# 7 Design Rules to Prevent Acid Condensation in Flue Gas Ductwork Seven design rules prevent acid condensation in flue gas ductwork: design to the minimum metal temperature, insulate continuously including flanges and stiffeners, limit air in-leakage, detail expansion joints, remove dead zones, drain condensate, and specify protection such as FlueGard-225SQC at the design stage. Protection matters because shutdowns still take duct steel through the acid dew point. **Category:** Listicles **Written by:** Edita Rojasova **Last updated:** Oct 27, 2026 **Source:** https://signals.polylloycoatings.com/listicles/flue-gas-ductwork-design-rules --- ## Flue Gas Duct Design Rules: Key Points - Acid condenses wherever duct steel runs below the acid dew point of the flue gas, typically 120 to 160 °C in cement plant flue gas depending on sulfur trioxide (SO3) and moisture, so duct design is mainly a metal-temperature problem. - Design to the coldest metal at minimum load, start-up and shutdown, not to the design-point gas temperature. - Insulate continuously, including flanges, stiffeners and supports, because heat sinks run cooler than the casing around them. - Limit air in-leakage at doors, ports, flanges and expansion joints: false air cools the steel and adds oxygen that can promote SO3 formation. - Remove dead zones and deposit traps, and drain the condensate that will still form at shutdowns. - Specify protection in the duct package at the design stage, for example FlueGard®-225SQC to 225 °C for baghouses, electrostatic precipitators (ESPs) and ducts, applied by certified partners under Polylloy supervision. Seven design rules prevent acid condensation in flue gas ductwork: design to the minimum metal temperature, insulate continuously including flanges and stiffeners, limit air in-leakage, detail expansion joints, remove dead zones, drain condensate, and specify protection such as FlueGard-225SQC at the design stage. Protection matters because shutdowns still take duct steel through the acid dew point. ## Acid Condensation in Flue Gas Ductwork Starts with Metal Temperature Acid condensation in flue gas ductwork is decided by steel temperature, which is set largely on the drawing board. Flue gas duct design for acid condensation control is the set of design-stage decisions on metal temperature, insulation, air tightness, joints, geometry, drainage and corrosion protection. It keeps duct steel above the acid dew point where possible and protects the steel that will still meet condensate. ### Why Duct Design Decides Where Acid Condenses Sulfur dioxide (SO2) in the gas partly oxidizes to SO3, which forms sulfuric acid (H2SO4) with water vapor. Polylloy's May 2026 International Cement Review (ICR) article puts the resulting acid dew point typically at 120 to 160 °C in cement plant flue gas, depending on SO3 concentration and moisture. Acid condenses where the metal surface, not the bulk gas, falls below that range. Symptoms of existing damage are covered in the [acid dew point warning signs](https://signals.polylloycoatings.com/listicles/acid-dew-point-corrosion-signs) guide. ### Why the Design Margin is Shrinking Energy-efficiency projects and waste heat recovery (WHR) systems lower flue gas temperature and narrow the margin to the dew point. High-efficiency filtration also removes the alkaline dust that used to buffer condensate downstream. Polylloy's view is that the industry solved an emissions problem and created a corrosion one. A duct designed for the old gas temperature may now spend more time near or below the dew point. ## How the 7 Flue Gas Duct Design Rules Were Selected Each flue gas duct design rule in this list meets all five of these criteria: - It is decided at the design or fabrication stage, not at an outage. - It is tied to a condensation mechanism Polylloy has published. - It applies to carbon steel ducts in cement, waste-to-energy, power and similar sulfur-bearing service. - It gives a concrete instruction for the duct package. - It is qualitative, with no unsourced numerical design margin. The order follows the design sequence: thermal design, envelope tightness, joints, geometry, drainage, then material and protection. It is not a ranking. ## Flue Gas Duct Design Rules at a Glance The table summarizes the seven rules in the order they are described. | Rule | Condensation driver it controls | Design-stage action | Where it shows up later | | --- | --- | --- | --- | | Minimum metal temperature | Low load and shutdowns | Map coldest metal | Cold sections, outlets | | Continuous insulation | Heat loss at sinks | Insulate flanges and