Question 430Steam Plants - Assistant Engineer

The correct answer is A) it increases the volume and temperature of the furnace gas leaving the stack. Carrying too much excess air in a boiler system reduces the boiler's efficiency. This is because the excess air increases the volume and temperature of the furnace gases leaving the stack, which results in more heat being lost rather than being transferred to the water in the boiler. The higher temperature of the exhaust gases indicates that less heat is being absorbed by the boiler, lowering its overall efficiency. The other answer choices are incorrect because they do not accurately describe the impact of excess air on boiler efficiency. Excess air does not decrease the volatility of the fuel (B), nor does it necessarily lower the flame temperatures (C). Excess air can also vary the degree of deposits on heat absorbing surfaces (D), but this is not the primary reason for the reduced efficiency.