Sentence:  One consequence of poor gas flow is hopper <WSD>sweepage</WSD>. High gas flow near the bottom of the precipitator and improper baffling can cause gas flow into the hopper area. 
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.