first build need some help

I didnt do the math, but 27' wide by 9" tall half moon sounds about right for that size smoker, why dont you make the firebox the same width as the cooking chamber ?

I am not sure what pit calculator you are using, but if it's the one at http://www.feldoncentral.com/bbqcalculator.html I am puzzled too. Taking your firebox dimensions as input does indeed give a half moon diameter of 21.15 inches from the calculator.

 

Crunching the math gives an answer close to a 27.5 inch wide by 9 inch tall half moon. For instance, from http://en.wikipedia.org/wiki/Circular_segment we get some nice formulas.

 

Assuming a 9 inch half moon height, h, and your cooking chamber radius, R, of 15 inches we would get an circle sector angle, phi, corresponding to

phi = 2*ARCCOS((15-9)/15) = 2.31855...

 

This would give the circular segment (half moon) area, A, corresponding to

A = R*R*(phi - sin(phi) ) / 2 = 178.5 sq inches

 

With a 9 inch half moon height you will indeed get 27.5 inch half moon diameter, c, corresponding to:

c = 2*R*sin(phi/2) = 27.5

 

The FB to cooking chamber opening should be approx 0.008 times the firebox volume. With your firebox dimensions 28*28*28*0.008 = 175.62, which is the same as the calculator. I based my RFS on the 0.008 factor and it works nice.

 

Cheers /Wes

Dave posted this link on another post. I have not figured it out yet, but you should be able to figure the area of a partial circle. I hope it helps.


http://www.1728.org/circsect.htm

using a tank with the round end, it will be easier to cut the tank if your firebox is as wide or more than the cooking chamber( less angles to cut) 

 

Also , I think it looks better, gives it a better blend into the tank.  Give the firebox plenty of height as well, by the time you install the fire grate and leave plenty of space below it for an ash pan, you dont want the flames to be hitting the top of the box. Id just go 30x30x30 and you should have no problems.

 

Remember, on the pit calculator, those are minimum requirements, going bigger hurts absolutely nothing. Just dont go smaller. You will be controlling the air and heat from your firebox vents, so focus on making them function good.

It should not have increased the hole size. The cook chamber is the same size and still only needs the same amount of air.

 

The top of the firbox is usually just a little higher than 1/3 the height of the tank, I suggest a 2nd plate 1/2  inch from the top of the firebox to help from getting a hot spot at the beginning of the reverse flow plate. So go 3/4" higher than that, draw your line and measure it out. You should be just about right sq. inches.

 

With 30 inch cook chamber, bottom of door should be 16" from bottom of tank. so thats putting your rack at 16 inches also, you dont need more than 4" between the bottom of the rack and the reverse flow plate, so that will still give you a half moon of 11'x 30" just about.( my mind is calculating thickness of material in there as well so dont critic my math, just my spelling, ok?)

if you can get a drawing of the dia of your tank size, say 30" circle....draw a line where you think the top of square fire box might go....then use tracing paper with 1" square lines on it....count all the full 1" squares that cover your opening you have drawn....guess at areas that are half covered with a 1" square and double how many for one side to match/mate the other side....easy basic square inch coverage....

Here is a link to the circle calculator.  It is not too difficult to figure out and it will allow you to determine the size and height of the opening under the RF plate.  Assume that the area under it should be 1.5 times the FB opening and you should be golden and have plenty of room above the plate.  Also I prefer to keep the firebox as low as possible while maintaining the correct opening size, but it is just a preference.  

 

http://www.1728.org/circsect.htm