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Chapter 10

Chapter 10. Heat Exchangers. Primary Contents. Analysis and calculation of overall heat transfer process. Types of heat exchangers. The log mean temperature difference. 对数平均温差. Thermal calculation. Enhancement and weakening. Key Points.

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Chapter 10

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  1. Chapter 10 Heat Exchangers

  2. Primary Contents • Analysis and calculation of overall heat transfer process • Types of heat exchangers • The log mean temperature difference 对数平均温差 • Thermal calculation • Enhancement and weakening

  3. Key Points • Analysis and calculation of overall heat transfer process • Critical thickness of insulation 临界热绝缘直径 • The log mean temperature difference • thermal design for heat-exchanger • Enhancement and weakening

  4. 10-1 Analysis and calculation of overall heat transfer process(传热过程的分析和计算)

  5. heat exchanger 换热器 overall heat-transfer coefficient 传热系数 critical thickness of insulation 临界热绝缘直径

  6. Φ • The Overall Heat-Transfer Coefficient 传热系数 • plane wall For stead-state heat transfer Φ

  7. The Overall Heat-Transfer Coefficient 非常有用的一个公式,快记住！ Compared with The overall heat-transfer coefficient for plane wall

  8. Φ • The Overall Heat-Transfer Coefficient • cylinder Φ

  9. overall fin surface efficiency • The Overall Heat-Transfer Coefficient • fin 肋面总效率

  10. The Overall Heat-Transfer Coefficient k based on Ai 肋化系数 β- the ratio of the total surface area and the wall area β>>1 and ηoβ>1

  11. ho, to di do ti pipe The layer of insulation around the pipe causes decrease or increase in heat transfer? insulation • Critical thickness of insulation 临界热绝缘直径

  12. If , do , the heat transfer will be increased; • If , do , the heat transfer will be decreased; ho, to di do ti pipe insulation • Critical thickness of insulation

  13. Example Calculate the critical radius of insulation for asbestos (λ=0.17W/m•℃) surrounding a 200℃, 5.0-cm-diameter pipe and exposed to room air at 20 ℃ with h=3.0W/m2•℃. Calculate the heat loss from the pipe when covered with the critical radius of insulation and without insulation. 石棉

  14. Φ [Solution] asbestos λ=0.17W/m·℃ di=5.0cm ti=200℃ air t∞=20℃ ho＝3.0W/m2·℃ Calculated the critical radius of insulation the heat loss when covered insulation

  15. Φ di=5.0cm ti=200℃ air t∞=20℃ ho＝3.0W/m2·℃ Without insulation the convection from the outer surface of the pipe is

  16. （2）if [discussion] （1）the addition of 3.17cm of insulation actually increases the heat transfer by 25 percent; （3）if fiberglass (k=0.04W/m•℃) employed as the insulation material 玻璃纤维

  17. 10-2Types of heat exchangers (换热器的类型)

  18. direct-contact heat exchanger 混合式换热器 recuperator heat exchanger 间壁式换热器 double-tube heat exchanger 套管式换热器

  19. shell and tube heat exchanger 管壳式换热器 cross-flow heat exchanger 交叉流换热器 plate type heat exchanger 板式换热器

  20. spiral plate heat exchanger 螺旋板式换热器 regenerator heat exchanger 回热式换热器 parallel flow 顺流 counter flow 逆流

  21. Types of heat exchangers • Direct-contact Heat Exchanger 混合式换热器 • Recuperator Heat Exchanger 间壁式换热器 • Double-tube 套管式 • Shell and tube 管壳式 • Cross-flow 交叉流 • Plate type 板式 • Spiral plate heat exchanger 螺旋板式 • Regenerator Heat Exchanger 回热式换热器

  22. Recuperator Heat Exchanger • Double-tube Heat Exchanger

  23. Recuperator Heat Exchanger Counter Flow Parallel Flow 顺流 逆流

  24. Recuperator Heat Exchanger • Shell and Tube Heat Exchanger

  25. Recuperator Heat Exchanger • Shell and Tube Heat Exchanger

  26. Recuperator Heat Exchanger • Shell and Tube Heat Exchanger

  27. Recuperator Heat Exchanger • Shell and Tube Heat Exchanger

  28. Recuperator Heat Exchanger • Cross-flow Heat Exchanger

  29. Recuperator Heat Exchanger • Plate type Heat Exchanger

  30. Recuperator Heat Exchanger • Spiral Heat Exchanger

  31. Development of Shell and Tube Heat Exchanger

  32. 10-3The Log Mean Temperature Difference(对数平均温差)

  33. Simple parallel flow and counter flow

  34. Simple parallel flow and counter flow • Parallel flow hot fluid cold fluid

  35. Simple parallel flow and counter flow • The log mean temperature difference

  36. Figure 10-23 ～ 10-26 • Complex heat flow • Calculate the LMTD for a simple counter flow with the same hot and cold fluid temperature; • Using a correction factor.

  37. parallel flow and counter flow

  38. Example Cold fluid is heated from 50℃ to 100℃ and hot fluid enters the exchanger at 300℃ and leaves at 150℃. (1) Compare the LMTD for a counter flow with that for a parallel flow. (2) If the fluids are used in a cross-flow heat exchanger, and the cold fluid is mixed, please calculate the LMTD.

  39. t 300℃ 150℃ 100℃ 50℃ A [Solution] (1) For parallel flow

  40. t 300℃ 150℃ 100℃ 50℃ A (2) For counter flow

  41. T 300℃ 150℃ 100℃ 50℃ A (3) For cross-flow heat exchanger from Figure 10-26, we have

  42. t t t1’ t1’ dt1’ t1’’ t1’’ △tmax △tmin dt1’’ t2’’ t2’’ dA’ dA’’ t2’ t2’ A A

  43. t t t dt1’ △tmin dt1’’ △tmax A A A t1’ t1’ t1’ t1’’ t1’’ t2’’ t2’’ t1’’ t2’’ t2’ t2’ t2’ dA’ dA’’

  44. 10-4Thermal design for recuperator heat exchanger(间壁式换热器的热设计)

  45. effectiveness of heat exchanger 换热器效能 传热单元数 Number of Transfer Unit thermal resistance of fouling 污垢热阻

  46. Calculation • Design Calculation known conditions： three of calculate： k & A • Check Calculation known conditions： calculate：

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