Degassing can only take place at the melt surface, with each screw rotation a defined area is present in the degassing zone. The renewal time oft he pushing flight indicates how long it takes for the melt to move out and back into the degassing zone:

$$t_{pf} = \frac{h}{D \cdot π \cdot N \cdot sinφ}$$

The degassing surface $A_{pf}$ is the surface of the melt which is in contact with the air and can degas, divided by the renewal time this gives the surface renewal rate:

$$A_{pf} = \frac {A_{pf}}{t_{pf}} = i \cdot D \cdot π \cdot N \cdot I$$