Thermodynamic Cycles

1. Thermodynamic Cycles • Objective • Classification of Thermodynamics Cycles • Analysis & Calculation of Thermodynamic Cycles • Carnot Vapor Cycle, Rankie Cycle, Regeneration Rankie Cycle,Reheat Rankie Cycle • Cogeneration • Gas Refrigeration Cycle • Vapor-Compression Refrigeration Cycle • Refrigerant • Other Refrigeration Cycles

2. Classification of Thermodynamics Cycles Power Cycle (+) Heat Energy Mechanical Energy Heat Pump Cycle (－) Refrigeration Cycle: keep low temperature of heat source with low temperature Heat Pump Cycle: keep high temperature of heat source with high temperature Working Fluid Gas Cycle: no phase-change of working fluid during cycle Vapor Cycle: phase-change of working fluid during cycle Combustion form

3. Carnot Vapor Cycle Several impracticalities are associated with this cycle: 1. It is impractical to design a compressor that will handle two phases for isentropic compression process(4-1). 2. The quality of steam decrease during isentropic expansion process(2-3) which do harm to turbine blades.

4. Carnot Vapor Cycle 3. The critical point limits the maximum temperature used in the cycle which also limits the thermal efficiency. 4. The specific volume of steam is much higher than that of water which means large amount of work and equipments input.

5. S 6 1 4 2 3 Rankine Vapor Cycle 4-6 Constant pressure heat addition in a boiler 6-1 to Superheated Vapor 1-2 Isentropic expansion in a turbine 2-3 Constant pressure heat rejection in a condenser 3-4 Isentropic compression in a pump

6. p S 6 p1 1 4 5 6 1 4 p2 3 2 v T 2 1 5 3 6 4 2 3 s Rankine Vapor Cycle

7. Thermal Efficiency of Rankine Vapor Cycle 4-5-6-1 Constant pressure heat addition in a boiler 1-2 Isentropic expansion in a turbine 2-3 Constant pressure heat rejection in a condenser 3-4 Isentropic compression in a pump

8. Thermal Efficiency of Rankine Vapor Cycle 1.Hard compressibility of water 2. Ek,Ep=0

9. Thermal Efficiency of Rankine Vapor Cycle Definition: d(汽耗率) — the heat rate required to generate work of

10. Increase Efficiency of Rankine Vapor Cycle

11. 1’ 1 5’ 5 4 3 2’ 2 Increase Efficiency of Rankine Vapor Cycle 1. － Pressure of Steam, Turbine Inlet －Unchange Two Cycles: ① 3-4-5-1-2-3 ② 3-4-5’-1’-2’-3

12. 1’ 1 5’ 5 4 3 2’ 2 Increase Efficiency of Rankine Vapor Cycle Disadvantages: 1. The presence of large quantities of moisture is highly desirable because it decrease the turbine efficiency and erodes the turbine blades. Increase of requirements on pressure vessels and equipment investment. 2.

13. 1’ 1 5 6 4 3 2 2’ Increase Efficiency of Rankine Vapor Cycle 2. － Temperature of Steam, Turbine Inlet －Unchange Two Cycles: ① 3-4-5-6-1-2-3 ② 3-4-5-6-1’-2’-3

14. 1’ 1 5 6 4 3 2 2’ Increase Efficiency of Rankine Vapor Cycle Advantages: i ii Superheating the steam to higher temperature is desirable because it decreases the moisture content of the steam at the turbine exit. Disadvantages: Superheating temperature is limited by metallurgical considerations.

15. 1 6 5 4 4’ 2 3 3’ 2’ Increase Efficiency of Rankine Vapor Cycle 3. － Condenser Pressure, Turbine Exit －Unchange Two Cycles: ① 1-2-3-4-5-6-1 ② 1-2’-3’-4’-5-6-1

16. 1 6 5 4 4’ 2 3 3’ 2’ Increase Efficiency of Rankine Vapor Cycle i Disadvantages: ii i Condense pressure is limited by saturation pressure corresponding to the temperature. ii It increases the moisture content which is highly undesirable.

17. Increase Efficiency of Rankine Vapor Cycle Example • Consider a steam power plant operating on the ideal Rankine cycle. • The steam enters the turbine at 2.5MPa and 350℃ and is condensed • in the condenser at pressure of 70kPa. Determine • The thermal efficiency of this power plant • The thermal efficiency if steam is condensed at pressure of 10kPa • The thermal efficiency if steam is superheated to 600 ℃ • The thermal efficiency if the boiler pressure is raised to 15MPa • while the turbine inlet temperature is maintain at 600 ℃

18. 3 2.5MPa 2 70kPa 1 4 State 1: State 2: Ideal Rankine Cycle State 3:

19. 3 2.5MPa 2 70kPa 1 4 State 4:

20. State 1: State 2: (b) Lowing the pressure of Condenser State 3:

21. 2.5MPa State 4:

