Abstract: The invention concerns an apparatus and a process for the periodic cleaning out of the pipes of heat exchangers to remove deposits of finely-divided solids, in particular carbon black or pyrogenically produced inorganic oxides. The apparatus consists of jet nozzles, which are arranged at a spacing above the gas entry openings of the pipes and are supplied with a cleaning gas, having excess pressure with respect to the process gas, via ducts provided with shut-off elements. The jet nozzles can be stationary or arranged for displacement along a row of the heat exchanger pipes. The process envisages flushing the pipes to be cleaned preferably in succession during operation of the heat exchanger, periodically, with a gas stream released with an impact and maintained for a short time at high speed.