Economizers are used as heat recovery equipment in packaged boilers instead of air heaters because of NOx concerns as discussed in Chapter 4. They are also less expensive and have lower gas pressure drops across them. Economizers for gas firing typically use serrated fins at four to five fins per inch. For distillate fuel, about […]

High alloy steel tubes used in superheaters and reheaters, unlike carbon steel, fail by creep rupture. Creep refers to the permanent deformation of tubes that are operated at high temperatures. Carbon steel tubes operate in the elastic range where allowable stresses are based on yield stresses, whereas alloy tubes operate in the creep-rupture range, where […]

Figures 3.19a and 3.19b Show an inverted loop superheater commonly used in packaged boilers, and Fig. 3.19c Shows a horizontal tube design with vertical headers. Superheaters operate at high tube wall temperatures; hence their design should be carefully evaluated. The convective superheater design located behind several rows of screen section operates at lower tube wall […]

The steam temperature in packaged boilers is often controlled from 60% to 100% load by using a two-stage superheater design with interstage attemperation as shown in Fig. 3.18. Steam temperature can also be maintained from 10% to 100%; however, this calls for a much larger superheater surface area. Deminer­alized water should be used for attemperation, […]

The superheater is an important component of a packaged boiler. The degree of superheat could be very high, with steam temperatures up to 1000°F, or as low as 50°F. With a very low degree of superheat, one can locate the superheater behind the evaporator and ahead of the economizer. In this case, the superheater may […]

Packaged steam generators of today use a single fan for up to 250,000 lb/h of steam. The furnaces of oil — and gas-fired boilers are pressurized, hence the fan parameters should be selected with care. Estimating the flow or head inaccurately can force the fan to operate in an unstable region or result in the […]

The function of a combustion control system is to ensure that the steam generation matches the steam demand. When the demand exceeds the supply, the steam pressure will decrease and vice versa. Although a few utility boilers generate steam at sliding pressures, packaged boilers typically generate steam at fixed pressure. The control system immediately adjusts […]

The fuel burner is an important component of any boiler. Burner designs have undergone several iterations during the last decade. Burner suppliers such as Coen and Todd are offering burners that result in single-digit NOx emissions and very low CO levels, competing with the SCR system presently used in the industry for single-digit NOx emissions. […]

Boiler efficiency calculations are done using ASME PTC 4.1 methods, as shown in Q6.19. When a combination of fuels is fired, the calculations can be involved. The results from a program developed are shown in Fig. 3.14. They show the performance of a boiler firing two different fuels at the same time. Based on the […]

The boiler efficiency is an important variable that is impacted by the type of fuel, its analysis, the exit gas temperature, excess air used, and ambient reference conditions. The major losses due to flue gases and the method of computing efficiency are discussed in Q6.19. With rising fuel costs, plant engineers should try to aim […]