Post by hpmer on Sept 20, 2008 16:01:52 GMT -8
Donkey,
I thought your post on how to calculate the gap was VERY helpful, but I had a devil of a time finding it again when I needed it. Here it is again:
"You can figure out if the gap is enough by doing the math.. Normally I like to avoid the numbers but here it's just too helpful.
I'll have to make some assumptions to do this myself for your system, sight unseen.. Here goes:
The 55 gallon barrels that I've got in my yard here are 22" diameter inside dimension.. You are saying that there is a 1.5" gap (all around??) between the large barrel and the small.. This indicates that the inner barrel is 19" diameter, outside dimension..
First figure out the cross sectional area of the two barrels, inside dimension of the large one, outside of the small..
The formula for figuring the area of a circle is PI times Radius Squared.. Or, put simply, the radius (half the diameter) times itself, times 3.14..
Large barrel = 22" diameter = 11" radius.. 11 x 11 = 121 x 3.14 = 379.94 square inches.
Same operation on the small barrel gives 283.38 inches square.
Subtract the inner volume from the outer to tell us the area of the channel made.. The area I get is about 96.56 square inches.
An eight inch stove pipe has an area of 50.24 square inches.. Looks to me like you've got enough and then some to spare.
I forgot to mention...
Steel barrels have those ridges running around them to add strength.. You should measure the inside barrel at its WIDEST POINT, and the outside barrel at its NARROWEST POINT..
The place where I have messed up in the past is in the gap above the heat riser, between it and the barrel.
The best (and least math-y) way to get it right (IMHO) is fire up the stove, place the barrel over it and move it up and down till you like it.
Otherwise you have to visualize an invisible cylinder over the top of the riser, as wide as the opening of the riser (8") and as tall as the gap above.. make sure that the imaginary surface area of this cylinder is equal to or greater than that of your size choice.. For an 8" system that would be 50.24 sq. inches.
To figure for the size of the cylinder you multiply circumference of the riser times the height of the gap above..
Or, actually a better way to say that would be, to figure out minimum gap size divide the circumference by the minimum area..
So:
The circumference of an 8" pipe is PI times Diameter, or 3.14 times 8 = 25.12"
Divide the area of our system size by the circumference, 50.24 / 25.12 = 2 inches.
For 8" systems, you need a gap above of no less than 2 inches."
I thought your post on how to calculate the gap was VERY helpful, but I had a devil of a time finding it again when I needed it. Here it is again:
"You can figure out if the gap is enough by doing the math.. Normally I like to avoid the numbers but here it's just too helpful.
I'll have to make some assumptions to do this myself for your system, sight unseen.. Here goes:
The 55 gallon barrels that I've got in my yard here are 22" diameter inside dimension.. You are saying that there is a 1.5" gap (all around??) between the large barrel and the small.. This indicates that the inner barrel is 19" diameter, outside dimension..
First figure out the cross sectional area of the two barrels, inside dimension of the large one, outside of the small..
The formula for figuring the area of a circle is PI times Radius Squared.. Or, put simply, the radius (half the diameter) times itself, times 3.14..
Large barrel = 22" diameter = 11" radius.. 11 x 11 = 121 x 3.14 = 379.94 square inches.
Same operation on the small barrel gives 283.38 inches square.
Subtract the inner volume from the outer to tell us the area of the channel made.. The area I get is about 96.56 square inches.
An eight inch stove pipe has an area of 50.24 square inches.. Looks to me like you've got enough and then some to spare.
I forgot to mention...
Steel barrels have those ridges running around them to add strength.. You should measure the inside barrel at its WIDEST POINT, and the outside barrel at its NARROWEST POINT..
The place where I have messed up in the past is in the gap above the heat riser, between it and the barrel.
The best (and least math-y) way to get it right (IMHO) is fire up the stove, place the barrel over it and move it up and down till you like it.
Otherwise you have to visualize an invisible cylinder over the top of the riser, as wide as the opening of the riser (8") and as tall as the gap above.. make sure that the imaginary surface area of this cylinder is equal to or greater than that of your size choice.. For an 8" system that would be 50.24 sq. inches.
To figure for the size of the cylinder you multiply circumference of the riser times the height of the gap above..
Or, actually a better way to say that would be, to figure out minimum gap size divide the circumference by the minimum area..
So:
The circumference of an 8" pipe is PI times Diameter, or 3.14 times 8 = 25.12"
Divide the area of our system size by the circumference, 50.24 / 25.12 = 2 inches.
For 8" systems, you need a gap above of no less than 2 inches."