In the milling circuit with an electromagnetic mill, a pneumatic system is used to transport the material and to maintain several parameters of the process. The pneumatic system contains three air inlets with controllable flaps and each inlet stream serves a different purpose, however, they are strongly physically coupled through the rest of the installation. Thus, position of each control flap strongly and nonlinearly affects the operating point of each inlet stream. Dynamics of air velocity within the main inlet pipe with respect to the setpoint of the main flap's position has been identified for multiple operating points of the installation, and the estimated models were checked and assessed. The resultant models may then be used to tune controller parameters for subsequent operating points. Identification and verification of the derived models was performed on data measured at an experimental milling installation.