mass catering

[jono]

Hello everyone. First post full disclosure preamble: i built only one RMH, though it was my whole heat, hot water, cooking  and clothes drying facility in a very old static caravan for the colder half of a year and it performed unreasonably well, considering the cold, leaky external chimney i cobbled together from scrap metal.  It's a single rectangular bell with a 4mm mild steel cooking surface and vermiculite board lined riser & burn tunnel made out of almost 50 of those purple brics you get from electric storage heaters.

The design challenge i would like your advice on is: what is the best way to make wood fired catering equipment?

My first goal will be to feed 200 wedding guests from 2 100L pans on aprovecho institutional stove style things. Do people agree that the best way to hear a pot is to follow their formula for the pot skirt in a barrel with a chimney exiting the bottom and insulating that barrel with fibre glass? I have not found another design that looks cooperative, and i don't think i have the capacity to improve on it. However there are several reasons why i don't want to use their design for the combustion chamber: i want my twigs to be somewhat gravity feed (so in not convinced their horizontal feed is right for my needs) and i think it might be better to use some kind of cast light weight masonry (ideally, clay rich earth based) as opposed to steel (partly to save the cost of the expensive stainless, partly for ease of manufacturing, but mostly to get to the higher temps necessary for complete combustion). I hope i don't end up burning a hole in the bottom of my pan, but as long as the cook is stirring the lentils and the food is not burning, then i don't think the stainless could be overheating. So, i think my first step is to buy those big expensive pans, follow the instructional video linked below then make a big batch of different shaped combustion chamber from clay/ash/sawdust (square and rounded j and sock shapes, a symmetrical v shapes, maybe a batch box style venturi slot, etc) probably around 4" diameter, then test to see which heats water the quickest.

Does this seem like a good plan? Anyone got any suggestions about my commission chamber? I started thinking about having a single 8" batch box and baffles to balance heat between different pans/ovens/hotplates. But the potential for heat loss, the complexity, and control now led me to believe that each oven or pan should have is own little fire chamber

[Daryl]

The first step is to see what is approved by your local county health department. Some places won't care, other places will. That is where you should start the design.

[jono]

Ah, beurocratic tyrannies, yes. I'm in North Wales.

Well, to install in a barn on a farm and use it to cook for my wedding, i think i can safely ignore the government, likewise for any future private events and for taking the stove to festivals or to feed the troops at civil disobedience events. Having said that, i think it good that the stove will be cool to touch and have a chimney.

In the long run i would like to get a certified commercial kitchen running on these things. I dont think the food standards agency inspectors are going to be interested in the stove so long as it looks clean. If i were putting these things in on a build and wanted building control to sign off the project they would be very interested in the details of the surrounding installation, but i doubt they would have any interest in the internals of the stove. Maybe I'm wrong. I think it only really becomes an issue if i want to sell them as HETAS certified stoves, which i won't. Though i imagine if there is an issue it will be to do with the tenuous seel between the pot and the barrel, and gases that could leak out there.

[jono]

For anyone who doesn't know what I'm talking about, i want to build a cook stove like in the following video, except i want to redesign the combustion chamber

www.youtube.com/watch?v=buPtpr-0sqA

[jono]

I've been thinking, the aprovecho styly heat exchanger uses almost the opposite principle to the bell (strictly minimize CSA to maximize gas to surface contact. Is it better? Compare these two designs

[jono]

[satamax]

Well Jono, the aprovecho isn't too far from a bell either. You could consider the skirt around the pot, a heat riser extension. And the space around a bell too. Tho you know that in thoses directly fed "chambers" there's no stratification, unlike a bell.

Have you seen this series of videos?

www.youtube.com/watch?v=VdhLWMW7IXA

[jono]

I know what you mean, the aprovecho style heat exchanger does have some bell like things, such as the flue exiting the cavity from below the height of the fire. But that thing about giving space for stratification is what i was thinking about. If you had to bet a thousand pounds which of the above designs would perform better, which one would you pick and why?

I'd bet on aprovecho style, though not because I understand why it's better, but just because i know how much lab time they put into refining that design. If i didn't know that, and i saw those two drawings i expect i would apply bell theory and bet my ton on the bell style. So, i assume that the reason the above aprovecho style would work better than the above bell style is that neither increase the surface area of the pot, and without that the additional volume given for stratification does not compensate for the efficiency lost by not having that very close contact between pot and gas when the heat is most concentrated immediately after the commission chamber, and if that is right, then the general design principles that apply to hear exchangers where space is very limited are very different to the design principles that apply when space is not limited.

Yes, i really like that set of videos, especially for the fabrication methodology it conveys. I also spent a good amount of time reading through parts of this aprovecho research recource
www.aprovecho.org/lab/rad/rl
And more of my life than i should admit reading this forum.

[satamax]

Jono, a bell has to have the gases slowing down considerably for any stratification to happen. On your above drawing, it wouldn't work, the heat riser would act as a gas stirer.

I like the aprovecho. And i don't think it can be improved uppon much. Tho, you could argue that the mass's flywhell effect could be desireable for some type of cooking. But if you need fast heat, then slow heat, then fast again. The aprovecho is best.

[jono]

I agree with you about the aprovecho, i am not trying to improve on it, i want to understand why it works so that i can apply the principle to other applications (maybe an oven, or a flash boiler to run a steam engine or for on demand hot water)

Im not sure that you are entirely correct about the bell. I have good stratification of hat in my single bell RMH (the top row of bricks are piping hot when the bottom row is still cold) and my heart rise vents directly into a cavity that is only 800mm high, 800 long and 200 deep, and peter often use a bell where the riser vents directly into the centre of a barrel and figures hot gas streams up the middle whilst cool gas flows down the inside surface, and though this sounds like a lot of potential for turbulence, if wager the top of that tower of barrels gets substantially hotter than the bottom. Having said that, i agree that the gas flow of the above design does not look very well designed

[jono]

How about this?
My concern is that i am missing something obvious which means that i will be waisting my time if i try and use an insulated refractory j to power an aprovecho

[peterberg]

The institutional stove works wonders, I've built two of those, downscaled. The crux of such a stove is as follows. When the gases go over the skirt they heat it from the outside thereby losing very little heat from inside. When the temperature inside and outside the skirt would be the same, more heat is available to the pot. That's why the barrel itself is insulated.

You need a pot of aluminum or a triple skin stainless steel pot which have the same capacity to transport heat all around.

[jono]

Ah, i was thinking i should avoid aluminium because of its low melting temperature, it hadn't occurred to me but of course, its much higher thermal conductivity makes it a good choice for this purpose. I don't know what triple skin stainless steel is. Would this pot work? (If not I'll be very grateful as you will save me a lot of money and waisted time as that is what i was planning to get!)

powellbrewing.co.uk/index.php?route=product/product&path=61&product_id=52

[peterberg]

The pot won't melt, the contents of the pot (soup?) won't go over boiling temperature. Triple skin stainless is a technique to ramp up the conduction quality of the pot. Between two stanless layers there's an aluminum layer. I'd suspect the pot you are planned to buy only has this facility in the bottom of it. Get an all alu pot, that's what they have at Aprovecho and there's a reason for it.

[jono]

Peter, did you put the testo on your institutional stoves? Did you make any changes to the combustion chamber? Is there a reason why I probably shouldn't bother experimenting with small cast refractory downdraft j style combustion chambers in institutional stoves? Or maybe with a venturi slot kind of shape? I wonder because the institutional stoves seem to burn a little over 1100c, and i have this idea that if i could get one to run at 1200c+ with a bit more mixing and secondary burn length it might be a tad better, and cooking might be easier with gravity feed fuel