Centrifugal Blowers for Low-Pressure Pneumatic Conveying and Air Slides

Moving material with air is one of the oldest uses of a blower, and one of the most misunderstood. Pneumatic conveying comes in several forms, and they differ enormously in the air pressure they need. Centrifugal blowers suit the low-pressure end: dilute-phase conveying over short distances with light solids loading, air slides that let bulk powder flow by gravity on a cushion of air, and vacuum hold-down. Higher-pressure conveying belongs to other machines. This page explains where centrifugal blowers work, where they do not, and how to size one honestly.

Three Conveying Duties

Dilute-phase conveying carries material suspended in fast-moving air, at high air velocity and a low ratio of solids to air. It is simple and gentle on the equipment, and a low-pressure centrifugal blower can supply it where the pipe is short and the loading is light.

Air slides are slightly inclined conveyors with a porous fabric bed. A small flow of low-pressure air fluidises the powder so it flows downhill by gravity. They are used for cement, alumina, lime, fly ash, and other fine powders, and for hot granular and biosolid materials in mining and chemical processing. Air-slide blowers are among the smallest blowers in a cement plant.

Vacuum service includes vacuum hold-down of parts and low-vacuum suction, where the blower runs on the suction side.

Sizing the Airflow

Dilute-phase conveying needs a minimum air velocity to keep the particles moving, and velocities around 20 m/s are common for many materials, more for coarse or heavy ones.

Example. A conveying line has a 200 mm internal diameter, so the pipe area is 0.0314 m².

Airflow = 0.0314 × 20 = 0.628 m³/s, or about 2,260 m³/h.

The air mass flow is 0.628 × 1.2 = 0.754 kg/s. With a solids loading of 5 kg of material per kg of air, the line carries about 3.8 kg/s, roughly 13.6 t/h. The airflow sets the capacity of the line, and the loading ratio sets how much pressure the blower must provide.

Why Pressure Is the Limit

The pressure needed grows with the length of the line, the number of bends, the lift, and above all the solids loading. Dense loading needs high pressure. A low-pressure system carries material at low loading over a modest distance, and a centrifugal blower works well there. Once distance or loading grows, the pressure climbs past what a centrifugal machine at our 500 mm WC (about 4.9 kPa) standard maximum can supply.

Blowers divide into centrifugal and positive-displacement types. Positive-displacement blowers, such as rotary lobe machines, are built for higher pressure at lower flow and hold their flow as pressure changes, which suits longer or denser conveying. So the first question for any conveying job is what pressure the line really needs, and the answer often decides the machine type. See centrifugal versus positive-displacement blowers.

Where the Blower Sits

• Blowing (positive-pressure) systems put the blower upstream of the feeder, so the blower handles clean air and the material never passes through it. That protects the wheel from abrasion.
• Vacuum systems place the blower at the end of the line, after a filter that separates the material. Handled properly, the blower still sees only clean air, but if material reaches the blower, wear can be severe.

In either case, keep the material out of the blower. If a duty needs the material to pass through the fan wheel, the abrasion is heavy and radial-blade wheels are typical. Radial wheels are not part of our range.

Air Slides: Low Pressure, Steady Air

An air slide needs a modest, steady air pressure to lift the powder off the fabric, enough to overcome the fabric’s resistance and the depth of powder. It also needs air that is dry and clean, because damp air makes powder stick and dust clogs the fabric. The blower is small, runs continuously, and should give steady pressure without pulsation. Confirm the pressure the air-slide manufacturer specifies, since it depends on the powder, the slide length, and the fabric.

Hot Materials

Hot granular material on an air slide, such as cement clinker fines or calcined products, needs air and equipment that tolerate temperature. The blower supplies ambient air, so it is ordinary, but the fabric and casing carry the heat. Where the blower handles hot air itself, see furnaces, kilns and ovens.

Combustible Powders

Fine organic and metal powders, coal, grain, and sulfur can burn or explode when suspended in air, and pneumatic conveying suspends them by design. That is a hazardous-area matter with its own standards and equipment. We do not currently manufacture ATEX-certified or AMCA 99 spark-resistant blowers, so conveying combustible material in a classified zone needs a specialist supplier. See combustible dust and hazardous atmospheres.

Control and Noise

Conveying blowers often run at a fixed duty, but speed control allows the air to be adjusted for different materials and rates. Blowers running against high pressure at high speed are noisy, so inlet silencers and acoustic covers are common. See blower noise and silencers.

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 air-slide blowers, low-pressure dilute-phase conveying over modest distances with light loading, and vacuum hold-down. Conveying that needs more than 500 mm WC, including dense-phase and long-distance dilute-phase systems, is outside our range and is better served by positive-displacement blowers or compressors. We will say so early, and explain which machine suits the duty.

What to Specify

1. Material, particle size, and bulk density.
2. Conveying rate, line size, and length, with bends and lift.
3. Air velocity and solids loading proposed, and the resulting pressure.
4. Air-slide length, width, and fabric specification, if applicable.
5. Air cleanliness and dryness.
6. Whether the material is combustible, and any hazardous-area classification.
7. Blower position: blowing or vacuum.

Get a Blower Selection for Your Conveying Duty

Send your material, line size and length, and conveying rate, and we’ll tell you whether a centrifugal blower suits the duty and size it if so. Contact us for a free quote.

Frequently Asked Questions