A stone grinding station can pass a desk review — adequate collector CFM, appropriate filter rating, reasonable duct layout — and still fail to control dust at the point where it matters. The failure rarely shows up until commissioning, when a pitot traverse at the hood face reveals capture velocity well below the design target, and the cause turns out to be duct losses and filter resistance that were never subtracted from the rated airflow figure. Correcting it at that stage typically means duct redesign, fan upsizing, or both — none of which were budgeted. What resolves this early is treating capture velocity, static pressure, and duct transport velocity as three separate specified quantities, not consequences of a single CFM number, and requiring all three to appear explicitly in the quotation before a purchase order is issued.
Define the dust source distance and capture opening
Capture velocity is a property of the airflow at a specific point in space — the location where dust first becomes airborne — not a property of the collector. Specifying it without fixing the reference geometry makes the number unverifiable. For a downdraft grinding table, the relevant distance is from the worksurface to the hood face or grated work surface opening; for a side-draft or backdraft hood configuration, it is the perpendicular distance from the grinding contact point to the hood opening plane. Both the distance and the opening area must be stated before the velocity figure has any design meaning.
Practitioner guidance from ACGIH and HSE HSG258 frames two different target ranges, and the gap between them is not an error — it reflects the difference between low-energy abrasion and high-energy grinding processes.
| Standard | Process/Hood Type | Recommended Capture Velocity |
|---|---|---|
| ACGIH | Tool hood, stone grinding & cutting | 100–200 fpm |
| HSE HSG258 | Large hood faces, high‑energy grinding | 2.5 to >10 m/s (≈492–1968 fpm) |
For most stone grinding and cutting operations involving hand-held or bench-mounted tools at a downdraft table, the ACGIH 100–200 fpm range applies at the hood face when the source is positioned at or near the worksurface. The HSE HSG258 upper range — up to and beyond 492 fpm — applies where the process itself generates turbulence that displaces dust laterally before it can settle into the capture zone. Angle grinders on hard stone, high-speed cutting wheels, and operations that produce visible airborne plumes rather than directed chips are candidates for the upper end of that range. The specifier’s decision is to place the process in the right category, because undershooting the velocity for a turbulent source means the hood geometry may be correct on paper while failing in practice.
ASHRAE Handbook Chapter 33 provides useful reference principles for hood geometry and effective capture distance — specifically, that capture velocity decays sharply with distance from a plain opening, and that even small increases in source-to-hood distance can require disproportionate airflow increases to maintain the same velocity at the source. This is why fixing the source distance before calculating airflow is a prerequisite, not a refinement.
Calculate airflow after duct and filter losses
Rated collector CFM is the airflow the fan can move against a specific static pressure — typically the resistance of the filter at one loading condition. Once that airflow enters a real duct network with bends, transitions, length, and branch splits, what arrives at the hood face is always less. The question is how much less, and whether the remainder still meets the capture velocity requirement.
Three parameters carry most of the design risk and must be calculated together as a system rather than applied independently.
| Paramètres | Minimum Requirement | Risk if Ignored |
|---|---|---|
| Duct transport velocity | >20 m/s for grinding dust | Dust settles in ducts, increasing fire risk and reducing airflow |
| Filter resistance (HEPA) | Include pressure drop of HEPA filter (99.97% at 0.3 μm) in static pressure calc | System airflow at hood falls short; nominal CFM becomes misleading |
| Dimensionnement des gaines | Size to minimize friction loss across branches | Inadequate capture velocity at distant workstations despite collector rating |
The duct transport velocity constraint creates a design tension that is worth naming directly: sizing ducts large enough to reduce friction loss — and thereby preserve more airflow for capture — can drop transport velocity below the ~20 m/s threshold needed to keep grinding dust suspended in the duct stream. Below that threshold, heavier particles begin settling at low points and bends, progressively narrowing the effective duct cross-section and reducing flow further. In a worst case, dry silica-containing dust accumulating in horizontal duct runs also represents a fire and deflagration risk. The result is that duct diameter cannot be freely traded against friction loss; both transport velocity and friction loss must be satisfied simultaneously, which constrains the solution space more than either criterion alone would suggest.
For silica dust applications, HEPA-rated filtration — typically specified at 99.97% efficiency at 0.3 µm — is the appropriate design target given the fine respirable fraction involved. The planning implication is that HEPA filter resistance must be included in the total static pressure budget from the start. A filter in clean condition has a certain initial pressure drop; a filter approaching its change-out point has a substantially higher one. Sizing the fan only against the clean-filter condition means the system will progressively underperform as the filter loads, and capture velocity at the hood will drift downward between maintenance intervals. The design-conservative approach is to size against a partially loaded filter resistance and verify that the fan curve still delivers adequate airflow at that resistance.
