Abstract: To purify smoke from a liquid fuel (in particular heavy fuel oil) boiler by eliminating unwanted components including acids, the smoke, before it is exhausted to the atmosphere, is caused to pass along a vertical path in a heat exchanger adapted to recover the latent heat of condensible components together with the substantive heat of the smoke. Water is sprayed at the top of the vertical path and collected at the bottom in the form of an effluent comprising the water, condensates and soot. A reserve water supply is provided. Water is fed from this reserve supply to the top of the heat exchanger where it is sprayed after adding to it an alkaline solution in sufficient quantity to neutralize acid components of the smoke. The effluent is recovered at the bottom of the heat exchanger. An effective dose of a floculating agent is added to it. The effluent is fed into a settling tank where is separates into a soot sludge and a clarified effluent which is fed to the reserve water supply.