A plant adds a bag dump station and needs to know how to size a dust collector before ordering: sum the inlet airflow, check duct velocity and static pressure, then convert airflow to filter area.
Quick answer: total inlet airflow in CFM comes first, built from the hood and duct flows that feed the collector. Filter area then equals that airflow divided by the air-to-cloth ratio, and the fan must deliver the airflow at the system’s static pressure. Stop and escalate if the dust is sticky, fibrous or wet.
This procedure carries a single chain of reasoning from hood to duct to filter to fan. It sits within the wider topic of industrial dust collection systems, where collector type and dust hazards are covered in more depth.
Sum the airflow at the collector inlet
Total inlet airflow is the starting figure for every other calculation. Villotech’s cartridge collector guide puts it first: define airflow in CFM at the collector inlet by summing extraction points and ducts. A dust collector sizing calculator can speed up the arithmetic, but it needs this summed figure as its input, so complete this step before opening one.
- List every extraction point that will connect to the collector, including each capture hood.
- Take the design airflow for each hood from a recognised hood design method.
- Add the branch flows to get the CFM at the collector inlet.
- Record that figure as the base case for duct, fan and filter checks.
Hood airflow and capture velocity
The airflow for a capture hood depends on the hood style and the air speed needed at the point where dust is released. Capture velocity for dust hoods therefore varies by process, and it should be read from design guidance, not guessed. The reference is ACGIH’s Industrial Ventilation: A Manual of Recommended Practice for Design, 31st Edition, and ACGIH teaches exhaust hood, duct and fitting selection using its calculation methods. An undersized hood fails at the source, and no later sizing step repairs that.
Check duct size and transport velocity
Duct velocity follows from airflow and duct diameter, and it has to stay high enough to carry dust. Woodweb’s component sizing guide notes that less volume through a given pipe diameter means slower airspeed, and gives an example of 400 cfm in a 6-inch duct moving only 2,037 fpm. The same guide treats 4000 fpm as an apt design rate for dust collection ducts.
The judgement for a designer is simple. A main duct that is too large for its flow runs slow, and slow air is a poor transport medium. A duct that is too small raises resistance. Use the 4000 fpm figure as a reference point for a woodworking-type dust, and confirm the transport velocity for your own material against the ACGIH manual before fixing diameters.
Add up static pressure and choose the fan
Static pressure measures the resistance created by moving air through a duct path, and it decides what the fan has to overcome. According to Woodweb, longer runs, elbows, wyes, blast gates and hoods all increase static pressure and decrease CFM. A bottleneck anywhere in a run limits the air volume the collector can pull.
Fitting choice therefore matters during sizing. The same guide favours a smooth duct interior, large-radius elbows and Y connections, and warns against sharp-radius bends and T connections. Count every fitting in the longest, most restrictive run and treat that run as the one that sets the pressure the fan must handle.
Reading the fan curve
A fan curve plots the airflow a fan delivers against the static pressure it works against. Woodweb cautions that a headline rating such as 2100 CFM at 10 in. static water pressure describes the very upper and lower limits of a machine’s performance curve. The airflow actually delivered depends on where the installed system’s resistance falls along the curve. Select the fan so that it delivers the inlet airflow at the system’s total static pressure, not at its free-air rating. ACGIH’s ventilation course covers fans and air cleaners including the Fan Laws, which relate fan performance changes to speed.
Convert airflow to filter area
Once inlet airflow is known, filter area follows from one formula. Villotech gives it as filter area in ft² equals CFM divided by the air-to-cloth ratio. In its worked example, 10,000 CFM at 2.0 ft/min gives a required filter area of 5,000 ft².
The choice that moves the answer is the ratio itself. For cartridges, Villotech advises aiming for 1.5–2.5 ft/min depending on dust type, so the same airflow can need noticeably different filter area at each end of that range. The air to cloth ratio is the main lever between a compact unit and an oversized one. A lower ratio means more media and a larger collector.
To turn area into hardware, divide the required filter area by the area per cartridge, which Villotech puts at roughly 200–350 ft² each. Round up to a whole cartridge count, and check that the housing and pulse-cleaning system can support it.
Validate the inlet before ordering
A calculated size is only a proposal until the collector’s own limits are checked. Villotech’s checklist is to confirm pulse air capacity, fan static pressure, hopper discharge and noise limits. Can velocity is a related check: it describes the upward air speed through the housing around the filters, and the manufacturer should state how the unit performs at your design airflow and inlet arrangement. Dust must be able to fall into the hopper against the air rising past it.
When to stop and escalate
Stop and change approach when the dust itself rules out the chosen collector. Villotech says cartridge collectors do not handle sticky, fibrous or wet dust well, and suggests baghouses or pre-separators for those cases. Where the material is wet or sticky enough that fabric filters struggle, wet scrubber dust collection is another technology to weigh against them.
Escalate to a specialist review when an existing system does not deliver its calculated airflow. ACGIH’s ventilation training covers troubleshooting and testing of existing systems, which is the right route when measured performance and calculated performance disagree. The table collects the checkpoints from this procedure.
| Checkpoint | Value or formula | Planning use |
|---|---|---|
| Filter area | CFM ÷ air-to-cloth ratio (ft²) | Applies to the summed inlet airflow, never to a single hood. |
| Worked example | 10,000 CFM at 2.0 ft/min gives 5,000 ft² | Shows the size of the result before choosing cartridges. |
| Cartridge ratio range | 1.5–2.5 ft/min, by dust type | Pick the end of the range that suits the dust, not the smallest unit. |
| Area per cartridge | About 200–350 ft² each | Divide required area by this to count cartridges. |
| Duct design rate | 4000 fpm cited as apt | Reference point for woodworking-type dust duct sizing. |
| Slow duct example | 400 cfm in a 6-inch duct is 2,037 fpm | Shows how low flow in a large duct slows the air. |
| Fan rating | 2100 CFM at 10 in. static water pressure | Limits of a curve, not the system’s operating point. |
Figures are from Villotech’s cartridge collector guide and Woodweb’s component sizing guide.
Frequently asked questions
What is the smallest duct I should use?
Woodweb gives a rule of thumb that minimum duct size is the fan’s impeller inlet opening, or larger if feasible. Check it against the velocity and pressure calculation for your own layout.
Which filter suits fine dust?
Woodweb’s guidance is to use a good quality cartridge filter, because a bag will not be effective at removing the fine dust. Match that to the dust type when choosing the air-to-cloth ratio.
Where can designers learn the ACGIH calculation methods?
ACGIH offers a ventilation course on industrial process exhaust system design that includes its calculation methods, and a webinar on the velocity pressure calculation method. Both sit alongside the manual itself on the ACGIH ventilation page.
