Calculating the Flowrate Requirement for a
Pipe
The velocity of the air leaving the mouth is a function of the pressure only.
This may seem counterintuitive at first because everyone knows that when you
knozzle the end of a hose by reducing the area of the exit hole, you increase
the velocity of the water. This is true but what you're actually doing is
increasing the pressure when you knozzle something. The velocity is calculated
by the following equation:
V = [2 * P / Ρ]^0.5
-
V is the velocity of the air leaving the mouth
-
Ρ is the density of air
-
P is the pressure
The flowrate Q is equal to the area times the velocity:
Q = A * V
Where the area is the internal width of the pipe times the windsheet thickness:
A = IW * WST
Put it all together and you get:
Q = IW * WST * [2 * P / ρ]^0.5
Assuming the density of air is 1.2 kg/m3 and adding a factor for units
gives you:
Q = 27.86 * IW * WST * [P^0.5]
The equation assumes that the pipe is unrestricted at the foot.
Here's an example. My CCC Soubasse pipe has an internal width of 4.724, the
windsheet thickness is 0.060 and the pressure is 2 inches of water.
Q = 27.86 * 4.724 * 0.060 * [2^0.5]
Q = 11.2 CFM