Heat transfer & heat exchangers - Lecture 1: Thermodynamics - Khoa Ta Dang

Nội dung text: Heat transfer & heat exchangers - Lecture 1: Thermodynamics - Khoa Ta Dang

1. HEAT TRANSFER & HEAT EXCHANGERS Dr. Khoa Ta Dang khoatadang@hcmut.edu.vn
2. Assessment 40% • Midterm (references allowance) test 60% • Final (references allowance) exam
3. Outline Lecture 6 •Thermal design Lecture 7 •Heat exchangers Lecture 8 •Evaporation Lecture 9 •Crystallization Lecture 10 •Refrigeration cycles
4. Content 1 • Basic concepts 2 • State variables 3 • Form of Energy & Energy transfer 4 • State Function & Processes
5. Basic concepts
6. Systems A system is a quantity of matter or a region in space chosen for study, called thermodynamic system Closed (control mass, nonflow) Surroundings Open (control volume) System Isolated (nonflow, no energy transfer) Boundary Adiabatic (nonflow, no heat transfer)
7. Phases of substance Solid Liquid Gas
8. Phases of substance 푭 ퟒ P = constant 푫 푬 vaporization evaporating melting Temperature warming 푪 melting Solid Liquid Gas cooling freezing condensing Energy input
9. Pressure Pressure is the force exerted by a fluid per unit area, this is applied to gas and liquid. For solid, this concept is called as stress bar Pa (N/m2) kPa atm kgf/cm2 lbf/in2 (psi) mmHg (torr) 1 105 100 1,01325 101325 101,325 1 14,696 760 0,9807 9,807 x 104 0,9679 1 14,223 133,3 1 3 푃 = 푃 − 푃 휌 = 13595 / 푒 푠 푡 tại 0℃ 푃푣 = 푃 푡 − 푃 푠 = 9,807 /푠2
10. Temperature SI system English system A. Celcius scale 1701–1744 (oC): G. Fahrenheit scale 1686–1736 (oF): ice (0oC) and steam point (100oC) ice (32oF) and steam point (212oF) 9 ℉ = ℃ + 32 5 Lord Kelvin scale 1824–1907 (K) William Rankine scale 1820–1872 (R) 퐾 = ℃ + 273.15 푅 = ℉ + 459.67 ቊ ቊ ∆ 퐾 = ∆ ℃ ∆ 푅 = ∆ ℉ 9 푅 = 퐾 5
11. Forms of Energy & Energy transfer
12. Entropy • Entropy is a measure of molecular disorder, or molecular randomness. The entropy of a system is related to the total number of possible microscopic states of that system • Entropy is transferred across a boundary by heat or mass 훿푞 푠 = 푒푣
13. State Functions & Processes
14. States State is the condition of a system not undergoing any change gives a set of properties that completely describes the condition of that system. At this point, all the properties can be measured or calculated throughout the entire system
15. State functions 푃 3 ∆ = 3 ; 푊 = 8 퐽 3 1 ∆ = 3 ; 푊 = 12 퐽 2 2 3 5 3
16. Processes 1: reversible 푃 Final state 2: irreversible 2 Process path 3: isothermal Initial state 4: isobaric 1 5: isochoric (isometric) 6: polytropic 2 1 7: adiabatic 8: isenthalpic 9: isentropic 10: cycle
17. Cycles 푃 푄푛푒푡 = 푊푛푒푡
18. th • The concept of thermal equilibrium and 0 definition of temperature 1st • The conservation of energy principle nd • The increase of entropy principle or the 2 destruction of quality energy principle rd • Definition of absolute entropy or the 3 reference point of entropy