stiffeners | Flanges, supports | | Air in-leakage | False air and oxygen | Fewer, tighter penetrations | Doors, ports | | Expansion joints | Leaks and cold frames | Seal and insulate joints | Joint frames | | Dead zones | Deposits holding moisture | Remove ledges, allow cleaning | Low-velocity pockets | | Drainage | Pooling condensate | Drain low points | Low points, stack base | | Protection at design | Residual condensation | Specify coating in package | Whole duct | ## Design to the Minimum Metal Temperature, Not the Design-Point Gas Temperature The first rule sets the thermal basis that every other rule depends on. ### The Rule Size insulation and select materials for the coldest metal the duct will see at minimum load, start-up, shutdown and upset, and record the hottest excursions as well. Polylloy's coating evaluation asks for the normal gas temperature range, maximum peaks, and the duration and frequency of those peaks, because both ends of the range matter. ### Why It Prevents Condensation Corrosion follows the coldest metal surface, not the gas reading on the control screen. Low load, start-stop operation and oversized sections enlarge the cold area. Every shutdown still takes the steel through the dew point, so good design shortens condensation time but cannot remove it. How coatings cope with repeated heating and cooling is covered in [hot flue gas duct protection through thermal cycling](https://signals.polylloycoatings.com/use-cases/hot-flue-gas-duct-protection). ### Applying It in the Duct Package Record the minimum metal temperature for each duct zone on the design basis, alongside the gas chemistry: SO2 and SO3, hydrogen chloride (HCl), hydrogen fluoride (HF) and moisture. ## Insulate Ductwork Continuously, Including Flanges, Stiffeners and Supports The second rule turns the thermal basis into an unbroken insulation envelope. ### The Rule Design external thermal insulation and weatherproof cladding as a continuous envelope. Flanges, stiffeners, supports and other heat sinks should be covered, not left as gaps. ### Why It Prevents Condensation Heat sinks such as fins and flanges run cooler than the surrounding casing, which is why Polylloy's coating evaluation asks about external thermal insulation and possible heat sinks. Damaged or wet insulation creates new cold spots and can hide corrosion underneath. ### Applying It in the Duct Package Detail insulation at stiffener rings, flange pairs, support points and access doors on the drawings, and specify cladding that keeps water out. In-service insulation walk-downs belong to the operating routine described in the [baghouse corrosion maintenance checklist](https://signals.polylloycoatings.com/articles/baghouse-corrosion-maintenance-checklist). ## Limit Air In-Leakage at Doors, Ports and Flanges The third rule keeps cold, oxygen-rich air out of the gas path. ### The Rule Keep penetrations such as access doors, inspection ports, instrument nozzles and dampers to those needed for operation and inspection. Specify seals and flange joints that stay tight across the duct's temperature range. ### Why It Prevents Condensation False air drawn through seal, joint or duct leaks cools the steel locally and raises oxygen, which can promote SO3 formation at the same time. The ICR article describes the result as highly localized corrosion zones. ### Applying It in the Duct Package Place doors and ports where they can be insulated and reached for seal maintenance. Call out gasket and seal replacement in the operating manual. ## Detail Expansion Joints for Tightness and Temperature The fourth rule treats expansion joints as the weak points they usually are. ### The Rule Treat every expansion joint as both a potential leak path and a potential cold spot. Select the joint for the gas temperature and chemistry, insulate the joint frame, and plan access for joint inspection and replacement. ### Why It Prevents Condensation Expansion joints are typical of the seal and joint leaks the ICR article identifies as false-air paths, and duct flanges and expansion joints are common places for this localized corrosion to appear. Joint frames and the steel either side are also discontinuities in the insulation envelope, so they behave like heat sinks. ### Applying It in the Duct Package Show the protection boundary at each joint on the coating drawing: which steel is coated, how the frame is prepared, and who inspects it. ## Remove Dead Zones, Ledges and Deposit Traps The fifth rule addresses geometry that lets dust settle against the steel. ### The Rule Avoid geometry where dust settles and stays: low-velocity