1. Overview

High and low pressure feedwater heaters are critical components in thermal power plants for utilizing extraction steam to heat feedwater. By doing so, they enhance the thermal cycle efficiency, reduce fuel consumption, and contribute to the overall economic performance of the power generation system.

Based on the pressure level of the extraction steam, feedwater heaters are categorized as:

• Low Pressure Heaters (LP Heaters)

• High Pressure Heaters (HP Heaters)

Though they are positioned at different stages in the water-steam cycle and endure different pressure levels, both types operate under the same principle — indirect heat exchange between steam and feedwater.

2. Working Principle

Feedwater heaters work by allowing steam extracted from various stages of the turbine to enter the shell side of the heater, where it condenses and releases heat. This heat is transferred to the feedwater flowing inside the tubes, increasing its temperature before entering the boiler or economizer.

The process ensures:

• Efficient energy recovery

• No direct mixing between steam and water

• Proper removal of condensate via a drainage system

3. Structural Types

Most feedwater heaters adopt shell-and-tube heat exchanger design. Common structural forms include:

Typical components:

• Shell (pressure vessel)

• Tube bundle (U-tube or straight)

• Tube sheets and baffles

• Steam inlet, water inlet/outlet

• Drainage and safety protection systems

4. Functions and Benefits

• Preheating of feedwater, reducing boiler load

• Improved heat rate of the entire system

• Thermal shock protection for boiler and turbine equipment

• Reliable operation as part of thermal regulation systems

5. Application Scenarios (Detailed)

5.1. Thermal Power Plants

Feedwater heaters are an essential part of the regenerative Rankine cycle.

• LP heaters are located between the condensate pump and the feedwater pump

• HP heaters are positioned between the feedwater pump and the economizer or boiler inlet

Benefits:

• Reduce specific fuel consumption

• Improve overall cycle efficiency by 2–5%

• Protect system components from thermal fatigue

5.2. Combined Heat and Power (CHP) Systems

In cogeneration plants, feedwater heaters not only serve the boiler cycle but also handle part of the heating load for industrial or residential use.
By adjusting the extraction steam distribution, they help maximize dual-purpose output—electricity and heat.

5.3. Nuclear Power Plant – Conventional Island

In nuclear plants, feedwater heaters operate in the non-nuclear "conventional island" to reheat return water from the steam generator.
Due to safety and material requirements, these heaters are usually constructed from stainless steel and designed for radiation resistance and high reliability.

5.4. Chemical and Metallurgical Industries

Feedwater heaters or their customized variants are widely used in:

• Ammonia synthesis feedwater preheating

• Ethylene cracking heat recovery

• Hot blast stoves in iron & steel production

These industries require heaters with excellent corrosion resistance, compact structures, and reliable long-term performance under harsh operating conditions.

6. Key Selection Considerations

1. Thermal matching with steam and feedwater pressure, temperature, and flow

2. Structure type, based on fouling tendency, temperature difference, and maintenance needs

3. Material selection, especially for corrosion or high-pressure environments

4. Drainage and safety design, including trap systems and pressure relief devices

5. Thermal and mechanical design, involving detailed performance calculation and stress analysis