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Unread 01-29-2013, 07:31 PM   #46
Bbq Bubba
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Quote:
Originally Posted by landarc View Post
A note about the nature of intake, exhaust, drafting and how it all balances...

In any system with a passive flow of air, that is to say where there is nothing either pushing air in, or pushing air out, the balance of the intake and exhaust in going to naturally balance. In a cooker, this relies upon the basic principal of heated air rising. Thus, if there are four one inch holes as exhaust, and one four inch hole as intake, the rather complex calculations for balancing these areas of flow do not apply. And no, it is not as simple as calculating the area of four little circles versus one large circle.

Rather directly, the chamber will exchange air only as rapidly as it exhausts air first. Other wise, pressure must build up in the chamber, which it clearly cannot in a PBC or most any other cooker. Thus, the amount of air in the cooker is controlled by how much air can escape the four exhaust holes. The intake could be any size, it will only draft as much as it can exhaust. Subtle pressure variations can occur at both locations based upon heat, as hotter air rises faster. In a way, the PBC seems to be self-regulating. Like most of our cookers. The biggest difference is that the air moves over the meat better in a PBC, due to the meat hanging.

I would add, this does not apply quite the same in an offset, as if the exhaust does not draft properly, the air will not move across the chamber correctly. This is why when designing an offset, the calculations should be done. Or you slap a 6" exhaust on it, like a Jambo, and let that control intake, which is why a Jambo is controlled from the exhaust and not the intake grate. Or so it said at the Holiday Inn I stayed at once,
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