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Rocket Mass Heaters And Perlite  RSS feed

 
pollinator
Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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et al and the 'pClould' : Just lately we are dealing with a lot of well - failures and intractable problems, I am wondering if during this new wave of experimentation some of our
problems are due to wet perlite, If due to the way the rocket mass heaters are Laid out- Water first from the Cob, and then running back down hill from the 'mostly' horizontal
run of our RMHs could be delivering even more condensed water vapors (produced from liberating the % of water water by weight in our fire wood, and of course the water
produced in burning the Hydrocarbons !) and there soaking some section of Cob located above our supposed to be insulating Perlite!

Like a fellow member once said in these Threads ''I am having a little trouble wrapping my head around Thinking like Fire'', ''and flowing like a gas'' second opinions please !

I expect that this is a possible source of some of the problems people are having, but is it possible this is way more common than we think ! For the craft Big AL !
 
gardener
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Location: Okanogan Highlands, Washington
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allen lumley wrote:et al and the 'pClould' : Just lately we are dealing with a lot of well - failures and intractable problems, I am wondering if during this new wave of experimentation some of our
problems are due to wet perlite, If due to the way the Rocket Mass Heaters are Laid out- Water first from the Cob, and then running back down hill from the 'mostly' horizontal
run of our RMHs could be delivering even more condensed water vapors (produced from liberating the % of water water by weight in our fire wood, and of course the water
produced in burning the Hydrocarbons !) and there soaking some section of Cob located above our supposed to be insulating Perlite!

Like a fellow member once said in these Threads ''I am having a little trouble wrapping my head around Thinking like Fire'', ''and flowing like a gas'' second opinions please !

I expect that this is a possible source of some of the problems people are having, but is it possible this is way more common than we think ! For the craft Big AL !



You can definitely get reduced performance due to wet perlite. Some symptoms can be quick initial heat transfer to the outside (the insulation isn't working); smoke (the fire isn't hot enough); and stagnation and smoke-back (the system stops drafting about 45 minutes to 1.5 hours into the burn, if the insulation of the heat riser is not adequate to maintain a difference in temp from heat riser to barrel).

We often make the cleanout just after the manifold the lowest point in the system, to provide for drainage of condensation during initial firing-in. You might be able to divert moisture with small dams of foil tape, if you need to corrall it to a specific exit point. Usually, though, the horizontal runs would dump into the manifold, not the firebox itself, so the impact should be relatively minimal.

A good system will generally work even while wet; performance definitely improves after a week or two of drying out.

As a side note:
Everyone at our workshop in NY knew all about wet firewood vs. dry wood - yet on arrival, we saw that the fuel was stored in woodpiles under scanty cover, growing mushrooms. Some owners make "dry" fuel by staging it to a covered porch area, then hauling it indoors a few days to a week before burning. This isn't actually dry - it takes months for wood to fully give up its moisture content, as it dries by wicking toward the end grain. (Though I do admit the NY owner's fuel-drying process produced better results than the frozen-solid punk I've burned on desperate occasions myself.)
Try weighing the wood, baking in a low oven for a day or two, then weighing again. If it's losing a significant amount of weight, find drier fuel, and improve the storage. Using a small amount of very dry wood to help heat up and dry out a fussy system can make a huge difference, as the steam in the wood itself can sometimes be enough to put out a fire.

Does that help at all?
The other thing to check on a sluggish system is the overall dimensions.
- Are there any points where the clearance is less than 50 square inches? (A barrel placed too low over the heat riser, choked manifold where the barrel meets the bench pipes, or choked exit are common issues).
- Is the heat riser tall enough, and the feed short enough? We usually do a 48" heat riser and a 16" feed, with about 24" burn tunnel (10" bridge over 24" total opening length).
- Is there adequate exit chimney height for the system? Sometimes just adding another 5 or 10 feet is all it needed. We usually do a vertical chimney to above the roof ridge, indoors until it meets the roof. Cold outdoor chimneys create tricky problems. Horizontal exits are often tricky too.
- Is the system too long, or has too many elbows? We recommend max. of about 50 feet horizontal run on an 8" system, and every 90 degree turn reduces this by about 5 feet. (Example: for a down-and-back bench, with one U-turn, the max would be 40 linear feet or about an 18-foot bench with 2 runs of pipe. For an L-shaped bench, with 2 90-degree turns and one 180-degree at the end, you would not want the total length to be more than 30 feet). If in doubt, cut the system down to about 20 feet (e.g. use one of the cleanouts to create a temporary exit) and see if it does better.
- Is there any corrugated material in the system, or anything else that could cause excessive drag?


Yours,
Erica W
 
pollinator
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Location: south-central ME, USA - zone 5a/4b
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I'll chime in with a few points I've learned and a discovery last night of a major problem that was most definitely causing serious issues with the system performance over the last week. Hope it helps others

#1 - we used the clay slip method for making our cob - it's been cold and our nearly pure clay was coming in as chunks of nearly pure ice for some of the build, so using some boiling hot water really helped get things going. The problem here was that the water content in the cob is way higher than traditional cob and on top of this, due to frozen hand syndrome, we went even wetter than recommended. The wetter the cob is, the longer it takes to get things running optimally, so fire early, fire hot, and fire long. Fire for 24 hours at a time if you have enough coffee on hand And, for the love of God, build your RMH during warmer, drier weather!

#2 - even quarter-inch thick saplings take quite some time to season in a wet climate like central Maine. 3 months wasn't enough and much of it is still green and springy once it thaws out, still full of moisture that you can hear hissing out as your RMH chokes on the steam. Our dead standing trees are even loaded with moisture in this climate. Dry wood is VERY important - more important than I ever thought when I ran a wood stove.

#3 - in a wet climate, with wet soil, building your RMH directly on the ground inside a polytunnel is going to just add to your battles with water. If I rebuild next year, a bed of gravel is a VERY worthy investment and building before the plastic goes on the polytunnel is just plain smart.

And for the feature of this post, last night Jerry the RMH was not lighting up - no draft. I checked first for an ash buildup in the back of the burn tunnel (had just cleaned it the day before so didn't expect much) and instead found a quickly growing pile of loose clay and perlite under the heat riser!

15 minutes later, I had a bucket of cob chunks ready to reconstitute, the barrel off by the door, a mountain of dry crumbly perlite chunks and a seriously deformed 8" galvanized sheet metal that used to resemble ducting. Evidently, the ducting, from the last few weeks of extreme heating and cooling, had deformed dramatically at the base of the heat riser, folding inward at the seam nearly 4" and completely choking off the heat riser!

This deformation also caused/allowed the perlite to fall out into the burn tunnel - serious hole in the bean bag had started happening. With the clay slip/perlite, I would have expected the perlite to have held its own structurally, but it was pretty much all loose and dirty looking. There were some chunks of still bound up perlite, but these were very fragile and crumbled with any amount of pressure (couldn't even pick them up), so I'd imagine the thermal expansion/contraction is what worked the majority of the perlite loose.

I rebuilt the heat riser by first inserting the round ducting into the square hole in the top of the burn tunnel, ensuring this deformation wont happen again. The perlite obviously didn't have nearly enough slip to give it structure, so I added more and repacked it into the 14" surround. I got the barrel back on and Jerry fired up. He ran like a champ for about 45 minutes...but then no draft! That wet perlite was allowing the stagnation Erica mentioned so every 15 minutes or so, draft was gone again and the fire was out.

So, #4 - don't choose galvanized ducting for the inside of your heat riser if you have a choice of more solid material, and make sure you use enough clay slip on your perlite if you're going that route. Use as much insulation as you can, but expect to have issues while it dries.

 
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