## Calculating free area of duct as a function of blast gate

### Calculating free area of duct as a function of blast gate

A couple of other engineers and I are working on an exhaust system and think it would be handy to be able to calculate the free area, crescent shaped of course, created by the operation of a blast gate/knife gate to a partial position inside a round exhaust duct. We can assume that the frontal edge of the gate will have the same radius as the duct. We are wanting to create a formula that will allow us to figure the free area for any given position of the damper/gate. Does anyone know of such an equation?