Dust Collection System Design for Blower Selection: Capture, Convey, Collect and Fan Placement

A dust collection system is only as good as its weakest link, and the blower is often blamed for problems that begin at the hood. Poor capture, ducts that let dust settle, a filter that loads faster than expected, or a blower in the wrong place: each shows up as low airflow or a dusty plant, and each is fixable in the design. This guide follows the air through the system, from hood to stack, and shows where the blower fits and what it has to overcome.

Capture, Convey, Collect

1. Capture. A hood draws dusty air in at the source.
2. Convey. Ducts carry the air to the collector at a velocity high enough to keep the dust suspended.
3. Collect. A filter, cyclone, or scrubber separates the dust from the air.
4. Discharge. The blower pushes the clean air out through the stack.

Capture Velocity Is Not Transport Velocity

Two different velocities matter, and they are often confused.

• Capture velocity is the air velocity at the point where the dust is generated, which must be high enough to draw the dust into the hood.
• Transport velocity is the air velocity in the duct, which must be high enough to keep the dust moving and stop it settling.

They are set independently. A hood with the right capture velocity, feeding a duct with too low a transport velocity, will clog with dust.

How Distance Wrecks Hood Performance

The airflow needed to hold a given capture velocity grows very fast as the hood moves away from the source. For a plain, unflanged opening, a widely used relationship gives the airflow as the capture velocity times the sum of ten times the distance squared plus the opening area.

Example. A hood opening is 0.3 m by 0.3 m, an area of 0.09 m², and the required capture velocity is 0.5 m/s.

Distance from source	Airflow needed
0.15 m	0.157 m³/s
0.45 m	1.057 m³/s
0.90 m	4.095 m³/s

Moving the hood from 0.15 m to 0.9 m raises the airflow needed by 26 times. The blower, ducts, and collector all have to be sized for that airflow, so the cheapest way to shrink a dust collection system is to bring the hood as close to the source as possible, or to enclose the source. Flanges and enclosures cut the airflow further.

Duct Velocities

Transport velocity depends on the dust. Light fume needs lower velocity than heavy, dense dust. As a guide, welding fume is commonly carried at about 10 to 13 m/s (roughly 2,000 to 2,500 feet per minute), while many general dusts are carried at around 20 m/s, and heavy or wet dusts need more. Do not go far above about 20 to 23 m/s in abrasive duty, because wear in ducts and bends rises rapidly with velocity. The ACGIH industrial ventilation manual is the standard reference for design velocities.

Example. A duct carrying 12 m³/s at 20 m/s needs a cross-section of 12 ÷ 20 = 0.6 m², a diameter of about 0.87 m. A smaller duct would raise the velocity, resistance, and wear.

Adding Up the Resistance

The blower must overcome every element in the path.

Element	Illustrative resistance
Hood entry	500 Pa
Duct and fittings	700 Pa
Filter (loaded)	1,500 Pa
Stack and outlet	300 Pa
Total	3,000 Pa (about 306 mm WC)

At 12 m³/s, the air power is 36 kW, and at 75% efficiency the blower needs 48 kW at the shaft. These figures are illustrative, so use the equipment suppliers’ resistance data for real designs. See how to size a centrifugal blower.

Where the Blower Goes

Clean side (after the filter). This is the common arrangement. The blower handles clean air, so a high-efficiency backward-curved wheel can be used and wear is minimal. The ducting between the hoods and the filter runs under negative pressure, so leaks draw air in, not dust out.

Dirty side (before the filter). The blower handles dusty air. It needs abrasion protection, heavy construction, and careful attention to build-up, and radial-blade wheels are typically used for heavy dust and material handling. Radial wheels are not part of our range. A dirty-side position is sometimes used after a cyclone, where some dust still gets through.

Our dust collection blowers are the clean-side and light-dust type.

Filter Loading and Control

As the filter loads, resistance rises, and the operating point drifts to lower flow and higher pressure. Size for the loaded condition, and where capture depends on steady airflow, use speed control to hold it. Pulse-cleaning cycles cause the resistance to swing up and down, so control loops should not chase every pulse. See blower curves, system curves and operating point drift.

Margins

Some designers add around 10 to 15% on airflow and more on pressure to cover leakage and loading, and practitioners disagree about how much is sensible. Too little margin leaves the system short as filters age, and too much wastes energy and can overload the motor when the real resistance is lower than calculated. Calculate the worst case, add a modest margin, and control the flow.

Balancing the System

Multiple hoods on one duct system need balancing so each gets its share of air. Blast gates and balancing dampers do this, and they should be set at commissioning and checked whenever the system changes. A system that was balanced for five hoods will not work for seven without rebalancing.

Combustible Dust and Explosion Protection

Many dusts burn or explode when suspended in air. Collectors handling such dust need a dust hazard analysis, explosion venting or suppression, isolation, and equipment suited to the zone. See combustible dust and hazardous atmospheres.

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. That covers clean-side dust collection blowers up to 500 mm WC. Radial-blade dirty-side duty, and blowers for classified hazardous zones, are outside our range.

Get a Blower Selection for Your Dust System

Send your airflow, resistance, and dust type, and we’ll size the blower and confirm whether it is within range. Contact us for a free quote.

Frequently Asked Questions