In HVAC, it’s sometimes useful to calculate the velocity of air in the ducts. All central air conditioners, for example, require ductwork with the specified size of ducts and airflow (measured in CFM).
To calculate the speed of air in the ducts, we use a duct velocity calculation (you can find the calculator below).
Let’s first look at how you can calculate the air velocity in the ducts. We have to use this equation for airflow in restricted spaces (such as ducts):
V (air velocity) = Q (airflow) / A (duct cross-section)
V represents the air velocity and is expressed in FPM (feet per minute). Q is airflow and is expressed in CFM (cubic feet per minute). A is the duct cross-section; basically, the area of the ducts. The bigger the ducts, the more airflow they can handle.
In short, air velocity in the ducts is calculated by dividing airflow by duct cross-section.
Airflow is expressed as a simple number. Example: Air conditioner has a max. airflow of 600 CFM.
To adequately calculate the speed of air in ducts, we have to primarily calculate the cross-section of the duct. Shape-wise, ducts can be:
- Round. The area of round flex ducts is calculated as π*R2 where R is the radius of the duct. Example: 10-inch round duct has a radius of 5 inches. Its cross-section is calculated as 3.14*52 = 78.5 sq inches or 0.545 sq ft.
- Rectangular. The area of rectangular ducts is calculated as x*y where x and y and the length of the sides of the ducts. Example: 10×12-inch rectangular duct has a cross-section of 10*12 = 120 sq inches or 0.833 sq ft.
With this in mind, we can use these handy duct velocity calculators to accurately estimate how fast the air is moving in our ducts. Below you will find 2 calculators; 1st one for round duct and the 2nd one for rectangular ducts:
Air Velocity Calculator For Round Ducts
Example: Let’s say you have circular flex ducts with a 12-inch diameter. The airflow is 500 CFM. If you put both of these numbers in the duct velocity calculator above, you get a 636.94 FPM. That’s a little more than 7 mph (miles per hour).
Air Velocity Calculator For Rectangular Ducts
Example: Let’s say you have rectangular ducts with 12-inch height and 12-inch width. The airflow is 500 CFM. What is the air velocity in the ducts? If you input all these numbers in the calculator above, you get 500 FPM (that’s 5.7 mph).
You can use both of these calculators freely to help you design ductworks for central air conditioning systems, for example. If you need more help determining the airflow in the ducts, you can use these duct sizing charts for adequate ductwork sizing.