For buyers evaluating a table de broyage à courant descendant paired with a dedicated collector, the sizing calculator referenced at Calculateur de débit (en CFM) pour tables de meulage à aspiration par le bas provides a workpiece-dimension and material-type framework for establishing a starting airflow figure — but the duct and filter losses described here still need to be subtracted before that figure translates into a hood-face velocity.
Check cross-drafts workpiece size and operator position
A system that achieves adequate capture velocity at a static test point — no operator, standard workpiece, no ambient airflow — can still fail to control dust under real operating conditions. This is not a system deficiency; it is a specification gap that shows up only when the test conditions diverge from actual use.
Cross-drafts are the most common environmental factor. Air movement from open doors, HVAC supply diffusers, adjacent equipment, or forklift traffic as low as 30–50 fpm can deflect the dust plume away from the capture zone before the hood velocity reaches it. The effect is amplified when the grinding operation generates turbulent jets — the grinding wheel itself throws dust outward, and a lateral cross-draft intercepts it at a distance the hood was not designed to reach. Specifying capture velocity without documenting ambient air movement at the workstation leaves this variable uncontrolled.
Workpiece size directly affects the effective source distance. A small workpiece positioned close to the downdraft surface presents the grinding contact point within the design capture zone. A large slab or irregularly shaped stone that overhangs the table perimeter can move the active grinding location significantly farther from the suction surface — sometimes far enough that the velocity at that distance has already decayed below the capture threshold. This is a dimensional boundary condition that belongs in the specification, not something left to the operator to manage case by case.
Operator position matters for side-draft and backdraft hood configurations more than for downdraft tables, but it is relevant in all layouts. A worker’s torso positioned between the dust source and the hood face creates a wake region that disrupts the capture airstream, pushing dust laterally rather than drawing it in. This effect is intermittent and rarely traced back to the original design — it surfaces as unexplained exposure variability during hygiene monitoring and is difficult to correct without revisiting hood placement.
A separate risk is resuspended dust from dried slurry or settled grinding debris on floors and horizontal surfaces. Even when active capture is functioning correctly, foot traffic and air currents can lift settled fine particles back into the breathing zone, increasing airborne silica concentration without any direct connection to the grinding operation itself. Housekeeping protocol and wet suppression at the worksurface are operational controls that the capture system cannot substitute for. Treating these as outside the specification scope can undermine measured capture performance.
Match fan collector and duct system as one design
In a single-station layout — one grinder, one collector, one short duct run — the fan and duct sizing are relatively straightforward. The more common procurement pattern in fabrication shops involves multiple grinding stations on a shared central system, and that layout introduces a design-coordination requirement that is frequently treated as a detail rather than a governing constraint.
A centralized system’s total airflow is fixed at the fan. How that airflow distributes across simultaneously operating stations depends on the resistance balance of the branch duct network — not on the nominal CFM rating of the collector. If branch ducts are sized for individual station airflow without accounting for simultaneous demand, opening multiple stations at once reduces the available airflow at each station below its design value. The stations that experience the greatest drop are typically those farthest from the fan, where branch resistance is highest and velocity decay is steepest.
The corrective design approach is to treat simultaneous operation as the baseline condition, not a contingency. Duct sizing must maintain adequate transport velocity and adequate hood-face capture velocity across all active stations at once, under the worst-case combination of stations in use. This means the duct network and the fan-collector unit must be specified together from the start, with the simultaneous demand assumption explicitly stated. A dépoussiéreur à cartouche rated for the aggregate demand of all stations does not automatically distribute that capacity correctly unless the duct network is sized to match — and that sizing work belongs in the design scope, not in post-installation troubleshooting.
This coordination obligation applies to any multi-station layout, not only centralized systems. Even decentralized arrangements with individual collectors can interact if they share supply air pathways, and the fan curve of each collector must be verified against the actual resistance of its branch, not the aggregate system resistance.
Verify static pressure and airflow during startup
Startup is the point where design assumptions meet real conditions, and the most common finding in underperforming systems is that nominal CFM was treated as a delivered performance guarantee rather than a fan rating that still needs to be confirmed against the installed system’s resistance. These two things are rarely identical.
| Éléments à vérifier | Pourquoi c'est important | Risk if Skipped |
|---|---|---|
| Actual capture velocity at hood | High CFM rating alone does not guarantee effective capture; duct/filter losses must be accounted | System may be non‑compliant; dust escapes capture zone |
| Static pressure at key points | Must match design curve incorporating filters, hoses, bends, and duct length | Insufficient static pressure reduces airflow, causing capture failure |
The practical startup procedure is to measure capture velocity at the hood face under representative operating conditions — not just at the collector inlet or at a convenient duct traverse section — and to record static pressure at defined measurement points in the duct network. ISO 10780 provides a testing framework for velocity and volumetric flow measurement in stationary source gas streams, which is useful as a methodology reference for pitot traverses during commissioning, even though it is not a governing compliance standard for grinding station installations. The measurement locations and acceptance thresholds should be defined in the specification before installation begins, so that startup verification is a contractual checkpoint rather than an ad hoc exercise.
