COND1'ppt Condensation of Single Components: Mechanisms

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COND1.ppt Condensation of Single
ComponentsMechanisms Basic Parameters(Chapt
er in Outline Notes Condensation Heat
Transfer )
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Mechanisms

• Homogeneous (e.g. fog) undesirable, since want
  condensation to occur on surface with controlled
  temperature
• Film-wise reproducible, so design for this
• Drop-wise higher heat-transfer coefficients, but
  hard to reproduce, hence undesirable

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Film-Wise
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Drop-Wise
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Single Component

• Key distinctions
• gravity-controlled vs flow-shear controlled
• laminar or turbulent flow
• vertical or horizontal surfaces
• So what? Affect calculation of heat-transfer
  coefficient

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Gravity or Flow Shear Controlled

• Need to estimate shear stress at interface
• Gravity liquid density x acceleration due to
  gravity x liquid film thickness
• Flow shear proportional to pressure drop

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Laminar/Turbulent

• Key parameter Reynolds number

• Viscosity of liquid used
• Gamma condensation rate per unit circumference
  (of tube), width of plate

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Vertical/Horizontal Surfaces

• Horizontal better than vertical by 40 typically
• Also need to consider
• liquid sub-cooling
• superheated vapour
• vapour inertia
• all affect heat-transfer coefficient calculation

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Condensation Heat-Transfer Coefficients Single
Component

• Typically higher than for single-phase transfer
• transfer of latent heat helps, because latent
  heat is large compared with specific heat
  capacity x temperature change for most materials,
  particularly water
• Values from 500 to 5000 W m-2 K-1 common

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Conclusions

• Three main mechanisms - we assume that filmwise
  condensation occurs, do not want fog, dropwise is
  irreproducible
• Have reviewed basic parameters representative
  numbers