowed to be single-skin of thick steel? Or do they back them with removable bricks or something? Seems like you would get better airtightness from a gasketed steel door than from a few loose bricks, but there's clearance to consider too.
I wonder if there is some testing we could do with the barrels under stringent supervision, and see if its performance surprises the skeptics. There used to be a lot more metal-masonry hybrid stoves in the Victorian era and onward, I've seen some lovely antique examples in New England fireplace shops.
Or we could play within the current guidelines, deliberately positioning cleanout doors where they could release some quicker radiant heat to target areas with good clearances near the downdraft bell, with the bell made of double-skin masonry otherwise.
The performance would change substantially, however, if instead of a thin conductive/radiant heat shedding barrel or single-skin bell, we had a bell that was double-skin and does not shed heat quickly. We've seen folks have draft problems when they insulated and added masonry around their steel barrels. We can adjust for this with shorter benches, or bells with less drag, but it does mean a substantial redesign and recalibration of performance (length, draft, drag, etc).
One thing I really like in the RMHs that I have not seen from tower-style masonry heaters is getting most of the heat from a low masonry bench. This is great for human comfort (full body heating pad, or lumbar heating, are much appreciate by farmers and builders). It's also good for conductive efficiency, seismic stability, cheaper footing requirements, and for separating the high-stress firebox and low-stress secondary heat capture for ease of building. I would love to see this approach given more consideration. I think we may be able to achieve something similar, with the bulk of the heat-extraction or at least half of it in the mass bench, if we go to single-skin with plaster where we now use barrels. I doubt it will work out that way for double-skin with insulating gaps. The lack of downdraft cooling on the outside channels may require shortening the benches beyond where they'd be full-length human comfort machines, esp. on smaller 6" channel models.
It is almost like we have separated the double skin, with firebrick and refractory for the heat riser, and are using the gap between this and the outer skin for downdraft smoke channels. Steel handles these temperature gradiants beautifully, and we have had some very good results bedding the lower-temperature base of the steel barrel in 4" of masonry, or using a little bit of ceramic wool to give it some expansion joint if using modern cements that can't be hot-wet cured for thermal expansion.
That's the other thing I doubt most of the conventional code folks have seen; hot-wet curing with clay masonry (that contains burnout/tensile strength fibers) gives you crack-resistant, thermal-expansion-tolerant results that are difficult to replicate with cementitious mortars. Well-made earthen masonry is just a lot more resilient in some ways, its 'softness' makes it less brittle. Whereas if you try to hot-wet-cure any of the commercial refractory cements, or Portland cement for that matter, you will be sorry.
Pat is probably right that the trick is going to be getting a reliable standard for "well-made cob" vs a novice's ignorant approach to "Clay!". Because clay, without aggregate or fiber, gives about as poor results as you would expect. Same is true for most cements; binders need fillers to function for gap-filling and surface sealing applications. Pure clay can be used as thin-set in the firebox but not really anywhere that gap-filling or crackproof construction are critical.
Lot easier to un-do and recycle novice clay than novice cement "seascapes," but it's still ugly and unpleasant work.
Anybody recommend facilities or qualified experts to supervise a Heat Til You Cain't Heat No More safety test? This might be something we can fund-raise for through some current connections. WA state would be ideal, or nearby if possible.
Yours, -Erica Wisner
|
|
|
Post by Orange on Jan 10, 2018 13:01:27 GMT -8
We have started isolating the J-style fireboxes almost all the time now. Hi ekw, do you have any info on combustion efficiency with and without isolated firebox?
|
|
|
Post by patamos on Jan 11, 2018 20:17:25 GMT -8
Thanks for your thorough input Erica. I appreciate all that you have done over the years to promulgate RMH natural building techniques. It is interesting to consider all the possible definitions of 'double skin', especially the heat riser and barrel combo. Perhaps it is safe to say that the primary purpose of any outer skin is to create an air tight seal (?) While also offering structural integrity where necessary (some places more than others)… With fibrous cob plasters covering a brick and clay-sand mortar lay up in the sprawling bench (and sometimes backrest) designs, i think we have an effective double skin meeting the objectives of the ASTM. Same goes for the RMH technique involving ducting encased in specified layers of cob. Especially when hot cured to give a bit of wiggle room And interesting to note that unfired cob handles thermal shocks quite well. Much in the way softer clay bricks fired at lower temperatures do. I guess the crux of the matter is making a case that 'cementitious mortars' (as specified in ASTM 1602-03) are really only applicable to the outer shiner (brick on edge) or stone or similar facings of heaters with a separate inner refractory core. The strength of these mortars is necessary given such a thin free standing wall lay up. And resilience is not necessary But we know we cannot use such mortars with cob due to mixing setting issues, and we cannot directly parge a clay brick core with them because they are too rigid and brittle to move/breathe with the more resilient core… So some of the issue lay in finding a common language to describe the apples and oranges of differenct techniques. Regarding cob materials, i agree that a fair level of expertise in needed when mixing specific materials for specific applications. So standardizing the language here will also be important. thoughts for now… pat
|
|
|
Post by patamos on Jan 12, 2018 11:03:01 GMT -8
Hi Serg, I tried opening this a couple of times. Downloaded fine but drop box is telling me i no longer have an application to open it. Could be my computer is getting old (and I am pissy about having to buy a new one…) Can you point to another source for the file? thanks
|
|
serg247
Junior Member
The mountain can not be conquered, it can allow it to ascend...
Posts: 111
|
Post by serg247 on Jan 12, 2018 11:47:48 GMT -8
|
|