Abstract: Each precipitation chamber of an electrostatic precipitator has an airlock provided by closable damper means and located in the path of the gas flow through the chamber between the inlet plenum and outlet plenum of the precipitator. Apparatus is provided in association with each airlock for establishing a gas pressure differential between the airlock and the rest of the chamber so that essentially no uncleaned gas can pass by leakage through the airlock to the precipitator outlet. The airlock is established during normal precipitator cleaning (rapping), thereby enhancing the effectiveness of cleaning by stopping gas flow through the chamber during cleaning, increasing particulate fall into the hoppers and permitting optimization of rapping intensity, rate and frequency. Precipitator efficiency is increased significantly by preventing outflow of re-entrained particulates during cleaning.