A direct flame burner unit for furnaces for the thermo-chemical treatment of steel strips in continuous hot-dip galvanizing plants, wherein the direct flame burner comprises a burner comprising:
- a combustion head wherein a combustion chamber is obtained, said combustion chamber being provided with an exit opening for the combustion flame;
- a body to which said combustion head is fixed and comprising:
- a first chamber provided with at least an inlet opening which is connectable to a supply source of a pre-heated comburent, wherein said first chamber is in fluid communication with said combustion chamber;
- at least a first lance for the injection of a fuel into said combustion chamber and having an inlet end which is connectable to a first supply source of a fuel and an outlet end in fluid communication with said combustion chamber;
- a mixing chamber provided with at least a first inlet opening which is connectable to a supply source of a not pre-heated comburent, with at least a second inlet opening which is connectable to a second supply source of a fuel and with at least an outlet opening of the mixture formed inside said mixing chamber;
- at least a second lance for the injection of said mixture into said combustion chamber and having an inlet end in fluid communication with said at least an outlet opening of said mixing chamber and an outlet end in fluid communication with said combustion chamber;
- said burner being alternatively and selectively operable in two distinct operating modes:
- a diffusive flame combustion mode, with lambda ranging between 0.9 and 0.98, wherein said combustion chamber is fed only through said first chamber and said first lance respectively with said preheated comburent and with said fuel, and
- a premixed flame combustion mode, with lambda ranging between 1 and 1.25, wherein said combustion chamber is fed only by means of said second lance with said mixture formed inside said mixing chamber,
- wherein the burner comprises a supply unit comprising:
- at least a first supply conduit for supplying said preheated comburent and which is connected to said inlet opening of said first chamber and which is connectable to said preheated comburent source, at least a first shut-off valve being provided along said first supply conduit;
- at least a second supply conduct for feeding said fuel and which is connected to said inlet end of said first lance and which is connectable to said first fuel source at least a second shut-off valve being provided along said second supply conduct;
- at least a third supply conduct for feeding said not-preheated comburent and which is connected to said first inlet opening of said mixing chamber and which is connectable to said not-preheated comburent source, at least a third shut-off valve being provided along said third supply conduct;
- and
- at least a fourth supply conduct for supplying said fuel and which is connected to said second inlet opening of said mixing chamber and which is connectable to said second fuel source at least a fourth shut-off valve being provided along said fourth supply conduct;
- wherein in said diffusive flame combustion mode said first shut-off valve and said second shut-off valve are open, said third shut-off valve and said fourth shut-off valve being closed, and in said premixed flame combustion mode, said third shut-off valve and said fourth shut-off valve are open, said first shut-off valve and said second shut-off valve being closed.