I understand that flow of exit gases at preheater may be calculated by Q = velocity * cross sectional area of duct But I do not know the formula for velocity and density. Please explain the same. Secondly, please explain how an EP functions.

You need to study the Ideal Gas Laws to understand the relationships between gas volume, density and velocity. An EP works by inducing a strong electrical field between discharge and collection electrodes. Dust carried in the gas stream passing between these oppositely charged electrodes itself becomes charged and is collected on the collection electrode from where it is periodically knocked off into the collection hopper at the bottom of the EP by the rapping devices.

Please tell me, what are the standard norms of the gas velocity in kiln, at kiln inlet, riser pipes and cyclones at different stages of a preheater? Also, please give the formula or method to calculate the gas velocity in kiln, at kiln inlet and different stages of the preheater.

The gas velocity at the kiln inlet should not be greater than 10m/s and ideally significantly less than that. you can calculate the velocity by using the Ideal Gas Laws to convert the normal cubic metres of exhaust gas at the kiln inlet to the actual cubic metres at the inlet temperature. Division by the cross-sectional area of the kiln inside the refractory will then give you an estimate of the actual velocity.