|Mini Mouse, exterior|
|Mini Mouse interior - 4" J-style firebox.|
|Fat Rabbit - and |
what is Peter looking at?
(see end for answer)
The Minnie (Mini) Mouse - 4" J-style Dragon Heaters core, sawn-off heat riser, in a 55-gal (200-liter) drum, with 5" stovepipe.
The Fat Rabbit - 4" batch-box made by Peter from vermiculite board, in a custom rolled-steel cylinder designed to mimic a larger-size shipping barrel that's commonly available in Europe (but not in Montana, perhaps due to the very long distance between the Rockies and most shipping ports).
So Ernie named his project with the recycled quartz-glass test tube,
|Dis-assembled Glow Bug, showing nice simple manifold. The special glass tube, blackened with smoke, is in the background.|
|The Kitchen Rack|
center: hot water,
right: covered griddle / BBQ smoker
I suspect it will just be called "The Kitchen," soon enough.
I spent the whole time noodling on two designs problems - one a submersible heater for a rocket-fired hot tub, the other a portable, small-space heater suitable for boats and camper-caravans.
I got paralyzed not knowing how small I could make things, and decided to take some time off and just make something TOO small.
|Improvised 2" rasp for interior of heat riser.|
The first version of the firebox, we cut into 4 bricks; the second version (shown) was extended a bit using scraps of insulation board, for a longer firebox and longer chimney.
Names for this 2" batch box ranged from Peter's offer to transfer the title of Minnie Mouse; or to christen it with an even-smaller new name such as "The Roaring Flea," or "Barking Flea," to which Tim Barker could not resist returning,
"More like 'the farting gnat.'"
|If you look closely you can see |
the intact ash shape of
a hemlock cone
just inside the mouth.
|Big ambitions, small but shiny achievement.|
The 2" core burned surprisingly well (that is, it did actually burn at all), and we learned some very useful things about what does and doesn't scale.
Insulation doesn't scale. I carved a cute little pinch-point handle in the vermiculite-board-scrap door, and almost burned my fingers trying to use it. Turns out, 1/4" of vermiculite between you and a 1200 degree fire is still 1/4" of vermiculite board, no matter how cute the firebox is. In fact, you could argue that insulation scales inversely: if you want to make a functional firebox half the size, and you would usually use 1" of insulation, you may need 1.5" or 2" to get the same performance from the smaller firebox because it will lose heat more rapidly due to greater surface area compared with the volume (and weight) of the fuel.
Fire doesn't scale. You still have to light a real fire even in a miniature firebox. We got it down to about 1/4 sheet of newspaper, a full load of matchstick-sized kindling, and then promptly refilling with "big" wood (short-sawn pieces of medium kindling, less than one stick in some cases made a full fuel load). Not surprisingly, fire also doesn't burn very clean when you only use 15 minutes of kindling, the dirtiest phase of the fire.
I suspect the flame length doesn't really scale, either, or at least not linearly. We did a to-scale 9" heat riser at first, and the flames came out the top a couple of inches. Then we put on a couple more feet of various-sized heat riser using the smallest stovepipe on hand (3"), and the flames filled the whole stack and came out the top by about a foot. I think there may be some maximum flame length that all fires can achieve, forest fire fighters will estimate flame lengths at about 4x the height of the fuel, and putting a chimney on it draws this up a bit longer. But I don't think even a small fire can be expected to burn clean with less than about 2 feet of total flame length, or some equivalent stay-time at adequate temperature.
Wood racks do scale nicely, however. You can stack a lot of 4" to 6" kindling in a cute little tangle of wire, and it's very nice to have it handy as your matchstick-tinder "kindling" burns down very fast before you can prepare "big"-kindling-wood.
And Peter's double-vortex, or rams-horns, does seem to scale even down to this tiny size. We were able to get it going in the bottom of the heat riser, but even that wasn't enough to guarantee a clean burn.
I spent some more time noodling on the boat-stove design, and didn't make much progress.
I laid up an elaborate design including a 1-loaf oven, tilted heat riser, 4-french-toast griddle, and double insulation on the sides. I was enjoying the symmetry and multi-function, and excited to see if it worked as a cooker/baker that could burn clean. I considered calling it The Shrimp, as the firebox-and-kicked-heat-riser reminded me of a curled whale-tale only much smaller.
But it seemed like even so, it was getting too big, with multiple insulating bricks on each side, and more courses in height than I had hoped for. It seemed it would be difficult to stabilize and make shock-proof.
I invited three of the experienced sailors in the group to critique it. After beating about the bush a bit, they basically agreed that in order to survive life on a boat, the stove should basically be able to be bolted down, turned completely upside-down and end for end, and continue to function (ideally throughout the process, but certainly after being turned back right-side-up).
