1. 6. Thermodynamic Cycles Objective
Classification of Thermodynamics Cycles
Analysis & Calculation of Power 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. 6.1 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
Inner Combustion
Outer Combustion
Combustion occurs in system
Combustion occurs out of system
Gas is also the working fluid.
The heat is transferred to working fluid through heat exchanger.

3. 6.2 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. 6.2 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 needs big equipments and large amount of work input.

5. 6.2 Carnot Vapor Cycle

6. 6.3 Rankine Vapor Cycle Principle S 4-6 Constant pressure heat1 2 3
4-6 Constant pressure heat
addition in a boiler
6-1 to Superheat Vapor
1-2 Isentropic expansion in
a turbine
2-3 Constant pressure heat
rejection in a condenser
3-4 Isentropic compression in
a pump

7. 6.3 Rankine Vapor Cycle

8. 6.3 Rankine Vapor Cycle S p v 1 6 5 4 3 2 p1 p2 4 6 1 2 3 T s 1 6 5 4

9. 6.3 Rankine Vapor Cycle Efficiency
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

10. 6.3 Rankine Vapor Cycle
Because of uncompressibility of water

11. 6.3 Rankine Vapor Cycle Definition:
d — the steam required to generate work of

12. 6.3 Rankine Vapor Cycle
Influencing factors

13. 6.3 Rankine Vapor Cycle 1. － Pressure of Steam, Turbine Inlet4 5 5’ 1’ 1 2 2’
－Unchange
Two Cycles: ①
② 3-4-5’-1’-2’-3

14. 6.3 Rankine Vapor Cycle Disadvantages: 1. 2. 3 4 5 5’ 1’ 1 2 2’
decrease the turbine efficiency and
erodes the turbine blades.
Increase of requirements on pressure
vessels and equipment investment. 2.

15. 6.3 Rankine Vapor Cycle 2. － Temperature of Steam, Turbine Inlet4 5 1 1’ 2’ 2 6
－Unchange
Two Cycles: ①
② ’-2’-3

16. 6.3 Rankine Vapor Cycle Advantages: i ii Disadvantages:4 5 1 1’ 2’ 2 6 i ii
it decreases the moisture content
of the steam at the turbine exit.
Disadvantages:
Superheating temperature is limited
by metallurgical considerations.

17. 6.3 Rankine Vapor Cycle 3. － Condenser Pressure, Turbine Exit4 5 1 3’ 2’ 2 6 4’
－Unchange
Two Cycles: ①
② 1-2’-3’-4’-5-6-1

18. 6.3 Rankine Vapor Cycle i Disadvantages: ii4 5 1 3’ 2’ 2 6 4’ i
Disadvantages: ii
i Condense pressure is limited
by the sink temperature.
ii It increases the moisture
content which is highly
undesirable.

19. 6.3 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 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 ℃

20. State 1:
State 2:
Ideal Rankine Cycle

21. State 3:
State 4:

22. 6.3 Rankine Vapor Cycle Actual cycle Irreversibility Flow friction
Heat transfer under temperature
difference
Heat loss to the surroundings

23. 6.3 Rankine Vapor Cycle Actual Rankine Vapor Cycle Turbine Efficiency2’
3(4) 2 1 5 6
Turbine Efficiency
Ideal Cycle
Consumed Steam kg/h
Actual Cycle

24. 6.3 Rankine Vapor Cycle Mechanical Efficiency
Relative Effective Efficiency
Effective Power
Boiler Efficiency
Equipment Efficiency

25. 6.4 Improvement to Rankine 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.
3(4) e 2 7 1 d 5 6 T s
Disadvantages:
It is difficult to control the temperature
The dryness is small

26. 6.4 Improvement to Rankine Cycle
Ideal Regenerative Cycle
3(4) e 2 7 1 d 5 6 T s
Regenerative Cycle: d
General Carnot Cycle: d-3
Ideal Carnot Cycle: e-5
Same
Efficiency

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

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

29. Ideal Regenerative Cycle1 3 2 7 1 6 5 T s 4 8 9
Turbine
Boiler
Regenerator 7
Mixing
Chamber 2 8
Cond-
enser 9 4 6 5 3
Pump II
Pump I

30. 6.3.2 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。

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

32. 6.4 Improvement to Rankine Cycle
Extracting
Regeneration
Boiler
Turbine
Regenerator
Condenser
Mixing Chamber
Pump II
Pump I 1 2 7 3 4 5 6

33. 6.4 Improvement to Rankine Cycle
Cogeneration
Definition
Cogeneration is the production of more than one
useful form of energy from the same energy source.
electric power
heat in low quality

34. 6.5 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

35. 6.5 Gas Refrigeration Cycle
Turbine
Compressor
Condenser
Cold
Source 1 2 3 4
1-2 Isotropic Compress
2-3 Isotonic Heat Rejection to Surrounding
3-4 Isotropic Expansion
4-1 Isotonic Heat Absorption

36. 6.5 Gas Refrigeration Cyclep v 1 2 3 4
Cp— Constant, Ideal Gas
Heat Absorbed from Cold Source
Heat Rejected to the condenser 1 T s 2 3 4 T1 T3
Work of Compressor
Work of Turbine

37. 6.5 Gas Refrigeration Cycle1 T s 2 3 4 T1 T3 2’ 3’ 4’

38. 6.5 Gas Refrigeration Cycle
Turbine
Condenser
Cold
Source
Compre-
ssor 1 2 3 4 5 6 T 3’ 3 4 5’ 2 5 1 6 g k m n s

39. 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 changed 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.

