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.

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.

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.

I have not observed this phenomenon, however it is perhaps connected with the additional draft that is required to keep the kiln hood under suction when there is inleak in the bottom stage of the preheater. This would cause the thermal energy from the calciner to be drawn up the preheater. Conversely when the inleak is in the cyclones above the calciner then the inleak makes no difference to the draft on the calciner.