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Heat Exchangers: The Effectiveness – NTU Method

Heat Exchangers: The Effectiveness – NTU Method. Chapter 11 Sections 11.4 through 11.7. General Considerations. General Considerations. Computational Features/Limitations of the LMTD Method :. The LMTD method may be applied to design problems for

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Heat Exchangers: The Effectiveness – NTU Method

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  1. Heat Exchangers:The Effectiveness – NTU Method Chapter 11 Sections 11.4 through 11.7

  2. General Considerations General Considerations • Computational Features/Limitations of the LMTD Method: • The LMTD method may be applied to design problems for which the fluid flow rates and inlet temperatures, as well as a desired outlet temperature, are prescribed. For a specified HX type, the required size (surface area), as well as the other outlet temperature, are readily determined. • If the LMTD method is used in performance calculations for which both outlet temperatures must be determined from knowledge of the inlet temperatures, the solution procedure is iterative. • For both design and performance calculations, the effectiveness-NTU method may be used without iteration.

  3. Heat exchanger effectiveness, : Definitions Definitions • Maximum possible heat rate: • Will the fluid characterized by Cmin or Cmax experience the largest possible temperature change in transit through the HX? • Why is Cminand not Cmax used in the definition of qmax?

  4. Definitions (cont.) • Number of Transfer Units, NTU • A dimensionless parameter whose magnitude influences HX performance:

  5. Cr HX Relations Heat Exchanger Relations • Performance Calculations:

  6. For Cr = 0, to all HX types. HX Relations (cont.) • Design Calculations: • For all heat exchangers,

  7. Analysis based on method • Convection (and friction) coefficients have been determined for selected • HX cores by Kays and London . Compact HX Compact Heat Exchangers Proprietary data have been obtained by manufacturers of many other core configurations. • Results for a circular tube-continuous fin HX core:

  8. Problem: Twin-Tube Heat Exchanger Problem 11.28: Use of twin-tube (brazed) heat exchanger to heat air by extracting energy from a hot water supply.

  9. Problem: Twin-Tube Heat Exchanger (cont.)

  10. Problem: Twin-Tube Heat Exchanger (cont.)

  11. Problem: Twin-Tube Heat Exchanger (cont.)

  12. Problem: Twin-Tube Heat Exchanger (cont.) and from Eq. (1) the effectiveness is

  13. Problem: Heat Transfer Enhancement Problem 11.65: Use of fluted spheres and solid spheres to enhance the performance of a concentric tube, water/glycol heat exchanger.

  14. Problem: Heat Transfer Enhancement (cont.)

  15. Problem: Heat Transfer Enhancement (cont.)

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