tube and increases in relation to the heat conducted to a maximum at the top of the quartz tube with the ratios prevailing between liquid and gas at the top of the tube unit the flow speed i e the gas speed at which flooding can take place with 25 mm packings is 2 5 3 0 m sec with 40 mm packings 3 4 m sec and with 50 mm packings 4 5 m sec in the present case the gas speed is calculated to be approx 2 5 m sec at the top of the tube unit 13 at total evaporation of the liquid fed in however the gas speed may increase to twice this speed however total evaporation takes place only in exceptional cases in case packings are to be placed in the entire quartz tube in the embodiment mentioned above the packings at the top should not have a diameter of less than 40 mm up to half of the height of the quartz tubes packings with a diameter of 25 mm can be used in certain embodiments it is also possible to limit the height of the packing layer so that at the top of the packing layer a maximum gas speed of 1 5 to 2 0 m sec will be obtained the liquid is then sprayed against the walls of the quartz tube with the aid of the spreading device 26 in the top of the quartz tube water.