A box-type FRP centrifugal fan is a corrosion-resistant centrifugal fan housed in a sealed enclosure made of fibre-reinforced plastic (FRP), built for aggressive gases that would destroy a steel or even stainless unit. The FRP shell, impeller, and casing resist acids, alkalis, and salts across a pH range of about 1 to 9 and at temperatures from −10°C to 60°C for standard resin, with special vinyl-ester grades reaching 90°C. It is the default choice for electroplating, chemical, and wastewater plants needing 200 to 50,000 m³/h of foul air extracted safely.
Why FRP Instead of Steel
Ordinary carbon or 304 stainless steel corrodes quickly in acid mist, chlorine, and hydrogen sulphide found in scrubbers and pickling lines. FRP is inert to most chemicals, weighs about 30% less than steel for the same strength, and needs almost no painting. The trade-off is lower temperature tolerance and impact sensitivity, so FRP fans are specified where corrosion—not heat or abrasion—is the dominant threat to service life.
Construction and Performance
The box enclosure wraps the centrifugal wheel and motor in an FRP casing with access doors, drain points, and an FRP or PVC impeller. Standard units develop 200 to 3,000 Pa of static pressure with efficiency of 60% to 80%, driven by a motor from 0.55 to 55 kW mounted outside the airstream for cooler running. Where the gas is explosive, an explosion-proof motor and anti-static resin are added, and the unit is grounded per local electrical code.
Typical Duty Numbers
A plating-shop scrubber exhaust might need 8,000 m³/h at 1,200 Pa through a wet scrubber, handled by a 5.5 kW FRP box fan. Continuous chemical duty runs 24/7, so the 10%–15% efficiency gap versus a steel wheel matters to energy cost; however, the avoided replacement every 1 to 2 years makes FRP far cheaper over a 10-year life. Filter load and duct loss must be counted in the system resistance.
FRP vs Other Corrosion Solutions
| Option | Chemical range | Max temp | Notes |
|---|---|---|---|
| FRP box fan | pH 1–9 | 60°C (90°C special) | Best all-round corrosion |
| 304 stainless | Mild, no chloride | 300°C | Fails in acid/chloride |
| 316 stainless | Better, still limited | 400°C | Costly, pitting risk |
| PP / PVC fan | Strong acids | 40–60°C | Lower mechanical strength |
Selection Checklist
- Choose an FRP box fan when gas is acidic, chlorinated, or salty with pH 1 to 9.
- Choose stainless only when temperature exceeds 90°C but corrosion is mild.
- Specify vinyl-ester resin when duty approaches 90°C or needs extra chemical proof.
- Mount the motor outside the airstream for cooler, longer motor life.
- Add explosion-proof motor and grounding for solvent or flammable-gas extraction.
FRP Inspection and Maintenance
FRP hides damage, so inspect the casing and impeller every 3 to 6 months for crazing, blistering, or chemical staining that signals resin attack. A blistered lining can let the acid reach the structural layer within months, so repair with the matched resin as soon as found. Keep the impeller clean; a 200 g build-up of slurry at 1,450 rpm shifts balance and can raise vibration by 20% to 40%. Because FRP is less impact-tolerant than steel, protect the inlet with a mesh and avoid hammer blows during any service work on the fragile shell of the fan unit.
Specifying Resin and Lining
Choose polyester for general acid mist up to 60°C, vinyl-ester for stronger chemicals or 90°C, and add a double liquid-applied lining for very aggressive duty. The liner thickness is usually 2 to 4 mm and must be spark-tested for pinholes before shipment. For abrasive slurry, cap the leading edges with ceramic or novolac. Getting the resin right is the difference between a fan that lasts 10 years and one that fails in 18 months, so always send the gas analysis (pH, temperature, contaminants) to the supplier for a written compatibility confirmation and warranty.
Comparing Life-Cycle Cost
A steel fan may cost 20% to 30% less upfront but in a plating line it can corrode through in 12 to 24 months, while FRP runs 8 to 10 years with only lining touch-ups. Over a 10-year horizon the FRP unit is often 40% to 60% cheaper once you add the lost production, labour, and safety risk of repeated replacements. Factor also the 30% lighter weight, which cuts foundation and crane cost, and the near-zero painting programme compared with a steel unit that needs recoating every 2 to 3 years in a corrosive atmosphere.
Safe Operation Notes
FRP is non-conductive, so a rotating wheel in flammable vapour must use anti-static resin and be grounded to prevent static sparks; pair it with an Ex-rated motor for any zone with explosive gas. Do not exceed the rated temperature, because above 60°C to 90°C the resin softens and the impeller can deform, throwing the balance out and shaking the casing. Fit temperature and vibration trips that stop the fan before a small fault becomes a full structural failure costing weeks of downtime and safety exposure.
Summary for Specifiers
For corrosive exhaust, FRP is usually the lowest life-cycle-cost answer despite a higher first price. Send the gas analysis, pick the right resin and lining, and add anti-static and Ex protection for flammable duty. Respect the temperature limit and fit vibration trips. Done well, an FRP box fan runs 8 to 10 years in acid, chlorine, or salt-laden air that would destroy a steel unit in under two, protecting both the maintenance budget and site safety every single day.
Quick Reference Numbers
FRP planning figures: 200 to 50,000 m³/h, 200 to 3,000 Pa, pH 1 to 9, 60°C standard rising to 90°C for vinyl-ester, and motors 0.55 to 55 kW mounted out of the airstream. Efficiency is 60% to 80%, life 8 to 10 years in acid versus 1 to 2 for steel, and a 5 g imbalance can raise vibration 30% to 50%. Keeping these in mind lets you defend the higher first cost with a clear, documented 40% to 60% life-cycle saving over the equipment's full service horizon on the plant.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What chemicals can an FRP fan handle?
Standard FRP with polyester or vinyl-ester resin resists most acids, alkalis, and salts across roughly pH 1 to 9, including acid mist, chlorine, and hydrogen sulphide. It is not suitable for strong solvents like ketones or aromatic hydrocarbons that attack the resin. For aggressive solvent duty, specify a proven chemical-grade lining and confirm compatibility with the supplier before purchase.
What is the temperature limit of an FRP centrifugal fan?
Standard polyester FRP is rated to about 60°C, while vinyl-ester resin grades reach 90°C; beyond that the resin softens and the structure weakens. If gas exceeds 90°C, switch to a stainless or special high-temp design, or cool the airstream with a pre-scrubber. Never run a standard FRP fan above its rated temperature or the impeller may deform.
Is an FRP fan efficient enough for continuous duty?
FRP box fans reach 60% to 80% efficiency, a little below the best steel wheels, but the energy gap is small against the huge saving from avoiding corrosion failure. For 24/7 chemical extraction the avoided cost of replacing a steel fan every one to two years makes FRP the lower life-cycle-cost choice despite the modest efficiency difference.
When do I need an explosion-proof FRP fan?
Specify explosion-proof construction whenever the extracted gas contains flammable vapours or dust above the lower explosive limit, such as solvent lines or flammable-waste scrubbers. This means an Ex-rated motor, anti-static resin, conductive grounding, and spark-proof impeller options, all wired and installed to the local hazardous-area electrical code for safe operation.