1. Definition and Basic Principle

In a Heat Recovery Steam Generator (HRSG), the economizer is a heat exchanger that uses low-grade heat from exhaust gases to preheat boiler feedwater before it enters the evaporator section. By raising the feedwater temperature, the economizer reduces the energy required in the main steam generation process, thereby improving overall thermal efficiency.

2. Operating Conditions and Flue Gas Characteristics

The exhaust gases entering the HRSG economizer typically originate from gas turbines, and they exhibit the following features:

• High temperature and fast flow velocity: which imposes thermal stress on the heat transfer surfaces.

• Corrosive composition: containing SOx, NOx, and moisture, which may form acidic condensates leading to low-temperature corrosion.

• Dust content (in some industrial applications): which may lead to fouling and ash deposition.

Therefore, the design of an economizer must consider corrosion resistance, abrasion durability, and heat transfer effectiveness.

3. Types and Structural Forms

Economizers in HRSGs are commonly categorized by their tube structure:

1. Plain tube economizers – simple design, suited for clean flue gas applications.

2. Finned tube economizers – enhanced surface area for improved heat transfer, commonly used where higher efficiency is required.

In terms of arrangement, they can be installed in horizontal or vertical configurations. Horizontal units facilitate easier maintenance and soot-blowing, while vertical units are preferred where space is limited.

4. Functions Within the HRSG System

The economizer contributes to HRSG performance in several key ways:

• Boosting thermal efficiency: By preheating feedwater with residual heat in exhaust gases, the steam generator reduces fuel consumption and load on other sections.

• Lowering flue gas outlet temperature: Enhances overall energy recovery and provides favorable conditions for downstream components such as SCR (Selective Catalytic Reduction) systems and dust collectors.

5. Design and Operational Considerations

When designing or operating an economizer in HRSG, several key issues must be addressed:

• Low-temperature corrosion: Ensure feedwater exit temperatures are above the acid dew point, or use corrosion-resistant alloys.

• Ash accumulation and erosion: Maintain optimal flue gas velocities, and consider soot-blowing systems or self-cleaning design features.

• Thermal stress: Design for thermal expansion and contraction with flexible supports or expansion joints to avoid structural damage.

6. Conclusion

The economizer is an essential component of an HRSG, playing a vital role in energy conservation and system efficiency. Its performance directly affects the overall heat recovery capacity and the lifecycle of the HRSG system. In an era where environmental regulations and fuel costs continue to rise, the efficient design and operation of economizers will remain a focal point for energy-intensive industries and power producers alike.