How do I size the combustion air intake for a custom made barbeque pit?
The pit was a 250 gallon propane tank. It is 30" in diameter and approximately 8 feet long. In most cases I will use about 30 lbs of charcoal and the charcoal bed dimension is 18" X 92" and about 2" deep. I am planning on putting (2) 4" pipe stacks for exhaust which will give about 32 square inches of exhaust area. I haven't put the exhaust stacks in yet so I'm still flexible and can change the size if necessary.
Unless you are using 4" nominal, like conduit, you need to check your calcs, because 4" actual in two pipes gives 25.2" by my thoughts. 4" EMT is 4.334" ID giving just over 29" area in two. http://www.home4c.com/conduit.htm (down the page)
The suggestion of oversized intakes with dampers is a requirement since you want to be able to shut them down to reduce the fire.
By the way, is it necessary to say that you have to be extremely careful in going into the tank to cut it? Like fill it with water and drain to clear out any propane vapor.