PURPOSE:To reduce the power consumption and at the same time improve the efficiency of units in operation by a constitution wherein a plurality of oxygen enriched air generating units are selectively put into operation in proportion to combustion load. CONSTITUTION:The air in a casing 7 is sucked through oxygen diffusion membranes 3 in order to enrich the oxygen concentration in air up to the order of 30%, for example, by putting the suction blower 4 of the oxygen enriched air generating unit U1 into operation. The oxygen enriched air is pressurized and supplied through a control valve 6 to a piping 1 by means of a fan 5 connected through a piping 11 to the outlet of the suction blower 4. A control means 2 calculates total combustion load based upon combustion load signals sent from respective heating furnaces F1, F2...Fi and issues control signals to start or stop to the respective oxygen enriched air generating units U1, U2...Ui in conformity with a predetermined priority correspondent to the total combustion load. Due to the constitution as mentioned above, the units can be efficiently put into operation correspondent to load.
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