Workshop Ventilation and Dust Collection System Design: Airflow, Duct Sizing, and Hood Types - GreenEngGuide - Industrial Engineering & Environmental Compliance Insights

A dust collection system that doesn’t capture dust at the source is just an expensive fan moving dirty air around the workshop. The hard part isn’t sizing the fan or the baghouse—it’s getting the contaminant into the system in the first place.

This article covers the front end of industrial dust collection: capture hoods, ductwork design, and system balancing. If you get this part wrong, no filter on the back end will fix it.

The Capture Velocity Principle

Dust collection starts with a simple question: how fast does the air need to move at the point of dust generation to pull particles into the hood?

Recommended Capture Velocities

The key word is at the point of generation. Capture velocity decays rapidly with distance from the hood opening. Doubling the distance from the hood reduces capture velocity by roughly a factor of 4 (for a plain opening hood). This is why hood position matters more than fan horsepower.

The Distance Problem

For a plain opening hood (no flange), the centerline velocity at distance x from the opening:

“`
Vx = V₀ × (1 / (1 + 10x²/A))
“`

Where V₀ is the face velocity, A is the opening area, and x is the distance.

The takeaway: if you move the hood from 150 mm to 300 mm away from the dust source, you need roughly 4× the airflow to maintain the same capture velocity. Before you upgrade the fan, move the hood closer.

Hood Types: What to Use When

Enclosing Hood (Best)

The process is enclosed inside the hood. Only openings are for material entry/exit and operator access. Required airflow is minimal because you’re not reaching out to capture distant dust—you’re just keeping the enclosure under negative pressure.

Enclosing hoods use 50-80% less air than capturing hoods for the same dust source. If you can enclose it, enclose it.

Capturing Hood (Most Common)

The hood is positioned near—but not enclosing—the dust source. Dust-laden air must be drawn across open space into the hood. This is the default for operations where the worker needs access: grinding stations, welding benches, drum filling.

Design rules:
– Position the hood as close as practical. Every 100 mm closer is worth a 20-30% airflow savings.
– Use the natural direction of particle motion. Hot processes generate upward thermal currents. Grinding wheels throw particles tangentially. Align the hood with the natural throw direction.
– Add a flange. A 100 mm flange around the hood opening reduces “pull” from behind the hood, directing all airflow from the front. A flanged hood needs 25% less airflow for the same capture.

Receiving Hood (Special Case)

Used when the process itself imparts velocity to the dust—grinding, spraying, high-speed transfer. The hood “catches” particles that are already moving. It’s not capturing from still air—it’s intercepting a particle stream.

The hood must be large enough to accommodate the spreading particle plume. A grinding wheel throws particles in a 30-45° arc from the contact point. The receiving hood must cover that arc. If the hood is too small, the fastest particles bounce off the back wall and escape.

Duct Design: Transport Velocity

Once dust is captured, the ductwork must keep it airborne. If velocity drops below the transport threshold, particles settle in the duct, restricting airflow and creating a fire hazard (accumulated combustible dust in a duct is an explosion risk).

Minimum Transport Velocities

| Dust Type | Minimum Duct Velocity (m/s) |
|———–|—————————|
| Fine, light dust (wood sanding, grain, cotton) | 15-18 |
| Medium dust (grinding, limestone, coal) | 18-22 |
| Heavy dust (sand, metal turnings, lead) | 22-25 |
| Wet or sticky dust | 25-30 |
| Fume (welding, soldering)—very fine | 10-15 |

These are minimums at any point in the system. If you have a long horizontal run where velocity might drop at the far end, design for the velocity at that point, not at the fan inlet.

Duct Sizing Method

The standard method for industrial dust collection is velocity pressure design:

1. Determine required airflow for each hood (m³/s)
2. Select duct diameter to maintain transport velocity at that airflow
3. Calculate pressure drop through each branch
4. Balance branches to within 10% of each other (or use blast gates)

Branch imbalance is the #1 cause of “this hood doesn’t pull” complaints. If Branch A has 500 Pa resistance and Branch B has 800 Pa, Branch A gets more air than designed and Branch B starves. You balance by:
– Adding resistance (blast gate) to the lower-resistance branch
– Or upsizing the higher-resistance branch duct

Duct Construction

System Balancing

Once the system is installed, it must be balanced. The design calculations tell you what the airflow should be. Field balancing makes it real.

Balancing Procedure

1. Run all hoods open. Measure static pressure and airflow at each hood.
2. Identify the governing branch (highest resistance). This branch controls the fan pressure requirement.
3. Adjust blast gates on lower-resistance branches to increase their resistance until each branch pulls its design airflow.
4. Adjust fan speed (VFD or pulley change) to achieve the design total airflow at the required static pressure.

The Critical Measurement

At each hood, measure:
– Static pressure (before the blast gate)
– Velocity pressure (with a Pitot tube traverse across the duct)
– Hood static pressure = static pressure at the hood throat

If hood static pressure increases over time, the duct is accumulating dust or the filter is loading. If it decreases, there’s a leak or a blast gate has been opened.

The Most Common Failures

1. “We added a new machine, just teed into the existing duct”

Dust collection systems don’t scale by adding branches. Every new branch steals air from existing branches. If you add a grinding station without recalculating the system, every existing hood gets less air. Somewhere in the plant, dust that was previously captured is now escaping—and the operator won’t notice until the filter blinds or the air quality complaint comes in.

Fix: Rebalance the entire system. Or install a dedicated collector for the new machine.

2. “We turned up the fan to pull harder”

A higher fan speed increases airflow—but also increases duct velocity, which increases pressure drop (ΔP ∝ V²), which means more power consumption (P ∝ V³). A 20% increase in fan speed costs 73% more power. Worse, higher duct velocities increase abrasion at elbows and can actually reduce capture efficiency by creating turbulence at the hood face.

Fix: Fix the hood position first. Increase fan speed only after confirming hoods are optimally positioned and ducts are clean.

3. “The filter is new, but the hoods still don’t pull”

If hood performance dropped suddenly, check:
– Is a blast gate closed that shouldn’t be?
– Is there a hole in the duct? (Listen for whistling or feel for suction where there shouldn’t be any)
– Did someone open an access door on the duct or collector?
– Is the fan belt slipping? (Check fan RPM vs nameplate)

Nine times out of ten, it’s a blast gate someone closed “temporarily” and forgot to reopen.

Summary

1. Move the hood closer before upgrading the fan. Distance is the enemy of capture.
2. Use the right hood type. Enclose if you can, capture if you can’t, receive if the process throws particles.
3. Maintain transport velocity throughout the duct run. The lowest velocity anywhere determines whether dust settles.
4. Balance the system. An unbalanced system over-ventilates some hoods and starves others.
5. Don’t tee into an existing system without rebalancing. Every new branch affects every existing branch.

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