PURPOSE: To prevent silica scale from being adhered to a heat exchanger by a method wherein either cold water or hot water is mixed with furnace slag and they are rapidly cooled and in turn steam is generated, either cold water or hot water is directly contacted with the steam to manufacture hot water and this hot water is utilized as a heat source.

CONSTITUTION: Molten slag at a furnace 15 is fed into a slag cooling layer 1, mixed with hot water there and rapidly cooled, thereby water crushing is formed. After the water crushing containing water is fed into a water crushing layer 2, water is dehydrated by a dehydrating machine 3, the dehydrated water crushing is transported out by a transporting conveyor 4 and in turn water from the dehydrating machine 3 is returned back to a hot water tank 5, thereafter the water is sent to a cooling tower 7 by a pump 6, cooled there and then the water is circulated again for the manufacturing of crushed water. Steam separated at the water crushing layer 2 is fed into a barometric condensor 8 released to the surrounding atmosphere acting as a direct contact type heat exchanger through a duct, contacted with cooling water dispersed from the nozzle, the water after being heated is utilized for heating of thermal medium of the heat utilizing equipment 11 at the heat exchanger 10.