Depending on the type of system, the water is distributed to its intended use as either hot water or steam. Figure 1 shows a fire-tube boiler, a boiler in which the hot combustion gases travel through tubes that are immersed in water to be heated or converted to steam. A water-tube boiler, a boiler in which water to be converted to steam is contained within tubes and hot combustion gases are outside the tubes. Most burners also inject air in proportion to the amount of fuel and ensure mixing of the fuel and air. In fire-tube boilers, steel conduits through which the hot gases of combustion pass.
Tubes separate the hot gases from the water and transfer heat to the water.
Water passes through the tubes in a water-tube boiler. The stack is a passage through which the combustion gases are exhausted to the atmosphere. The gases in the stack are referred to as stack gases or flue gases. Common Boiler Types Boilers can be categorized by any of a number of characteristics. A boiler can combust natural gas, oil, coal, pulverized coal, wood, or wood waste to heat water.