Class 2 (2nd Engineer) MEKG 📅 Oct 2022

Exam Question

Discuss, with reference to the superheater outlet temperature of the main boiler operating at a constant load, the following statements:

(a) An increase in excess air will tend to cause a decrease in superheater outlet temperature due to the cooling effect of more air being introduced.

(b) A decrease in economizer inlet temperature will tend to cause a decrease in super heater outlet temperature due to the cooling effect of more water being introduced.

(d) Excessive amounts of total dissolved solids in the boiler water will cause variations in the super heater outlet temperature

Reference Answer

This response analyzes the factors affecting the superheater outlet temperature in a marine main boiler operating under a constant load, which implies a constant firing rate and steam demand.
### (a) An increase in excess air will tend to cause a decrease in superheater outlet temperature.
This statement is **correct**. The reasoning involves the principles of combustion and heat balance within the boiler.
* **Combustion Stoichiometry:** For complete combustion, a specific amount of oxygen (air) is required per unit of fuel, known as the stoichiometric ratio. In practice, to ensure complete combustion and prevent soot/smoke formation, more air than is theoretically required is supplied. This additional air is termed 'excess air'.

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