22. Actual Rankine Vapor Cycle Irreversibility • Fluid friction • Heat transfer under temperature • difference • Heat loss to the surroundings

23. 1 5 6 3(4) 2 2’ Actual Rankine Vapor Cycle Turbine Efficiency Ideal Cycle Actual Cycle

24. Actual Rankine Vapor Cycle Mechanical Efficiency Relative Effective Efficiency Effective Power Boiler Efficiency Equipment Efficiency

25. T 1 6 5 7 2 3(4) e d s Ideal Regenerative Cycle 预热锅炉给水，使其温度升高后再进入锅炉， 可提高水在锅炉内的平均吸热温度，减小水与 高温热源的温差，对提高循环效率有利。 利用汽轮机中的蒸汽预热锅炉给水，称为回热。 Transfer heat to the feedwater from the expanding steam in a heat exchanger built into the turbine ,called Regeneration. Regenerative Cycle: 1-7-d-3-4-5-6-1 General Carnot Cycle:3-4-5-7-d-3 Ideal Carnot Cycle: 5-7-2-e-5 Same Efficiency

26. 1 Turbine Boiler 7 2 Regenerator Mixing Chamber Condenser 4 6 5 3 Pump II Pump I Ideal Regenerative Cycle Extracting Regeneration

27. T 1 1kg 6 akg 7 5 (1-a)kg 3(4) 2 s Ideal Regenerative Cycle >0

28. T 1 9 8 6 7 4 5 3 2 s Ideal Regenerative Cycle 1 Turbine Boiler Regenerator 7 2 Mixing Chamber 8 Cond- enser 9 4 6 5 3 Pump II Pump I

29. Ideal Reheat Cycle 蒸汽经汽轮机绝热膨胀至某一中间压力时全部引出，进入锅炉中特设的再加热器中再加热。温度升高后再全部引入汽轮机绝热膨胀做功。称为再热循环。

30. 1 a 5 b 6 4 3 c 2 Ideal Reheat Cycle intermediate pressure

31. Cogeneration • Definition • Cogenerationis the production of more than one • useful form of energy from the same energy source. • electric power • heat in low quality 背压式热电联供 抽气式热电联供

32. Gas Refrigeration Cycle Ideal Reversed Carnot Cycle T1 — Temperature of heat source with high temperature, surrounding temperature T2 — Temperature of heat source with low temperature, cold source q1 — Heat rejected to the surroundings q2 — Heat absorbed from cold source w0 — Work input

33. Condenser 3 2 Compressor Turbine 1 4 Cold Source Gas Refrigeration Cycle 1-2 Isotropic Compress 2-3 Isotonic Heat Rejection to Surrounding 3-4 Isotropic Expansion 4-1 Isotonic Heat Absorption

34. p 3 2 4 1 v T 2 3 T3 T1 1 4 s Gas Refrigeration Cycle Cp— Constant, Ideal Gas • Heat Absorbed from Cold Source • Heat Rejected to the condenser • Work of Compressor • Work of Turbine

35. Gas Refrigeration Cycle

36. Gas Refrigeration Cycle

37. Vapor-Compression Refrigeration Cycle • Shortcomings of Gas-Compression Refrigeration Cycle • 1.small Refrigeration-Coefficient because heat absorption • and rejection are not isothermal process; • 2.Lower refrigeration capability of refrigerant (gas) • So…refrigerant is change to Vapor • The highest efficiency is that of Vapor Carnot Reverse Cycle Impracticalities: 1.Large moisture content is highly undesirable for compressor and turbine. 2.Work output is limited by liquid expansion in the turbine.

38. Vapor-Compression Refrigeration Cycle • So…practical vapor-compression refrigeration cycle is: 2 2 3 4 3 4 1 1 6 5

39. 2 4 3 1 6 5 Vapor-Compression Refrigeration Cycle

40. 2 4 3 1 6 5 Vapor-Compression Refrigeration Cycle Throttle: ① fluid with low quality is difficult to be compressed. ② work loss is relatively small ③ easily adjust pressure of fluid and temperature of cold source Work difference between Turbine and throttle

41. Vapor-Compression Refrigeration Cycle Regeneration — more realistic cycle Advantages: 1. 2. 3.Superheated vapor is desirable T 2 Super- cooled Liquid 3 4 4’ Superheated Vapor 1’ 5’ 5 1 s

42. Condenser 2 4 1’ Compressor Regenerator Throttle Valve 4’ 1 Cold Source 5’ Vapor-Compression Refrigeration Cycle Conditions:

43. 4 3 2 2’ 5 1 Vapor-Compression Refrigeration Cycle Irreversibility 1-2’ Isotropic Compress Efficiency 制冷机的制冷能力是随 工作条件不同而变化的， 因此，给出制冷能力时， 必须指明相应的工作条件。