A process of quenching an effluent containing materials having a tendency to coke is disclosed, the process being characterized by the quenching of the effluent in a quench zone wherein heat is transferred from the effluent through at least one heat exchange tube wall, and through a solid heat transfer medium surrounding the tube and in contact with the tube, to water, the temperature of the wall being maintained above the dew point of materials having a tendency to coke partly by the provision of the solid heat transfer medium which increases gradually in thickness from a point at or near the entry of the tube to a point at or near the exit of the tube.