Chapter 1 STEAM CYCLES

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1 Chapter 1 STEAM CYCLES Assoc. Prof. Dr. Mazlan Abdul Wahid Faculty of Mechanical Engineering Universiti Teknologi Malaysia 1 Chapter 1 STEAM CYCLES 1

Chapter Objectives To carry out 1 st law and 2

completes a thermodynamic cycle Steam/Vapour

to produce mechanical power output from energy

2 Chapter Objectives To carry out 1 st law and 2 nd law thermodynamic analysis on a vapour power plant in which the working fluid is alternatively vaporized and condensed as it completes a thermodynamic cycle Steam/Vapour Power Plant Is a thermodynamic heat engine used to produce mechanical power output from energy sources such as fossil fuel, nuclear. 3 Various type of Steam Power Plant 4 2

3 Simplified Model for Analysis A Energy conversion process occurs B Energy required to vaporize the liquid water C Cooling water circuit D Electric power generation 5 Simplified Model for Analysis A Energy conversion process occurs B Energy required to vaporize the liquid water C Cooling water circuit D Electric power generation 6 3

Simplified Model for Analysis 7 Basic Components in a Steam Cycles 1. Boiler: to transform liquid water into vapour (steam) of high pressure and temperature. 2.

4 Simplified Model for Analysis 7 Basic Components in a Steam Cycles 1. Boiler: to transform liquid water into vapour (steam) of high pressure and temperature. 2. Turbine-Generator: to transform kinetic energy of the vapour into mechanical power (rotating shaft). The mechanical power is used to drive an electric generator. 3. Condenser: to cool off the wet vapour exiting the turbine and transform it back into the liquid water 4. Feed-water pump: to deliver the water exiting the condenser back into the boiler, thus completing one thermodynamic cycle 8 4

5 9 10 5

6 11 Cycle for Vapour Power Plant The Rankine Cycle Basic With superheat Reheat cycle Regenerative cycle with open-type feedwater heater Regenerative cycle with closed-type feedwater heater *Thermodynamic heat engine ideally working in a Carnot cycle, any comment? 12 6

7 The Carnot Vapour Cycle The Carnot cycle is the most efficient cycle operating between two specified temperature limits but it is not a suitable model for power cycles. Because: Process 1-2 Limiting the heat transfer processes to two-phase systems severely limits the maximum temperature that can be used in the cycle (374 C for water) Process 2-3 The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear. Process 4-1 It is not practical to design a compressor that handles two phases. The cycle in (b) is not suitable since it requires isentropic compression to extremely high pressures and isothermal heat transfer at variable pressures. T-s diagram of two Carnot vapor cycles. 1-2 isothermal heat addition in a boiler 2-3 isentropic expansion in a turbine 3-4 isothermal heat rejection in a condenser 4-1 isentropic compression in a compressor 13 Cycle for Vapour Power Plant The Rankine Cycle Basic With superheat Reheat cycle Regenerative cycle with open-type feedwater heater Regenerative cycle with closed-type feedwater heater *Thermodynamic heat engine ideally working in a Carnot cycle, any comment? 14 7

Rankine Cycle: The Ideal Cycle for Vapour Power Cycles Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it

8 Rankine Cycle: The Ideal Cycle for Vapour Power Cycles Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser. The cycle that results is the Rankine cycle, which is the ideal cycle for vapor power plants. The ideal Rankine cycle does not involve any internal irreversibilities. 1-2 Isentropic expansion in a turbine 2-3 Constant pressure heat rejection in a condenser 3-4 Isentropic compression in a pump Constant pressure heat addition in a boiler The simple ideal Rankine cycle 15 Energy Analysis of Basic Rankine Cycle (ideal) The steam flows round the cycle and each process is analyzed using steady flow energy equation. Using energy balance for a steady flow system For single stream (one-inlet-one-exit) systems, mass flow rate remains constant. If kinetic and potential energy are negligible, the energy equation becomes 16 8

