What is a gas recirculation fan, and how is it different from an ID fan?
A gas recirculation fan takes a controlled slice of hot flue gas off the boiler, kiln or furnace and pushes it back into the process — into the windbox, burner zone or gas duct — to reshape the temperature profile and hold NOx down. An ID fan, by contrast, pulls all the flue gas out to the stack. The GRF fan works on a closed recycle loop at positive delivery pressure, typically handling 10 to 25 percent of the total gas flow. Because it recycles the gas rather than exhausting it, the corrosion, ash and heat concentrate on the loop, so the survival engineering matters even more than on a once-through ID duty.
The recycled gas sits below the acid dew point. What materials do you use?
Recirculated gas is often tapped at a cooler point in the flue-gas path, so it can sit near or below the SO₃ dew point, typically around 120 to 150 °C, where sulfuric acid condenses and attacks the wetted metal. We hold the casing wall above dew point with insulation and heat tracing, and select corrosion-resistant metallurgy for the gas: Corten, 316L, or higher alloys such as Hastelloy or Inconel on request. The right grade depends on your SO₂/SO₃ and moisture, so we size both the material and the dew-point margin to your gas analysis, not a default.
How hot can the recycled gas be, and how do you manage the heat?
Continuous duty up to 600 °C across the envelope, with most boiler recirculation loops running 150 to 400 °C and kiln and furnace loops reaching the higher end. Above 400 °C we fit a shaft cooling disc as standard, available from 350 °C on request, to keep the heat out of the bearings. We upgrade the casing to IS 2062 or 16Mo3, add metal or fabric expansion joints sized for the thermal growth, up to about 25 mm on a long run at 400 °C, size the shaft for expansion and select bearings for a sustained 80 to 100 °C housing temperature. The fan is built for your stated gas temperature and excursion case, not a generic rating.
How do you protect the impeller from recirculated fly ash?
Even a slice of flue gas carries residual ash, and on a recycle loop that ash passes the wheel every cycle, so uneven erosion throws a curved wheel out of balance before it wears through. We default to a radial-tipped or straight-radial wheel that rejects dust from the blade root rather than packing it in, add chrome-carbide hard-facing on the blade leading edges for severe ash, and bolt in AR400 wear plates at the volute throat and outlet with access doors so they can be replaced in place, not cut out and re-welded. The wear package is sized to your recycle-tap loading, and where the take-off is downstream of an ESP or bag filter it is usually much lighter.
How do you keep the recirculation ratio steady across boiler load?
The temperature profile and NOx setpoint you are targeting move with boiler load, so the recycle split has to track rather than drift. We engineer the duty point onto the stable, best-efficiency region of the selected wheel so the fan does not wander as furnace draft and gas density shift underneath it, and we make VFD speed control the default because it holds the ratio across the range without the throttling loss of a damper at part load. Inlet vane dampers remain available for legacy retrofit. We verify the curve on the 200 HP VFD test rig at your gas density before dispatch.
Do you have a track record on gas-recirculation duty specifically?
We engineer gas-recirculation fans to your duty rather than claiming a long named reference list for this specific loop. The underlying engineering — hot, ash-laden, acid-dew-point flue gas on radial and radial-tipped wheels with shaft cooling, wear plates and dew-point protection — is the same survival engineering we apply across our induced-draft and hot-gas work, which is well proven. Specify your recirculation duty: gas analysis, temperature, ash load, recycle ratio and re-injection point, and we engineer and quote to it. We would rather show you the design basis than dress up a reference.
What is the lead time for a gas-recirculation fan?
A standard engineered GRF fan runs roughly 9 to 14 weeks order-to-dispatch: offer in 3 to 5 working days, GA drawing approval 2 to 3 weeks from PO, manufacture, balance and paint 6 to 10 weeks, and performance test plus FAT about a week. A high-temperature build with special metallurgy and dew-point scope, or a shutdown replacement matched to an existing footprint, is quoted with a dated commitment against your window rather than a placeholder. Send the recycled-gas analysis, temperature and duty point and we scope it precisely.
Do you build GRF fans to API 673, CE and ATEX requirements, and what do you actually test?
Yes. We design and build to API 673 for refinery and fired-heater recirculation service as project-specific scope, allowing 7 to 10 working days for the offer. CE is self-declared per 2006/42/EC and 2014/35/EU, and ATEX Zone 2/22 is self-declared per 2014/34/EU, Category 3, where the area classification calls for it. To be precise: those are self-declarations of conformity, not third-party certifications. Performance is tested in-house to the AMCA 210 / ISO 5801 method on our 200 HP VFD test rig, not AMCA-certified, and we are not an AMCA member. Balance is to ISO 21940 G6.3 as standard, with G2.5 or G1.0 on application. Our only third-party certification is ISO 9001:2015.
