What is an economizer? Are you familiar with economizers?

What is an Economizer?

An economizer is a heat exchange device installed in the tail-end flue of a boiler. It utilizes the waste heat from the high-temperature exhaust gases discharged by the boiler to preheat the boiler feedwater. This reduces the amount of heat input required by the main boiler body, thereby achieving the goal of "saving coal."

How does an Economizer Work?

Heat Recovery Process:

The temperature of boiler exhaust gases typically ranges from 200°C to 400°C and still contains a significant amount of waste heat.

Before entering the main boiler body (the steam drum), the boiler feedwater is first pumped into the economizer. The economizer uses metal piping (often cast iron or steel tubes) to facilitate counter-flow heat exchange between the low-temperature feedwater and the high-temperature exhaust gases. The water temperature increases before entering the steam drum.

Energy Transfer:

The exhaust gases are cooled before being discharged, reducing heat loss.

After absorbing heat, the feedwater temperature is significantly raised. Consequently, the main boiler body requires only a relatively small additional amount of heat to generate steam.

Why Use an Economizer?

• Significantly Improves Boiler Efficiency: For every 20-30°C reduction in exhaust gas temperature, boiler efficiency can increase by approximately 1%. In large boilers, installing an economizer can improve efficiency by over 5%.
• Reduces Fuel Consumption: Recovering waste heat directly reduces the consumption of coal or gas, leading to a substantial decrease in operating costs.
• Mitigates Thermal Stress: Preheated feedwater reduces the temperature difference with components like the steam drum and furnace, lowering thermal stress and extending boiler lifespan.
• Environmental Benefits (Emission Reduction): Lowering exhaust gas temperature helps reduce emissions of particulate matter and acidic gases (e.g., SO₂). It also makes it easier to condense and treat some harmful substances in the flue gas and reduces the power consumption of the induced draft fan.

Economizers Used in Heat Recovery Steam Generators (HRSG)

In HRSGs, the economizer plays the role of preheating boiler feedwater to maximize the recovery of waste heat from exhaust gases.

Installation Location: The economizer is located at the most downstream point in the flue gas path and at the starting point of the water/steam circuit.

• Flue Gas Side: High-temperature exhaust gases (approximately 400-600°C), after passing through the superheater and evaporator and releasing most of their heat, enter the economizer located at the downstream end. Here, the gases transfer their remaining waste heat to the feedwater, cooling down to about 90-150°C before being discharged to the chimney.
• Water Side: Low-temperature feedwater from the feedwater pump first enters the economizer for preheating before proceeding to the evaporator to absorb heat and begin boiling. This design maximizes the reduction of exhaust gas temperature and improves overall system efficiency.

Types of Economizers

1. By Material

• Cast Iron Economizer: Advantages include good resistance to sulfuric acid corrosion. However, it has low pressure-bearing capacity, is bulky, and prone to air leakage. It was once an option for small industrial boilers but is now being phased out.
• Steel Tube Economizer: The absolute mainstream choice for modern boilers. Made from carbon steel or alloy steel seamless tubes, it offers high pressure-bearing capacity, a lightweight structure, and high heat transfer efficiency. Almost all economizers in power plant boilers and HRSGs are of this type.

2. By Working Fluid (Water) State

• Non-Boiling Economizer: The outlet water temperature is below the saturation temperature at the corresponding pressure; the water does not boil. It operates safely and reliably and is the mainstream choice for modern medium/low-pressure boilers and HRSGs.
• Boiling Economizer: The outlet water temperature reaches saturation temperature, with a portion (typically ≤10%-20%) vaporized into steam. It is primarily used in high-pressure and above power plant boilers to more fully utilize waste heat from flue gases. However, its design and control are more complex, requiring consideration of issues like steam-water separation.

3. By Structural Form

• Bare Tube Economizer: The simplest structure, consisting of smooth steel tubes bent into serpentine coils. Advantages include ease of soot cleaning, but it has a relatively small heat transfer area and is bulky.
• Finned Tube Economizer: The mainstream configuration for high-efficiency boilers. By welding or rolling extended fins onto the outside of steel tubes, the heat transfer area on the flue gas side is greatly increased. This makes the equipment more compact and efficient, especially suitable for HRSGs using clean gas turbine exhaust (low dust). Main types include:
  • H-Fin Tube Economizer
  • Spiral/Helical Fin Tube Economizer
  • Studded Pipe Economizer

