To calculate duct sizes use the equation Quantity = Area X Velocity. By calculating duct sizes with this equation it is possible to change duct sizes and maintain the same quantity (CFM) and velocity (FPM) in the duct.
However, the pressure loss changes each time the duct size changes. The equation Q = A X V does not take this into account. The duct designer needs to know what the total pressure loss is for a duct run in order to select the proper size fan. The static pressure at the fan outlet must be equal to the resistance of the duct system.
Using Q = A X V to calculate duct size changes with each change in CFM maintains the same velocity, but the friction loss for each size will not remain the same. This is not to say that the equation Q = A X V is not an important equation. Understanding the relationships of this equation is essential to understanding airflow in duct.
The industry has generally adopted the equal friction loss method of sizing duct. This gives the equivalent duct size based upon maintaining the same friction loss. The equal friction loss method varies the velocity but maintains the same friction loss per 100 feet of duct run. By maintaining the same friction loss per 100 feet, it is only necessary to determine the total length of the duct run to determine the total friction loss.
Understanding aspect ratio is important for the equal friction loss method. Choosing the best aspect ratio of a duct can reduce friction loss.
Friction loss in a duct is the result of the air molecules rubbing against the inside of the duct. For the same air quantity and velocity, a duct with a greater surface for the air to rub against will develop more friction loss. This means that if the quantity of air and the area of the duct remain the same:
The greater the perimeter (distance around) of a duct, the more friction loss there will be.
Aspect ratio is a way to determine the best practical perimeter for a duct. Aspect ratio is the ratio between the width and height of a duct. Divide the width by the height to find the first number of the aspect ratio:
Aspect ratio = width divided by height
A square duct has an aspect ratio of 1 to 1:
Aspect ratio = 12” divided by 12” = 1 divided by 1
Aspect ratio = 1:1
A 24” X 8” duct has an aspect ratio of 3 to 1:
Aspect ratio = 24” divided by 8” = 3 divided by 1
Aspect ratio = 3:1
A 30” X 12” duct has an aspect ratio of 2.5 to 1:
Aspect ratio = 30” X 12” = 2.5 divided by 1
Aspect ratio = 2.5:1
Figure 1 compares the aspect ratios of different size ducts that have nearly the same area (about 200 sq. ins.) but increasing perimeters. (Actual duct is sized in even numbers. Odd numbers are used in Fig. 1 in order to maintain about the same area.) note that the aspect ratio increases as the length of the perimeter increases. Notice also that the round pipe has the shortest perimeter and therefore the least friction loss for a given CFM and velocity.
Size Area (sq. in.) Perimeter Aspect ratio
16” dia. 201 50” NA
14” X 14” 196 56” 1:1
17” X 12” 204 58” 1.42:1
20” X 10” 200 60” 2:1
25” X 8” 200 66” 3.12:1
29” X 7” 203 72” 4.14:1
33” X 6” 198 78” 5.5:1
Fig. 1: Perimeters and aspect ratios of ducts with almost the same area.
When using aspect ratio to choose duct sizes, follow these general principles:
cRound duct has the least friction loss.
cNext to round duct, square duct is best.
cAs the aspect ratio increases, the friction loss increases.
cAvoid using duct with an aspect ratio greater than 3 to 1, if possible. In addition to increasing friction loss, it costs more to fabricate and install – more labor, more material, and heavier gages of metal.
(The 122-page softcover “Airflow in Ducts” is available for $18.50 plus tax and shipping from LAMA Books, 20956 Corsair Blvd., Hayward, Calif. 94545-1002; phone 888-452-6244 or fax 510-785-1099; www.lamabooks.com.)