Centrifugal Blowers for Industrial Drying and Process Air

Drying is a job about moving air, because air carries the water away. A dryer or oven works by heating air, passing it over or through the product, and taking the moisture-laden air out. The centrifugal blower is the machine that keeps that air moving against the resistance of heaters, product beds, plenums, and ducts. The same family of blowers also handles blow-off, air flotation, and other process air duties, and in many plants these blowers replace compressed air, which is one of the most expensive ways to move air.

What Drying Air Has to Do

To dry a product, air must supply heat to evaporate the water, and then carry the vapour away. The amount of air depends on how much water you must remove and how much moisture each kilogram of air can take up on its way through.

Example. A dryer must evaporate 500 kg/h of water. The air picks up 0.02 kg of moisture per kg of dry air as it passes through the product.

Air mass flow = 500 ÷ 0.02 = 25,000 kg/h, or 6.94 kg/s.

At the dryer’s operating temperature of 80 °C, air density is about 1.0 kg/m³, so the volume flow is about 6.95 m³/s, or about 25,000 m³/h. Drier exhaust air takes up more moisture per kilogram but means more energy is lost with the exhaust, so drying design is a balance between airflow, temperature, and how humid the air is allowed to become.

Where Process Air Blowers Are Used

• Dryers and ovens, both continuous conveyor types and batch.
• Blow-off systems, which remove moisture from parts before painting, coating, or packing.
• Air flotation, which supports webs and sheets on a cushion of air, in printing and metal-finishing lines.
• Circulating air in ovens and dryers, so temperature and humidity stay uniform.
• Vacuum hold-down and low-vacuum service, where the blower runs on suction.
• Food processing, including drying and cooling.
• Curing ovens and heat-treatment lines, where heated air circulates over the work.

Blow-off and flotation duties sit in the high-pressure class, where a low-pressure fan cannot deliver enough. Single-stage backward-curved blowers can supply these duties oil-free, with stable airflow, and with efficiencies up to about 80%, and companies have often used expensive compressed air for drying, blow-off, air flotation, and cleaning that a blower can do at far lower operating cost.

Recirculation and Exhaust

An oven or dryer normally has two air streams. A recirculation blower moves hot air around the chamber to spread heat and take moisture off the product. An exhaust fan removes a portion of the air to hold humidity or fumes down, and make-up air replaces it. Ovens with burners are usually run under positive pressure, and an exhaust fan keeps smoke, fumes, and combustion products from bleeding into the room. That exhaust fan can be quite small, and is sized to match the fresh air the combustion blower supplies. See combustion air.

Balance matters. If exhaust exceeds make-up, the chamber goes to negative pressure and cold air leaks in, hurting temperature uniformity. If make-up exceeds exhaust, hot air leaks out. Set the exhaust flow, the make-up flow, and the chamber pressure together.

Forward-Curved or Backward-Curved

Forward-curved wheels suit oven and dryer exhaust, process drying, and cooling where air is clean and pressure is modest. They are compact, quiet, and common in air-handling arrangements. Backward-curved wheels suit higher-pressure duties, such as blow-off, high-velocity impingement dryers, and beds of product with real resistance, and they are more efficient and non-overloading. See backward-curved versus forward-curved blower wheels.

Heat, Humidity, and Materials

Hot, humid air changes what the blower is made of and how it is built.

• Temperature. Blowers in hot air need materials, bearings, and motor positioning suited to it. Our high-temperature blowers cover ovens up to 500 °C. See furnaces, kilns and ovens.
• Humidity and condensation. Moist air can condense inside a cooler housing and corrode steel, and it forms a paste with dust. Provide drains and consider corrosion-resistant materials.
• Product carry-over. Lint, dust, and volatile matter from the product can foul blades, so filtration and cleaning access matter.
• Solvents. Drying with flammable solvent vapour is a hazardous-area duty and needs specialist equipment. We do not currently manufacture ATEX-certified or AMCA 99 spark-resistant blowers.

Plenum and Packaged Arrangements

Many ovens and dryers use a plenum blower arrangement, where a wheel discharges into a chamber that distributes air. Packaged air kits combine the blower, motor, and housing for a plenum system, which simplifies installation. Confirm the arrangement, the plenum pressure needed, and the position of the motor relative to the hot air.

Control

Drying loads change with product, moisture, and production rate. Speed control lets the airflow follow the load and saves energy, since power varies with the cube of speed, and it also allows airflow to be tuned during commissioning. Dampers are simple but waste energy. See flow control for blowers.

Efficiency: Blower Versus Compressed Air

Where a plant uses compressed air for drying, blow-off, or cleaning, replacing it with a blower usually cuts operating cost a great deal, because a blower moves large volumes at low pressure far more efficiently than a compressor. The limit is pressure: a blower cannot deliver the very high pressure of a compressor, so the application must fit within its range. Our standard maximum is 500 mm WC.

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, single-inlet and double-inlet designs, and high-temperature blowers for ovens up to 500 °C, kilns up to 650 °C, and furnaces up to 800 °C. That covers dryer and oven recirculation and exhaust, blow-off, air flotation, and process air within those limits. Airflow and drive arrangement are confirmed for each selection.

What to Specify

1. Water to evaporate, or the heat and airflow the process needs.
2. Air temperature and humidity through the chamber.
3. Resistance of heaters, product bed, plenum, and ducts.
4. Recirculation versus exhaust duty, and chamber pressure.
5. Wheel type, materials, and motor position for the temperature.
6. Product carry-over, solvents, and hazardous-area classification.
7. Control approach and noise limits.

Get a Blower Selection for Your Dryer or Oven

Send your moisture load, temperature, and resistance, and we’ll size the blower and confirm the arrangement. Contact us for a free quote.

Frequently Asked Questions