Good morning everyone... sorry its been so long since my last post.
William asked me how it all went in the end...
Stephen, how did this oven turn out?
Pretty damn well is the answer... See figure 1 at the end. The batch rocket works well with no smoke except when lighting - the smoke only lasts a short while before the batch rocket really starts to roar and then all smoke disappears...
I've now had it running now around 10 times... I've been learning how it performs:
Performance
The parrilla works a treat with a very hot zone above the batch rocket which then reduces in heat towards the front where it is not uncomfortable to stand. I tend to put pots on the super hot part for boiling things. In the middle I sear steak/tuna, at the front I cook eggs etc. See Figure 2. This redirects the exhaust gases from the burn tube around to the front of the hot plate before they pass down the sides and then finally up the chimney. There are baffles in the parrilla to make this work.
The pizza oven works well. I load it with one batch to get up to heat, then start cooking pizza on the second load. The door remains closed except when I remove or put in new pizza. Each pizza takes about 10 minutes with a turn half way through. i can cook 2 pizzas at a time. See Figure 4
For a 2 pizza cook and a dog-food cook-up (I have a very lucky dog)... I boil up 2 chickens in a pot with rice and veggies... which creates about 15L of stew... this uses 2 batches of wood
When hosting a large group of people (say 12 guests x 4 pizzas plus steaks etc on parrilla) I get through maybe 4 batches of wood... say 60L.
With a ventilation fan going everyone can sit in the outdoor kitchen without it being uncomfortable - if this was in the UK, I would guess that you'd not need the fan
The front and tops of the oven get warm to the touch but nothing more than maybe skin temperature
In terms of thoughts and modifications,
I suspect that my secondary air pipes will need replacement within a year or two. See Figure 5
Keeping the heat in is critical to pizza temperatures, making a better seal would be a sensible task
Finding a way to transmit/store more of the heat into the pizza chamber might be a good idea perhaps with an extra turn in the burn tube before it exits the pizza chamber
Having thicker insulation under the floor might be a good idea. Presently it has 4" of EPS then perlite concrete base then some loose fill.
Finding a low cost supplier of bulk perlite is critical... just buying from the garden centre is way too expensive
Tasks still to do:
Put handle and insulation on "secondary" batch rocket door
Find an oven thermometer that works
Cast a chimney for the back of the parilla
Comment on permie member concerns:
Getting to a high enough temperature: I don't have temperature data yet, but the fact that the pizza is delicious is the best data point that I could have. The objective was to get the pizza oven up to temperature. Rather than make it a dirty black oven I was hoping I could use other design concepts to reach that objective. These concepts were: (i) insulating the hell out of all parts of the oven (ii) trying to minimise any thermal bridging (iii) minimise hot air leaks from the pizza oven (iv) allow the hottest "exterior" parts of the batch rocket to heat the air in the oven (this concept arose after some advice from Fox James ... so many thanks to him). To be clear on that last point the hottest part of the batch oven is at the back, sides and top of the burn tube. These parts are exposed to air that can freely circulate up into the pizza oven. The batch rocket itself sits on 2 thermal insulating bricks. The front of the batch rocket and underneath is partially blocked off with loose perlite. So in a thermal sense the batch rocket kind of sits inside the pizza oven.
Losing too much heat up the burn tube: By swapping out a perlite insulating tube for a piece of well pipe the hottest gases coming out of the batch rocket now passes some of their heat into this thick cast iron tube which sits inside the pizza oven. At the top of this tube is an insulating piece of perlite to minimise heat loss via conduction from this tube up to the parilla. I figured the parilla would get hot enough with just the exhaust gases.
I will post more pictures and provide temperature data as soon as I have some.
One other point... here in Costa Rica, temperatures don't drop below 20 degrees ever. Even the ground temperature stays above that. I'm wondering whether this might play a role in enabling this oven to work so well.
All the best
Stephen