If measured capture velocity falls short at startup, the diagnostic path is to work backward through the resistance budget: measure static pressure at the filter inlet and outlet to quantify filter resistance, traverse the main duct to confirm transport velocity, and identify whether the shortfall is concentrated at one branch or distributed across all stations. Each finding points to a different corrective action — filter stage undersizing, duct diameter mismatch, fan curve mismatch — and knowing which one is the cause prevents the common mistake of adding duct area to solve a problem that is actually a filter resistance issue, which drops transport velocity without recovering capture velocity.
Set maintenance triggers for pressure drop changes
Capture velocity is not a fixed property of an installed system — it changes over time as filter loading increases, duct surfaces accumulate deposits, and fan performance drifts. A system that passed startup verification will not maintain that performance without a monitoring framework that detects and responds to the changes that cause airflow to decrease.
Filter clogging is the most common cause of progressive airflow reduction in dry dust collection systems. As a cartridge or bag filter loads, its resistance increases, and the fan — operating on a fixed curve — responds by moving less air. The capture velocity at the hood drops in proportion, often gradually enough that the change is not noticeable without measurement. Monitoring differential pressure across the filter stage provides a leading indicator: a rising pressure drop signals increasing resistance before the reduction in capture velocity becomes significant. Establishing a defined threshold — for example, a pressure drop increase of a specified percentage above clean-filter baseline — and treating it as a filter service trigger keeps the system operating within its design airflow band.
Pressure drop changes can also signal issues other than filter loading. A sudden drop in differential pressure may indicate a filter failure or seal bypass rather than a clean filter — airflow appears to recover while filtration efficiency collapses. A pressure drop increase concentrated in a duct branch rather than at the filter stage may indicate a partial blockage from settled dust, which reduces both transport velocity and capture velocity at that branch’s stations. Treating pressure drop monitoring as a single-cause diagnostic — “high pressure drop means change the filter” — misses these compound indicators. The maintenance trigger should initiate a brief diagnostic check before defaulting to filter replacement, particularly when the pressure change pattern is inconsistent with normal filter loading curves.
For facilities running wet stone grinding processes, where the dust collector handles both dry dust and mist or slurry carryover, filter loading behavior can differ significantly from dry-only installations, and the pressure drop trigger thresholds may need to be recalibrated based on actual operational experience rather than transferred from a dry-dust reference installation.
Put airflow assumptions into the quotation
A quotation that specifies only collector CFM and filter rating leaves the buyer without any contractual basis for demanding correction when the installed system underperforms at the hood face. The gap between rated CFM and delivered capture velocity is not a vendor fabrication — it reflects real losses from duct friction, filter resistance, and source distance — but if those losses are not documented in the quotation, there is no agreed reference point for measuring whether the delivered system matches what was sold.
Three categories of assumptions carry the highest risk when excluded.
| Assumption Item | Description | Risk if Excluded from Quotation |
|---|---|---|
| Duct losses | Friction loss from duct length, bends, transitions | System may fail to deliver specified capture velocity, leading to costly rework |
| Résistance du filtre | Pressure drop of HEPA filter (99.97% at 0.3 μm) | Actual airflow at hood lower than design; capture velocity insufficient |
| Workstation distance & capture opening | Distance from hood to dust source, opening area | Capture zone may not reach source; capture velocity falls below standard |
The underlying procurement principle is straightforward: a quotation that states rated CFM without also stating the static pressure against which that airflow is delivered, the filter resistance assumed at service condition, the duct transport velocity maintained at design flow, and the capture velocity at a defined hood-face distance cannot be evaluated as a performance commitment. It can only be evaluated as a component specification. Those are different things, and the difference surfaces at commissioning rather than at purchase order.
OSHA 1910.94, as a process-reference point, reflects the regulatory intent that local exhaust ventilation must achieve effective capture at the source — not simply move a volume of air through the system. That intent requires the quotation assumptions to support a defined capture velocity at a defined location, not just a fan rating at a test point. Buyers who treat these as post-order details to be worked out during installation are accepting a procurement risk that is avoidable at the specification stage.
