Condensation Heat Transfer

Definition

Condensation heat transfer is a phase-change process that occurs when a saturated vapor comes into contact with a surface maintained at a temperature lower than its saturation temperature. During this process, the vapor releases its latent heat of vaporization, converting into a liquid state while transferring significant thermal energy to the solid surface.

Main Content

1. Filmwise Condensation

• In this mode, the condensed liquid forms a continuous, thin film that covers the entire surface area of the cooling body.
• The liquid film acts as a thermal resistance, impeding further heat transfer from the vapor to the solid surface.

2. Dropwise Condensation

• The vapor condenses into discrete, individual droplets that form on the surface rather than a continuous film.
• This mode is significantly more efficient than filmwise condensation because the bare surface area remains exposed to the vapor, allowing for much higher heat transfer coefficients.

3. Governing Factors

• The temperature difference between the saturated vapor and the solid wall significantly affects the rate of condensation.
• Surface geometry (vertical vs. horizontal) and surface tension properties determine whether the condensate forms a film or droplets.

[Vapor Phase]
      |
      | Heat Transfer (q)
      V
   (Droplets) ----> High heat transfer (Dropwise)
   (Continuous Film) ----> Lower heat transfer (Filmwise)
   __________________________________________
   |          Cold Surface                 |

Working / Process

1. Vapor-Surface Interaction

• Saturated vapor molecules collide with a cooling surface that is below the saturation temperature.
• Energy is extracted from the vapor molecules, leading to a loss of kinetic energy and a change in phase from gas to liquid.

2. Nucleation and Growth

• Small nuclei of liquid form on the surface, often triggered by surface roughness or impurities.
• These nuclei grow as additional vapor molecules lose their latent heat and join the liquid mass.

3. Condensate Removal

• Under the influence of gravity or shear forces, the liquid droplets or film begin to move off the surface.
• In filmwise condensation, the film flows downward; in dropwise, droplets coalesce and roll off, clearing space for new condensation.

Advantages / Applications

• Power Generation: Used extensively in steam condensers within thermal power plants to recycle water and maintain vacuum conditions for turbines.
• Refrigeration Systems: Essential in evaporators and condensers where refrigerants change phases to facilitate heat exchange.
• Chemical Processing: Utilized in distillation columns and heat exchangers to recover valuable solvents and process vapors efficiently.

Summary

Condensation heat transfer is a highly effective thermodynamic process where vapor transitions to liquid upon contact with a cold surface, releasing latent heat. It is categorized primarily into filmwise and dropwise condensation, with the latter offering superior thermal performance. It plays a critical role in industrial cooling and power cycles.

Important terms to remember: - Latent Heat: The energy absorbed or released during a phase change. - Saturation Temperature: The temperature at which a substance transitions between liquid and gas at a given pressure. - Condensate: The liquid formed from the cooling of vapor.