Boiling & condensation

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Boiling & condensation

  1. 1. PRESENTED BY : Ziad ZohdySUBMITTED TO : Dr. Mohamed abo el-azmBOILING & CONDENSATION
  2. 2. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tubeOut lines
  3. 3. What is meant by Boiling• the transition of a liquid into vapor .• Boiling begins when the liquid is heated to the point atwhich the pressure of the saturated vapor above itssurface becomes equal to the external pressure
  4. 4.  To keep a liquid boiling , the heat that is expendedin the formation of bubbles must be supplied. Thus,boiling is inseparably associated with heat transfer,which results in the transmission of heat from theheating surface to the liquid. The heat exchangeduring boiling is one of the forms of convective heattransfer.
  5. 5. Different boiling regimes in poolboilingNucleateboilingTransition boilingFilm boiling
  6. 6. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tubeOut lines
  7. 7. Types of boilingPool boilingFlow boiling
  8. 8. Pool boiling The fluid isn’t forced to flow by a mover like a pump Any motion of the fluid is due to natural convection The surface Temperature is greater than thesaturation temperature so the boiling will occur
  9. 9. Flow boiling Liquid is forced to move by an external source like apump Liquid is forced through the tube with temperatureTs Heat is added through the wall of the tube ( hotliquid passes outside or electrical heater)
  10. 10. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tubeOut lines
  11. 11. Boiling curve
  12. 12. Natural convection A super heated liquid rising to the surface andevaporation takes place from the free surface The fluid motion in this mode of boiling is governed bynatural convection and the heat transfer from the heatedsurface to the fluid also by natural convection.
  13. 13. Nucleate Boiling From A-B : isolated bubbles are formed on theheated surface, but these bubbles are dissipated inthe liquid shortly after they separate from thesurface. From B-C : the heater temperature is increased, sowe get large number of nucleation sites and large
  14. 14.  These bubbles move up to the free surfacewhere they break up and release their vaporcontent. Nucleate boiling is the most desirable boilingregime in practice because high heat transferrates can be achieved in this regime
  15. 15. Nucleateboiling
  16. 16. Transition Boiling The temperature difference is increased at pointC, the heat flux decreases. This is because a large fraction of the heatersurface is covered by a vapor film, which acts as aninsulation due to the low thermal conductivity of thevapor
  17. 17. TransitionboilingNucleate boiling
  18. 18. Film Boiling In this region the heater surface is completelycovered by a continuous stable vapor film. Point D, where the heat flux reaches a minimum, iscalled the Leidenfrost point.
  19. 19. Film boilingNucleate boiling Transition boiling
  20. 20. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tubeOut lines
  21. 21. Heat transfer correlationsHEATTRANSFERCORRELATIONSNucleateboilingMAXheat fluxMINheat fluxFilmboiling
  22. 22. Nucleate boilingThe rate of heat transfer depends on : the number of active nucleation sites on thesurface. the rate of bubble formation at each site It is observed that irregularities on the heatingsurface, including roughness and dirt, serve asadditional nucleation sites during boiling
  23. 23. MAX heat flux maximum heat flux found at point CCcr is a constant whose value depends on the heatergeometry.
  24. 24. MIN heat flux The minimum heat flux found at D
  25. 25. Film boiling Cfilm= 0.62 for horizontal cylinders Cfilm= 0.67 for spheres
  26. 26. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tubeOut lines
  27. 27. CONDENSATION
  28. 28. Condensation Condensation occurs when the temperature of avapor is reduced below its saturation temperature. Ts > TwVapor is going to condense on the surface
  29. 29. Types of condensationDrop wisecondensationHigh rateof heattransferFilmcondensationLow rateof heattransferU0 = 1/(Ao*((1/(h0 * Ao)) + Rw + (1/(hi * Ai)))– Uo = Overall heat transfer coefficient– hi= Tube side heat transfer coefficient– ho = Condensation heat transfer coefficient–Rw = Wall resistanceIf the area of condensation decreases, coefficient willincrease, therefore drop wise condensation have morecondensation than film wise condensation
  30. 30. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tubeOut lines
  31. 31. The rate of heat transfer for the same temperature difference, dropcondensation get high heat flux more than the filmcondensation. Drop condensation is desirable but it difficult tomaintain.
  32. 32. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tubeOut lines
  33. 33. Film condensation on horizontaltubes A saturated vapor at temperature (Ts ) condensing on the outside of ahorizontal tube maintained at ( Tw ) Liquid film has go down along thecircumference & finally falls off atthe bottom of the tube.
  34. 34. The average heat transfer coefficientgiven by : λ : The latent heat ρ : Density K : Thermal conductivity μ: Kinematic viscosity D : Diameter
  35. 35. The film reynolds number at thebottom of the tube
  36. 36. • what is meant by Boiling• Types of boiling• Boiling curve• Heat transfer correlations• Condensation• Types of condensation• Rate of heat transfer• Film condensation on horizontal tube• problem as an exampleOut lines
  37. 37. Problem on film condensation onhorizontal tubes : Saurated R-22 vapor at 262 K condenses as a filmon a horizontal tube ( 22 mm OD ) whose outer wallis at a temperature of 258 K Calculate :-1- The average heat transfer coefficient. “ h “2-The film Reynolds number at the bottom of the tube“Re “
  38. 38. We had to calculate the mean temprature first to get thepropertise of R-22 from the table at the mean temperature MEAN FILM TEMP = ( Ts + Tw)/2 Mean Temp = (262 + 258 ) / 2 = 260 K
  39. 39.  m/L : condensation rate per meterlength
  40. 40. References http://dspace.mit.edu/bitstream/handle/1721.1/61501/HTL_TR_1962_019.pdf?sequence=1 http://www.thermopedia.com/content/990/ Heat Transfer by Cengel 2nd Ed
  41. 41. Ziad zohdyTHANK YOU 

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