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Questions and Answers
What are the two most important examples of convection processes associated with a change of phase of a fluid?
What are the two most important examples of convection processes associated with a change of phase of a fluid?
Which phase change process involves the change of a vapor to the liquid state?
Which phase change process involves the change of a vapor to the liquid state?
What sustains the change from liquid to vapor state during boiling?
What sustains the change from liquid to vapor state during boiling?
Which type of heat transfer involves heat transfer to the solid surface during a phase change?
Which type of heat transfer involves heat transfer to the solid surface during a phase change?
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In the context of boiling and condensation, what is significant about latent heat effects?
In the context of boiling and condensation, what is significant about latent heat effects?
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What type of heat transfer involves changing a vapor to a liquid or a liquid to a vapor?
What type of heat transfer involves changing a vapor to a liquid or a liquid to a vapor?
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When a smooth film is formed on a surface during condensation, what is this process called?
When a smooth film is formed on a surface during condensation, what is this process called?
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What happens when the liquid does not wet the surface during condensation?
What happens when the liquid does not wet the surface during condensation?
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In film-condensation, what represents a thermal resistance to heat transfer?
In film-condensation, what represents a thermal resistance to heat transfer?
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What is the process called when droplets are formed during condensation that fall down the surface in some random fashion?
What is the process called when droplets are formed during condensation that fall down the surface in some random fashion?
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Why must a chemical engineer understand boiling and condensation processes?
Why must a chemical engineer understand boiling and condensation processes?
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What is the Reynolds number calculated to be in the given scenario?
What is the Reynolds number calculated to be in the given scenario?
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What assumption was confirmed to be correct based on the calculated Reynolds number?
What assumption was confirmed to be correct based on the calculated Reynolds number?
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How much heat transfer rate is calculated between the surface and the fluid for the given scenario?
How much heat transfer rate is calculated between the surface and the fluid for the given scenario?
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What is the film temperature evaluated at in the solution provided?
What is the film temperature evaluated at in the solution provided?
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What are the physical properties of water at a film temperature of 99 °C according to Appendix A of the heat transfer book (J.P.Holman)?
What are the physical properties of water at a film temperature of 99 °C according to Appendix A of the heat transfer book (J.P.Holman)?
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In the given scenario, what is the mass of steam condensed per unit length of tubes for a tube wall temperature of 98 °C?
In the given scenario, what is the mass of steam condensed per unit length of tubes for a tube wall temperature of 98 °C?
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Study Notes
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.
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Description
Explore the concepts of condensation and boiling heat transfer in chemical engineering with a focus on convection processes associated with a change of phase of a fluid. Presented by Dr. Abbas J. Sultan from the University of Technology, Baghdad, Iraq.
