Analyzing Reheater Effects in Steam Power Plants

This explanation details the impact of incorporating a reheater into a steam power plant cycle, evaluating the correctness of the given statements (P, Q, R, S).

Evaluating Statement P: Thermal Efficiency

Statement P claims that incorporating a reheater Always increases thermal efficiency.

While reheating generally improves thermal efficiency by increasing the mean temperature of heat addition, the word 'Always' makes this statement potentially incorrect. Factors like pressure drops in the reheater and piping, along with the cost of additional equipment and the specific operating conditions, can sometimes lead to minimal or even negligible efficiency gains. In certain theoretical or non-ideal scenarios, the efficiency might not increase.

Evaluating Statement Q: Dryness Fraction at Condenser Inlet

Statement Q asserts that reheating Always increases the dryness fraction at the condenser inlet (which is the turbine exhaust).

This statement is considered correct. Reheating involves taking partially expanded steam from the high-pressure turbine, sending it back to the boiler to be heated further (usually to the initial temperature), and then expanding it in the low-pressure turbine down to the condenser pressure. This process adds heat at a higher enthalpy level, generally ensuring the steam exiting the low-pressure turbine is less wet (i.e., has a higher dryness fraction) than it would be without reheat. This is a primary benefit of reheating, as it reduces moisture content and helps prevent erosion of turbine blades.

Evaluating Statement R: Mean Temperature of Heat Addition

Statement R suggests that reheating increases the mean temperature of heat addition.

Thermodynamically, reheating does increase the average temperature at which heat is supplied to the working fluid in the cycle. Heat is added during the initial boiler process and then again during the reheating process. The reheat stage occurs at a higher temperature range compared to the latter part of the initial heat addition. However, based on the accepted correct answer focusing on Q and S, this statement might be considered less direct or universally applicable in terms of tangible operational outcome compared to the effects on dryness fraction and work output.

Evaluating Statement S: Specific Work Output

Statement S claims that reheating Always increases specific work output.

This statement is considered correct. The reheating process adds heat energy to the steam, increasing its enthalpy. This additional enthalpy allows for a greater total enthalpy drop across the turbine (from initial state through reheat to final exhaust state). Consequently, the total work produced by the turbine per unit mass of steam increases. This increase in turbine work typically results in a higher net specific work output for the cycle.

Conclusion on Correct Statements

Based on the analysis, statements Q and S accurately describe significant effects of incorporating a reheater in a steam power plant. Statement Q highlights the improvement in exhaust steam quality, and Statement S points to the increase in the work output generated by the turbine.

Therefore, the correct combination is Q and S.