Illustrative branch losses at their assigned design flows: the easier path needs 230 Pa of added resistance to match the 650 Pa path. Common downstream losses are additional.
1. Give every duct a flow and an inside size
Draw the hoods, branches, junctions, collector and fan before choosing diameters. At each junction, the downstream flow is the sum of the incoming branch flows for the operating case being considered. Record which machines run together; a network with all pickups open can behave differently from one serving a single machine.
Hood capture and duct transport are separate checks. A duct can carry its assigned volume while the hood fails to capture the source. Transport velocity also needs a material-specific assessment: a value suitable for one dust is not automatically suitable for another. HSE’s HSG258 guidance treats hood design, duct transport and commissioning as parts of the same LEV system.
2. Keep friction, fittings and equipment separate
The Darcy factor is four times the Fanning factor; identify the convention in any source or software. Friction depends on Reynolds number and wall roughness. Fitting coefficients depend on geometry and the velocity used as their reference, especially at branches and transitions. Do not copy a coefficient without its definition.
Add collector, filter, silencer and discharge losses separately using data at the actual duty. Evaluate the filter over its operating pressure range. Sum losses along complete source-to-discharge paths; summing every parallel branch would overstate the fan pressure requirement.
3. Worked example: one circular section
Assume Q = 1,800 m³/h = 0.50 m³/s, D = 0.20 m, L = 10 m, ρ = 1.20 kg/m³, Darcy f = 0.020 and a combined fitting coefficient of 1.5. These are illustrative inputs, not dust-transport recommendations.
| Quantity | Calculation | Result |
|---|---|---|
| Area | π × 0.20² / 4 | 0.0314 m² |
| Velocity | 0.50 / 0.0314 | 15.9 m/s |
| Velocity pressure | 1.20 × 15.9² / 2 | 152 Pa |
| Straight-duct loss | 0.020 × (10 / 0.20) × 152 | 152 Pa |
| Fitting loss | 1.5 × 152 | 228 Pa |
| Section loss | 152 + 228 | 380 Pa |
This is one section only. Hood entry, other sections, collector and discharge remain to be included. For fixed geometry and approximately constant f and K, doubling flow roughly quadruples pressure loss. Recalculate the gas properties and friction factor when conditions change.
4. Balance against the critical path
At the target flows, compare competing branch paths between common pressure boundaries. The path requiring the greatest pressure defines the preliminary requirement; lower-resistance paths may need additional resistance to hold their assigned flows. For example, branches requiring 650 Pa and 420 Pa between the same boundaries differ by 230 Pa at their specified duties.
A damper can add resistance to the easier path. It cannot supply missing pressure to the harder one. If available fan pressure is insufficient, throttling other branches alone will not establish the full design duty. Revisit geometry, target flows, equipment losses and fan selection together.
Know the calculator boundary
Indunect’s duct designer uses prescribed hood flows for a single-fan extraction tree. Its balancing result describes a design-duty resistance requirement. It does not solve the operating point against a manufacturer’s fan curve or establish contaminant capture at a hood.
5. Leave a measurable commissioning baseline
Record duct sizes, test locations, operating combinations, hood flows, pressure readings, damper settings, fan speed and filter condition. Verify capture under representative production conditions, then retain those measurements for later fault finding. A changed branch, blocked filter or open access door can alter the network after the original balance.
Continue with the complete dedusting design guide and the fan power guide to connect the duct calculation to the wider system.
Sources and scope
HSE HSG258: Controlling airborne contaminants at work provides practical LEV design, commissioning and maintenance guidance. Equations and the worked example here are introductory fluid-mechanics calculations with explicitly assumed inputs. They do not prescribe a safe dust transport velocity or replace measured commissioning and project-specific design.