# Free cross-sectional area inside duct

Cross-sectional area of empty duct minus the cross-sectional area of all objects inside the duct.

Symbol
$A_{d_{fill}}$
Unit
mm$^2$
Formulae
 $\frac{\pi \left(- D_{e}^{2} + Di_{d}^{2}\right)}{4}$ $N_{c} \leq 1$ $\frac{\pi \left(- 2 D_{e}^{2} + Di_{d}^{2}\right)}{4}$ $N_{c} \leq 2$ $\frac{\pi \left(- 3 D_{e}^{2} + Di_{d}^{2}\right)}{4}$ $N_{c} \leq 3$ $\frac{\pi \left(- 4 D_{e}^{2} + Di_{d}^{2}\right)}{4}$ $N_{c} > 3$
Related
$D_{e}$
$Di_{d}$
Used in
$Q_{d_{fill}}$