9 Energy Analysis of Basic Rankine Cycle (ideal) 1) The cycle analysis i) Boiler Since there is no work interaction between the working fluid and surrounding, W=0. Thus, heat addition to the working fluid kj/kg ii) Turbine Since the expansion process is assumed to be isentropic (reversible adiabatic), then Q=0. Thus, amount of work produced by turbine kj/kg 17 Energy Analysis of Basic Rankine Cycle (ideal) 1) The cycle analysis iii) Condenser No work interaction between the working fluid and surrounding, W=0. Heat rejected from working fluid to the cooling water kj/kg ii) Feed-water pump Since the pumping process is assumed to be isentropic,then Q=0. Thus, amount of work required by feed-water pump kj/kg 18 9

10 Energy Analysis of Basic Rankine Cycle (ideal) 2) Performance of steam plant i) Specific steam consumption (ssc) Define as the steam flow rate in kg/hr required to develop 1 kw of power output. The lower the ssc the more compact the steam plant kg/kw.s kg/kw.hr ii) Work ratio (wr) Define as the ratio of the net work produced by the plant to the work produced by the turbine 19 Energy Analysis of Basic Rankine Cycle (ideal) 2) Performance of steam plant iii) Thermal efficiency (η th ) Defined as the ratio of net work produced by the plant to the amount of heat added to the working fluid iv) Isentropic efficiency (η is ) The actual expansion and pumping processes are adiabatic but nor reversible. Thus, they are not isentropic

Energy Analysis of Basic Rankine Cycle (ideal) 2) Performance of steam plant v) Back work ratio Defined as the ratio of the work supplied to the feed-water pump to the work produced by turbine iv)

11 Energy Analysis of Basic Rankine Cycle (ideal) 2) Performance of steam plant v) Back work ratio Defined as the ratio of the work supplied to the feed-water pump to the work produced by turbine iv) Efficiency ratio 21 Example 1 A steam power plant operates between a boiler pressure of 50 bar and a condenser pressure of 0.03 bar. Calculate for these limits the thermal efficiency, the work ratio and the specific steam consumption: a) For a Rankinecycle with dry saturated steam at entry to the turbine b) For a Rankinecycle with the turbine isentropic efficiency of 85%. Sketch the cycle on a T-s diagram 22 11

Cycle for Vapour Power Plant The Rankine Cycle Basic With superheat Reheat cycle Regenerative cycle with open-type feedwater heater Regenerative cycle with closed-type feedwater heater *Thermodynamic

12 Cycle for Vapour Power Plant The Rankine Cycle Basic With superheat Reheat cycle Regenerative cycle with open-type feedwater heater Regenerative cycle with closed-type feedwater heater *Thermodynamic heat engine ideally working in a Carnot cycle, any comment? 23 Rankine cycle with Superheat Why superheat? Improvement in the basic Rankine cycle Steam temperature at inlet to the turbine is increased at boiler pressure, thus increasing the average temperature of heat addition. Increase the cycle efficiency Steam exits the turbine is more dry Specific steam consumption drops The technique The saturated steam exiting the boiler is passed through a second bank of smaller tubes located within the boiler, heated by the hot gas from the furnace 24 12

13 Rankine cycle with Superheat Degree of superheat 25 Rankine cycle with & without Superheat Basic Rankine Cycle Rankine Cycle with superheat 26 13

14 Example 2 Reconsider the vapourpower cycle of Example 1. Calculate it s thermal efficiency and s.s.cif the steam exiting the boiler is heated to 500 C before entering the turbine. Assume the pump work is small and can be neglected. Sketch the cycle on a T-s diagram 27 Cycle for Vapour Power Plant The Rankine Cycle Basic With superheat Reheat cycle Regenerative cycle with open-type feedwater heater Regenerative cycle with closed-type feedwater heater *Thermodynamic heat engine ideally working in a Carnot cycle, any comment? 28 14

