Why put the air vent in AHU U trap LINE?

: A discharge line trap has important functions both during the compressor on period and during its off period. During the on period, the trap acts as a turbulator to entrain oil droplets and carry them efficiently up the elevated discharge line. Therefore, traps would be most useful in unloading or multi-compressor systems to better ensure proper oil circulation during low system loads. Also, you are required to change the size of the pipe after the "P" trap.