| abstract |
Boiler water is heated in a steam boiler while feed water, which has a hardness of 5 mgCaCO 3 /L or less and is used as the boiler water, is supplied to the steam boiler. A part of the boiler water is appropriately disposed of. A chemical aqueous solution containing an alkaline metal silicate, an alkaline metal hydroxide and at least one kind of scale preventive agents of ethylenediamine tetraacetate, and its alkaline metal salt, is supplied to the feed water so that a concentration of the scale preventive agent is at least 1.5 mol equivalent times relative to the hardness of the feed water. In the steam boiler, a condensation rate of the boiler water is set so that the concentration of the scale preventive agent in the boiler water is 40 mg EDTA/L or less. |