Lecture Notes on Thermodynamics 2008

1. Lecture Notes on Thermodynamics 2008 Chapter 10 Steam Power Cycles Prof. Man Y. Kim, Autumn 2008, ⓒmanykim@chonbuk.ac.kr, Aerospace Engineering, Chonbuk National University, Korea

2. Carnot Cycle – Review Reversible work done at the moving boundary : Ideal gas : and Assuming no changes in KE and PE : Let’s integrate the above equation : ① → ②(Isothermal Heat Addition Process) : ② → ③(Adiabatic Expansion Process) : ③ → ④(Isothermal Heat Rejection Process) : ④ → ①(Adiabatic Compression Process) : From the Equations in adiabatic process : or We can find . Finally, Comments on efficiency :

3. Rankine Cycle – Ideal Steam Power Cycle ③ Saturated or superheated vapor • ideal, 4 steady-state process cycle Expansion • utilizing a phase change between vapor and liquid to maximize the difference in v during expansion and compression • idealized model for a steam power plant system ① →② : Pump – reversible adiabatic pumping (compression) process ②→ ③: Boiler – constant pressure heat transfer ③→ ④: Turbine – reversible adiabatic expansion ④→ ①: Condenser – constant pressure heat transfer • Heat transferred to the working fluid ( ): area a-2-2’-3-b-a ① Saturated liquid Compression • Heat transferred from the working fluid ( ): area a-1-4-b-a • Thermal efficiency (neglecting changes in KE and PE) • Temperature between 2-2’ is less than the temperature during evaporation • Why Rankine than Carnot ? • pumping process • superheating the vapor • see Example 10–1

4. Rankine Cycle – Efficiency ★ Effect of Superheating the Steam in the Boiler ★ Effect of Maximum Pressure of the Steam ★ Effect of Exhaust Pressure Drop • w ↑ by area 3-3’-4’-4-3, qH↑ by area 3-3’-b’-b-3 → cycle efficiency↑ • Average T↑ at which heat is transferred to the steam → cycle efficiency↑ • quality ↑ • If single cross-hating area = double cross-hatching area, average T↑ at which heat is transferred to the steam → cycle efficiency↑ • quality ↓ • T↓ in which heat is rejected → cycle efficiency↑ • moisture content ↑ → erosion of the turbine blade • see Example 10–3

5. Real Steam Power Cycle – Losses Turbine Losses • major loss • 3-4s : ideal isentropic turbine expansion process • 3-4 : actual irreversible process in the turbine • flow of working fluid through the turbine blades and passages • heat transfer to the surroundings Pump Losses • 1-2s : ideal isentropic pump compression process • 1-2 : actual irreversible process in the pump • irreversibility with the fluid flow of working fluid Piping Losses • a-b : entropy increase due to friction • b-c : entropy decrease due to heat transfer Condenser Losses • Heat transfer by raising water to its saturation temperature

6. Rankine Cycle – Reheative and Regenerative Cycle • Reheative Rankine Cycle • 2-2’ : working fluid is heated while in the liquid phase, 2’-3 : vaporization process → The process 2-2’ cause the average T at which heat is supplied to be lower than in the Carnot cycle 1’-2’-3-4-1’ → Efficiency of Rankine cycle < corresponding Carnot cycle → In the regenerative cycle the working fluid enters the boiler at some state between 2-2’, and consequently the average T at which heat is supplied is higher • Regenerative cycle : FWH (Feedwater Heater) → Average T at which heat is supplied has been increased • Open / Closed Feedwater Heater • see Examples 10–4, 10–5, and 10–6

7. Homework #10 Solve the Problems 10–1C, 2C, 3, 5, 21