--- title: "Thermal plant advantages, superheating and reheating" subject: "Power Plant Equipment" chapter: 7 key: "semester_7/power_plant_equipment:thermal-superheating-reheating" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/thermal-superheating-reheating site: BCT Engineering --- # Thermal plant advantages, superheating and reheating Power Plant Equipment · Chapter 7: Thermal (Steam) Power Plant - Asked in: 2080 Bhadra, 8 marks - Video: [Reheat Rankine Cycle || Reheat Cycle || Reheating of Steam || Advantage of Reheat Rankine Cycle](https://www.youtube.com/watch?v=WjJTdFM9ECE) — Power Plant Guide - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/thermal-superheating-reheating - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7 ## Question What are the advantages of thermal power plant? Explain superheating and reheating with T-S diagram. ## Short answer Thermal plants offer readily available fuel, lower initial cost, load-centre siting, less space, and short construction. Superheating heats saturated steam above saturation, raising mean heat-addition temperature and exhaust dryness. Reheating reheats partially expanded steam, further increasing mean temperature, net work, and exhaust dryness. ## Answer #### Advantages of thermal (steam) power plant - Fuel (coal, oil, gas) is readily available and easy to transport, and the initial cost is lower than that of hydro and nuclear plants. - Can be located **near the load centre**, so transmission cost and losses are small. - Needs **less space** than a hydro plant of the same capacity. - **Construction time is short** compared to hydro plants. - Output does not depend on rainfall; it can run as a base-load plant all year. - Unit sizes up to about 1000 MW are possible; the steam can also be used for process heating (cogeneration). - Overload capacity is good. #### Superheating Superheating is heating dry saturated steam at constant boiler pressure, in the **superheater**, to a temperature above its saturation temperature (process g-1). ```text T ^ 1 | /| superheat g-1 | f---------g | | / boiling | | / | | 3 | | 4---------------2 +-----------------------> s 3-4 pump, 4-f-g-1 boiler (heating, boiling, superheating), 1-2 turbine, 2-3 condenser (4 and 3 almost coincide) ``` Benefits: - Heat is added at a higher temperature, so the **mean temperature of heat addition and cycle efficiency increase**. - **Work per kg of steam increases** (area of the T-s diagram grows), so steam consumption falls. - Exhaust steam (state 2) is **drier**, reducing erosion and friction loss in the last turbine stages. - Superheated steam has no moisture, so losses in pipes are lower. Limit: maximum temperature is fixed by metallurgy of tubes and blades (about 540-600 °C). #### Reheating In reheating, steam that has partially expanded in the HP turbine is returned to the boiler and reheated at constant (intermediate) pressure to about the original temperature, then expanded in the LP turbine. ```text T ^ 1 3 | /| /| | / | / | | f--------g' | / | | / 2------' | | / | | 6 | | 5--------------------------4 +--------------------------------> s 5-6 pump, 6-f-g'-1 boiler (heat, boil, superheat), 1-2 HP turbine, 2-3 reheater, 3-4 LP turbine, 4-5 condenser ``` Benefits: - Higher mean temperature of heat addition (reheat at 20-25% of boiler pressure), so efficiency increases by about 4-5%. - Work output per kg increases, steam rate falls. - Exhaust dryness increases (above about 0.88), reducing blade erosion; allows use of high boiler pressure. Drawbacks: higher cost and more complex piping and control; economic only for large units. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is an advantage of a thermal power plant over a hydro plant? A. Zero fuel cost B. Can be located near the load centre and built quickly C. No air pollution D. Very long plant life Answer: B. Thermal plants need less space, have shorter construction time, and can be sited near the load, cutting transmission cost. (Key point: Advantages of thermal plant) 2. What is superheating? A. Heating saturated water to boiling at constant pressure B. Heating dry saturated steam at constant pressure above its saturation temperature in the superheater C. Heating steam after the HP turbine in the reheater D. Compressing steam to higher pressure Answer: B. Superheating is constant-pressure heating of dry saturated steam above saturation temperature (process g-1). (Key point: Definition of superheating) 3. Which is a benefit of superheating? A. Lower mean temperature of heat addition B. Higher mean heat-addition temperature, more work per kg and drier exhaust C. Wetter exhaust steam D. Lower turbine inlet enthalpy Answer: B. Heat is added at higher temperature, work per kg rises and exhaust is drier, reducing erosion. (Key point: Benefits of superheating) 4. What fixes the maximum superheat temperature? A. Condenser pressure B. Metallurgy of tubes and blades (about 540-600 C) C. Feed pump capacity D. Cooling water temperature Answer: B. Material strength at high temperature limits steam temperature to about 540-600 C. (Key point: Limit of superheat) 5. In reheating, to what is the steam reheated and at what pressure? A. To about the original temperature at constant intermediate pressure B. To condenser temperature at constant volume C. To a lower temperature at boiler pressure D. To saturation at condenser pressure Answer: A. Partly expanded steam is reheated at constant intermediate pressure to about the original temperature. 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