The answer is in the name. Boilers are made with a "tube - in - shell" construction, and the name implies what is in the tube. In a water tube boiler, the water circulates through the tubes, and the burner is usually under them. Most of the time, the "shell" of this style of boiler would not be watertight. A fire tube boiler has a watertight outer jacket with tubes through the center of it. The flame is shot through the tubes.
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