A boiler is a device that converts the chemical energy of fuel into useful heat output. Typical heat outputs are steam (saturated or superheated), hot water or thermal fluids like mineral oil.
There are many different types of boiler but all of them can be classified in two basic groups:
(a) Water tube boiler where water is contained in tubes and flames and hot combustion gases pass around them,
(b) Fire-tube or shell boiler where combustion gases pass through a furnace tube and after that enter tube bundles immersed in water within the shell.
Applied Industrial Energy and Environmental Management Zoran K. Morvay and Dusan D. Gvozdenac © 2008 John Wiley & Sons, Ltd
Most boilers have a furnace chamber where the majority of the heat is transferred directly from the flame by radiation. After the furnace chamber the flue gas goes through passages where heat is transferred primarily by convection. Two-thirds of heat transfer takes place in the furnace and the remaining third in the flue gas passages.
Most of the boilers in industry are used for the production of saturated low and medium pressure steam. If operated correctly, all types of modern boilers are more or less equally efficient at converting fuel into steam, hot water or hot oil. Table 2.1 shows the expected thermal efficiencies obtainable for different boiler types, based on the Gross Calorific Value (GCV) of the fuel.
More detail on various boiler types and designs can be found in the literature (see References).
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