Short Answer:

Boiling is the process of rapid vapor formation that occurs when a liquid is heated to its boiling point, where its vapor pressure becomes equal to the surrounding pressure. During boiling, bubbles of vapor form inside the liquid and rise to the surface, converting the liquid into vapor.

This process requires the addition of heat energy known as latent heat of vaporization. Boiling is an important heat transfer mechanism used in power plants, refrigeration systems, and various industrial processes where efficient heat removal or phase change is required.

Detailed Explanation :

Boiling

Boiling is a phase change process in which a liquid transforms into vapor when its temperature reaches a certain point called the boiling point. At this temperature, the vapor pressure of the liquid equals the external or surrounding pressure acting on it. When this condition is achieved, vapor bubbles start forming inside the liquid, not just on its surface, and rise upward to escape into the surrounding atmosphere.

Unlike evaporation, which occurs only at the surface of a liquid at any temperature, boiling takes place throughout the entire volume of the liquid once the required conditions are met. It is an important mechanism in heat transfer and thermodynamics because large quantities of heat can be transferred during boiling without a large rise in temperature.

Process of Boiling

When heat is supplied to a liquid, its temperature begins to rise. As the liquid temperature increases, so does its vapor pressure. The boiling point is reached when the vapor pressure of the liquid becomes equal to the surrounding pressure. At this moment, bubbles form inside the liquid at different locations.

These bubbles contain vapor of the liquid and are surrounded by the liquid itself. When the vapor pressure inside the bubbles is strong enough to overcome the external pressure and surface tension forces, the bubbles grow and move upward to the surface. On reaching the surface, they burst and release vapor into the surrounding air. This process continues as long as heat is supplied, and the temperature remains constant during the phase change.

The amount of heat energy required to convert a unit mass of liquid into vapor without changing its temperature is known as the latent heat of vaporization. This heat helps break the molecular bonds that hold the liquid molecules together.

Types of Boiling

Pool Boiling:
This occurs when a liquid is heated on a stationary surface, such as water boiling in a kettle or pan. In pool boiling, the liquid is not forced to move by any external means.
Flow Boiling:
In this type, the liquid flows over a heated surface while boiling occurs simultaneously. This is common in heat exchangers, boilers, and cooling systems where the fluid is in motion.

Each of these types can be further classified based on heat flux and temperature difference between the surface and the fluid, such as natural convection boiling, nucleate boiling, transition boiling, and film boiling.

Boiling Regimes

The boiling process can be divided into several regimes based on the heat transfer rate and surface temperature difference:

Natural Convection Boiling:
Occurs at low temperature differences. Here, bubbles form occasionally, and heat transfer is mainly by natural convection currents.
Nucleate Boiling:
This is the most efficient and desirable regime where numerous small vapor bubbles form at active nucleation sites. The heat transfer rate is very high, and this mode is used in most practical applications.
Transition Boiling:
This is an unstable phase between nucleate and film boiling. In this region, vapor films form and collapse rapidly, causing fluctuations in heat transfer.
Film Boiling:
At very high surface temperatures, a continuous vapor layer forms between the surface and the liquid. The vapor film acts as an insulating layer, which drastically reduces heat transfer efficiency.

The graph of heat flux versus surface temperature difference is known as the boiling curve, which illustrates these regimes.

Factors Affecting Boiling

Surface Temperature:
Higher surface temperature increases the rate of bubble formation and thus increases the rate of boiling.
Surface Roughness:
Rough surfaces provide more nucleation sites for bubble formation, enhancing the boiling process.
Pressure:
The boiling point of a liquid depends on pressure. At higher pressure, boiling occurs at higher temperatures, while at lower pressure, boiling occurs at lower temperatures.
Fluid Properties:
Liquids with low viscosity and high thermal conductivity boil more efficiently.
Surface Material and Condition:
Clean and thermally conductive surfaces promote efficient boiling. Contaminated surfaces reduce heat transfer.
Flow Velocity (in flow boiling):
Higher velocity helps remove bubbles quickly, maintaining an active boiling surface and improving heat transfer.

Applications of Boiling

Boiling is widely used in mechanical and thermal engineering applications because it allows high heat transfer rates with small temperature differences. Some key applications include:

Boilers:
Used in power plants where water is converted into steam to drive turbines for electricity generation.
Refrigeration Systems:
Boiling of refrigerant in the evaporator helps absorb heat from the surroundings, enabling cooling.
Heat Exchangers:
In various industries, boiling is used to control and transfer heat efficiently between fluids.
Cooking and Sterilization:
Boiling is used in domestic and industrial applications for heating, sterilizing, and food preparation.
Nuclear Reactors:
Boiling is a key process in boiling water reactors (BWRs), where water turns into steam to generate electricity.

Difference Between Boiling and Evaporation

While both involve the conversion of liquid into vapor, there are key differences:

Boiling occurs at a fixed temperature (boiling point) throughout the liquid, while evaporation happens only at the surface and at any temperature.
Boiling is a rapid process involving bubble formation, whereas evaporation is a slow process without bubbles.
Boiling requires external heat energy, while evaporation can occur naturally.

Conclusion

Boiling is a phase change process where a liquid turns into vapor when its vapor pressure equals the external pressure. It involves the formation and release of vapor bubbles within the liquid and is influenced by factors like temperature, surface conditions, and pressure. Boiling plays a vital role in many industrial and engineering systems due to its high heat transfer capability and is a key concept in thermodynamics and heat transfer.