3 is deflected into passage 10 and by means of the container 12 and the exchanger 15 it reaches the burner 16 that contains the heating element 17 and the catalytic mass 18 the air is outlet from the burner hotter but depleted of oxygen due to the effect of the combustion process it cools in the exchanger 15 and again in the water refrigerator 19 before it returns to the space 1 through passage 20 the modifications added to this invention compared to the prior are configuration include first and foremost the addition of at least one oxygen concentration sensor in the diagram of enclosed fig1 two probes are illustrated with numbers 21 and 22 respectively the consequent modifications are listed throughout the description the air to be conveyed to the burner is deflected into passage 10 immediately downstream from the fan 3 without crossing the coils 5 and 6 cold and hot respectively it has been noticed that thanks to the control achieved by the oxygen sensors the slight temperature variations to which the air drawn and pushed by the fan 3 may be controlled and the consequent difference in the solvent vapour concentration within it can now be tolerated consequently the function of the carburetor previously assigned to the tank 12 is outdated and therefore it is no longer necessary to flow into this some liquid solvent by means of a pump the tank 12 is however still useful as a separator of any drops of liquid pulled in by the air the solvent thus held back is then returned to space 1 through passage 14 finally it is now preferred that passage 20 through which the burnt air runs leads out directly in space 1 rather than.