When it comes to designing a duct system, many experts recommend a pressure drop of between 0.05 and 0.20 inches for every 100 feet of duct length. This value can be adjusted to meet the specific needs of the project, but it's important to remember that the type of duct material used will also affect air flow and static pressure. As a result, the size calculations for flexible ducts may differ slightly from those of sheet metal ducts.One way to select an appropriate friction loss per unit of length is to determine an initial speed and size the first section for this speed. This will help ensure that the pressure drop is within the desired range.
It's also important to consider other factors such as air velocity, air temperature, and duct insulation when selecting a pressure drop value. In conclusion, it's essential to take all of these factors into account when selecting a pressure drop value for your duct system design. By considering the type of duct material, air velocity, air temperature, and insulation, you can ensure that your system is designed to meet your specific needs.