Dew Point Corrosion in Flue Gas Heat Recovery: Material Selection for Platular Heat Exchangers

Flue gas heat recovery improves industrial energy efficiency by reducing fuel use and exhaust temperature, but low-temperature operation increases the risk of flue gas dew point corrosion, especially in gases containing sulfur, chlorides, moisture, dust, biomass exhaust, waste gas, or chemical process emissions. For Platular heat exchangers, or welded gas plate heat exchangers, this risk should be assessed early because compact welded plate channels may create local cold surfaces. If the metal wall temperature drops below the acid dew point, sulfuric acid, hydrochloric acid, or other acidic condensates can form and cause rapid corrosion.

Key Takeaway

● Acid condensation starts below acid dew point.

● Minimum wall temperature matters more than average gas temperature.

● SO₃, HCl, HF, moisture, oxygen, dust, and deposits increase corrosion risk.

● Cold-end plates, welds, and low-flow areas are key risk zones.

● Material selection must match gas chemistry and condensate severity.

● 316L, duplex steel, high-alloy steel, and nickel alloys suit different risks.

● Wall-temperature control is as important as corrosion-resistant material.

● Plate spacing, flow distribution, drainage, and cleaning access affect service life.

● Customized design is usually required for corrosive flue gas recovery.

What Is Flue Gas Dew Point Corrosion?

How Acid Dew Point Forms in Flue Gas

Flue gas dew point corrosion occurs when acidic vapor in flue gas condenses on metal surfaces and forms a corrosive liquid film. In sulfur-bearing combustion systems, sulfur is mainly converted into SO₂, and part of it may oxidize into SO₃. When SO₃ reacts with water vapor, sulfuric acid vapor forms and may condense at a temperature much higher than the normal water dew point.

In chloride-containing exhaust, hydrochloric acid may also exist as vapor or condensate. When HCl, HF, SO₃, and water vapor coexist, the condensate can become highly acidic and corrosive to carbon steel and even some stainless steels. Therefore, acid dew point should be evaluated based on actual flue gas composition, not only water vapor content.

Why Average Gas Temperature Is Not Enough

A heat recovery system may appear safe when the average flue gas outlet temperature is above the acid dew point, but corrosion is controlled by the actual metal surface temperature. In compact welded plate exchangers, plate wall temperature can be lower than bulk gas temperature, especially near the cold end or areas strongly cooled by the cold-side gas.

Vulnerable areas include cold-end plates, inlet corners, flow maldistribution zones, low-velocity passages, and surfaces close to cold air channels. Low-load operation, winter conditions, excessive cold-side flow, start-up, and shutdown can further lower wall temperature. Therefore, evaluation should focus on the minimum plate wall temperature under both normal and transient conditions.

Common Damage Patterns

Typical signs of flue gas dew point corrosion include pitting, wall thinning, plate perforation, acidic deposits, leakage, and increased pressure drop from corrosion products or fouling. Localized pitting is especially dangerous for thin heat transfer plates because it can penetrate the wall faster than uniform corrosion.

In Platular heat exchangers, weld seams, plate edges, cold-end zones, drainage points, and deposit-covered areas need special inspection. Acidic condensate may be trapped in crevices or under deposits, causing long acid-metal contact and severe under-deposit or crevice corrosion.

Why Platular Heat Exchangers Need Special Attention

Compact Welded Plate Structure

A Platular heat exchanger uses welded metal plates to form gas channels, enabling efficient gas-to-gas heat transfer through thin plate walls. This compact structure improves heat recovery efficiency and reduces equipment size, making it suitable for industrial waste heat recovery. However, strong cold-side cooling may lower local wall temperature below the acid dew point, so efficiency must be balanced with corrosion protection.

Welded Construction and Corrosion Sensitivity

Prandtl gas plate heat exchangers use all-welded construction and pressure testing to ensure long-term sealing between gas streams. In acid dew point conditions, weld quality and material compatibility are critical because weld toes, heat-affected zones, corners, and plate edges may corrode first. Design should consider base material, welding consumables, welding procedures, surface condition, inspection methods, temperature distribution, and condensate drainage.

Thermal Expansion and Structural Reliability

High-temperature gas heat recovery equipment must withstand thermal expansion, thermal stress, and repeated load changes. Prandtl’s gas plate heat exchanger design considers structural reliability under high-temperature service to reduce deformation, weld fatigue, and leakage risks. In dew point corrosion applications, corrosion-resistant materials, structural flexibility, proper support, and controlled operation are needed because corrosion and stress may accelerate damage together.

Fouling and Under-Deposit Corrosion

Dust, ash, soot, catalyst powder, salts, and sticky particles can accumulate on heat transfer surfaces and absorb acidic condensate. These deposits may keep the metal wet and create a corrosive microenvironment even after gas temperature rises above the dew point. Therefore, plate spacing, gas velocity, pressure drop, cleaning access, and fouling characteristics should be optimized according to dust loading, particle properties, corrosion risk, and maintenance conditions.

