Determine the actual gas flow rate, expressed in ACFM (actual cubic feet per minute), and the diameter of the stack's opening.
Calculate the area of the stack's cross section, at the opening. As an example, suppose the diameter of the opening is 6 feet, and pi is a known constant at 3.14. You would calculate the area as follows:
area = pi * radius * radius
And since:
diameter = 2 * radius
Then:
radius = diameter / 2
Therefore:
area = pi * (diameter / 2) * (diameter / 2)
Now, we'll repeat the calculation with known variables:
area = 3.14 * (6 / 2) * (6 / 2)
Therefore:
stack area = 28.3 square feet
Calculate the stack exit velocity using the formula:
velocity = (actual gas flow rate) / (stack area)
Based on the previous example, assume the actual gas flow rate is 60,000 ACFM:
velocity = 60,000 / 28.3
Therefore:
stack exit velocity = 2,120 feet per minute