Boiling and Condensation

1. Introduction

Boiling and condensation are important modes of heat transfer that involve a phase change of a fluid.

• Boiling: liquid → vapor
• Condensation: vapor → liquid

During phase change, a large amount of heat is transferred at almost constant temperature, making these boiling and condensation processes extremely efficient and widely used in engineering applications such as boilers, condensers, power plants, and refrigeration systems.

2. Boiling

2.1 Definition

Boiling is the process in which a liquid changes into vapor when its temperature reaches the saturation temperature corresponding to the surrounding pressure.

Boiling occurs when:$$T_{surface} > T_{sat}$$

2.2 Types of Boiling

a) Pool Boiling

• Liquid is stationary
• Motion is caused only by buoyancy forces due to bubble formation

Examples: Water heated in a vessel, electric kettle

b) Flow Boiling

• Liquid is in forced motion over a heated surface
• Heat transfer is influenced by both boiling and convection

Examples: Boilers, evaporators, heat exchangers

2.3 Modes of Pool Boiling

The behavior of boiling is best explained using the boiling curve, which relates heat flux to excess temperature $(T_s – T_{sat})$(Ts​−Tsat​).

1. Natural Convection Boiling

• Occurs at low surface temperatures
• No bubble formation
• Heat transfer mainly by convection

2. Nucleate Boiling

• Formation of vapor bubbles at nucleation sites
• Bubbles grow and detach from the surface
• Very high heat transfer rate
• Most desirable boiling regime

3. Critical Heat Flux (CHF)

• Maximum heat flux point
• Beyond this, heat transfer suddenly drops
• Also called burnout point

4. Transition Boiling

• Unstable regime
• Partial vapor film formation

5. Film Boiling

• Continuous vapor film forms between surface and liquid
• Heat transfer occurs mainly by radiation and conduction through vapor
• Poor heat transfer
• Example: Leidenfrost effect

2.4 Factors Affecting Boiling Heat Transfer

• Surface roughness and material
• Pressure and saturation temperature
• Nature of liquid
• Degree of superheat
• Presence of impurities

3. Condensation

3.1 Definition

Condensation is the process in which vapor changes into liquid when it comes in contact with a surface at a temperature below the saturation temperature.$$T_{surface} < T_{sat}$$

During condensation, the latent heat of vaporization is released.

3.2 Types of Condensation

a) Filmwise Condensation

• Vapor forms a continuous liquid film on the surface
• Film acts as a thermal resistance
• Lower heat transfer rate

b) Dropwise Condensation

• Liquid forms discrete droplets on the surface
• Droplets fall off due to gravity
• Very high heat transfer coefficient (5–10 times higher than filmwise)
• Difficult to maintain in practice

3.3 Modes of Condensation

• Condensation on vertical plates
• Condensation on horizontal tubes
• Condensation inside tubes

Each mode affects the thickness of the condensate film and heat transfer rate.

3.4 Heat Transfer in Condensation

Heat transfer during condensation depends on:

• Temperature difference
• Properties of vapor and liquid
• Orientation of surface
• Flow regime of condensate

Condensation heat transfer coefficient is generally much higher than convection.

4. Comparison Between Boiling and Condensation

Aspect	Boiling	Condensation
Phase change	Liquid → Vapor	Vapor → Liquid
Heat absorbed/released	Absorbed	Released
Temperature	Nearly constant	Nearly constant
Heat transfer rate	Very high	Very high
Common devices	Boilers, evaporators	Condensers

5. Engineering Applications

Boiling

• Steam generation in boilers
• Evaporators in refrigeration systems
• Nuclear reactor cooling
• Chemical processing equipment

Condensation

• Steam condensers in power plants
• Refrigeration and air-conditioning systems
• Distillation columns
• Heat recovery systems

6. Advantages and Limitations

Advantages

• Extremely high heat transfer rates
• Efficient use of latent heat
• Compact equipment size

Limitations

• Surface damage at critical heat flux
• Control of boiling regime is difficult
• Dropwise condensation is hard to sustain

7. Summary

• Boiling and condensation involve phase change
• Boiling absorbs heat; condensation releases heat
• Nucleate boiling and dropwise condensation offer maximum heat transfer
• Widely used in thermal engineering applications

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