Condensation and Boiling Heat Transfer in Chemical Engineering

Reynolds Number and Heat Transfer

• The Reynolds number can be calculated using the equation: Re = (Ts - Tw) * 4hL / (μf * hg)
• In this case, the Reynolds number is 49.6, which indicates that the laminar assumption is correct.
• The heat transfer rate can be calculated using the equation: q = hA * (Ts - Tw)
• The heat transfer rate is 2367 W.

Example 2: Condensation on Tubes

• One hundred tubes of 0.50-in (1.27-cm) diameter are arranged in a square array and exposed to atmospheric steam.
• The tube wall temperature is 98 °C.
• The film temperature is 99 °C, which is the average of the tube wall temperature and the steam temperature.
• The physical properties of water at a film temperature of 99 °C are: ρl = 960 kg/m³, μl = 2.82 × 10⁻⁴ kg/m.s, kl = 0.68 W/m.

Condensation and Boiling

• Boiling and condensation are classified as forms of convection mode of heat transfer.
• They involve fluid motion and are used to change a vapor to a liquid or a liquid to a vapor in power or refrigeration cycles.
• The chemical engineer must understand the processes involved in order to design the appropriate heat transfer equipment.

Condensation Heat-Transfer Phenomena

• Condensation occurs when a vapor comes into contact with a surface at a temperature below the saturation temperature of the vapor.
• The condensate forms on the surface and flows down the surface under gravity.
• If the liquid wets the surface, a smooth film is formed, and the process is called film condensation.
• If the liquid does not wet the surface, droplets are formed, and the process is called dropwise condensation.

Comparison between Film and Dropwise Condensation

• In film condensation, the surface is blanketed by the film, which grows in thickness as it moves down the plate.
• A temperature gradient exists in the film, and the film represents a thermal resistance to heat transfer.

Introduction to Condensation and Boiling

• Condensation and boiling are convection processes associated with a change of phase of a fluid.
• These processes occur at a solid-liquid or solid–vapor interface and involve latent heat effects associated with the phase change.
• The change from the liquid to the vapor state due to boiling is sustained by heat transfer from the solid surface; conversely, condensation of a vapor to the liquid state results in heat transfer to the solid surface.