Fire + technology = better fire!
My name is Matthew and I am a pyromaniac
Like any sensible, well-adjusted human male, I love playing with fire. Perhaps it's some kind of inbuilt caveman instinct that harks back to the days when fire was the zenith of human technology. Perhaps men never really mature past the age of 10, and are forever as fascinated by the dancing flames as the countless children who huddle around bonfires every November, gleefully poking the embers with sticks (wooden sticks: that's another thing guys inexplicably love - but that's a different article). Grown men temper their pyromania with a veneer of civilisation, and act in a restrained manner around the bonfire only to sacrifice their dignity on the smouldering coals of the barbecue, where every summer they can be seen huddled round, beer in hand, proffering sage titbits of wisdom on how to get the bugger started, or why that's the wrong kind of charcoal, or why firelighters are better than bunched up bits of newspaper (or vice versa, of course), until one of them loses patience and gets out the petrol, and they all go home with full bellies, happy hearts and, if they're lucky, intact eyebrows.
Now when something burns it is simply reacting with the air. Give the fire more air and it will burn harder and hotter. So I decided to make a device for burning garden waste, cardboard, wood and any other combustible household waste in a constant stream of air. Read on...
The container
.jpg) | | This is where the fire happens |
Here we have a bog-standard galvanised steel garden incinerator. It's a bin with holes around in the bottom and a chimney in the lid. These work by drawing air through a fire, using the natural convection of the rising hot gas to draw fresh air in through the holes at the bottom. This makes them more efficient than, say, just arranging everything in a pile and lighting it. But we have our sights set higher.
.jpg) | | Alex dremelling a hole for the air inlet |
Alex cuts a 100mm round hole near the base of the incinerator using a rotary tool - also known as a "dremel", after a common manufacturer of high-speed multi-purpose rotary tools, even though there are many other manufacturers of such gadgets (like the way vacuum cleaners are referred to as "hoovers"). This is where the air will be blown in. Cutting holes in sheet metal using a rotary tool is botchy at best, but I don't have a 100mm holesaw and in any case this hole doesn't have to be neat - it just has to be a hole.
The air duct
.jpg) | | The duct, ready to be riveted to the incinerator |
The air is blown in through a 2 metre long section of steel duct, 100mm wide. The end has been cut into strips and splayed out, and holes drilled to accommodate the pop-rivets which will secure the duct to the main container. Alex can be seen in the background, somewhat compromising my assertion that men don't think of almost everything as a symbolic phallus.
.jpg) | | Drilling holes into the incinerator for the pop rivets |
Marking out, measuring and precision work have no place here. The duct end is offered up to the hole in the incinerator, holes are drilled through and rivets put in place. Airtightness is neither possible nor necessary.
.jpg) | | The world's biggest pipe? |
Having assembled the device we were struck by its resemblance to a pipe, and considered smoking it. But it was a bit early in the day, and in any case we both prefer cigars. Also, as you can see, it's a little unwieldy, and not easy to get one's mouth round a (rather sharp) 100mm duct.
The finished furnace
.jpg) | | Assembled and ready for use |
So here's the finished article, ready to run. The air blower is an Axminster AWDE5002 coarse dust extractor, which is basically just a centrifugal blower. They blow just as well as they suck, so in this case I've hooked the outlet of the blower up to the duct. This will blow air into the furnace at 500m³/hr. This is fast enough that it tries to lift the lid off the incinerator, despite the fact that air can escape out of the chimney.
On a side note, the AWDE5002 is a curious device. It's clear from the very fast startup and the ugly click-whine when powered down that this thing uses a cap start/cap run (or possibly cap-start/induction-run) motor, with a centrifugal switch to turn off the startup cap when the motor gets up to speed. Surely in an air blower, of all things, this is totally unnecessary? There is zero startup torque in an air blower, after all. Surely a permanent split cap motor is ideal for this application, being quieter, more reliable and with lower startup surge current? Well, anyway, if anyone can enlighten me on this, please do.
Starting the fire
.jpg) | | Getting things going - the fan isn't turned on yet |
We started things off in the obvious way - shove all the cardboard boxes the stuff was delivered in into the incinerator, dump meths all over it (impatient, see) and apply a flame. Before long the cardboard is burning away nicely, if somewhat smokily. Hmm, if only there was a way of getting more air to it...
