For designing the ductwork system for a project, the first things should be done is to providing the duct distribution system layout.
There are different methods for sizing the ducts but the most common method is constant pressure drop. The good practice for low pressure ductwork is to size the duct at 0.08” (inch of water column) pressure drop per 100 feet of the length but always the air velocity need to be checked as well to not exceeding the maximum allowed number for each application. For sizing duct, can either use a duct sizing chart (in ASHRAE Fundamental or other HVAC handbooks) or the duct design calculator (below image).
There are 3 ductwork design systems:
Low pressure system - In this system the air velocity in duct is below 2,000 fpm and the system static pressure would be 1” to 2” W.C.
This system is for all interior ductwork for commercial and residential spaces. The noise level for each space should not exceed the allowed level, like in office spaces the maximum noise level would be NC 35. The best practice for air velocity in office spaces is between 1,200 to 1,400 fpm. The air velocity at diffusers shall not exceed 500 fpm to keep the noise level lower than NC 35. The below chart can be used for the office spaces and residentials.
Medium pressure system - In this system the air velocity in duct is between 2,000 fpm to 2,500 fpm and the system static pressure would be 3” to 4” W.C.
This range velocity is using for the main duct risers in tall buildings but when it arrives to the space then need to design the low-pressure ductwork.
High pressure system - In this system the air velocity in duct is 2,500 fpm and higher and system static pressure would be 6” to 10” W.C.
This system is for industrial applications and tall buildings duct risers.