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Condensation and Boiling Heat Transfer

Condensation and Boiling Heat Transfer

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Condensation and Boiling Heat Transfer

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  1. Condensation and BoilingHeat Transfer • boiling, condensation : high heat transfer rates • understand the processes to design the appropriate heat-transfer equipment Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman

  2. Condensation Heat Transfer

  3. Modes of condensation • Dropwise/filmwise condensation • Direct/Indirect/homogeneous condensation

  4. Modes of condensation In vertical flat plate, Tw < Tsat : condensate will form at surface. • Dropwise condensation: liquid does not wet the surface, droplets are formed. • Filmwise condensation: liquid wets the surface, smooth film is formed. The surface is blanked by the film, which grows in thickness as it moves down the plate.

  5. Filmwise Condensation

  6. Dropwise Condensation

  7. Homogeneous Condensation

  8. Direct Contact Condensation

  9. Condensation • In the remaining lecture we now focus on indirect contact filmwise condensation

  10. General approach to condensation

  11. General approach to condensation

  12. Condensation on Flat Plate

  13. Nusselt Analysis - Assumptions

  14. Mass flow of condensate Heat transfer at wall

  15. Amount of condensate added between x and x+dx Thus

  16. Heat transfer coefficient

  17. In term of Nusselt number For vertical plates and cylinders and fluids with Pr > 0.5 and cT/hfg ≤ 1.0

  18. For non-linear temperature profile For laminar film condensation on horizontal tubes

  19. To determine flow (laminar or turbulence) use Renolds number Critical Re is 1800 For Vertical plate of unit depth, P = 1 For Vertical tube, P = d

  20. Relate mass flow with total heat transfer and heat transfer coefficient

  21. Using 20 % safety factor in design problems For inclined surfaces

  22. Condensation number (Co)

  23. For condensation of refrigerants at low vapor velocities inside horizontal tubes For higher flow rates

  24. A vertical square plate, 30 by 30 cm, is exposed to steam at atmospheric pressure. The plate temperature is 98C. Calculate the heat transfer and the mass of steam condensed per hour. Example 1

  25. Example 2 • One hundred tubes of 1.27 cm diameter are arranged in a square array and exposed to atmospheric steam. Calculate the mass of steam condensed per unit length of tubes for a tube wall temperature of 98C. (use condensate properties from Ex.1)