Thanks for taking time to respond to my post. I hope I'm not being a pest, but I really want to understand this notion of running the chimney back over the top of the oven. I'm still dubious about its value, but am willing to be convinced otherwise and to, perhaps, modify my oven accordingly. This debate has already taught me a great deal by causing me to think more deeply about these matters.
I have no problem accepting the fact that a hot chimney results in better combustion. But facts do not always represents important considerations. If you don't mind, I will explore this fact. The reason this is a fact is pretty easy to understand. The walls of the chimney, which are heated by conduction from the dome of the oven, conduct that heat to the air transitting the chimney. This raises the temperature of that air causing more rapid convection out of the chimney. This draws used, hot air out of the oven which in turn draws more fresh air into the fire resulting in better combustion. Si?
But this is only a fact if the walls of the chimney are warmer than the air passing through the chimney. If they are the same temperature, no benefit is derived. If the walls are cooler than the passing air, it actually cools down the air and slows down convection up the chimney which is detrimental to the combustion. Clearly, the hotter the walls of the chimney are in relation to the air, the more the benefit.
So the big question here is what is that temperature differential and what differential is needed for there to be a demonstrable benefit. I'm going to do a little experiment next time I fire up my oven (Friday, I hope). I have a thermometer with a dial gauge embedded in the dome of my oven, about one inch inside the brick. The temperature there will give an approximation of the temp of the wall of a chimney that is routed back over the dome. Then I will measure the temperature of the air just as it enters the chimney. Unfortunately this isn't so easy. My thermocouples (type T) have a maximum range of
662F. But I have a friend who might be able to lend me some Type J's which go up 1382F. Although my oven isn't a true Neapolitan pizza oven, these two temps should be interesting. At the very least, they will help me see if rerouting the chimney back over the dome would be of any benefit. Stay tuned.
If you are willing, I would appreciate your comments on the fact the chimney length is a very important consideration in the amount of draft (sucking) applied by the chimney. There are formulas that demonstrate this fact, but perhaps it is easier to imagine the following: a chimney that is only 1cm long. It is easy to see that air moving such a short distance before being released into the open produces very little draft. Then imagine a chimney with 1m of length. It should be intuitively obvious that there is much more air moving through the chimney, drawing in more air behind it. And then imagine a chimney twice that length - with twice the volume of air moving through it there would propotional increase in draft force. Right?
Anyway, sorry to have gone for so long. It would be a great privilege if we are back in Naples at the same time to visit the pizzeria to which you refer, especially if their pizza is any good