Design and operation A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat for the steam requirements of process industries or for power generation. Combustion boilers are designed to use the chemical energy in fuel to raise the energy content of water so that it can be used for heating and power applications. Many fossil and non-fossil fuels are fired in boilers, but the most common types of fuel include coal, oil and natural gas. During the combustion process, oxygen reacts with carbon, hydrogen and other elements in the fuel to produce a flame and hot combustion gases. As these gases are drawn through the boiler, they cool as heat is transferred to water. Eventually the gases flow through a stack and into the atmosphere. As long as fuel and air are both available to continue the combustion process, heat will be generated. Boilers are manufactured in many different sizes and configurations depending on the characteristics of the fuel, the specified heating output, and the required emission controls. Some boilers are only capable of producing hot water, while others are designed to produce steam. Boilers can burn coal, oil, natural gas, biomass as well as other fuels and fuel combinations. Most boilers are classified as either watertube or firetube boilers, but other designs such as cast iron, coil type, and tubeless (steel shell) boilers are also produced. Components of a boiler system The main components in a boiler system are boiler feedwater heaters, deaerators, feed pump, economiser, superheater, attemperator, steam system, condenser and condensate pump. In addition, there are sets of controls to monitor water and steam flow, fuel flow, airflow and chemical treatment additions.
More broadly speaking, the boiler system comprises a feedwater system, steam system and fuels system. The feedwater system provides water to the boiler and regulates it automatically to meet the steam demand. Various valves provide access for maintenance and repair. The stem system collects and controls the steam produced in the boiler. Steam is directed through a piping system to the point of use. Throughout the system, steam pressure is regulated using valves and checked with steam pressure gauges. The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system.
Feedwater system The water supplied to the boiler, which is converted into steam, is called feedwater. The two sources of feedwater are condensate or condensed steam returned from the process and makeup water (treated raw water) which must come from outside the boiler room and plant processes. Feedwater heater Boiler efficiency is improved by the extraction of waste heat from spent steam to preheat the boiler feedwater. Heaters are shell and tube heat exchangers with the feedwater on the tube side (inside) and steam on the shell side (outside). The heater closest to the boiler receives the hottest steam. The condensed steam is recovered in the heater drains and pumped forward to the heater immediately upstream, where its heat value is combined with that of the steam for that heater. Ultimately the condensate is returned to the condensate storage tank or condenser hotwell. Deaerators Feedwater often has oxygen dissolved in it at objectionable levels, which comes from air in-leakage from the condenser, pump seals, or from the condensate itself. The oxygen is mechanically removed in a deaerator. Dearators function on the principle that oxygen is decreasingly soluble as the temperature is raised. This is done by passing a stream of steam through the feedwater. Deaerators are generally a combination of spray and tray type. One problem with the control of deaerators is ensuring sufficient...
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