Acid mist, solvent vapour and overspray decide whether a finishing-line fan lasts one season or many years. What to specify, with the arithmetic.
Reviewed by Jitamitra application engineering
Share on LinkedInA plating exhaust at 35 to 45 °C looks like a ventilation job and usually gets bought like one. It is not. The gas leaves the tank close to saturated with acid, the booth next door is moving solvent vapour that can sit inside its flammable range, and overspray and plating salt are building up on every wheel in the shop. A fan chosen for the duty point alone typically corrodes through, sparks or shakes itself apart within 6 to 18 months; one engineered for the air it actually moves runs 8 to 10 years and more. Here is what decides which one you get — with the arithmetic, and a checklist you can copy into your RFQ.
A plating, pickling or paint shop runs a family of fan duties, not one. Lip and hood extraction pulls hydrochloric, sulphuric, chromic and nitric acid mist off the tanks. Booth exhaust moves solvent-laden air from liquid-paint, dip-coat and powder-coat booths. A booster fan pulls the combined acid stream through a wet scrubber, commonly against 150 to 600 mmWC of packed-bed resistance. Around them sit fume extraction on buffing and grinding stations, general dilution ventilation and bath-laboratory fume hoods.
None of these fans is hot, and that is the trap. The gas leaves the tank close to saturation, so it sits at or below its acid dew point all the way to the scrubber: every cold surface becomes a condensing surface, and the condensate is acid. And when a booth exhaust or scrubber booster stops, the line stops. Work in a booth or a plating sequence does not wait for a spare, part-coated components go to rework, and a scrubber that loses draught while the tanks are still fuming is an emissions exceedance.
Saturated acid aerosol attacks wetted metal from the inside. Mild steel is consumed; 304 stainless pits and perforates in chloride service. Even a sound casing fails if the wheel thins unevenly, because metal lost from one blade is imbalance long before it is a hole. The usual history is a wheel that corrodes out of balance within a single season of 6 to 12 months, helped along by condensate pooling at the volute low point and the shaft penetration.
Booth and dip-coat exhaust can carry solvent vapour inside the flammable range, especially in flash-off zones and curing-oven exhaust, where air change is kept low to hold heat. A wheel rubbing its inlet cone, a static discharge from an unbonded assembly, or a tramp part drawn into the wheel is an ignition source inside a duct full of fuel and air. Powder-coat booths add a combustible-dust atmosphere.
Overspray solids and crystallised plating salts deposit on the wheel and casing, and never evenly. The rotor drifts out of balance, vibration climbs, and a bearing or a seal fails. On a belt-driven fan the first symptom is often bearing noise, which is easy to misread as a lubrication problem.
There is no single corrosion-resistant fan. The right material for chloride is the wrong one for chromic, so the wetted material has to come from the chemistry, concentration, temperature and saturation of each stream. The principle: no bare steel in the airstream, and a material whose weakness the stream cannot reach. For the stainless question in more depth, see SS304 vs SS316 for fan construction.
| Stream | Wetted construction | Watch-point |
|---|---|---|
| Hydrochloric pickling, mixed-acid and chloride-bearing exhaust | PP casing and wetted parts, with your choice of SS 316L or FRP-coated impeller; or FRP-lined carbon steel | We state the expected impeller life on your duty in the offer, and supply the standby. Thermoplastics lose stiffness as temperature rises: confirm the maximum gas temperature, including upset. |
| Sulphuric (anodising), chromic (hard chrome), nitric (passivation, bright dip) | SS 316L, or a higher alloy against your specification | Stainless suitability depends on concentration and temperature; decided per stream, never by default. |
| Solvent-laden paint and dip-coat booth exhaust | Coated carbon steel with non-sparking wheel-to-inlet construction | Corrosion is mild; the governing requirement is ignition control. |
| Powder-coat booth and recovery | Coated carbon steel, bonded and earthed | Combustible dust: classify the zone and build to it. |
Whatever the material, three details decide service life: internals fully coated or lined with no bare steel exposed, a drain at the volute low point so condensate never pools, and a wash-down connection so the casing can be flushed in place. On a belt-driven arrangement the motor and bearings stay out of the airstream, and the shaft penetration carries a seal rated for the gas.