CHRIST ALMIGHTY
.jpg) | | Embers are forced out of the exhausts by the air pressure |
The first time we turned the power on, the combination of air pressure from the fan and expanding gas from the suddenly-fed fire blew the lid clean off the incinerator, and what can only be described as a pillar of solid fire rose up into the night sky, followed a fraction of a second later by a massive wump of glowing embers, sparks and ash, which spread out and settled across both adjoining gardens as well as my own. You know the good bit at the end of Raiders of the Lost Ark? It was not unlike that. So you can see here a massive block of lead sat atop the chimney, holding the lid down. This is why sparks are being forced out from under the lid and down the sides.
.jpg) | | Remember those air inlet holes in the base of the incinerator? Well they're not inlets any more! |
Turning the fan on produces these spectacular plumes of flame out of the holes in the incinerator. It also flings sparks everywhere, although this depends on the material being burnt - wood doesn't spark much but cardboard produces tons of bright, light embers which flood out like crazy. Even with the bright light from the flames, the steel incinerator walls can be seen glowing red hot here. It's also incredibly loud - the combination of motor spinning and air blowing and fire roaring and fuel crackling is like being stuck between a rocket launch and a busy airport. A treat for all the senses!
.jpg) | | A particularly violent blast spews yellow-hot flames and sparks from the bin at a terrifying rate |
A "pure cardboard" run. Cardboard produces the most impressive sparks and embers. It looks like I'm at a safe distance, doesn't it? It didn't feel safe.
.jpg) | | After being blown for a while, the fire momentarily becomes absurdly bright |
If the incinerator burns without forced air, it burns relatively calmly. If you blow air in, it burns violently but the flames aren't actually very bright. This is because the combustion is quite "clean", due to the availability of oxygen, and that produces "roaring blue" flames, instead of "smoky yellow" ones (ahh, memories - remember being taught how to use bunsen burners?) But when you turn off the blower, all of a sudden the fire is once again oxygen-starved but it's at a very high temperature. The result is a quick burst of incredibly bright yellow flames, as seen here.
.jpg) | | More very bright post-blow flames |
Another shot of the very bright fire that follows an air blast. In this picture and the one above, the big lead brick holding the lid on is clearly visible. Incredibly, it never even became warm to the touch.
.jpg) | | I have no idea what caused these patterns to form inside the incinerator, but they sure are purty |
After one of the air blasts we took off the lid to see what was left, and we noticed these crazy patterns on the inside of the bin. I have no idea what causes them - something about the way the galvanising is applied, perhaps? If anyone knows the answer I'd love to hear it.
.jpg) | | The poor thing has earned a night's rest |
After a few hours of hard use, the incinerator is no longer shiny. It is covered in a white powdery coating of what I can only assume is zinc oxide. Unbelievably, the aluminium pop rivets which secure the duct to the incinerator have held fast; only one melted. In this shot the fan is turned off and the incinerator, although clearly glowing cherry red, is calmly and steadily burning what's left of the night's fuel supply. Incidentally, we burned an enormous mountain of cardboard and scrap wood in a tiny fraction of the time it would have taken the old-fashioned way. A less powerful (i.e. less loud) fan would genuinely make this a very efficient, clean, neighbour-friendly way of burning waste.
Videos
Firstly an apology: these videos are shocking quality because they were shot on the only video-capture device I have - my mobile phone. But not to worry, as soon as I get a decent video camera (i.e. when I next alleviate my boredom at work with online shopping), I shall be putting high-quality videos up.
Here is a video of a blast shot by Alex (you can hear him maniacally cackling in the background); and here is another. In both cases the bin glows purple - that's due to the way extreme levels of infra-red radiation affect digital sensors which aren't properly shielded. In real life it was glowing red/orange.
So - successful experiment! Many thanks to Alex, who spent his weekend with me cutting, filing, hammering, drilling and riveting this device into shape, and of course risking life and limb merely by being present at the great switching-on. The tricky bit is: what can we do next that's even more insane...
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