Feature	Spiral Fin Tube Economizer	H-Fin Tube Economizer	Studded Pipe Economizer
Design Concept	Maximize extended heat transfer area for high efficiency and compactness.	Balance enhanced heat transfer with anti-fouling/anti-wear properties for reliability and durability.	Enhance flue gas turbulence; studs disrupt the boundary layer.
Key Advantages	1. High heat transfer efficiency (large fin ratio, high heat exchange per unit volume). 2. Compact structure (smaller volume for equivalent heat exchange). 3. Cost-effective.	1. Excellent anti-fouling capability (flue gas flows through straight channels, minimizing eddy-induced ash buildup). 2. Superior anti-wear performance (uniform flow, no severe local scouring; thickness can be increased at wear-prone points). 3. Easy to clean (straight profile allows effective soot blower operation).	1. Very strong anti-fouling capability, especially against sticky ash. 2. Intense flue gas turbulence leads to high convective heat transfer coefficient.
Main Disadvantages	1. Poor anti-fouling performance (ash tends to accumulate in eddies on the lee side of spiral fins). 2. Wear risk (ash accumulation can lead to localized wear).	1. Relatively lower heat transfer efficiency (fin ratio typically smaller than spiral fins). 2. More complex structure and higher cost (demanding welding process, more material). 3. Relatively larger volume (slightly more space required for equivalent heat exchange).	1. High flue gas resistance increases energy consumption. 2. Relatively small extended heat transfer area (approx. 5-8 times). 3. Complex manufacturing process and high cost.
Manufacturing Process	High-Frequency Resistance Welding (mainstream), Integral Rolling.	Flash Butt Resistance Welding.	Resistance Welding.
Suitable For	Clean or low-dust flue gas, e.g., Gas Turbine HRSGs, Gas/Oil-Fired Boilers, some industrial furnaces with clean exhaust.	High-dust, high-wear, or sticky ash flue gas, e.g., Coal-Fired Boilers (especially tail-end), Sintering Machine/Cement Kiln Waste Heat Boilers, Waste Incineration Boilers.	Flue gas containing sticky fly ash prone to severe fouling, e.g., Biomass Boilers (straw, etc.), some Oil/Waste Liquid Incineration Boilers.

4. By Flue Gas & Working Fluid Flow Direction

• Counter-Flow Arrangement: The most classic and efficient method. Low-temperature feedwater enters at the economizer outlet, flowing opposite to the flue gas direction, maintaining a large mean temperature difference for heat transfer.
• Parallel-Flow Arrangement: Water and flue gas flow in the same direction. This results in a smaller heat transfer temperature difference and lower efficiency, but provides more uniform tube wall temperature. It is often used in areas with extremely high flue gas temperature to protect the tubes.

5. Special Types of Economizers

• Heat Pipe Economizer: Utilizes the principle of thermal superconductivity in heat pipes, completely isolating the flue gas side from the water side. It offers excellent corrosion and leakage protection, making it very suitable for highly corrosive flue gases or retrofit projects with space constraints.
• Condensing Economizer: Cools the flue gas below its dew point (typically <90°C), recovering not only sensible heat but also the latent heat of condensation from water vapor in the flue gas. This offers significant energy-saving potential (can further improve efficiency by 5-10%), but requires materials with high acid corrosion resistance.

How to Select an Economizer Type?

• Boiler Pressure & Parameters: Determines whether a boiling or non-boiling type is suitable.
• Fuel & Flue Gas Characteristics: High sulfur content and strong corrosivity necessitate a focus on corrosion-resistant materials (e.g., ND steel) or structures (e.g., heat pipes). High dust content requires a structure that facilitates cleaning.
• Space Constraints: For compact spaces, finned tube types are preferred.
• Economics & Retrofit Needs: Condensing and heat pipe economizers have higher initial investment but also high returns, often used in energy-saving retrofits.

High-Temperature vs. Low-Temperature Economizers

• High-Temperature Economizer: Located in the high-temperature flue gas zone, directly after the boiler furnace or superheater. Its main function is to recover high-grade sensible heat, heating the feedwater to a temperature close to saturation. It faces challenges like high-temperature corrosion and ash fouling. Materials require good high-temperature strength and oxidation resistance.
• Low-Temperature Economizer: Located at the very end of the flue gas system, after the air preheater. It aims to deeply reduce the exhaust gas temperature, sometimes cooling it below the acid dew point. Its core challenge is low-temperature (acid dew point) corrosion. The use of corrosion-resistant materials (e.g., ND steel, fluoroplastics, glass flake coatings) or anti-corrosion design is essential.