Where your area classification is ATEX Zone 2 (gas) or Zone 22 (dust), the fan is built spark-resistant to the AMCA Type B/C rubbing-clearance approach: non-sparking materials where the wheel could contact the inlet, adequate running clearance, the whole assembly bonded and earthed against static, and a motor rated to the classified zone. We self-declare this construction to 2014/34/EU, Category 3, for Zone 2/22. It is a self-declaration of conformity, not a third-party certificate, and it does not cover Zone 1 or Zone 21, which need Category 2 equipment. The zone is your classification; the fan is built to it. The construction types are set out in Spark-resistant fans: AMCA Type A/B/C.
A non-stick coated wheel and casing lets deposit shed rather than key on. Access and cleanout doors and an optional wash-down spray allow cleaning without dismantling. The rotor is dynamically balanced to ISO 21940 grade G 6.3 as standard, with G 2.5 on application, so it keeps running inside its vibration limits between wash cycles. In service, judge it against ISO 14694: for a rigidly mounted fan in category BV-3, 4.5 mm/s is acceptable, 7.1 mm/s is the alarm and 9.0 mm/s the shutdown level (6.3, 11.8 and 12.5 mm/s on flexible mounts).
A backward-curved plate wheel is the workhorse for booth exhaust and scrubber boosters: best efficiency on clean, higher-pressure air. An aerofoil wheel suits general and local exhaust ventilation, where the air is largely clean and noise matters. A radial wheel is the rugged choice where scrubber carry-over or wet solids are in the stream, because it sheds solids that would load a backward-curved blade. We would not put a forward-curved wheel on acid duty: its many shallow blades are hard to coat completely and harder to clean. The general map is in Choosing the wheel.
Booth airflow is set by the capture velocity across the open face; the flammability limit is then checked against it. Take a booth with a 3.0 m × 2.5 m open face, a design face velocity of 0.5 m/s, and a spray rate that evaporates 10 kg/h of xylene (molar mass about 106 kg/kmol, lower flammable limit about 1.0% by volume).
In an open spray booth the capture airflow is roughly fifteen times the flammability requirement, so the booth fan is sized by capture, and the spark-resistant build protects against local pockets, upsets and the duct rather than the bulk mixture. The picture reverses in a curing oven or an enclosed flash-off tunnel, where air change is minimised to save heat and the same solvent load can approach the limit. There the exhaust fan is a safety device, sized to the flammability calculation with margin and interlocked with the burner.
A booster draws 40,000 m³/h of saturated acid fume through hoods and ducting (100 mmWC), a packed-bed scrubber (150 mmWC when clean) and a mist eliminator and stack (50 mmWC): 300 mmWC in total. Fan static efficiency is taken as 70% for illustration.
Sized on the clean pressure drop, a 55 kW motor looks comfortable and runs out of power the month the packing loads. Sized on the fouled figure, the motor is 75 kW and the fan runs on a variable-frequency drive that adds speed as the bed loads. Density pushes the same way: saturated gas at about 40 °C is near 1.10 kg/m³ against 1.20 kg/m³ for standard air, so a fan tested on standard air at the same volume flow develops about 9% more pressure and absorbs about 9% more power than it will on the line. Specify the duty at site gas conditions, and state which condition the motor must cover, including a cold start on dry ambient air. The fan laws behind these sums are in The cube law.
Copy this into a tender or RFQ as it stands.
A surface-coating systems provider in western India reported abnormal vibration and noise on a blower serving an engineering customer's line: 15,500 m³/h at 60 mmWC on a 10 HP motor, several years into service. The site's diagnosis was imbalance, and the order raised was for balancing.
Vibration together with noise pointed past imbalance to the rotating assembly. Process and aerodynamic causes were ruled out first, since the line was still making its air; the motor and drive next. Our technician went to site carrying two bearings to fit if required. Both halves of the job were needed: bearings replaced, then the rotor balanced in situ, in that order, because a rotor re-bearinged after balancing is no longer the rotor that was balanced. Reported 29 December; offer issued the next working day; technician on site 4 January.
