What is Superheater for HRSG Boiler System?
What is a Superheater?
A superheater is a critical component in a boiler system used to heat saturated steam into superheated steam. In a heat recovery steam generator (HRSG), it utilizes the waste heat from high-temperature flue gas to further increase the steam temperature and energy grade, thereby enhancing the overall systems thermal efficiency and economic performance.
Why Heat Saturated Steam into Superheated Steam?
Improves Steam Work Capacity
During expansion and work in a turbine or steam engine, superheated steam is less prone to condensing into water droplets, which protects the blades and improves thermal efficiency.
Reduces Pipeline Corrosion
Superheated steam does not condense during transport, avoiding internal pipe erosion and corrosion caused by water.
Difference Between Saturated Steam and Superheated Steam?
The core difference lies in the "presence of liquid water" and the "form of energy."
| Feature | Saturated Steam | Superheated Steam |
|---|---|---|
| Definition | Steam produced when water is heated to its boiling point (saturation temperature) at a given pressure, existing in a co-existing gas-liquid phase state. | Steam produced by further heating saturated steam at the same pressure, raising its temperature above the saturation temperature for that pressure. |
| Physical State | Wet steam (may carry fine water droplets). At the endpoint of vaporization / starting point of condensation. | Dry steam (pure gaseous phase, contains no liquid water droplets). |
| Temperature | Temperature equals the saturation temperature at the current pressure (e.g., 180°C at 1MPa). Pressure and temperature have a fixed relationship. | Temperature is higher than the saturation temperature at the current pressure (e.g., can be superheated to 300°C at 1MPa). Pressure and temperature are no longer bound. |
| Energy Composition | Primarily contains latent heat of vaporization (heat absorbed during the phase change from liquid to gas). | Contains all the latent heat of vaporization plus additional sensible heat (heat absorbed as temperature rises). |
| Enthalpy (Total Energy) | Relatively lower. | Significantly higher. The added superheat greatly increases the total energy. |
| Application & Characteristics | Suitable for heating and humidification processes (e.g., space heating, cooking), as it releases large amounts of latent heat upon condensation. Prone to condensing into water droplets during work in turbines, which can impact and damage blades. | Suitable for power and work applications (e.g., power generation turbines, industrial drives). Less prone to condensation during expansion, improving heat engine efficiency and protecting equipment. No condensation loss during transport. |
Structural Components of a Superheater
A superheater typically consists of a set of parallel serpentine tube bundles. Core components include:
Tubes
Made of high-temperature resistant alloy steel (e.g., 12Cr1MoVG, TP347H) to withstand high temperatures and pressures.
Headers (Inlet/Outlet)
Distribute and collect steam, usually located at both ends of the tube bundle.
Support Structure
Secures the tube bundle and accommodates thermal expansion.
Insulation Layer
Reduces heat loss.
Why are Superheaters Typically Built with Bare Tubes Instead of Finned Tubes?
High-temperature flue gas transfers heat through the outer wall of the bare tubes to the water or steam flowing inside, completing the heating, vaporization, and superheating process. The smooth surface of bare tubes is less prone to ash deposition and helps reduce flue gas flow resistance, making them suitable for the high-temperature, high-velocity flue gas environment inside boilers.
The presence of fins would significantly exacerbate heat transfer non-uniformity. Heat accumulates at the fin root, causing its temperature to be much higher than the tube wall temperature of the bare section. At high temperatures, the allowable stress of the material decreases sharply. This "hot spot" at the fin root becomes a structural weakness, highly susceptible to creep deformation, accelerated oxidation, and even tube failure.
For superheaters, especially high-temperature sections: because the working fluid (steam) temperature is highest, operating conditions are most severe, and safety requirements are most stringent, priority must be given to ensuring material strength and temperature uniformity. Therefore, bare tubes are almost exclusively used. Enhancing heat transfer is not the primary design goal; safe and reliable long-term operation is.
Main Types of Superheaters
1. Classified by Heat Transfer Method
- Convection Superheater: Placed in the flue gas duct, relying on flue gas冲刷 (scouring) the tube bundle for convective heat transfer. This is the most common type.