15 Rankine cycle with Reheating Improvement in the superheat Rankine cycle The average heat addition is increased in another way Usually, steam is reheated to the inlet temperature of the high-pressure turbine The dryness fraction of the steam exiting the turbine stages is further increased, which is the desired effect Specific steam consumption is improved (decrease) The steam is reheated at constant pressure 29 Rankine cycle with Reheating Improvement in the superheat Rankine cycle The average heat addition is increased in another way Usually, steam is reheated to the inlet temperature of the high-pressure turbine The dryness fraction of the steam exiting the turbine stages is further increased, which is the desired effect Specific steam consumption is improved (decrease) The steam is reheated at constant pressure 30 15

16 Rankine cycle with Reheating 31 Rankine cycle with Reheating The cycle analysis i) Heat input..? ii) Work output.? iii) Work input? 32 16

17 Example 3 33 The enthalpy-entropy (h-s) chart Also known as Mollier diagram or h-s diagram The chart contains a series of constant temperature lines, a series of constant pressure lines, a series of constant quality lines and a series of constant superheat lines 34 17

18 Mollier diagram 35 Cycle for Vapour Power Plant The Rankine Cycle Basic With superheat Reheat cycle Regenerative cycle with open-type feedwater heater Regenerative cycle with closed-type feedwater heater *Thermodynamic heat engine ideally working in a Carnot cycle, any comment? 36 18

19 The Regenerative Cycle What is regeneration process? In a regenerative cycle, the feed-water is preheated in a feed-water heater (FWH), using some amount of steam bled off the turbine, before it is delivered back into the boiler. The preheating process occurs in the FWH at a constant pressure. The steam required for heating the feed-water is bled off the turbine at certain bleeding pressure, P bleed. 37 The Regenerative Cycle Purpose of regeneration process The main purpose of regeneration process is to increase the thermal efficiency If the feed-water is preheated before entering the boiler, then less heat will be required to transform the feed-water into steam, in the boiler As a result, thermal efficiency of the plant increases 38 19

The plant can use more than one open feed-water heater Each open-type FWH requires one extra pump 2) Closed-type Feed-water heater An closed-type FWH is basically a heat exchanger The feed-water

20 The Regenerative Cycle Types of Feed-water Heater (FWH) There are 2 types of feed-water heater; an open-type and a closed-type. 1) Open-type Feed-water heater An open-type FWH is basically a mixing chamber The feed-water is preheated by direct mixing with the steam extracted from the turbine. The plant can use more than one open feed-water heater Each open-type FWH requires one extra pump 2) Closed-type Feed-water heater An closed-type FWH is basically a heat exchanger The feed-water does not mix freely with the bled off steam, hence both fluids can be at different pressure. The condensate exiting the closed-type is throttled back into condenser and mix with the feed-water in the condenser 39 The Regenerative Cycle: Open-type FWH Ideal regenerative cycle using open-type FWH 40 20

21 The Regenerative Cycle: Closed-type FWH Ideal regenerative cycle using closed-type FWH 41 The Regenerative Cycle: Open-type FWH 42 21

22 The Regenerative Cycle: Open-type FWH 43 The Regenerative Cycle: Open-type FWH 44 22

23 The Regenerative Cycle: Open-type FWH 45 The Regenerative Cycle: Closed-type FWH Ideal regenerative cycle using closed-type FWH 46 23

24 The Regenerative Cycle: Closed-type FWH 47 The Regenerative Cycle: Closed-type FWH 48 24

25 The Regenerative Cycle 49 The Regenerative Cycle 50 25

26 The Regenerative Cycle 51 The Regenerative Cycle 52 26

Chapter 10 VAPOR AND COMBINED POWER CYCLES

Thermodynamics: An Engineering Approach, 6 th Edition Yunus A. Cengel, Michael A. Boles McGraw-Hill, 2008 Chapter 10 VAPOR AND COMBINED POWER CYCLES Copyright The McGraw-Hill Companies, Inc. Permission