PURPOSE: To increase the heat exchange efficiency by providing each tube bundle with a means for adjusting the flow distribution and flow rate of the hot gas stream between tube bundles in a boiler evaporating water introduced into a shell by the hot process stream introduced into heat exchanging tubes.

CONSTITUTION: A plurality of heat exchanging tubes having the inlet and outlet ends are mounted in a cylindrical shell, and heat is exchanged between water introduced into the shell and the hot process stream introduced into the heat exchanging tubes to evaporate water, while cooling the hot process stream. In a waste heat boiler having this structure, the heat exchanging tubes are mounted as at least two tube bundles, and each tube bundle is provided with a means for adjusting the flow distribution and flow rate of the hot gas stream between the tube bundles to control the production of steam and the cooling of the process stream. Whereby the flow of hot process stream can be optimized on the different tube bundles, and the heat exchanging efficiency can be improved.