What does CFM stand for and how is it used in duct design?

What does CFM stand for and how is it used in duct design?

• A. Cubic feet per minute; used to size ducts and achieve target room ventilation.
• B. Kilowatts per hour
• C. Pounds per square inch absolute
• D. Meters per second.

Answer: (A)

CFM stands for cubic feet per minute. It is the volumetric rate of air flow, indicating how much air is moving through a duct or space each minute. In duct design, you first determine the required ventilation and thermal loads for the space, which tells you the target airflow in CFM needed to meet comfort, indoor-air quality, and code requirements. With that target, you size the ducts so they can carry that amount of air without excessive resistance or noise. The basic relationship used is Q = V × A, where Q is the flow rate in CFM, V is the air velocity in feet per minute, and A is the duct cross-sectional area. This lets you choose a duct size that yields the desired CFM at an acceptable velocity and pressure drop. If the duct is too small, velocity and noise increase; if too large, energy efficiency can suffer. Other units mentioned—kilowatts per hour (energy use), pounds per square inch absolute (pressure), and meters per second (velocity)—do not define the flow rate the way CFM does.

CFM stands for cubic feet per minute. It is the volumetric rate of air flow, indicating how much air is moving through a duct or space each minute. In duct design, you first determine the required ventilation and thermal loads for the space, which tells you the target airflow in CFM needed to meet comfort, indoor-air quality, and code requirements. With that target, you size the ducts so they can carry that amount of air without excessive resistance or noise. The basic relationship used is Q = V × A, where Q is the flow rate in CFM, V is the air velocity in feet per minute, and A is the duct cross-sectional area. This lets you choose a duct size that yields the desired CFM at an acceptable velocity and pressure drop. If the duct is too small, velocity and noise increase; if too large, energy efficiency can suffer. Other units mentioned—kilowatts per hour (energy use), pounds per square inch absolute (pressure), and meters per second (velocity)—do not define the flow rate the way CFM does.