Our record of that job holds a balancing sheet and a site report, but no signed post-repair vibration figure and no customer acceptance rating, so we claim neither. That is why every field-balancing visit we make now records before-and-after readings against ISO 14694, with the bearing and its failure mode logged. The lesson for a finishing line: the fan that has run faultlessly for years is the fan nobody measures. Read the full case →
We build the fans across the whole finishing line — acid-fume exhaust, booth exhaust, local exhaust ventilation, general ventilation and the scrubber booster — each engineered to its own chemistry and area classification, with the material, coating and spark-resistance choices documented on the GA drawing you sign off before we cut metal. 405 surface-coating and plating fans delivered, across 34 customers, within an envelope of up to 2,25,000 m³/h, 2,000 mmWC, 425 HP and 500 °C. Every fan is performance-tested in-house on our 200 HP VFD test rig to the IS 4894 / ISO 5801 / AMCA 210 method; fans above the rig's 200 HP are run at derated speed and scaled to the specified duty by the fan laws. The full range is on the surface coating & plating page.
Further reading. The duty pages for corrosive gas exhaust, paint-booth / ATEX exhaust and local exhaust ventilation; then Field fan vibration diagnostics for the deposit-and-bearing story, and CE and ATEX fans: a buyer's guide for the conformity paperwork.
Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.
Sources & basis. Standards referenced: ISO 21940 (rotor balance quality), ISO 14694 (industrial fan vibration), the ATEX equipment directive 2014/34/EU, AMCA spark-resistant construction types A/B/C, and the IS 4894 / ISO 5801 / AMCA 210 performance test methods. Physical properties in the worked examples (xylene molar mass and lower flammable limit, saturated-air density) are standard handbook values; the airflows, pressures and efficiency are illustrative and describe no specific installation. The field case is from our own service work, published anonymised.
For hydrochloric and mixed-acid streams we offer a PP casing and wetted parts, with your choice of SS 316L or FRP-coated impeller, and we state the expected impeller life on your duty in the offer and supply the standby. FRP-lined carbon steel is the alternative. Either way there is no bare steel in the airstream. Thermoplastics lose stiffness as temperature rises, so confirm the maximum gas temperature, including upset. A drain at the volute low point and a wash-down connection keep condensate from pooling.
It depends on the acid. 304 stainless pits and perforates in chloride service, so hydrochloric and mixed-acid streams get the PP construction instead. For sulphuric, chromic or nitric streams, SS 316L or a higher alloy against your specification holds where the concentration and temperature allow. The construction is decided stream by stream from the bath chemistry, concentration, temperature and saturation, never by default.
No. Zone 1 and Zone 21 need Category 2 equipment. Our spark-resistant construction for paint-booth exhaust is self-declared to 2014/34/EU, Category 3, for Zone 2/22: a self-declaration of conformity, not a third-party certificate. The zone is the plant owner's classification, and the fan is built to it.
Size it on the fouled pressure drop, at site gas conditions. In our worked example a booster needs 46.7 kW at the clean pressure drop and 58.8 kW once the packing loads, so the motor is 75 kW rather than 55 kW, and a variable-frequency drive adds speed as the bed loads. State the duty at site conditions and which condition the motor must cover, including a cold start on dry ambient air.
Usually two things together: uneven paint or salt deposit on the wheel, and bearing wear. Vibration with noise points to the rotating assembly. Replace the bearings first, then balance the rotor in situ, and judge the result against ISO 14694. A non-stick coated wheel, cleanout doors and a balance of ISO 21940 G 6.3 hold the fan inside its limits between wash cycles; a periodic vibration reading catches the drift before it becomes a breakdown.
Flow, static, gas temperature, application — or attach a spec, GA drawing or a multi-fan schedule. Engineer to engineer.
ISO 9001:2015 quality system · performance-tested to IS 4894 / ISO 5801 / AMCA 210 method · witnessed FAT on request.
*For our standard range. ATEX and special projects need 5 to 7 working days.
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