the shape of an elongated square in the drawing which only shows a small section of the reactor chamber there is shown the farther wall 10 of the chamber and the side walls 11 12 the front wall of the reactor chamber has been omitted for sake of clarity in the drawing it is seen how the fluid arrives from the right end of the chamber and flows in a serpentine path in the channel which is defined by the units and the walls of the chamber the cavity 9 formed between the insert and the side wall 12 makes it possible for the fluid to pass on to the adjacent lower row in fig9 there is shown an embodiment of a unit with slightly different form giving another kind of insert the unit 1 1 is seen from one side and has also upper and lower limits 3 1 and 4 1 in fig1 there is shown a cross section of the unit 1 1 as may be seen in the drawing the unit 1 1 has a plane surface 2 1 and a cylindrical part 5 1 there is a softly bent or radiused change over from the cylindrical part to the plane part the upper and lower limits 3 1 and 4 1 have the shape of a truncated triangle with two parallel sides as may be seen in fig1 there is a space 6 1 in front of the cylindrical part 5 1 of the unit fig1 illustrates how the units may be arranged in order to form a part of the insert intended to be used in the reactor chamber every second unit being turned 180 u00b0 in relation to the other unit the part of the insert shown in fig1 may.