pockets, internal ledges, horizontal stiffener faces inside the gas stream, and crevices at welds, overlaps and fasteners. Provide access for deposit removal and inspection. ### Why It Prevents Condensation Dust and ash concentrate soluble salts, retain moisture and create shielded locations. Polylloy's technical paper describes the sequence: deposition in operation, cooling at shutdown, moisture uptake or condensation, salt dissolution, then localized corrosion beneath the deposit. The same paper notes that dry operation does not demonstrate that shutdown exposure is benign, and tells designers to evaluate details that create shielded locations. ### Applying It in the Duct Package Avoid crevice-forming details such as intermittent welds, lap joints and bolted overlaps on surfaces that will meet condensate or be coated. Smooth transitions in low-velocity zones, and mark cleaning access on the general arrangement drawing. ## Drain the Condensate That Will Still Form The sixth rule accepts that some condensate will form and gives it somewhere to go. ### The Rule Assume some condensate forms at every shutdown and start-up, and design low points so it drains away instead of pooling on steel. ### Why It Prevents Condensation Damage Gas keeps cooling as it moves toward the stack, and condensate runs down and collects at the base. Surjit Gill, Polylloy's co-founder, describes operators seeing rain inside a stack at the bottom access door. Poor drainage is also one of the root causes to fix on corroded stacks. ### Applying It in the Duct Package Identify low points, horizontal runs and stack bases on the layout. Specify drains with materials suited to acidic condensate, and include the drain nozzle area in the coating scope. ## Specify Corrosion Protection in the Duct Package at the Design Stage The seventh rule protects the steel that the first six rules cannot keep dry. ### The Rule Write the material and protection decision into the duct specification before fabrication: protected carbon steel or alloy, grade by service limit, surface preparation, inspection hold points and who applies. In Polylloy's experience the design stage is where the coating is easiest to add, and once the fabrication schedule is fixed it is the step most easily squeezed out. On the new flue gas desulfurization (FGD) ducting at PPL Brunner Island, Pennsylvania, StackGard®-255SQW was the selected system in 2009. New duct construction allowed the protective system to be incorporated before service exposure; the record is an application reference, not a performance result. ### Matching FlueGard Grades to Duct Temperature Polylloy's [FlueGard flue gas coatings](https://www.polylloycoatings.com/fluegard) are matched to duct metal temperature by service limit: - FlueGard-225SQC: to 225 °C (437 °F) for baghouses, ESPs and ducts, resisting SO3, SO2, HCl, carbon dioxide (CO2), HF and nitrogen oxides (NOx). - FlueGard-425S: to 425 °C (797 °F) with peaks to 500 °C, a single-component inorganic polymeric coating whose bulletin lists ducts in smelters, incinerators, power plants and refineries. - FlueGard-455CHT: to 455 °C (850 °F) with 20 mg Taber abrasion, where high heat and abrasion resistance are both needed, such as hot, dusty ducts. A design reviewer can check the evidence. FlueGard on coal mill baghouses at three cement plants in Mexico was inspected after about 20, 15 and 16 years, with no issues observed, excellent condition and excellent performance reported. FlueGard-225SQC showed no observed deterioration after six months in 56% H2SO4 at 60 °C in Polylloy's own sulfuric acid immersion test. A FlueGard-455CHT duct in a Swiss cement plant showed no visible widespread delamination in the areas that could be inspected at about 17 months. ### Justifying Protected Carbon Steel to a Design Review Protected carbon steel may offer a cost-effective alternative to stainless steel where the coating is suitable for the specified service conditions. 