conic
WFO Team Player
Posts: 186
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Post by conic on Aug 8, 2012 19:36:09 GMT
As much as I am all for technology, the point has been missed to rediscover the ancient art of fires. I can getv the fire going with no mechanical or electrical assistance its called trial and error and is more satisfying than using my blowtorch. I start at the opening and start to push back and spread sideways slowly, now I also make sure there is no kindling etc at the back of the fire as it will just not burn, even when the oven is up to 500c I was amazed by that.
I then spread the fire across the base then move it to the right hand side and cook on the middle and left.
Conic
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Post by tonyb on Aug 13, 2012 18:05:49 GMT
I have similar experience of little oxygen getting to the back of the fire, its either mostly used up or the air currents create some sort of dead patch at the rear. From this observation, I now make sure that the fire is just back of centre.
I also start the fire just at the entrance to the dome and gradually move it back and spread it out but find the spreading out takes longer than expected, so I've experimented with making a fire and pushing it right of centre and then making a second fire and pushing it left of centre which does seem to speed up the firing process a little.
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Post by turkey on Aug 14, 2012 15:52:20 GMT
Turkey, I've seen that video (I believe) - is it the guy with the walking stick and cup of coffee? I was unsure as to why he had the door on there with the holes, I thought it was more of a safety/asthetic thing rather than for performance reasons. I might be wrong but I can't see how reducing the apperture for which air has to be sucked through will increase overall air flow, it will just mean it's moving quicker through the initial hole in the door would it not? that was the video, this image is a link to it.  I think the door means like a pipe where you blow on the fire the incoming air is directed at the heart of the fire, it forces the air to be low, and also makes the flue effectively a lot longer and easier for the gas to go up and no out the door. I suspect it is almost like a turbo on the car, as it burns better it sucks better which means it burns better etc.... so simple I think it could work  |
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Post by faz on Aug 14, 2012 16:36:46 GMT
The air travels faster as it has to pass through the narow gap. This will make the air reach further into the dome, and also further into the embers/logs, meaning that combustion will be faster. The increased turbulence will also improve combustion of the gases and smoke, helping to release more of the fire's energy in the oven. Though when you get to the stage where flames are shooting out of the top of the oven, you're losing a lot of energy!!
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Post by Fat Bob on Aug 14, 2012 18:33:43 GMT
That's a very amusing picture Turkey. Must be burning like blazes with fire coming out the chimney!
That base is beautifully built and the oven has been positioned well to blend in with the landscaped garden.
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Post by spinal on Aug 14, 2012 22:14:34 GMT
Also, correct me if I'm wrong - but the door appears to bisect the flue in two. This means that you have fresh air added to the very hot gasses in the chimney, which effectively would allow them to combust fully in the chimney.
This in turn would mean you get a cleaner burn - but you also waste all that extra heat sadly...
Same concept as a rocket stove or a woodgas stove (very popular in US and Italy... never seen one in the UK until today when I looked for one on ebay...) I may build one out of some cans and post pics here... could be useful as an oven heater/drier...
M.
EDIT: Was thinking - with some pipework bringing fresh air to the top / apex of the inside of a conventional WFO, we may be able to create both a second burn as well as harnessing the energy.... hmmm....
M
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Post by faz on Aug 15, 2012 8:05:29 GMT
These thoughts come from my experience of engineering of combustion in incinerators and combined heat and power plants, which are significantly more high tech than a WFO - so while I think the points are valid I do not necessarily think they are required for a WFO!! Introducing additional (overfire) air into the roof of the oven would be effective when the gases air hot enough to autoignite, so will improve overall efficiency once the the oven is up to temperature. Earlier on, the ability to deliver additional air to the rear or directly underneath the woodpile through a tuyere is likely to reduce the time taken for the fire to get well established and I imagine will reduce the period where the fire is smoky. One potential issue is that adding too much additional air will have a cooling effect and will reduce the draw in the chimney and probably also the total airflow into the oven, and could therefore ironically have the reverse effect to the one you are trying to achieve. A second potential issue is that introducing air in this way is likely to create hot spots, which may increase the potential for cracking or brick damage. All high tech stuff, and I have to admit that I tend to lean a bit more towards conic's views about the WFO not really needing much technical assistance, and simply getting a bit of practice on the best way to start and tend to the fire in your own individual WFO is the way forwards!! Afterall, we are trying to cook pizzas, not destroy hazardous waste or smelt metal  |
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Post by spinal on Aug 15, 2012 8:15:30 GMT
faz - interesting! A query from someone that comes from a totally different field and who's only knowledge of combustion comes from a few burnt eyebrows while lighting BBQs...