There was much gesturing of the shimmying, juddering, jolting drops that happen when a boat is wallowing and running her nose ahead of herself in heavy seas. (And that's considered recreational sailing... a "bad day" at sea can include a complete capsize and self-righting, which is not the same as a shipwreck, on account of sometimes you can still collect the broken pieces and motor home by yourself, and on rare instances with over-built rigging and zealous furling, you may even be able to continue sailing after the storm passes).
So my design would need to be re-worked to meet the newly defined, upside-down-drop-kick standard of ruggedness for marine applications.
I think something similar may be required of stoves for campers, which, though they rarely undergo complete capsize, are sometimes seen to wallow and jolt their way up winding mountain trails where a sensible person would not take a donkey cart.
I switched tracks to the submersible heater, and may have accidentally solved both problems.
Peter helped simplify the design - I'd been thinking about a refractory-lined pocket rocket, maybe casting refractory into barrels, or using a combination of brick, cob, and a cast-refractory or tile feed.
Pocket rockets are a scrap-steel proof of concept for rocket thermosiphon fireboxes; their main drawbacks are that the fire is hot enough to chew through about a third of the metal feed tube in a matter of months, and few barrels last more than a year - I think about 3 or 4 years is the max. A stove that eats itself is not a safe thing to install in your home, nor is it particularly appealing to a dedicated environmentalist like Paul Wheaton, which is why they've been banned from the labs after a winter of far too many excited, and terrifying, experiments.
So my goal was to make 1) a pocket rocket that didn't eat itself, and 2) could be safely lowered into a container of water as a dead-simple immersion heater.
We did some preliminary figuring and realized:
1) it would be nice if, in case the barrel might rust through and admit some water leakage, nevertheless the lining material or metal outer layer should not explode from trapped steam when a fire is lit inside. This suggests either a double-lined metal construction with an easily-inspected-and-replaced outer barrel, or at the very least, not using any of the castable refractories that are noted for steam spalling.
2) It would be nice if, when lowered into the water tub, the heater did not float up and tip itself over, swinging its hot chimney about in a terrifyingly random orbit, and/or drowning its own fire. This means the weight needs to be about the same as the barrel would be, if filled with water - for a 120-lb grease can, about 100 to 120 lbs. (Peter calculated it independently with a metric tape measure at 50 to 52 kg, which is pretty darn close.)
That's a reasonable two-person lift, but tends to rule out doing any much bigger versions as a lift-in, lift-out heater.
Tim suggested we could do a couple simple things to get around this - mainly to make a separate side tank or barrel, and plumb it to the tub with a thermosiphon so that you would not need to move the immersion heater every time you wanted to warm up the tub. This is an excellent and civilized notion, which would require me knowing rudimentary plumbing skills, or assuming I could pick that up from present company, also would require me having the foresight to order some parts for the purpose. Neither of which were particularly the case, and I was getting tired of hanging around asking present company for ideas and help. So I regretfully rejected the solution. But I did take his idea far enough to use a barrel of water for the testing vessel, since we were also having difficulty re-assembling our second-hand redwood hot tub enough to re-create its alleged, former capacity to hold water.
Peter pointed out that casting things was a complicated way to make a first prototype, in any case.
Much simpler to line the thing with fire bricks, and see what happened. We tried a test-fit, and it looked like 8 or 9 split firebricks would line it pretty nicely, with room for a dividing wall of full brick. We cajoled Paul into buying a masonry wet-saw (at substantial expense) so I could cut 20-degree angles on the corners of a large collection of split firebrick, completing the shape with a couple of angled full-brick wedges. I would prefer a symmetrical feed and firebox, but this is a good tight fit and much simpler. (Peter kept encouraging a tighter fit, and suggested changing the orientation of the lapped joints so that the bricks really helped jam each other in there.)
|Larger opening on left is updraft, small on right is feed.|
|Detail showing brick joins (no mortar)|
The Caddisfly (or Nymph), named for a small elongated larval creature that glues rocks to itself and lives underwater.
We lit it, and it burned so clean, so quickly, that I did not look up in time to see any smoke. it's possible it smoked for a moment, but it was cleaner than most pocket rockets I've ever lit, which is already clean compared to most improvised wood-burning stoves.
The surface was a disappointingly warm temperature - hot enough to be quite comfortable standing next to, and even to touch briefly, but I was concerned that it would not put out enough heat to do much good in the big hot tub.
Peter was ecstatic, however, pointing out that this was a very excellent solution to his original problem, and my earlier one, of a small, compact space-heater with safe temperatures for reduced-clearance installations.
