A method and apparatus for preventing the vaporization of a cooling liquid used in a boiler which is heated by waste gases from a converter. A circulating pump pumps a constant amount of cooling liquid through a system which includes in addition to the boiler and the pump a heat exchanger in which the liquid is cooled after leaving the boiler and a reservoir which receives the cooled liquid from the heat exchanger and which supplies the liquid to the circulating pump. A compensating or equalizing expansion tank communicates with the reservoir and maintains over a liquid which is in this latter tank a cushion of a noncorrosive gas. As the temperature of the liquid rises, the pressure of the gas in this cushion also rises, so that the pressure of the liquid rises, thus raising the boiling point thereof in such a way as to prevent vaporizing of the liquid. The liquid is continuously circulated during the off-blow as well as during the blow periods of each cycle, so that by the time the end of an off-blow has been reached, the cooling liquid has been cooled down to the temperature which it had initially at the beginning of the blow period.