Risk Areas in Platular Heat Exchangers

Main Factors Affecting Flue Gas Dew Point Corrosion

Flue Gas Composition

The severity of flue gas dew point corrosion depends on SO₂, SO₃, HCl, HF, water vapor, oxygen, NOₓ, dust, alkali salts, and other process components. SO₃ mainly affects the sulfuric acid dew point, while HCl and chloride salts increase pitting and crevice corrosion risks, especially on stainless steels. Therefore, material selection should be based on measured or reliably estimated gas composition.

Metal Surface Temperature

Flue gas dew point corrosion starts when the metal surface temperature falls below the acid dew point and acidic condensate forms. A safe design should evaluate the minimum wall temperature, especially at the cold-end plates of Platular heat exchangers, rather than only checking gas inlet and outlet temperatures. Low load, high cold-side flow, winter operation, start-up, and shutdown may require a temperature margin, bypass, staged recovery, flow control, or preheating.

Gas Flow and Deposits

Gas flow affects heat transfer, pressure drop, fouling, erosion, and corrosion. Low velocity may cause dust settlement and acid retention, while excessive velocity may increase erosion and fan power. Flow distribution, header design, guide plates, and inlet/outlet configuration should be optimized to avoid overcooled zones and deposit-prone low-flow areas.

Cleaning and Maintenance Conditions

Maintenance conditions directly influence corrosion control because acidic deposits can cause premature failure even on corrosion-resistant alloys. Inspection doors, cleaning ports, drainage points, soot removal methods, and accessible duct arrangements should be considered during the layout stage. If shutdown time is short or access is limited, the design should emphasize fouling prevention, easier cleaning, and more conservative material selection.

Material Comparison Table

Design Strategies to Reduce Corrosion Risk

Keep Critical Surfaces Above Acid Dew Point

The most effective way to reduce corrosion is to keep critical metal surfaces above the acid dew point. Acid dew point and minimum plate wall temperature should be evaluated together during thermal design, because continuous acidic condensate can still damage high-grade alloys. If outlet temperature is too low, bypass control, staged recovery, adjusted cold-side flow, recirculation, or minimum temperature control may be required.

Optimize Plate Spacing and Gas Velocity

Plate spacing affects heat transfer, pressure drop, fouling, and cleaning. Narrow channels improve heat transfer and compactness but may increase blockage risk, while wider channels improve fouling tolerance but require more heat transfer area. Prandtl gas plate heat exchangers can be customized to balance heat recovery efficiency, fouling control, pressure drop, and corrosion protection.

Improve Flow Distribution

Good flow distribution helps maintain uniform temperature and reduce corrosion risk. Uneven gas distribution may create overcooled channels or low-velocity fouling zones where condensate and deposits accumulate. Flow arrangements such as U-type, W-type, S-type, I-type, L-type, or customized structures can be selected according to duct layout and process needs.

Provide Cleaning, Drainage, and Inspection Access

Acidic condensate and deposits should not remain inside the exchanger for long periods. Drainage points, inspection openings, removable duct sections, and suitable cleaning methods should be considered during the layout stage. For dusty or corrosive gas, the exchanger structure and material should match the planned cleaning method, such as manual cleaning, soot blowing, air pulsing, or water washing.

Control Start-Up and Shutdown Conditions

Start-up and shutdown are often the most corrosive periods because cold metal or cooling surfaces can promote acid condensation. Operating procedures should include controlled heating, dry-out operation, condensate drainage, and avoidance of long wet stagnant periods. In some systems, corrosive flue gas should be bypassed until the exchanger reaches a safe temperature.

Conclusion

Flue gas dew point corrosion is a major risk in low-temperature flue gas heat recovery. Acidic condensate can attack heat transfer plates, welds, ducts, drainage areas, and cold-end surfaces, especially when SO₃, HCl, HF, moisture, dust, and deposits are present.

For Platular heat exchangers, reliable material selection requires acid dew point evaluation, minimum wall-temperature control, flow distribution optimization, fouling management, pressure drop review, cleaning access, drainage design, and operating procedure control.

Carbon steel, 304, 316L, duplex stainless steel, high-alloy stainless steel, nickel-based alloys, and protective coatings each have application limits. The correct choice depends on real flue gas composition, condensate severity, operating temperature, maintenance conditions, and lifecycle cost. Nanjing Prandtl Heat Exchange Equipment Co., Ltd. can provide customized gas plate heat exchanger solutions based on actual process data for safe, efficient, and long-term operation.

FAQ

What causes flue gas dew point corrosion?

Flue gas dew point corrosion is caused by acidic vapors condensing on metal surfaces when surface temperature falls below acid dew point. Common condensates include sulfuric acid from SO₃ and water vapor, and hydrochloric acid from chloride-containing gas.

Which material is best for flue gas dew point corrosion?

There is no universal best material. 316L may suit moderate service, duplex or high-alloy stainless steel may suit stronger chloride or mixed acid exposure, and nickel-based alloys may be required for severe condensate conditions.

Can stainless steel fully prevent dew point corrosion?

No. Stainless steel can reduce corrosion risk, but chlorides, low-pH condensate, sulfuric acid, crevices, deposits, and low wall temperature can still cause pitting or crevice corrosion.