- Radiant Superheater: Arranged in the furnace or high-temperature radiant zone, primarily absorbing radiant heat from flames or high-temperature flue gas. Operates at higher temperatures.
- Semi-Radiant Superheater: Absorbs both convective and radiant heat (e.g., platen superheater).
2. Classified by Arrangement
- Vertical Type: Facilitates drainage but has a more complex support structure.
- Horizontal Type: Simpler support structure but more prone to ash accumulation.
3. Classified by Steam Flow Path Relative to Flue Gas
- Parallel Flow: Steam and flue gas flow in the same direction. Lower tube wall temperature, smaller heat transfer temperature difference.
- Counter Flow: Steam and flue gas flow in opposite directions. Higher heat transfer efficiency, but higher tube wall temperature.
- Mixed Flow: Combines advantages of parallel and counter flow, common in multi-stage superheaters.
4. Classified by Structural Features
- Serpentine Tube Superheater: Composed of numerous parallel U-shaped or W-shaped seamless steel tubes (serpentine tubes), with tube ends welded to inlet and outlet headers. This is the most classic and widely used form of convection superheater.
- Platen (Panel) Superheater: Formed by multiple tubes bent side-by-side into a "screen" or "curtain" shape.
- Wall Superheater: Tubes are closely spaced and arranged along the inner wall surface of the furnace.
- Enclosure (Wall) Superheater: Arranged on the walls of the horizontal convection pass or rear vertical shaft.
Application of Superheaters in Heat Recovery Boilers (HRSGs)
In most medium-to-large HRSG systems or those requiring higher parameters, it is common to install not just one, but multiple superheaters in a "staged arrangement." For example, medium-pressure and above boilers typically have two to three stages, while high-pressure and above boilers may have three to five stages.
This design is based on comprehensive considerations of thermodynamic efficiency, material safety, and operational flexibility.
1. Why Use a Staged Superheater Arrangement?
Matches Flue Gas Temperature Gradient
Flue gas temperature gradually decreases from inlet to outlet. Staging allows each superheater stage to operate within its optimal temperature range, improving overall efficiency.
Controls Metal Wall Temperature
Staging with inter-stage attemperators (desuperheaters) effectively controls the metal wall temperature of each heating surface, ensuring material safety.
Enables Precise Steam Temperature Control
Inter-stage attemperators offer fast response and precise control for regulating final steam outlet temperature.
Adapts to Complex Heat Sources
For flue gas with large temperature fluctuations or complex compositions, a staged arrangement allows flexibility in material selection and placement.
2. Typical Superheater Placement in HRSGs
Superheater placement strictly follows the sequence of decreasing flue gas temperature.
First Stage: Low-Temperature (Primary) Superheater
Location: Mid-to-rear section of the flue gas path, typically in the duct after the evaporator and before the economizer.
Role & Characteristics: The flue gas here is initially cooled, with moderate temperature (e.g., 450-600°C). It is responsible for the preliminary heating of saturated steam. The relatively lower flue gas temperature helps avoid fouling/slagging on heating surfaces and allows for more cost-effective materials.
Second (or Final) Stage: High-Temperature Superheater
Location: High-temperature zone of the flue gas path, typically at the furnace or radiant cooling chamber outlet.
Role & Characteristics: The flue gas temperature here is highest (can reach 650°C or higher). It is responsible for heating the attemperated steam to the final design outlet temperature. Due to the high heat load, higher-grade heat-resistant steels (e.g., TP347H, Super304H) are commonly used.
Special Stage: Platen or Radiant Superheater
Location: For very high-temperature heat sources (e.g., gas turbine exhaust, large incinerators), platen or wall radiant superheaters may be directly installed within the furnace or radiant chamber.
Role & Characteristics: Primarily absorbs heat from high-temperature flames or flue gas via radiant heat transfer. Operates in the most severe environment but effectively lowers the flue gas temperature entering the convection pass, preventing slagging on downstream convection heating surfaces.