--- title: "Regenerative scheme with open feed water heater" subject: "Power Plant Equipment" chapter: 7 key: "semester_7/power_plant_equipment:regenerative-open-heater-layout" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/regenerative-open-heater-layout site: BCT Engineering --- # Regenerative scheme with open feed water heater Power Plant Equipment · Chapter 7: Thermal (Steam) Power Plant - Asked in: 2072 Chaitra, 8 marks - Video: [Regenerative Rankine Cycle with Open Feed Water Heater explained with diagram & graph](https://www.youtube.com/watch?v=qZV4Lwjry2s) — Pratik S Pagade - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/regenerative-open-heater-layout - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7 ## Question List the common methods used for performance of a steam turbine power plant. Sketch layout for a regenerative scheme with an open feed water heater. Explain its working with corresponding processes on T-S diagram. ## Short answer Steam bled from the turbine heats condensate in a direct-contact open feedwater heater, raising mean heat addition temperature. The saturated mixture is pumped to boiler pressure; less heat is rejected in the condenser, improving efficiency. ## Answer #### Common methods of improving performance The efficiency of a Rankine (steam) cycle is $\eta = 1 - \dfrac{T_{\text{low}}}{T_{m,\text{in}}}$ roughly, so it improves by **raising the mean temperature of heat addition** $T_{m}$ or **lowering the temperature of heat rejection**. Common methods: 1. **Increasing boiler (steam) pressure** - raises the saturation temperature at which heat is added. 2. **Superheating the steam** (higher turbine inlet temperature) - raises mean temperature of heat addition and dryness at exhaust. 3. **Reducing condenser pressure** (better vacuum) - lowers heat rejection temperature. 4. **Reheating** - steam partly expanded in HP turbine is reheated in the boiler and expanded in LP turbine. 5. **Regenerative feed water heating** - steam bled from the turbine heats the feed water in feed water heaters. 6. **Using economiser and air preheater** - recover heat from flue gases. 7. **Supercritical / binary vapour and combined cycles** - use very high pressure, or a topping cycle (gas turbine) whose waste heat runs the steam cycle. 8. Reducing losses: better turbine blading, insulation, reduced throttling. #### Regenerative scheme with an open feed water heater An **open (direct contact) feed water heater** is a mixing chamber in which bled steam and feed water mix directly; the outlet is saturated water at heater pressure. ```text 1 +--------+------>+---------------+ +-----+ | Boiler | | Turbine |===| Gen | | + S/H | +---------------+ +-----+ +--------+ | m kg (2) | (1-m) kg (3) ^ 7 v v | +-----------+ +-----------+ +--------+ 6 | Open FWH | | Condenser | | Pump 2 |<---| (mixing) | +-----------+ +--------+ +-----------+ | 4 ^ 5 | +--------+ | | Pump 1 |<--------+ +--------+ ``` #### Working 1. Superheated steam from the boiler enters the turbine at state 1. 2. A fraction $m$ kg/kg is extracted at the intermediate pressure (state 2) and sent to the open heater. 3. The remaining $(1-m)$ kg expands to condenser pressure (3) and is condensed to saturated water (4). 4. **Pump 1** raises the condensate to heater pressure (5). 5. In the open heater the bled steam condenses and gives its latent heat to the feed water; the mixture leaves as saturated water at heater pressure (6). Energy balance: $m h_2 + (1-m) h_5 = h_6$. 6. **Pump 2** raises it to boiler pressure (7); the boiler heats it from state 7 instead of from cold condensate. Because feed water enters the boiler hot, the mean temperature of heat addition rises and less heat is rejected in the condenser, so the efficiency increases. A separate pump is needed after each open heater; the open heater also removes dissolved gases (deaerator). #### T-s diagram ```text T ^ 1 | /| | f----------g | | / | | 7 | | 6 . . . . . . . 2 bleed (m kg) | / | | 5 | | 4-----------------3 (1-m) kg +-------------------------> s 4-5 pump 1, 5 and 2 mix in FWH -> 6, 6-7 pump 2, 7-f-g-1 boiler, 1-2-3 turbine, 3-4 condenser ``` $$ \eta = \frac{(h_1 - h_2) + (1-m)(h_2 - h_3) - [(1-m)W_{p1} + W_{p2}]}{h_1 - h_7} $$ ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does regenerative feed water heating raise the efficiency of a steam cycle? A. It raises the mean temperature of heat addition and reduces heat rejected in the condenser B. It lowers the boiler pressure so the pump needs less work C. It increases the dryness of steam at the condenser inlet only D. It removes the need for a condenser Answer: A. Feed water enters the boiler hot, so heat is added at a higher mean temperature, and less steam reaches the condenser to reject heat. (Key point: Why efficiency improves) 2. In an open (direct contact) feed water heater, what is the state of the fluid leaving the heater? A. Superheated steam at boiler pressure B. Saturated water at the heater pressure C. Subcooled water at condenser pressure D. Wet steam at heater pressure Answer: B. Bled steam mixes with feed water directly and condenses, so the mixture leaves as saturated water at the heater pressure. (Key point: Open heater principle) 3. For m kg of bled steam (h2) mixing with (1-m) kg of feed water (h5) to give h6, and what does this arrangement need afterwards? A. m h2 + (1-m) h5 = h6; a second pump to raise the water to boiler pressure B. m h2 = (1-m) h5 + h6; no further pump C. m h5 + (1-m) h2 = h6; a second pump to condenser pressure D. h2 = h6; a throttle valve before the boiler Answer: A. The heater is a mixing chamber so energy in equals energy out, and since the outlet is at heater pressure, pump 2 is needed to reach boiler pressure. (Key point: Energy balance and pumps) 4. In the open-heater scheme, what fraction of steam reaches the condenser? A. m kg B. (1-m) kg C. 1 kg D. (1+m) kg Answer: B. m kg is bled at the intermediate pressure to the heater; the remaining (1-m) kg expands to condenser pressure. 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