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causes for changes in excess O2 are tube fouling, changes in fuel or in atmospheric conditions, or damage to the control system. The base for comparison should be established by the boiler service personnel or the boiler contractor at the time of initial installation of the boiler, and he or she should be consulted immediately if any unusual conditions are observed.

Excessive Stack-Gas Temperature

This problem can be caused by water-side or fireside fouling. If fire-side fouling (i.e., a buildup of soot, ash, or other particles) is taking place, the rate of heat transfer from the boiler to the water or steam it is heating is impeded, and the stack gas is correspondingly hotter. The effect of soot on fuel consumption is noticeable, according to the American Boiler Manufacturers Association (ABMA).10 Efficiency losses due to soot are approximately as given in Table 14.12. For this reason, the ABMA recommends that tubes be cleaned once every shift where practical. Where such cleaning is not practical, it is recommended that tubes be cleaned whenever the stack temperature rises 75°F.

Table 14.12 Typical fuel losses due to soot.

Table 14.13 Effect of water-side scaling.

Thickness

of Scale

Soft

Hard

Hard

(in.)

Carbonate

Carbonate

Sulfate

1/50

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