Steam Generators The boiler or evaporator is that part of the steam generator where phase change or boiling occurs from liquid (water) to vapour (steam), essentially at constant pressure and temperature. The term boiler is traditionally used to mean the whole steam generator.
Boilers Boiler may be defined as a closed vessel in which steam is produced from water by combustion of fuel. According to ASME a steam generating unit is defined as: “ A combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat so made available to the fluid being heated and vapourised ”.
Boilers Steam generated is employed for: (i.) Generating power in steam engines or steam turbines. (ii.) Heating the buildings in cold weather and for providing hot water supply. (iii.) In textile and chemical industries, sugar mills
Boilers Primary requirements of steam generators or boilers are: (i.) Water must be contained safely. (ii.) Steam must be safely delivered in desired condition (as regards its pressure, temperature, quality, required rate).
Classification of Boilers Horizontal, Vertical and Inclined Horizontal boiler – If the axis of the boiler is horizontal. Parts of horizontal boiler can be inspected and repaired easily but it occupies more space. The vertical boiler occupies less floor area. Fire Tube and Water Tube Fire Tube – the hot gasses are inside the tubes and water surrounds the tubes. Eg: Cochran, Lancashire, Locomotive boilers. Water Tube – water is inside the tubes and hot gases surround them. Eg: Babcock and Wilcox, Stirling, Yarrow boiler
Fire tube Boiler
Water tube Boiler
Classification of Boilers Externally and Internally fired Externally fired – if the fire is outside the shell. Eg: Babcock and Wilcox boiler, Stirling boiler Internally fired – furnace is located inside the boiler shell. Eg: Cochran, Lancashire boiler Forced and Natural circulation Forced circulation – water circulation is done by a forced pump. Eg: Velox, Lamont, Benson boiler Natural circulation – water circulation takes place due to natural convention currents produced by the application of heat. Eg: Lancashire, Babcock and Wilcox boiler
Classification of Boilers High and Low pressure High pressure – which produce steam at pressures of 80 bar and above. Eg: Babcock and Wilcox, Velox, Lamont, Benson boilers Low pressure – which produce steam at pressures below 80 bar. Eg: Cochran, Cornish, Lancashire, Locomotive boilers Single and Multi-tube Depending upon whether the fire tube is one or more than one. Eg: Cornish, Simple vertical boilers, etc are single tube and Benson, Cochran, Lancashire boilers, etc are multi tube type.
Classification of Boilers Stationary and Portable Stationary/land – used for steam power plants, central station utility power plants, plant process steam, etc. Portable/Mobile – include locomotive and other small units for temporary use.
Comparison between Fire-tube & Water-tube
Particulars Position of water and hot gases Mode of firing Operating pressure Rate of steam production Suitability Risk of bursting Floor area Fire-tube Hot gases inside the tubes and water outside the tubes Generally internally fired Limited to 16 bar Lower Not suitable for large power plants Involves lesser risk on explosion Water-tube Water inside the tubes and hot gases outside the tubes Externally fired As high pressure as 100 bar Higher Suitable for...