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

41. Vapor-Compression Refrigeration Cycle
1-2
Isotropic compress to superheated vapor
2-3-4
Isotonic condensed to saturated liquid
4-5
Isentropic expansion in a turbine
4-6
Isotropic expansion through throttle to humidity vapor
5-1
Constant pressure heat absorption in a cool source to dry saturate vapor 1 2 3 4 5 6

42. Work difference between Turbine and throttle
Vapor-Compression Refrigeration Cycle 1 2 3 4 5 6
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

43. 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

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

45. Isotropic Compress Efficiency
Vapor-Compression Refrigeration Cycle 1 2 2’ 4 5 3
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。

46. 6.7 Refrigerant Definition
The work fluid cycling flowing in refrigeration
system while transferring energy with surrounding
in order to refrigerate.
Thermodynamic Request
Critical temperature should be much higher than temperature
of surroundings.
① steam easier be condensed;
② larger range of latent heat;
③ heat absorption and heat rejection closer to
isothermal process

47. 6.7 Refrigerant Thermodynamic Request
Solidification temperature should be lower than evaporation
temperature to prevent blocking the pipes.
Larger latent heat is more desirable.
appropriate saturate pressure
small
being nontoxic ,non-corrosive, nonflammable, chemically steady;
low cost
Environment & Safety Request
Ammonia 氨 ， Feron 氟利昂

48. Water — lithium bromide
6.8 Absorption Refrigeration System
Definition
The form of refrigeration that inexpensive thermal energy
instead of mechanical energy or electric power is consumed to
transfer heat form low temperature to high temperature is
absorption refrigeration.
Geothermal Energy
Solar Energy
Absorption refrigeration system involves the absorption of
a refrigerant by a transport medium .
Ammonia — Water
NH3－ H2O
Water — lithium bromide
H2O － LiBr

49. 6.8 Absorption Refrigeration System
Principle
Condenser
Evaporation
Q－Solar Energy
Expansion Valve
Adjust
Valve
NH3-H2O
Absorber
Generator
pump
rectifier
Cooling
Water Q1 Q2 Q3 Q4
NH3
Weak
Rich
NH3

50. 6.8 Absorption Refrigeration System
Thermodynamic Analysis
Thermal Efficiency
Advantage:
A liquid is compressed instead of a vapor , and
thus the work input for absorption refrigeration system
is very small.

51. 6.9 Vapor-Jet Refrigeration System
Principle
Mixture
Condenser
Evaporation
Expansion
Valve
pump Q1 Q2
Boiler
Diffuser
Nozzle

52. 6.9 Vapor-Jet Refrigeration System
Condenser
Evaporation
Expansion
Valve
pump Q1 Q2
Boiler 1’ 1 2 3 4 5’ 5 2’ Q3 T 1’ 5’ 3 4 1 5 2’ 2 s

53. 6.9 Vapor-Jet Refrigeration System
Thermodynamic Analysis
Thermal Efficiency
Disadvantage:
Irreversibility such as mixture process and
heat transfer with temperature difference;
Large exergy loss

54. 6.10 Liquefaction of Gases 气体液化循环中的工质，在循环中即作为冷却剂使用， 同时本身又被液化并输出液态产品。
The liquefaction of gases has always been an important area of
refrigeration since many important scientific and engineering process
at cryogenic temperature depend on liquefied gas.
Example:
separation of oxygen and nitrogen from air
preparation of liquid propellants for rockets
the study of material properties at low temperature
the study of exciting phenomenon such as superconductivity
气体液化循环中的工质，在循环中即作为冷却剂使用，
同时本身又被液化并输出液态产品。

55. 6.10.1 Min. Work in Liquefaction of Gases2 8 6 4 5
Gas-Liquid Coefficient
Quality at State 4

56. 6.10.2 Linde Cycle Principle Condenser Expansion Valve Heat Exchanger
Separator
Liquid Removed
Compressor 1 2 3 4 5 6 P2 T s 1 2 6 4 5 3 7 P1

57. 6.10.2 Linde Cycle Thermodynamic Analysis Take the Heat Exchanger,
Expansion Valve,
Separator
as system.
Liquid: y kg ; gas: (1-y) kg T s 1 2 6 4 5 3 7 P1 P2
Heat of liquefaction y kg:

58. 6.10.2 Linde Cycle Irreversibility in liquefaction
Thermodynamic Analysis
Irreversibility in liquefaction
of gas:
① heat loss in heat exchanger q’
② non-adiabatic, heat addition from
surrounding q’’ T s 1 2 6 4 5 3 7 P1 P2

59. cannot be treated as Ideal Gas
Linde Cycle
Irreversibility in compression
of gas:
① isothermal compression 1-2
② isothermal efficiency (0.59)
Thermodynamic Analysis T s 1 2 6 4 5 3 7 P1 P2
Actual work consumption
cannot be treated as Ideal Gas

60. 6.10.3 Claude Cycle Thermodynamic Analysis a-y y Condenser Compressor
Liquid Removed 2
Compressor
HE1
HE2
HE3
Turbine
Expansion
Valve
Separator 1 3 4 4’ 6 7 8 9 y
a-y
Thermodynamic Analysis

61. Piston expander：
Turbine：
Considering mechanical efficiency