At what temperature would the gasses autoignite? Would the 60%-ish ratio of door be large enough to introduce air for this - so that when the oven is nice and hot, the gasses would autoignite anyhow inside the dome?
M.
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Post by faz on Aug 15, 2012 9:42:12 GMT
When you see the long flames travelling across the roof of your oven this is the gases given off by the wood combusting as they travel across the roof of the oven. As long as either the roof of the oven is hot enough to trigger autoignition (somewhere above 350 deg C based on a quick look on Wikipedia) OR there is a flame present, these gases will ignite. The easiest way to think of this is when you have large amounts of smoke coming off a wood fire, the gases are not getting hot enough (early stages of lighting the oven) or do not have enough oxygen to burn (poorly stacked fire). My feeling is that the door is plenty big enough to allow enough air in to the oven, the issue is whether it is able to penetrate deeply enough in to the fire, AND whether the hot gases have enough time to combust in the oven. I think this is why you'll get smoke from a fire located near the door, which then disappears or reduces when the fire is pushed to the back/side of the oven. The extra travel time in the hot environment of the oven allows for better combustion. The hotter the fire, the better the draw and greater the volume and speed of the airflow into the oven. This will lead to more efficient combustion once the oven warms up. But then we already knew that |
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Post by spinal on Aug 16, 2012 0:47:27 GMT
So would it help to have a set of curved pipes, each one roughly a semi-circle with a handle - like a sickle - to get air to the back of the oven?
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Post by faz on Aug 16, 2012 8:28:54 GMT
It might help early on in the lighting, but do you really want an incinerator with ugly pipes fitted in to it, or do you want a nice elegant simple pompeii oven?? If I built another oven I would increase the chimney height to improve the overall draw, but that would be it. I wouldn't try to make any technical improvements as I really don't think they're necessary.
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Post by turkey on Aug 16, 2012 9:51:17 GMT
bisects the flue in two? I think the top of the door has a slight lip to hold it in but what I think he is trying to achive with his primitive metal work (better than mine) is to seal the entry and just have that small air gap which is the same area as the flue to make is a 1 in 1 out system to really get the benefit of the draw. I saw rocket stoves prior to building my oven, and also saw a video where they had made a rocket stove effectively in the wood store under the oven venting into the oven, the idea being this will burn clean from the outset and the gasses will heat the oven to mean when they start the main fire the oven will be over the cold hump and be a lot less smokey. It was only my lack of space and metal work skills and tools that stopped me as I liked the idea, in practice i am not sure how long one of those stoves would take to warm the oven but living in a terrace smoke is a real issue, or will be if I keep producing it  |
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Post by faz on Aug 16, 2012 9:57:05 GMT
That's the same issue I have turkey, though I am finding that as my wood dries and I get a bit of practice firing the oven the issue with smoke is reducing greatly. I am not really looking for a technical fix, as I am sure I can resolve the majority of the problem with practice.
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Post by moorsy on Aug 16, 2012 11:43:44 GMT
100% agree with you faz. I have also found that dry wood and good fire management greatly reduce smoke. One mistake i was making was messing around with the fire too much, you have to set it up and let it do it's own thing, too much poking and moving logs only slows it up and causes more smoke.
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Post by turkey on Aug 16, 2012 14:48:13 GMT
do you guys go for small chunks of wood so they light easily or do you try and get up to decent size chunks asap to really make the fire large inside the oven?
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