Refresh and try again. Open Preview See a Problem? Ada marked it as to-read Jun 21, Lists with This Book. Piyush Agarwal added it Apr 18, No other book explains the fundame An excellent primer for power plant professionals who have to wear many hats and need a practical explanation of the design and basic operation of conventional bufcker generating boilers and HRSGs without having to wade through technical material.

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The author and publisher assume no liability whatsoever for any loss or damage that results from the use of any of the material in this book. Use of the material in this book is solely at the risk of the user. Convection occurs both naturally and mechanically on the combustion andwaterside of the boiler. Fans assist convection on the gas side, while waterside con-vection occurs both naturally and assisted by pumps. Waterside and combustion-side flow circulation are examined in more detail in this chapter and chapter 2.

Properties of water and steamIn addition to the reasons mentioned earlier for the selection of water as aheating medium, another is its excellent heat capacity. Other physicalaspects are also important. Between the freezing and boiling points, any heatadded or taken away directly changes the temperature of the liquid. However, atthe freezing and boiling points, additional mechanisms come into play.

Considerthe scenario in which water is heated at normal atmospheric pressure, and the tem-perature reaches F C. At this point, further energy input does not raisethe temperature, but rather is used in converting the liquid to a gas. This is knownas the latent heat of vaporization. Once all of the water transforms to steam, additional heating again results in adirect temperature increase. Likewise, when water is cooled to 32F, additionalcooling first converts the water to ice before the temperature drops any lower.

Thisis known as the latent heat of fusion. As a closed pressure vessel, a boiler allows water to be heated to temperaturesmuch higher than those at atmospheric conditions.

Thus, it is possible to add much more heat to the waterthan at atmospheric pressure. This in turn gives the fluid more potential for workin a heat transfer device. The following discussion of boiler designs illustrates howthe boiler components extract energy from burning fuel to produce steam. Fundamental boiler designFigure is the simple outline of a natural circulation, drum-type boiler. Steam generation begins in the waterwall tubes located within the furnacearea of the boiler.

As the boiler water flowing into the tubes absorbs heat, fluiddensity decreases and the liquid rises by convection. Conversion to steam begins asthe fluid flows upwards through the waterwall tubes, known as risers. AsAppendix outlines, a smooth transition of water to steam in the tubes isimportant. The increase in temperature to which steam is raised above the saturationpoint is known as the degree of superheat. Consider again a 2, psig Steam tables show that the saturation temperature at this pressure isF C.

If the steam is heated to 1,F C for use in a turbine, thenit has F C of superheat. Modern utility boilers typically have final steamtemperatures of 1,F to 1,F C, although some units have beendesigned with final steam temperatures of 1,F C and on a few occasions,1,F C. Higher steam temperatures are rare due to material performanceissues. Ash fouling of superheater tubes chapter 3 is a major concern during thedesign and operation of a boiler. Fouling potential is greatest in the hottest por-tions of the convection pass, so wider spacing between superheater tubes isrequired in these areas to prevent bridging of ash deposits.

Figure illustratesthe proportional spacing of superheater tubes as a function of flue gas temperature. Tighter tube bundles are possible further along in the flue gas path, where foulingpotential is lower. Reheat is a design modification to steam generating units that improves effi-ciency, and is standard with large boilers.

The general steam flow path in a unitwith a single-reheat loop is illustrated in Figure The reheater increases tem-perature, not pressure, but the temperature gain still improves efficiency. Commondesignations for boilers list both the superheat and reheat temperatures. Like superheaters, reheaters may be placed at various locations within the gas path. Control of steam temperature in the superheater is important to maximizeefficiency and prevent overheating of tubes.

The common method of steam tem-perature control is attemperation from a spray of feedwater introduced directly intothe steam. Included is a start-up steamnetwork Reproduced with permission from Steam, 40th ed.

Feed after the secondary superheater could potentially allow waterdroplets to enter the turbine. Some high-temperature units are also equipped withreheater attemperators, although this is not universal.

Direct attemperation requires a specialty nozzle Fig. Nozzle designand materials minimize the effects of thermal stress when the relatively cool feed-water enters the hot steam line.

Another type of attemperator found in older two-drum units is the cooling coil attemperator Fig. This is a non-contactattemperator in which a portion of the main steam is bypassed through tube bun-dles located in the mud drum and then reintroduced to the main steam. The cool-er boiler water lowers the temperature of the bypass steam. Control valves adjustthe flow rate of steam to the attemperator in accordance with main steam temper-ature. Figure also illustrates a piping arrangement to assist with unit start-up.

The key feature is the steam bypass directly to the condenser. As a unit starts, theFig. This is not ideal for the turbine. An initialbypass of steam to the condenser allows the unit to develop some heat before thesteam is introduced to the turbine. Many plants do not have this option, however,and must introduce wet steam to the turbine at start-up. EconomizerToward the further reaches of the convection pass sits the economizer.

Aswith reheaters, most modern, large steam generating boilers have an economizer. The economizer extracts additional heat from the flue gas, but in this case trans-fers the energy to the feedwater.

Figure outlines a typical economizerarrangement. Economizer tubes may be finned to provide additional heat transfer,although the fins increase the potential for fly ash accumulation.


Basics of Boiler and HRSG Design



Basics of Boilers and HRSG Design





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