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HEAT EXCHANGER DESIGN

HEAT EXCHANGER DESIGN. Heat Transfer Equipment Types. Double Pipe Heat Exchanger. Consists of two concentric pipes with one fluid flowing through the inner pipe while the other fluid flowing through the annular space. Shell and Tube Heat Exchanger.

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HEAT EXCHANGER DESIGN

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  1. HEAT EXCHANGER DESIGN

  2. Heat Transfer Equipment Types

  3. Double Pipe Heat Exchanger • Consists of two concentric pipes with one fluid flowing through the inner pipe while the other fluid flowing through the annular space

  4. Shell and Tube Heat Exchanger • Consists of tube bundles enclosed in a cylindrical shell with one fluid flowing through the tubes and the other flowing outside of the tubes

  5. Heat Transfer Equipment in Industries • Exchanger: heat exchanged between two process streams • Heaters and coolers: where one stream is plant service • Vaporiser: if a process stream is vaporised • Reboiler: a vaporiser associated with distillation column • Evaporator: if concentrating a solution • Fired exchanger: if heated by combustion gases • Unfired exchanger: not using combustion gases

  6. Heat Transfer Equipment in Industries MODES of HEAT TRANSFER • Conduction • Transfer of heat from one part of a body to another part of the same body or between two bodies in physical contact, without significant displacement of the particles of the two bodies • Convection • Transfer of heat from one point to another within a fluid or between a fluid and a solid or another fluid, by the movement or mixing of the fluids involved • Radiation • Transfer of heat by the absorption of radiant energy

  7. BASIC THEORY • General equation for heat transfer across a surface for DPHE is: • Q =heat transferred per unit time, W • U=the overall heat transfer coefficient, W/m2oC • A= heat-transfer area, m2 • Tm= the mean temperature difference,oC

  8. BASIC THEORY • General equation for heat transfer across a surface for STHE is: • Q =heat transferred per unit time, W • U=the overall heat transfer coefficient, W/m2oC • A= heat-transfer area, m2 • Tm= the mean temperature difference,oC • Y = geometric correction factor

  9. Tube-Side Passes • One tube pass • Two tube pass • Three tube passes

  10. Geometric Correction Factor Also refer to Figure 11-4, Perry 7th Edition

  11. Geometric Correction Factor For design to be practical, Y ≥ 0.85

  12. Logarithmic Mean Temperature Difference ΔT1 ΔT2 If ΔT1 < ΔT2 and (ΔT2/ΔT1) ≤ 2, then ΔTlm is the arithmetic mean temp difference

  13. Overall Heat Transfer Coefficient • Rearranging the General Equation in terms of driving force and total resistance: Driving Force Total Resistance

  14. Overall Heat Transfer Coefficient • The overall coefficient is reciprocal of the overall resistance to heat transfer, which is the sum of several individual resistances. Individual resistance is the reciprocal of individual HTC.

  15. Total Resistance • the sum of several individual resistances • Individual resistance is the reciprocal of individual HTC. Convection Conduction Convection inside

  16. Total Resistance Conduction Heat Transfer is governed by Fourier’s Law! k = thermal conductivity of the Solid (BTU/hr-ft2-(OF/ft)) A = Area perpendicular to the direction of heat transfer x = distance of heat flow

  17. Total Resistance At Steady State:

  18. Total Resistance If k is constant: Define R = Δx/kA Thus, q= - ΔT/R

  19. Total Resistance If k is not constant: If k varies slightly with Temp: **km is evaluated at the mean temperature

  20. Total Resistance If k is not constant: If A varies slightly with Thickness:

  21. Total Resistance Convection Heat Transfer q = hcA (T1 – T2) Where: hc- convection heat transfer coefficient, Btu/hrft2°F -similar to k/∆x A – Heat transfer Area T1 – temperature at surface 1 T2 – temperature at surface 2

  22. Total Resistance Convection Heat Transfer: Rearranging q = (T1 – T2)/(1/hcA) Where: hc- convection heat transfer coefficient, Btu/hrft2°F -similar to k/∆x A – Heat transfer Area T1 – temperature at surface 1 T2 – temperature at surface 2

  23. Total Resistance Convection Conduction Convection inside

  24. Total Resistance inside

  25. Typical Fouling Factor (Foust, 1980)

  26. Heat Transfer Without Phase Change

  27. Double Pipe Heat Exchanger

  28. Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Tube-side Applicabilty: Non-metallic fluid 0.5 < NPr < 100 NRE > 10,000

  29. Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Annular Space Applicabilty: Non-metallic fluid 0.5 < NPr < 100 NRE > 10,000

  30. Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Short Tube (L/D < 50)

  31. Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE Laminar Flow, Forced Convection

  32. Shell and tube heat exchanger

  33. Invidual Heat Transfer Coefficient HT w/o Phase Change: STHE, ho

  34. Invidual Heat Transfer Coefficient HT w/o Phase Change: STHE, hi

  35. Heat Transfer WITH Phase Change

  36. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface Assumptions: Pure vapor is at its saturation temperature. The condensate film flows in laminar regime and heat is transferred through the film by condensation. The temperature gradient through the film is linear. Temperature of the condensing surface is constant. The physical properties of the condensate are constant and evaluated at a mean film temperature. Negligible vapor shear exists at the interface

  37. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface, Laminar

  38. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface, Turbulent

  39. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface  If the amount of condensate is unknown For Nre > 40, h is multiplied by 1.2

  40. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface  If the amount of condensate is known For Nre > 40, h is multiplied by 1.2

  41. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes For Nre > 40, h is multiplied by 1.2

  42. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes

  43. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes  w/o splashing

  44. Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes  w/ splashing

  45. Invidual Heat Transfer Coefficient Film Temperature Condensate Properties are evaluated at the Film Temperature Tf = ½(Tsv + Tw) by Kern, D.Q., Process HT Tf = Tsv- 0.75ΔTby McAdams, W.H., Heat Transmission, 3rd. Ed. ΔT = Tsv - Tw

  46. Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere

  47. Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere

  48. Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere Nusselt-type Equation by Rohsenow: Cr varies from 0.006 to 0.015

  49. Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere Nusselt-type Equation by Forster and Zuber:

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