--- title: "Steam plant components and reheating" subject: "Power Plant Equipment" chapter: 7 key: "semester_7/power_plant_equipment:steam-components-reheating" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/steam-components-reheating site: BCT Engineering --- # Steam plant components and reheating Power Plant Equipment · Chapter 7: Thermal (Steam) Power Plant - Asked in: 2082 Baisakh, 3+2+3 marks - Video: [Steam Power Plant Layout & Working Principle](https://www.youtube.com/watch?v=ChvI2v85fsU) — Electrical4U - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/steam-components-reheating - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7 ## Question Sketch the main components of steam turbine power plant. Explain how reheating increases efficiency of the plant. Draw the layout and show the cycle in T-s diagram. ## Short answer A steam power plant comprises boiler, turbine, condenser, and feed pump. Reheating raises efficiency by reheating partly expanded steam to high temperature, which increases the mean heat-addition temperature, boosts net work per kg, and reduces exhaust wetness. ## Answer #### Main components of a steam turbine power plant A steam (thermal) power plant works on the **Rankine cycle**. Its main parts are: ```text superheated steam (1) +---------------+ -------------> +---------+ +-----+ | Boiler | | Steam |===| Gen | | (furnace, | | turbine | +-----+ | economiser, | +---------+ | superheater) | | exhaust (2) +---------------+ v ^ ^ | +-----------+ fuel| air| v flue gas | Condenser |<--cooling | | chimney +-----------+ water | | condensate (3) | feed water (4) +-----------+ | +------------------| Feed pump |<--+ +-----------+ ``` - **Boiler (steam generator):** burns coal/oil/gas and converts feed water into high-pressure steam. Includes furnace, water walls, drum, **superheater**, **reheater**, **economiser** (heats feed water by flue gas) and **air preheater**. - **Steam turbine:** expands steam and converts its heat energy into shaft work; drives the **alternator** (generator). - **Condenser:** condenses exhaust steam under vacuum using cooling water; the low back pressure increases turbine work. Cooling water is cooled in a **cooling tower** or taken from a river. - **Feed pump** (with condensate pump): returns the condensate to boiler pressure, usually through **feed water heaters** (regeneration). - **Fuel and ash handling, draught system** (FD/ID fans), **chimney**, **ESP/dust collector**. - **Water treatment plant** for make-up water. #### How reheating increases efficiency In a reheat cycle the steam leaving the boiler (state 1) expands in the **HP turbine** only up to an intermediate pressure (state 2). It is taken back to the **reheater** in the boiler and heated at constant pressure to nearly the original temperature (state 3). It then expands in the **LP turbine** to condenser pressure (state 4). How reheating increases efficiency and output: - **Higher mean temperature of heat addition:** heat added in reheating (2-3) is at a high temperature. If the reheat pressure is chosen properly (about 20-25% of boiler pressure), the average temperature at which heat is supplied rises, so $\eta$ rises by about 4-5%. - **More work per kg of steam:** the expansion 3-4 lies to the right of the original expansion line, so the specific work output increases. The steam consumption (kg/kWh) falls and the plant output for the same boiler size increases. - **Drier exhaust steam:** without reheat, expansion from high pressure ends in very wet steam (dryness below 0.85). Reheat gives exhaust dryness about 0.9 or more, which reduces blade erosion and friction loss, so the **turbine internal efficiency** improves. - It makes very high boiler pressures usable, which is itself a means of raising efficiency. Cycle efficiency with reheat: $$ \eta = \frac{(h_1 - h_2) + (h_3 - h_4) - W_p}{(h_1 - h_6) + (h_3 - h_2)} $$ Limitations: extra cost of reheater, piping and controls; useful only for large units (above about 100 MW) and high pressures. #### Layout and T-s diagram of reheat cycle ```text 1 +----------+----->+------+ +------+ | Boiler | | HP |=====| LP |===> Generator | + S/H | 2 | turb | | turb | | Reheater |<-----+------+ +->+------+ | |------------3---+ | 4 +----------+ v ^ 6 +-----------+ | +------+ 5 | Condenser | +------| Pump |<-------+-----------+ +------+ ``` ```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 ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. On which thermodynamic cycle does a steam turbine power plant work? A. Brayton B. Rankine C. Otto D. Diesel Answer: B. Steam (thermal) power plants work on the Rankine cycle. (Key point: Cycle of steam plant) 2. What is the role of the condenser? A. Generate superheated steam B. Condense exhaust steam under vacuum, giving low back pressure and more turbine work C. Pump water to boiler pressure D. Remove ash from the furnace Answer: B. The condenser condenses exhaust steam under vacuum; the low back pressure increases turbine work. (Key point: Role of condenser) 3. In a reheat cycle, what happens to steam after the HP turbine? A. It goes directly to the condenser B. It returns to the boiler reheater, is heated at constant pressure to near original temperature, then expands in the LP turbine C. It is mixed with feed water in a heater D. It is throttled to condenser pressure Answer: B. HP exhaust is reheated at constant intermediate pressure (2-3) and then expanded in the LP turbine (3-4). (Key point: Reheat process) 4. How does reheating benefit the turbine exhaust steam? A. It makes the exhaust wetter, improving lubrication B. It makes the exhaust drier, reducing blade erosion C. It makes no change to dryness D. It makes the exhaust superheated at condenser pressure always Answer: B. Reheat raises exhaust dryness to about 0.9 or more, reducing blade erosion and friction loss. (Key point: Exhaust dryness benefit) 5. What is the proper reheat pressure, and where is reheating economical? A. About 20-25% of boiler pressure; large units above about 100 MW B. About 80-90% of boiler pressure; any small unit C. About 1% of boiler pressure; small units D. Equal to condenser pressure; all units Answer: A. Reheat pressure about 20-25% of boiler pressure gives the best gain, and the extra cost makes it useful only for large high-pressure units. 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