immediately adjacent the burner outlet the present invention employs a ring 14 of refractory material of equal internal diameter which leads to an expanded chamber 16 of refractory material 17 in the prototype the inside diameter of the chamber 16 was 48 34 with a length of about eight feet to ten feet the chamber 16 is also encased by a cylinder 18 of metal adjacent the outer surface of the refractory material 17 and a second cylindrical wall 19 held in spaced relation to cylinder 18 by a plurality of spacers 21 each end of the annular space 22 formed between the cylindrical metal walls is closed by an annular plate 23 and 24 a cooling fan 26 which may be the same fan used in cooling the burner supplies forced air to the annular space 22 a portion of this forced air may be exhausted through vent 27 a plurality of tuyeres 28 are spaced equidistantly about the chamber and provide air injection from the annular space 22 into the chamber thus maintaining the cyclonic effect a row of refractory bricks 29 are used to reduce the outlet diameter of the chamber such that the gases therein are under some pressure as.