Sentence:  However, observation of the hopper area and the outlet field can usually detect the presence of heavy dust reentrainment either by hopper <WSD>sweepage</WSD> or by scouring of the plates. 
Meaning of the word marked with <WSD> tags:  The process by which gases passing beneath the electrodes of an electrostatic precipitator pick up dust from the hoppers, removing it from the precipitator exit.