Maximum operating Temperature of Stainless steel in Heat Exchanger
he maximum operating temperature for a stainless-steel shell and tube heat exchanger is not a single number but depends on a combination of factors, primarily the grade of stainless steel and the design pressure.
Here’s a detailed breakdown:
Quick Summary by Grade
| Stainless Steel Grade | Typical Maximum Continuous Operating Temperature (in Air) | Key Limiting Factor & Notes |
| 304 / 304L | ~800°C – 900°C (1472°F – 1652°F) | Oxidation resistance, strength loss, carbide precipitation (sensitization) in ~425-860°C range. |
| 316 / 316L | ~800°C – 900°C (1472°F – 1652°F) | Similar to 304 but better resistance to sensitization and some corrosives. Molybdenum adds strength. |
| 321 | ~810°C – 900°C (1490°F – 1652°F) | Stabilized with Titanium to resist sensitization, making it a good choice for this temperature range. |
| 347 | ~870°C – 900°C (1600°F – 1652°F) | Stabilized with Niobium, offering similar benefits to 321 with potentially better high-temp strength. |
| 310S | ~1100°C (2012°F) | High chromium and nickel content provides superior oxidation resistance and strength at high temps. |
Detailed Explanation of Limiting Factors
The maximum temperature is not just about melting. The following factors determine the practical limit:
- Oxidation Resistance: At high temperatures, metals react with oxygen, forming scale that weakens the material. Higher chromium content (e.g., in 310S) forms a more protective scale layer.
Strength Reduction (Creep): As temperature increases, the mechanical strength of the steel decreases. Creep—the slow, permanent deformation of material under
- stress—becomes the critical design factor. The design pressure drastically impacts the max temperature. A high-pressure exchanger will have a much lower max temperature rating than a low-pressure or vacuum exchanger made from the same material.
- Carbide Precipitation (Sensitization): For unstabilized grades like 304/316, exposure in the range of 425-860°C (800-1580°F) causes chromium carbides to form along grain boundaries. This depletes chromium, making the steel susceptible to intergranular corrosion. This is why stabilized grades (321, 347) are preferred for prolonged service in this “sensitization” range.
- Thermal Stability: Long-term exposure can cause microstructural changes (e.g., sigma phase formation), which embrittle the steel.
Practical Design Considerations
In real-world engineering, the maximum temperature is dictated by the ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Division 1.
- The code provides Maximum Allowable Stress Values (MAST) tables for each material at various temperatures.
- A mechanical engineer will use these tables to design the vessel thickness for the specified design pressure and temperature.
- The practical, safe operating limit for a standard pressure vessel made of 304/316 SS is often considered to be around 800°C (1472°F). Beyond this, the allowable stress values drop significantly, requiring extremely thick walls, which becomes impractical and uneconomical.
What about higher temperatures? (Beyond 1100°C)
For applications requiring temperatures beyond the capability of standard stainless steels (e.g., >1100°C), different materials are used:
- Nickel-Based Superalloys (e.g., Inconel 600, 625): Good up to ~1150°C.
- Refractory Metals (e.g., Tungsten, Molybdenum): Very high melting points but oxidize rapidly in air.
- Ceramics or Ceramic-Lined Exchangers: Used for the most extreme temperatures (e.g., >1500°C) in specialized applications.
Conclusion
For a stainless steel shell and tube heat exchanger:
- A safe, general guideline is ~800°C (1472°F) for standard grades like 304/316 under pressure.
- The exact maximum temperature must be determined by a mechanical engineer using the ASME code tables for the specific grade, considering the design pressure.