Short Answer:

An economizer is a heat recovery device used in boilers to preheat the feedwater by utilizing the heat from flue gases leaving the boiler. This process reduces fuel consumption and increases the overall efficiency of the boiler. It helps in saving energy by transferring waste heat to the incoming cold water before it enters the boiler drum.

In simple words, an economizer improves the performance of a boiler by reusing heat that would otherwise be wasted. By heating the feedwater using exhaust gases, it lowers fuel usage, saves cost, and enhances the overall thermal efficiency of the power plant or industrial system.

Detailed Explanation :

Economizer

An economizer is an important heat recovery component in a boiler system that is designed to improve energy efficiency and reduce fuel consumption. It is placed in the flue gas path, usually after the superheater and before the chimney. The function of the economizer is to utilize the remaining heat of exhaust gases to preheat the feedwater supplied to the boiler.

This process of preheating feedwater saves fuel because the water entering the boiler requires less additional heat to reach its boiling point. The economizer thus contributes significantly to fuel economy, better efficiency, and environmental sustainability by lowering the amount of heat lost through exhaust gases.

1. Purpose of Economizer

The main purpose of an economizer is to improve the efficiency of the boiler by recovering waste heat. When the flue gases leave the boiler furnace, they are still at a high temperature (around 300°C to 400°C). Instead of releasing this heat into the atmosphere, it is utilized to heat the feedwater.

The key purposes include:

1. To save fuel – by reducing the amount of heat needed in the boiler to convert water into steam.
2. To increase efficiency – by using waste heat energy.
3. To reduce thermal stress – as preheated water causes less temperature difference in the boiler shell and tubes.
4. To reduce cost of operation – due to fuel and energy savings.

2. Construction of Economizer

An economizer consists of a series of metal tubes or finned tubes through which the feedwater flows. These tubes are arranged in the path of hot flue gases coming from the boiler. The heat from the gases is transferred through the tube walls to the feedwater flowing inside.

The main parts of an economizer are:

1. Tubes:
   • Made of cast iron or steel to withstand high temperatures and pressure.
   • Arranged in a zig-zag or serpentine pattern for better heat transfer.
2. Headers:
   • Collect and distribute feedwater into the economizer tubes.
3. Casing:
   • Outer structure that encloses the economizer and directs the flow of flue gases.
4. Inlet and Outlet Connections:
   • Feedwater enters from the inlet header and leaves through the outlet after absorbing heat.
5. Bypass Dampers:
   • Provided to control the flow of flue gases when the economizer is not in operation.

These components work together to ensure maximum heat transfer efficiency from the exhaust gases to the water.

3. Working of Economizer

The working principle of an economizer is based on heat exchange between the hot flue gases and the cold feedwater.

1. The feedwater from the feed pump is passed through the economizer tubes before entering the boiler drum.
2. The flue gases coming from the superheater or boiler furnace flow around these tubes.
3. The heat from the flue gases is transferred to the feedwater through conduction and convection.
4. The flue gases lose part of their heat and become cooler before exiting through the chimney.
5. The feedwater, after absorbing the heat, becomes preheated and enters the boiler drum at a higher temperature.

This process helps in saving fuel, as the water entering the boiler requires less heat to convert into steam.

4. Types of Economizers

Economizers are classified based on their design and application. The main types are:

1. Non-condensing Economizer:
   • Operates with flue gases above the dew point temperature to avoid condensation.
   • Commonly used in industrial and power plant boilers.
2. Condensing Economizer:
   • Cools the flue gases below the dew point temperature, recovering latent heat of vaporization.
   • Provides higher efficiency but requires corrosion-resistant materials.
3. Integral Economizer:
   • Built within the boiler unit, designed as part of the boiler system.
4. Separate Economizer:
   • Installed externally in the flue gas duct, separate from the main boiler structure.

5. Advantages of Economizer

1. Improved Efficiency:
   • Increases the overall efficiency of the boiler by 10–15%.
2. Fuel Savings:
   • Reduces fuel consumption as feedwater enters the boiler at a higher temperature.
3. Reduced Heat Loss:
   • Minimizes the loss of heat in exhaust gases.
4. Reduced Thermal Stress:
   • Preheating water reduces temperature shock to boiler components, increasing their life.
5. Lower Operating Cost:
   • Economical operation due to better utilization of heat energy.
6. Environmental Benefits:
   • Less fuel usage means reduced carbon emissions.

6. Disadvantages of Economizer

1. High Initial Cost:
   • Increases the cost of boiler installation.
2. Maintenance Required:
   • Scaling and soot accumulation on tubes require regular cleaning.
3. Corrosion Issues:
   • Moisture and acidic gases may corrode metal tubes, especially in condensing economizers.
4. Pressure Drop:
   • Causes minor resistance to flue gas flow.

7. Applications of Economizer

Economizers are widely used in:

• Thermal power plants to improve steam generation efficiency.
• Industrial boilers for process heating.
• Marine boilers in ships.
• Central heating systems for energy recovery.
• Refineries and paper mills where large steam demand exists.

In modern power plants, economizers are a standard component of boiler systems and play a key role in achieving high efficiency.

8. Efficiency Gain from Economizer

The efficiency improvement achieved through an economizer can be calculated approximately as:

For every 20°C reduction in flue gas temperature, there is roughly 1% increase in boiler efficiency. This means an economizer can improve efficiency by up to 10–12%, depending on its design and operation.

Conclusion

An economizer is an essential heat recovery component of a boiler that utilizes waste heat from flue gases to preheat the feedwater. By doing so, it increases the overall efficiency of the boiler, saves fuel, reduces operational costs, and minimizes heat losses. Although it adds to the initial cost and requires maintenance, the long-term energy savings and efficiency improvement make it indispensable in modern thermal systems. Economizers thus play a vital role in achieving energy efficiency and sustainable operation in industrial and power plant boilers.