The practical requirement is to ask, before issuing a purchase order, that the quotation explicitly state: the design capture velocity at the hood face and the distance at which it is measured; the total static pressure budget including filter resistance at a defined loading condition; the duct transport velocity at design flow; and the simultaneous operating demand assumption for multi-station systems. A supplier who cannot or will not document these assumptions is effectively declining to commit to delivered performance — which is a different kind of information, and useful to have before the order is placed rather than after the system is installed.
The practical implication across all of these sections is that the specification document, not the collector’s nameplate, is where capture performance is either secured or left open. Rated CFM gives a starting point for evaluating whether a system is in the right size range; it does not confirm that capture velocity at the hood face will meet the design target once duct losses, filter resistance, cross-drafts, and simultaneous demand are accounted for. Those factors must be resolved at the design and quotation stage, because the cost of resolving them after installation — duct redesign, fan replacement, re-commissioning — is substantially higher than the cost of requiring the supplier to document them upfront.
Before accepting a quotation for a stone grinding dust control system, confirm that the following are explicitly stated: the assumed capture velocity at a defined hood-face distance, the static pressure budget at a defined filter loading condition, the duct transport velocity at design flow, and the simultaneous station demand assumption if more than one station shares the system. Any one of these left unspecified is a gap that the buyer will be asked to close later, typically on the supplier’s terms rather than their own.
Questions fréquemment posées
Q: What if the grinding work regularly extends beyond the downdraft table surface — does the capture velocity specification still hold?
A: No, it stops being valid at that point. Capture velocity is calculated for a fixed source distance, and once the active grinding contact moves beyond the table perimeter, the distance to the hood face increases and the available velocity at that location drops — often below the design threshold. Workpiece dimensions and the maximum likely grinding position should be defined before specifying capture velocity, not assumed to match the table footprint. If oversized workpieces are routine, either the hood geometry or the airflow target needs to be recalculated for the actual source distance, or a supplemental capture device positioned closer to the source should be specified.
Q: After startup verification confirms adequate capture velocity, what is the right interval for re-measuring it in service?
A: Re-measurement should be triggered by pressure drop change, not by a fixed calendar interval. Because filter loading is the primary driver of airflow reduction, differential pressure across the filter stage is a more reliable trigger than elapsed time — it reflects actual system resistance rather than assumed use patterns. Establish a baseline pressure drop at commissioning, define a percentage increase that initiates a service check, and include a hood-face velocity re-measurement as part of that check rather than as a separate scheduled activity. For wet-process grinding stations where slurry carryover affects filter loading unpredictably, the threshold may need to be tightened relative to a dry-process reference.
Q: Is a centralized multi-station collector more or less reliable than individual collectors at each grinding station?
A: Neither is inherently more reliable — the deciding factor is whether the duct network is designed to match whichever approach is chosen. A centralized system concentrates fan and filter maintenance at one point, which can be an advantage, but it requires the branch duct network to be balanced for simultaneous operation from the start; if it is not, distant stations will consistently underperform regardless of the collector’s aggregate rating. Individual collectors eliminate the branch-balancing problem but multiply the number of filter maintenance points and introduce the risk that portable units are moved or bypassed under production pressure. The more relevant question is whether simultaneous operating demand has been treated as the design baseline, because that assumption governs performance in both configurations.
Q: At what point does adding more duct length or bends make a system impractical to specify as a single unit?
A: When the cumulative static pressure losses from duct length, bends, and transitions consume a large enough share of the fan’s available static pressure that the remainder cannot simultaneously satisfy both the minimum transport velocity of ~20 m/s in the duct and the required capture velocity at the hood face, the layout has exceeded what a single fan-collector unit can cover without upsizing to a larger fan curve or splitting into separate systems. There is no universal distance threshold — it depends on duct diameter, number of fittings, and filter resistance — but the calculation becomes determinative rather than advisory at that point. If a proposed layout requires the fan to operate at the far edge of its curve to meet transport velocity alone, there is no margin left for filter loading or simultaneous demand, and that is a design boundary rather than a sizing adjustment.
Q: If a supplier provides all the specified documentation — capture velocity, static pressure budget, duct transport velocity, simultaneous demand assumption — what is the right way to confirm those figures are achievable before committing to the order?
A: Request that the supplier’s stated values be tied to a verifiable fan curve, not to nominal ratings. A fan curve plots airflow against static pressure across the operating range, and the design operating point — the specific combination of airflow and static pressure that the system must deliver — should fall clearly within that curve at a static pressure that includes both the duct resistance and the filter resistance at a defined loading condition. If the design point sits at or near the edge of the curve, there is no operating margin for filter loading, simultaneous demand variation, or minor duct deviations. A design point positioned in the middle third of the fan curve indicates that the system has been sized with realistic margin, and that condition is worth specifying as an acceptance criterion before the purchase order is issued.
