One of my friends often designs pipe system without reducing dimension of the main pipe. He also do this to duct system as well.

Is this a wrong design?

Besides, I was confused about static pressure and velocity pressure. So these are my opinions, pls let me know yours.

FAN -> A___________1___________2__________B

1. To move the air from A to B of a straight duct, fan must create the head pressure about 100 Pa. It's total head pressure but the main force that "move" the air is only static pressure. The velocity pressure is just make the air moves fast or slow. This duct is constant area, so the velocity pressure doesn't change but the static pressure will be decrease along the length. What if static pressure decrease to 0 when the air reach to position 2? The remain velocity pressure will convert to static pressure?

2. The outlet of Air Handling Unit often has a diverging section from smaller to larger area to raise static pressure which allow the air move farther? How about the outlet of FCU which is usually has a converging section?

3. Suppose there is one branch at position 1. The fan airflow is 1000 lps and branch airflow is 300 lps. Assuming the main duct is sized with 1.2 Pa/m friction loss, after position 1 the airflow is 700 lps and if I don't reduce dimension of main duct, the friction loss will decrease, e.g 0.9 Pa/m. So by not reducing duct size, the friction loss of the duct system is decrease and the head pressure is also decrease. If we ignore the cost and installation clearance, my friend's design is reasonable or there's something that I was wrong. The main reason of reducing duct or pipe size along the length is just about decrease the material cost ???