Boilers
Boilers are essential devices used to heat fluids, primarily water, to provide space heating, process heat, or generate electricity. They operate by utilizing a heat source, usually through the combustion of fuel, to raise the temperature of a fluid to or near its boiling point. The heated fluid is then circulated for various applications, while in closed systems, it is cooled and returned to the boiler for recycling. Types of boilers include fire-tube and water-tube configurations, with fire-tube boilers generally being more efficient for lower capacities, while water-tube boilers can handle higher pressures and capacities.
Additionally, boilers are categorized by their combustion methods, with stoker boilers being common in coal applications. Other modern approaches include fluidized bed combustion, which enhances efficiency by keeping fuel in a fluid state. Waste heat boilers capture excess heat from industrial processes, contributing to energy recovery. The efficiency of boilers is critical, with Energy Star certification indicating high performance and potential cost savings. Overall, the choice of boiler type and its operational efficiency play significant roles in both energy use and environmental impact.
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Boilers
Summary: Boilers are used for heating a fluid in order to provide space heating, to process heat, or to generate electricity through a turbine, and therefore have a wide range of applications.
A boiler is a device used to heat a fluid, generally water, in order to provide space heating or process heat directly, or to generate electricity. There are various types of boilers that vary in size and technical specification, depending upon their application, but the general principal of their operation is the same. A heat source is used, typically the combustion of a fuel, to heat a fluid to, or near, its boiling point. Water is most often used as the working medium because it is cheap, readily available, and has suitable properties in terms of thermal capacity and boiling point. The resulting hot fluid or vapor is then circulated as required in an industrial process, heating device, or turbine. Unless the system is open (in which case the hot water or steam exits, for example as hot tap water), the hot fluid is returned at a cooler temperature to the boiler and recycled. When the heat source is a furnace used for the combustion of fuel, hot exhaust gases may or may not be used to preheat the water, which is related to the concept of lower and higher heating values (LHV and HHV) for fuels, corresponding to the fraction of their energy content that is actually exploited.
The heating value or energy value of a fuel is the amount of heat released during the combustion of a specified amount of fuel, and is expressed in terms of the LHV or HHV. The HHV is defined as the return of the combustion products to the pre-combustion temperature, including any vapor produced. The HHV is the same as the thermodynamic heat of combustion and takes into account the latent heat of evaporation/vaporization of water in the combustion products, which is the regained energy relating to the change of phase from gas to liquid. The LHV, on the other hand, neglects the latent heat of vaporization and therefore assumes that water leaves the system in the gaseous state. The distinction between the two is that this second definition assumes that the combustion products are all returned to the reference temperature and the heat content from the condensing vapor is not considered useful.
Types of Boilers
Most boilers can be categorized into either fire-tube boilers, in which the hot gases from combustion flow through the inside of the tubes and heat is transferred to surrounding fluid; or water-tube boilers, in which the setup is reversed. The reason for the tubes is to maximize the surface area between the combustion zone and the fluid for heat transfer. Fire-tube boilers generally have lower initial investment and are more efficient, but are limited in terms of capacity and pressure. Water-tube boilers, on the other hand, can be built to any capacity and pressure, and have higher efficiencies. Other classifications of boilers include packaged boilers, in which the complete plant is enclosed in one modular unit, ready for connection onsite.
In addition, boilers can be categorized according to their method of combustion. A widespread type of boiler is the stoker boiler, in which coal is typically used as a fuel. One example is the traveling or chain grate stoker, in which a moving grate acts as a conveyor belt, carrying burning coal, while hot air is drawn through the grate to enable complete combustion. The coal should burn completely before passing off the end of the conveyor, which means that a precise control of conveyor speed, air flow, and coal feed rate is required. Another type of stoker boiler is the spreader stoker, in which coal is continuously fed in or “spread” into the furnace above a burning coal bed. Fine particles are burnt in suspension and larger particles fall to the bed to be burned. In this case, the combustion is very quick and this type of boiler responds very quickly to load fluctuations, hence it is common for some industrial applications.
Most modern coal-fired boilers use pulverized coal because the combustion is much more efficient than with larger lumps, due to the larger surface area. In fluidized bed combustion (FBC) boilers, the base of the combustion chamber is characterized by the fuel being in the fluid state, even though the fuels employed are solids such as coal and agricultural wastes. Small particles of sand are suspended in a stream of hot air, as when pulverized coal is fed into the furnace, which due to the high and uniform temperature of the bed combusts very quickly and efficiently. Generally, this approach is coupled with a conventional boiler at standard pressure, so that it is referred to as an atmospheric fluidized bed combustion (AFBC) boiler. If the combustion chamber is pressurized, however, which is similar to the FBC, the heat release rate can be increased and therefore the combustion efficiency increases. This is known as pressurized fluidized bed combustion (PFBC). Finally, some other variations include the waste heat boiler, which is used to recover waste heat, as from an industrial process, in order to provide another process with heat or to generate electricity.
For boilers raising steam for electricity generation, the main way of improving overall efficiency is to increase the turbine inlet temperature, which means higher temperatures and pressures. With inlet temperatures around 550 degrees Celsius (1,022 Fahrenheit), they already operate at the current technological limit, in what is referred to as the supercritical region—in which the working fluid ceases to have distinct phases. This places extreme requirements on the materials for boiler construction and it is ultimately these materials, and their ability to withstand such high temperatures, that represent one of the main challenges for the future in this area.
Boilers certified by the Energy Star program, a 1992 initiative from the US government that promotes energy-efficient products, have an efficiency rating of 90 percent for gas boilers and 87 percent for oil boilers. According to the US Environmental Protection Agency, an Energy Star-certified oil boiler can save about $420 in costs over the course of its lifetime, while a certified gas boiler can save $780.
Bibliography
"Boilers." Energy Star, www.energystar.gov/products/boilers. Accessed 30 July 2024.
Kohan, A. Boiler Operators Guide. 5th ed. New York: McGraw-Hill, 2021.
Rayaprolum, Kumar. Boilers for Power and Process. Boca Raton, FL: CRC Press, 2009.
Rogers, G., and Y. Mayhew. Thermodynamics: Work and Heat Transfer. 4th ed. Harlow, UK: Longmann Scientific, 1992.