Short Answer:

A feedwater heater is a device used in steam power plants to preheat the water before it enters the boiler. The heat for preheating is usually taken from steam extracted from the turbine. This process increases the efficiency of the power plant by reducing the fuel required to convert water into steam.

In simple words, the feedwater heater helps to improve thermal efficiency by using waste or extracted steam to raise the temperature of the feedwater. It also reduces the temperature difference between water and boiler tubes, minimizing thermal stress and extending the boiler’s life.

Detailed Explanation :

Function of a Feedwater Heater

A feedwater heater is an important heat exchange device used in steam power plants to heat the water (called feedwater) before it enters the boiler. Instead of using fuel to heat cold water in the boiler, part of the heat is recovered from extracted steam taken from intermediate stages of the turbine. This process reduces the overall fuel consumption and improves the thermal efficiency of the power plant.

The basic function of a feedwater heater is to raise the temperature of feedwater by transferring heat from steam or other heat sources, thereby increasing the efficiency and lifespan of the boiler system. Feedwater heating is a common practice in large thermal power plants and is an essential component of the Rankine cycle regeneration process.

Working Principle

The working of a feedwater heater is based on the principle of heat exchange, where a high-temperature fluid (usually extracted steam) transfers its heat to a low-temperature fluid (feedwater). The extracted steam condenses as it gives up its heat, and the feedwater absorbs this heat, becoming warmer before entering the boiler.

The typical working process is as follows:

Steam is extracted from a turbine at an intermediate pressure.
This extracted steam enters the feedwater heater.
The feedwater from the condenser or economizer passes through the heater tubes.
Heat is transferred from the steam to the feedwater.
The condensed steam (called drain water) is removed and sometimes reused in the cycle.
The preheated feedwater is then sent to the boiler for steam generation.

This process reduces the energy required by the boiler to convert water into steam and thus increases the cycle efficiency.

Purpose and Importance of Feedwater Heater

The main functions of a feedwater heater are as follows:

Improves Thermal Efficiency:
Preheating feedwater reduces the amount of fuel required in the boiler, improving the overall efficiency of the power plant.
Reduces Fuel Consumption:
As less fuel is needed to heat already warm water to its boiling point, it saves energy and reduces operational costs.
Minimizes Thermal Stresses:
By preheating the water, the temperature difference between the feedwater and boiler tubes is reduced. This minimizes expansion and contraction stresses, preventing tube damage.
Increases Turbine Efficiency:
Extracting steam at intermediate stages of the turbine for feedwater heating helps balance the load on turbine stages, improving turbine efficiency.
Improves Overall System Life:
Preheated feedwater reduces condensation shock in the boiler and ensures stable steam generation, increasing the overall reliability of the system.
Improves Condensate Quality:
In closed feedwater heaters, condensate and feedwater do not mix, maintaining clean and treated water throughout the system.

Types of Feedwater Heaters

Feedwater heaters are mainly classified into two types depending on whether or not the steam and water mix during heat transfer:

Open Feedwater Heater (Direct Contact Type)

In an open feedwater heater, the extracted steam and the feedwater come into direct contact with each other. The steam condenses while transferring its heat directly to the feedwater. The mixed water (now at a higher temperature) is then pumped to the boiler.

Features:

Simple in design and inexpensive.
High heat transfer efficiency due to direct mixing.
Requires good water quality to prevent contamination.

Example:
A deaerator used in power plants is a common type of open feedwater heater that also removes dissolved gases like oxygen and carbon dioxide.

Closed Feedwater Heater (Indirect Contact Type)

In a closed feedwater heater, the extracted steam and feedwater do not mix directly. The steam flows on one side of metal tubes, while the feedwater flows on the other side. Heat is transferred through the tube walls, and the steam condenses separately.

Features:

Maintains purity of feedwater.
More complex and costly than open type.
Suitable for high-pressure and large-capacity power plants.

Closed feedwater heaters can be arranged in series at different pressure levels to maximize heating efficiency.

Advantages of Using Feedwater Heater

Higher Plant Efficiency: Reduces the heat load on the boiler and fuel consumption.
Improved Boiler Life: Minimizes thermal shocks and metal stress.
Fuel and Cost Savings: Uses waste steam energy instead of fresh fuel.
Improved Steam Cycle: Enhances the regeneration process in the Rankine cycle.
Reduced Corrosion: Removes dissolved gases in open heaters like deaerators.

Applications of Feedwater Heater

Feedwater heaters are used in:

Thermal power plants to preheat water before boiler entry.
Industrial boilers where efficiency improvement is needed.
Marine steam systems to enhance engine performance.
Combined-cycle plants to recover waste heat and improve output.

Example for Better Understanding

Suppose feedwater enters a boiler at 30°C, and the boiler produces steam at 200°C. If feedwater is first heated to 80°C using a feedwater heater, the boiler only needs to raise the temperature by 120°C instead of 170°C. This means less fuel energy is required, resulting in significant fuel savings and higher system efficiency.

Typical Feedwater Heating in Power Plants

In large thermal power stations, several feedwater heaters are used in series, connected to different turbine extraction points. For example:

Low-pressure feedwater heaters receive steam from low-pressure turbine stages.
High-pressure feedwater heaters receive steam from intermediate turbine stages.
This multistage heating process further increases efficiency and reduces fuel usage.

Conclusion

In conclusion, the function of a feedwater heater is to preheat the water before it enters the boiler by using steam extracted from the turbine. This process increases thermal efficiency, reduces fuel consumption, and minimizes thermal stresses on the boiler. Feedwater heaters can be of open or closed types depending on whether the steam and water mix or not. They play a crucial role in improving the economy, reliability, and performance of power plants by recycling waste heat within the steam cycle.