19. Property Diagrams
20. − 푣 diagram , ℃ Critical point for water 373.95 Saturated vapor Saturated liquid 푣, 3Τ 0.003106
21. 푃 − 푣 diagram 푃 Critical point Superheated vapor region Compressed liquid region Saturated liquid–vapor region 푣
22. 푃 − diagram 푃푠 푡, 푃 600 for water 400 200 0 , ℃ 0 50 100 150 200 푠 푡
23. − 푆 diagram , ℃ Critical point 400 300 Saturated for water liquid line 200 Saturated vapor line 100 푠, 퐽Τ 퐾 0 1 2 3 4 5 6 7 8
24. Reference state Reference state is chosen to assign a value of zero for a convenient property or properties at that state • For water, the saturated liquid at 0.01℃ is taken at the reference state , 푠 = 0 • For refrigerant 134a, the saturated liquid at − 40℃ is taken at the reference state ℎ, 푠 = 0
25. Carnot cycle 푃 퐿 휂 푛표푡 = 1 − 1 푞 푖푛 푞 1 푖푛 2 2 4 퐿 3 푞표 푡 4 푞표 푡 3 푣 푠 Thermal efficiency increases with an increase in the temperature at heat supply or decrease in the temperature at heat rejection
26. Stirling & Ericsson cycles 푃 1 푃 1 푃 푞푖푛 4 1 푞푖푛 푞푖푛 2 4 4 2 푞 푞 표 푡 표 푡 푞표 푡 2 3 3 3 푣 푣 푣 Carnot cycle Stirling cycle Ericsson cycle 퐿 휂 푛표푡 = 휂푆푡푖 푙푖푛 = 휂 푖 푠푠표푛 = 1 −
27. Brayton cycle Fuel Combustion chamber 2 3 Compressor Turbine 푤푛푒푡 1 Opened cycle 4 Fresh air Exhaust gases
28. Vapor Power Cycles
29. Rankine cycle 푞푖푛 Boiler 3 2 푞 3 푤푡 푖푛 푤푡 Turbine 2 푤 푤 4 푞 1 4 표 푡 푞표 푡 Condenser 1 푠 푊푡 푄표 푡 휂푅 푛 푖푛푒 = = 1 − 푄푖푛 푄푖푛
30. Refrigeration Cycles
31. The concepts • Cooling capacity of a refrigeration system that is the rate of heat removal from the refrigerated space, in terms of tons of refrigeration • Tons of refrigeration that is the rate of heat can freeze 1 푡표푛 2000 푙 of liquid water at 0℃ 32℉ into ice at 0℃ in 24 hours, this amount is said to be 1 푡표푛. 1 푡표푛 표 푒 𝑖 푒 푡𝑖표푛 = 211 퐽Τ 𝑖푛 = 200 푡 Τ 𝑖푛 = 3.5 푊
32. ideal vapor–comp. refrigeration cycle Evaporator coils WARM environment Capillary tube Freezer isobaric 푄 compartment 푄 3 Condenser 2 푄 Compressor 퐿 −18℃ Condenser Expansion coils valve 푊푖푛 isentropic isenthalpic 4 Evaporator 1 3℃ 푄퐿 isobaric 푊푖푛 COLD refrigerated Compressor space
33. Refrigerant selection • Refrigerants: chlorofluorocarbons (CFCs), ammonia, hydrocarbons, carbon dioxide, air water • Requirement for refrigeration selection: temperatures of condenser and evaporator, then toxic, corrosive, chemical, Saturated liquid latent heat and the cost 2 • ∆ ≈ 10℃ 푄 1 • Lower , higher 푃 푃 = 3 • ∆ ≈ 10℃ − 1 푊푖푛 • Higher 퐿, higher 푃 퐿 • 푃 > 1 푡 1 4′ 4 푄퐿 Saturated vapor 푠
34. Reversed Brayton (gas) cycle 푞 푞 푃 = 퐿 = 퐿 푤푖푛 푤 표 ,푖푛 − 푤푡 ,표 푡 2 푊 푄 표 ,푖푛 1 3 푄퐿 푊푡 ,표 푡 4 푠 ℎ − ℎ 푃 = 1 4 ℎ2 − ℎ1 − ℎ3 − ℎ4
35. Summary Power Refrigeration cycles cycles 푠 푠
36. Example 1 WARM environment Saturated liquid 2 푄 푄 3 Condenser 2 3 isentropic Compressor Expansion 푊푖푛 valve 푊푖푛 1 4 Evaporator 1 4 푄퐿 Saturated vapor 푄퐿 푠 COLD refrigerated space