PURPOSE: To increase the thermal efficiency of a plant by a method wherein a heater, in which heat exchange between cooling water heated by a filling floor and induced mixture gas is effected, is provided in the lower zone in a vessel above the filling floor.

CONSTITUTION: The support 12 of filling materials 3 and a drain sump top plate 13, provided below the support 12, are provided in a vessel 1. The drain sump top plate 13 forms a mixed gas chamber 14, into which the mixed gas is introduced, together with the support 12. The mixed gas chamber 14 communicate with a gas inlet nozzle 4 and finned heat transfer tubes 15 are arranged in the chamber 14. The upper parts of the finned heat transfer tubes 15 are connected integrally to the floor surface 12c of the support 12 and communicate with a cooling water reservoir 12d formed by the support 12 while the lower parts of the same are connected to the drain sump top plate 13 and communicate with a drain sump 7. Cooling water, heated to a temperature near a value T3 by the mixed gas introduced from a gas inlet nozzle 4 in the filling floor of the filling materials 3, is collected into the cooling water reservoir 12d of the support 12 and, thereafter, flows through the finned heat transfer tubes 15 and drops into the drain sump 7. Heat exchange is effected between the cooling water and the mixed gas, introduced into the mixed gas chamber from the gas inlet nozzle 4, while the cooling water flows through the finned heat transfer tubes 15 whereby the cooling water is heated.