In a heat exchange unit that recovers waste heat from carbon black smoke by preheating oxygen-containing gas such as air being passed to a carbon black producing reactor, a control loop is provided that automatically diverts the oxygen-containing gas being preheated through a by-pass line if the temperature of the effluent smoke being removed from the heat exchange is below a minimum temperature value in order to minimize the deposition of carbon on the heat exchange surfaces in said heat exchange zone.