What is the velocity of air in an air duct if the flow rate is 340 cfm and the duct area is 4.5 square feet?

To find the velocity of air in an air duct, you utilize the formula that relates flow rate to duct area and air velocity. The equation can be expressed as:

Velocity (V) = Flow rate (Q) / Area (A)

In this scenario, the flow rate is given as 340 cubic feet per minute (cfm), and the duct area is specified as 4.5 square feet. To calculate the velocity:

1. Convert the flow rate into compatible units, which in this case is already in cfm.

2. Divide the flow rate by the area:

V = 340 cfm / 4.5 sq ft.

This results in:

V = 75.555... ft/min.

When rounding this number, it becomes approximately 75.5 ft/min. This corresponds to the correct choice, which reflects the basic principles of fluid dynamics that govern airflow through ducts.

Understanding this calculation allows professionals in fire alarm systems and HVAC to ensure that systems are designed for effective ventilation and smoke control, which are critical in fire safety scenarios.