mass catering

[jono]

Above shows the overall layout. bellow you can see the small shelf I made to create a venturi vacuum to draw in the secondary air

these pictures show the shape of the port


sealed up with clay


all wraped up in ceramic wool with a fire in


and here are some pics of the double vortex, first one is right at the beggining of the burn, the second image shows it once it has got going a bit




Though there are a lot of details that need to be tuned with this design, one thing was clear, it is very resistant to stalling with a restrictive heat exchanger. holding the steel plate about 1cm above the riser had no noticable effect on the combustion, it only started to struggle when I squeezed it down to about 5mm. other things i like about this design are that the angled floor of the fire box make it self feeding, though the fuel load is quite small. One of the problems was that it did start to burn the wrong way out of the fire box, though not very much and only towards the end of the burn. if you look at the first picture that shows the basic layout, it did supprizingly well. next I'll reduce the firebox opening by changing the shape and location of the secondary air feed tunnel, get some barrels for the stove at the weekend, and get myself a flue gas analiser

[jono]

should I put my secondary air in at the top or bottom of the port?





I think the design goal is to have a small responsive fire that burns well at a range of different temperatures. After I have stoked the thing to the max for 20 mins to bring 70L of food to boil, and I want to simmer it for an hour. when I put the minimum amounts of wood on the embers, I think Ill want to close the primary air down and leave the secondary air open. My instinct is that the bottom fed secondary air version would burn cleaner than the top fed when its closed down like that.

my friend started calling it a v tube

[jono]

I got a testo 305 for £20 on ebay, and some old barrels being delivered soon

[jono]

Here is my pan in a barrel. I dont think I'm going to fit a pot skirt in their! Oh well. Bellow is the internal shape and the external shape of my mold, made from all sorts of rubbish. I think I will put my internal mold shape in a black plastic bag then suck the air out with my hover, to keep the moisture off the card board. Right at the bottom are some details of my internal mold shape. I spent a good long time on the details of that shape. I'm going to the scrap yard tomorrow morning to see if they have a good long chimney pipe that is 80 to 100mm diameter, like a lamp post, or maybe an old iron drain pipe with a 90 elbow. That riser is almost 80mm (its not long enough yet, I know). I hope I'm right in assuming that my chimney should be the same diameter, or a little larger than my riser, which is 3.5", so I'm going for 3.5-4".

[jono]

oh. and my cheapo testo has a falty o2 sensor, and I'm not sure if I can find a replacement or fix it somehow.

[jono]

Oh, my plan is to cast it all in the barrel, with support rods in place. the cardboard tube that is going to define the feed pipe has a hole cut for it in the side of the barrel, and there will be a variety of rods and bits of metal that will connect the refractory shape to the barrel in place. the whole thing will be on a board on bits of wood, then I will hit that board with my hammer drill to help vibrate things into place a bit, though I am also going to fill and assemble the mold in layers, so the vibration is for detail, not for moving large amounts of material round corners. I'f I'm really lucky, this level of effort will be just adequate. If not, I got a whole bunch of people who are expecting a wood fired cooker to disapoint on satarday. feeling the pressure.

If anyone could comment on my chimney issue, same as riser? or 5" like a standard instove. I think same as riser, or somewhere inbetween. Help

[jono]

I decided on a 4" chimney. those pipes are just shy of 2m, heavy gauge, and cost £4 from the local scrap yard. Poor racing bike

[jono]

thats the mold full of refractory. I dont think Ill sleep tonight for the excitement of demolding in the morning

[jono]

The cast refractory came out a treat, all my chimney parts are cut and prepped for welding, I got all my ingredience, but its dark, getting later, and im waiting on a guy from ebay to deliver my new welder, so either my welder wont turn up in time, or my refractory will explode when i'm cooking tomorrow, or it will all work out great. this is all very exciting for me. is anyone reading this?

the refractory is quite soft and damp feeling still. I hope its going to be ok when I fire it up tomorrow morning. I think Ill give it some gentle burns tonight to hurry it up a bit. How long should it take? 

a replacement o2 sensor costs £50. might have to get one of those soon.

[jono]

the event went surprisingly. The combustion chamber was not drawing as well as i would like for the first hour of use, though I think this was just purging moisture from the concrete.

 

[jody]

Looks great jono, have you got an image of what it looks like on the inside of the combustion chamber?

[jono]

sorry jody, ill get a proper report on the whole process soon, i just need to find the time to finish some  details and document it properly

[esbjornaneer]

For sure there are people reading it, a shame you did not get any answers from the heavy weight contributors at the end... It will be great to hear how it all worked out in the end. I take it that it worked OK after the first hour.

[jono]

great timing because as you sent that message, the stove was on its second voyage feeding the troops of the london climate march.

so, the first firing (last month) was a gradual transition from working terribly to working acceptably by the end of the day. as I said, i think it was about the refractory concrete curing process requiring heat to purge the moisture, which sapped energy from the fire. after the first firing there was a really clear line on the refractory about half way down the fire box that marked the transition from the material that had been purged of residual moisture to material that had not. to prep for last nights event, i put the combustion chamber on the barbecue and built a fire beneath it and in it at the same time. this worked perfectly.

Yesterday the fire worked spectacularly well. I have no idea whether or not it would produce a stellar emissions graph (though I would love to get a flue gas analyzer on it) but attaining smoke free combustion was very quick on start up (maybe 30 seconds, using a petroleum firelighter and a handful of twigs), smoke free combustion was very reliable even using damp twigs collected in the park, all this is very important when wheeling the thing around in a dense crowd of people (oh the liabilities, boiling hot food, fire, and a huge weight of metal on wheels, not to mention food hygiene!). The temperature control I wanted is there too, i could occasionally add a couple of larger twigs (less surface area for more fuel) and maintain a simmering temperature, or bring the 50L+ of soup up to a vigorous (but not excessively so) boil by stuffing the combustion chamber full of small twigs, and though keeping it burning hot requires very regular refueling, when neglected it will hold a bed of embers from which it will quickly relight for ages, like maybe over an hour. I started the soup by bringing 5L of veg oil up to temp and deep fat frying pearl barley, potatoes and lentils. so, 80mm diameter riser seems like a very good size combustion chamber for my 77L pot.

Overall, I'm rely pleased with the functionality of the thing, though I would really like to get an analyzer so that i can hone the geometry to get it working at its very best. also, because the diameter of my pot is only about 5cm less than the diameter of the barrel, i don't have enough room to fit a pot skirt between the pot and barrel, so I have not really been able to test the resistance to stalling by a restrictive heat exchanger cavity, which was one of the main design goals. I might test it actually, ill end up with pretty much a 1cm gap all round, but i doubt this will be enough.

One other thing that i think will be of interest to other builders is how simple my mold making technique was, which worked fine. you just need to make your shape out of cardboard and duct tape dense enough to resist compression, and if i were to do it again i would try and make the blocks of card board that fill it to be a bit smaller, as this would make it easier to pull them out, but if time and money are limited and you just want to get a novel shape made up in a hurry, then I would say don't let the need for perfection stop you. that mess of whiskey tubes, cardboard, duck tape, glue and silicone pictured above worked out great for me.



That's the best picture I have of my setup yesterday. I'll upload some details of the combustion chamber when i get home.

[jono]

of the topic of rocket stoves, that trailer i made totally failed. I pushed it for ages with 2 flat tires before one of the wheels completely collapsed half way between Trafalgar square and the houses of parliament (within sight of the finish line!). So that is the kind of thing that might happen if you are as cheap and slap dash as me. still, we fed a huge number of people and had an excellent time.