When setting up a ventilation system, it is essential to take into account the amount of CFM (Cubic Feet per Minute) that is lost due to the length and shape of the ducts. Generally, a 7% loss in CFM is estimated for every 25 feet of ductwork. If flexible ducts are used, a 3% loss occurs, while metal ducts incur a 3% loss for each 90-degree curvature. Additionally, a 3-7% loss is expected for straight and hard casting or fluid conduction.
To accurately calculate the pressure drop due to friction over a straight length of duct, a pressure drop and duct size calculator can be employed. This calculator takes into account the absolute roughness (D2) of the duct material, as well as the duct height (D5). For instance, if a G. I.
300 x 300 duct is 10 meters long and has a capacity of approximately 1000 CFM, the pressure drop due to friction can be calculated. Calculating the size of the duct from the allowed pressure drop can be more complex. Excel Solver or tabulating large samples of pressure losses with pipes of different sizes and then using interpolation to obtain the correct size are two methods that can be used. Additionally, a premium version of the duct sizer is available which allows users to switch between rounded values of commonly available duct sizes or use exact values, calculate losses for rectangular or round ducts, and show/hide all working cells to solve problems.
The friction loss in a 20-inch duct with an airflow of 4000 CFM can be estimated at approximately 0.23 inches of water per 100 feet of duct. In summary, when configuring a ventilation system, it is important to consider the amount of CFM that is lost due to the length and shape of the ducts. A pressure drop and duct size calculator can be used to accurately calculate the pressure drop due to friction over a straight length of duct.