316 stainless is not immune to chloride pitting in acidic condensate: molybdenum improves its localized corrosion resistance but does not provide immunity. Compare like for like: same equipment, conditions, scope and period. Lifecycle evaluation considers preparation, application, maintenance, downtime and replacement alongside the initial material cost, and the full comparison sits in [carbon steel vs stainless steel for flue gas](https://signals.polylloycoatings.com/comparisons/carbon-steel-vs-stainless-steel). Extra steel thickness is weighed in [corrosion allowance or coating](https://signals.polylloycoatings.com/listicles/corrosion-allowance-vs-coating). Shop or field application is a separate decision, covered in [shop-applied vs field-applied coatings](https://signals.polylloycoatings.com/comparisons/shop-vs-field-applied-coatings). ## How to Choose Which Duct Design Rules to Apply First The seven rules are design-stage decisions tied to published condensation mechanisms, listed in design sequence rather than ranked, and that sequence also sets the order of work in a design review. ### Decision Path for the Design Review Fix the thermal and tightness rules (one to four) first, because no coating compensates for a leak that keeps feeding cold air onto the steel. Then settle geometry and drainage (five and six). Finally, protect the steel that will still cross the dew point (seven), matched by service temperature and gas chemistry. Mild service where steel stays well above the dew point may not need a severe-service coating, and a conventional coating can be enough. Surface preparation is critical: the grades require an SSPC-SP10 / NACE No. 2 near-white blast with a profile above 3 mils (75 µm). For heat-cured grades, final heat cured activation is completed during start-up using process heat. Certified partners apply; Polylloy formulates, supplies, specifies, trains applicators and supervises. To book a Coating Performance Review, [request a coating evaluation](https://www.polylloycoatings.com/contact) with duct drawings, equipment type, coated area, operating conditions (temperature, gas chemistry, abrasion), location and project or outage timing. ### FAQs **How Do You Prevent Acid Condensation in Flue Gas Ductwork?** Keep the duct steel above the acid dew point wherever possible and protect the steel that cannot be kept there. In practice that means designing to the coldest metal temperature, insulating flanges and stiffeners, limiting air in-leakage, detailing expansion joints, removing deposit traps, draining low points and specifying a coating matched to the duct's temperature and gas chemistry. **What Coating is Used for Hot Acid Dew Point Corrosion in a Dust Collector?** Choose by metal temperature and gas chemistry. FlueGard-225SQC is rated to 225 °C (437 °F) for baghouses, electrostatic precipitators and ducts and resists SO3, SO2, HCl and HF. Hotter ducts can use FlueGard-425S, to 425 °C with peaks to 500 °C. Certified partners apply both grades under Polylloy supervision after an evaluation of the specific equipment. **How Do I Justify Protected Carbon Steel to a Design Review?** Show the service conditions, the coating's published service limit and the field evidence for similar duty, then compare against 316 stainless on the same equipment, scope and period. Protected carbon steel may offer a cost-effective alternative to stainless steel where the coating is suitable for the specified service conditions. Include surface preparation and inspection hold points in the specification. **Does Insulating a Flue Gas Duct Stop Acid Dew Point Corrosion?** Insulation reduces condensation but rarely removes it. Gaps at flanges, stiffeners, supports and doors leave cold spots, wet or damaged insulation creates new ones, and every shutdown still takes the steel through the dew point. Insulate continuously as a design rule, then protect the steel that will still meet condensate during cool-down and start-up. **Why Do Flue Gas Ducts Corrode at Expansion Joints?** Expansion joints are both leak paths and discontinuities in the insulation. False air drawn in at a joint cools the steel locally and adds oxygen, which can promote sulfur trioxide formation, so acid condenses in a small area. Select joints for the gas temperature and chemistry, insulate the frames and include the joint steel in the protection scope. **Should New Flue Gas Ducts Be Coated in the Shop or After Erection?** Either can work, and the choice depends on section size, transport, schedule and site conditions. Shop coating gives full access and controlled blasting conditions; field coating suits site-fabricated or oversized sections and the joints between shop-coated sections. In both cases certified partners apply the coating and Polylloy specifies and supervises. Decide it at the design stage. **Can Good Duct Design Eliminate Acid Condensation Completely?** Usually not. Design can shorten the time steel spends below the acid dew point, but start-ups, shutdowns, low-load running and upsets still cool parts of the duct, and deposits absorb moisture as equipment cools. That is why design rules and corrosion protection are specified together, with the protection matched to the duct's temperature and gas chemistry.