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The kraft recovery boiler has many tubes located in the combustion gas stream that collect the heat of combustion and convert the energy into steam by heating boiler feedwater inside the tubes. In the heat transfer section, there are three main sections of tubes: the superheater, the generator or boiler bank, and the economizer. Feedwater enters through the economizer and then flows to the steam drum. The steam drum is a large cylindrical vessel situated horizontally across the boiler that acts as a reservoir of feedwater for the boiler bank. The waterwall tubes are part of the feedwater circulation fed from the mud drum and steam drum system. The steam drum is connected to the mud drum by multiple boiler tubes. The mud drum also provides the blowdown point, where feedwater impurities are continuously drained to the sewer through a small pipe.