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 for higher efficiency if the plant is base-loaded and operates continuously. Often less efficient and less expensive units are purchased owing to lack of funds, and this practice should be reviewed. One should look at the long-term benefits to the end user. Similarly, the fan operating costs should also be evaluated. A design with high gas pressure drop in the boiler may be less expensive, but if one considers the long-term operating costs, it may not be the better choice.

Table 3.5 Shows the effect of excess air and exit gas temperatures on boiler efficiency and cost of operation. It is important to operate at as low an excess of air as possible; however, as discussed in Chapter 4, limits on NOx and CO may force the burners to use higher values of excess air.

Table 3.5 Effect of Excess Air and Exit Gas Temperature on Efficiency3

As shown in Tables 3.4 And 3.7, the efficiency of packaged boilers varies with load. This information may be used as a planning tool as discussed, particularly when the plant has HRSGs in addition to steam generators.