Centrifugal Blowers for Fume Extraction and Scrubber Systems
Fume extraction and gas scrubbing put a blower in some of the most awkward air it ever meets: moist, corrosive, sometimes hot, and carrying droplets or sticky residue. The blower still has to do the ordinary job of moving air against resistance, but the design choices that matter most are about where it sits in the system, what it is made of, and how it copes with moisture. This page covers those choices, a worked pressure example, and the boundary of what standard blowers can do, particularly for flammable fumes.
Fume Extraction and Scrubbing: What They Involve
Fume extraction captures airborne contaminants such as welding fume, solvent vapour, acid mist, and process gases at or near the source, and carries them to a treatment device or a safe discharge point.
Scrubbers clean the gas by contacting it with liquid. Venturi, packed-tower, and spray scrubbers all add substantial resistance, and they leave the gas saturated with moisture and sometimes carrying entrained droplets.
The blower provides the pressure to pull the fume through hoods, ducts, and the scrubber, and to push the clean gas to the stack.
Where the Blower Sits
- Before the scrubber (dirty side). The blower handles the raw fume, which may be corrosive, sticky, or carrying particles. Materials and cleanability become the main concerns.
- After the scrubber (clean side). The blower handles saturated gas, possibly with droplets, and keeps the whole system under negative pressure so leaks draw air in. Moisture, drains, and corrosion become the main concerns.
Many systems place the blower after the scrubber for the same reason dust collectors do: it keeps the ducting upstream under negative pressure, so a leak does not push fume into the workspace.
Worked Example: Pressure for a Scrubber System
Illustrative example. A scrubber system extracts 8 m³/s. The pressure budget is a venturi scrubber at 2,000 Pa, plus ducts, hoods, and stack at 800 Pa, for a total of 2,800 Pa (about 285 mm WC).
Air power is 8 × 2,800 = 22.4 kW. At 70% total efficiency the blower needs 32 kW at the shaft. That sits in the high-pressure class and well inside our 500 mm WC standard maximum. The figures are illustrative, because real scrubber resistance depends on the design and the operating conditions, so use the equipment supplier’s figure.
Wet Gas Is Not Standard Air
Water vapour is lighter than air, so moist gas is less dense than dry air at the same temperature. Saturated air at 60 °C has a density of about 0.98 kg/m³, compared with about 1.06 kg/m³ for dry air at the same temperature, roughly 7% lower. Combined with the temperature itself, the blower develops less pressure than its standard-air rating. Select from a density-corrected curve, and use the actual gas composition. See gas temperature, altitude and density correction.
Moisture, Droplets, and Carry-Over
Gas leaving a scrubber can carry droplets, and droplets in a blower cause corrosion, imbalance from uneven wetting, and water hammer on the wheel. Practical protection includes a mist eliminator or droplet separator between the scrubber and the blower, drain connections at the lowest point of the housing and ducting, and a sloped duct so that condensate runs away from the blower. The blower should not have to stand in water. After a shutdown, drain and dry the housing, because standing water corrodes and grows deposits that unbalance the wheel.
Materials and Corrosion
The right material depends on the fume: acid, alkali, solvent, or a mixture, at a specific concentration and temperature. Coated steel suits mild conditions, stainless steel suits many others, and some aggressive fumes need special materials or linings. Material choice covers the wheel, the housing, the shaft seal, and the fasteners. Where fume passes through the blower, a motor outside the airstream, with a suitable shaft seal, avoids exposing it. Materials are confirmed for each duty on our blowers, so give us the gas composition and temperature.
Backward-Curved Wheels for Fume Duty
Backward-curved wheels give the best efficiency for clean or lightly loaded gas, a stable curve, and a non-overloading power characteristic. They also shed light deposits better than forward-curved wheels, whose narrow blade pockets collect them. See backward-curved versus forward-curved blower wheels.
Welding and Light Fume
Welding fume is fine and light, and for such fume, duct velocities of roughly 10 to 13 m/s (about 2,000 to 2,500 feet per minute) are commonly used, lower than for heavy dust. The blower’s job is to hold the capture velocity at the hood as the filter loads, so filtered systems should be sized for the loaded-filter resistance. See dust collection system design.
Flammable and Toxic Fume: Know the Limit
Solvent vapour, hydrogen, and many process gases are flammable, and some fumes are toxic. Extracting flammable vapour from a classified area is a hazardous-area duty, and the blower and motor must be suitable for the zone inside and outside the blower. We do not currently manufacture ATEX-certified or AMCA 99 spark-resistant blowers, so fume duties in classified zones, or handling flammable gas that needs such equipment, come from a specialist supplier. A standard blower is only suitable where the blower and its motor sit outside any classified zone and the gas handled is not explosive. See combustible dust and hazardous atmospheres.
Control and Energy
Scrubber and extraction duties often run for long hours at varying load. Speed control saves energy because power varies with the cube of speed, and it lets the airflow be tuned to the capture velocity actually needed. See flow control for blowers.
Where Our Range Fits
NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure, with backward-curved and forward-curved wheels and single-inlet and double-inlet designs. That covers extraction and scrubber blowers for non-classified areas within that pressure. Materials, seals, and drains are confirmed for the gas and temperature on each duty. Scrubbers needing more than 500 mm WC, and fume duties in classified hazardous areas, are outside our range.
What to Specify
- Airflow and how it was derived, for example capture velocity at hoods.
- Scrubber and ductwork resistance, and the basis for the figure.
- Gas composition, temperature, humidity, and droplet carry-over.
- Blower position: before or after the scrubber.
- Materials, shaft seal, and motor position.
- Hazardous-area classification and flammability of the gas.
- Drain arrangement, control approach, and noise limits.
Get a Blower Selection for Your Extraction or Scrubber System
Send your airflow, scrubber resistance, and gas details, and we’ll size the blower and confirm materials and arrangement. Contact us for a free quote.
Frequently Asked Questions
Should the blower go before or after the scrubber?
Often after, so the upstream ducting is under negative pressure and leaks draw air in. That puts wet gas through the blower, so drains and materials matter.
How does moisture affect blower pressure?
Moist gas is less dense than dry air, so the blower develops less pressure. Saturated air at 60 °C is about 7% lighter than dry air at that temperature.
How do I protect a blower from droplets?
Use a mist eliminator, drains at the lowest point, and sloped ducts, and drain and dry the housing after shutdown.
What pressure does a scrubber system need?
It is the sum of hood, duct, scrubber, and stack losses. An illustrative 8 m³/s system with a 2,000 Pa venturi totals about 2,800 Pa, roughly 285 mm WC.
Can you supply blowers for flammable fume extraction?
Not for classified areas. We do not currently manufacture ATEX-certified blowers, so those come from a specialist supplier.
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Call +91-9311805618 or use our contact form for a facility-specific recommendation.
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