It turned out, the bricks were fit so tight, that we could move it on a hand truck at 45- degrees or more, with no noticeable shifting. So with a good clamp-on lid and flexible top shim, we may be close to solving the upside-down-drop-kick criteria for boat and camper stoves, too.
The Caddisfly doesn't have the cooking or baking capacity I was hoping for in a tiny-space heater, at least not in this version, but it may be possible to add something on later. Some kind of plug or shelf in the feed tube, or even a cleverly designed feed door, could give some pot-heating capacity, and there might be a way to add a chimney-stack oven on to the back end.
The stove does put out too much heat out the chimney for true efficiency, so some kind of compact chimney-heat-extractor would complete the puzzle.
|Lasse, pronounced like NASA|
|Sidewinder with warm bench and happy campers|
Further out in the yard, while all this was going on, Lasse Holmes and his fellow Alaskan Greg were steam-rolling through a demonstration of his Cabin Heater which tried out a new stock door (not his favorite, as the air feed wasn't adiequate), and a new small warming-oven feature, and showcasing some prior innovations with adobe-block cob bench construction, and the side-winder adaptation of the batch box which allows a compact block design with a bigger cooktop.
Ernie had a simultaneous team working on a rocket kiln, or roasting oven, of some kind. I don't know if either of those had cute names, but they were impressive anyway. Nate, Steve, Thekla, and Glen worked on those with great dedication in teams over two weeks.
|Kiln in progress|
|Kiln completed- |
(and what happens when you thicken plaster with extra clay.)
And Weston was building timber-framed skids for the future hot tub platform.
And Randall was welding custom parts for Peter and Tim.
|Why is this handle on fire?|
On a different note, the morning after the longest night of work and play at the end of the week, we saw sobering evidence of a near escape:
At some point in the past year, someone attached wooden supports to the temporary door, made of refractory insulation, on Peter's 8" batch box beastie (a big heater prototype from last year's event). With this temporary door propped in place, it has been run as the main shop-heater for most of the year.
This event involved a lot of extra shop hours, plus evening hangout time in a warm space, and it may have been run as much as 16 hours on that last, long day and night of work.
(Peter generally recommends stopping after 4 hours or so; the permanent designs have thermal mass to carry you through between firings.)
Someone left it running with the door slightly off-center, which exposed a screw to the full heat of the fire, which resulting in this dramatic charring of the makeshift handle, which was still smoldering the next morning.
Jocelyn and I were very, very glad that the crew had swept up most of the sawdust piles after Tim's comments about the fire danger of combined wood-working and metal-working shops (sparks from grinders and welders, plus sawdust; and sawdust fires are notorious for subtly smoldering a long time before they finally catch).
A sobering reminder that just because a prototype works for a while without incident, doesn't mean it's "safe." Fire is never safe - but it can be used safely. It's like Gandalf: the most dangerous thing in the room. The important question is not whether it is dangerous - of course it is - but whether it is friendly. Or in the case of fire, whether you have reached a mutual understanding.
The 8" beastie needs a permanent door, usually a $500 or so job for a good craftsman. It's not an innovation, just a completion detail. Failing that, it needs a better temporary door (with no wood involved).
Or does it?
Traditional ovens had wooden doors that were soaked in a pail of water, then used just for the baking cycle, then replaced when they charred beyond usefulness. But traditional ovens were not left in the hands of inattentive visitors, who are accustomed to self-tending stoves and furnaces. And they're traditionally outdoors, away from other things that can burn.
When firing a heater long hours into the night, would you expect a gathering of "pyronauts," no matter how tired or distracted they may be, to have the common sense to notice the door is on fire?
Would you have noticed?
It may never have burst into flame; the fire would have started as an oxygen-starved coal right around the metal screw, and could have continued undetected long after the fire was out.
Our token efforts at cleaning up flammable debris are the only defence I can raise that we ought to be trusted again with someone else's shop space. Luckily, in this case, that token defence was sufficient to prevent the fire spreading.
And ... and ... and ...
It was impossible to take it all in, nearly impossible to concentrate with all the interesting things going on in every corner, yet all these new things were made which clearly required great concentration.
Some worked perfectly as planned, a few prototypes were discarded unfinished or fell short of their original goals. Both failures and successes also impressed us with new lessons and new discoveries (both promising and dire).
I give full credit to Daniel for trying to get cameras on everything at once. He even had a flying camera drone, and his pilot's license to operate it.
(And that's what I think had caught Peter's attention, way back at the beginning. Either that or he is doing math again - which is often accompanied by a tremendously amusing ticker-tape "blooplooplooplooplooploo" sound effect from his mouth, which probably stops people interrupting him while he is working on the answer to their prior question.)