3600

Alloy of tantalum and tungsten

Figure 8.27 Boiler economizer.

Figure 8.28 Fuel savings from a gas-fired boiler using economizer.

Figure 8.27 Boiler economizer.

steam demand or in case of a feedwater pump failure. This is usually obtained by controlling the amount of exhaust gases flowing through the economizer using a damper, which diverts a portion of the gas flow through a bypass duct.

The extent of heat recovery in the economizer may be limited by the lowest allowable exhaust gas temperature in the exhaust stack. The exhaust gases contain water vapor both from the combustion air and from the combustion of the hydrogen that is contained in the fuel. If the exhaust gases are cooled below the dew point of the water vapor, condensation will occur and cause damage to the structural materials. If the fuel also contains sulfur, the sulfur-dioxide will be absorbed by the condensed water to form sulfuric acid. This is very corrosive and will attack the breeching downstream of the economizer and the stack lines. The dew point of the exhaust gases from a natural-gas-fired boiler varies from approximately 138°F for a stoichiometric fuel/air mixture, to 113°F for 100% excess air. Because heat-transmission losses through the stack cause axial temperature gradients from 0.2 to 2°F/ft, and because the stack liner may exist at a temperature 50 to 75°F lower than the gas bulk temperature, it is considered prudent to limit minimum stack temperatures to 300°F, or no lower than 250°F when burning natural gas. When using the fuels containing sulfur, even greater caution is taken. This means that the effectiveness of an economizer is limited

Figure 8.28 Fuel savings from a gas-fired boiler using economizer.

unless the exhaust gases from the boiler are relatively hot. Figure 8.28 is a graph of the percent fuel saved plotted against percent excess air for a number of stack gas temperatures using natural gas as a boiler fuel. The plots are based on a 300°F hot-gas temperature leaving the economizer.

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