Local Exhaust Ventilation (Dust/Fume Extraction Plant)

Health and safety requirements

The primary legislation regarding ventilation systems is the Health and Safety at Work Act 1974 and the Management of Health and Safety at Work Regulations 1999. Mechanical ventilation systems including Local Exhaust Ventilation (LEV) are also subject to the Control of Substances Hazardous to Health Regulations.

All LEV assessments are carried out in accordance with COSHH regulations 8, 9 & 10 and also following the guidance of HSG 258 (2017) LEV – Controlling Airborne Contaminants at Work.

Generally, reports are valid for 14 months from the date of the assessment unless specified in HSG 258 Table 18. All reports should be held on record for a minimum of 5 years and be available near the LEV system for which it is written.

Our LEV assessments contain

All LEV systems are subjected to a thorough assessment to determine the control likely to be achieved. This comprising of;

  • Visual inspection
  • Measurement of plant performance and an assessment of control
  • Assessment of air cleaner, filter performance

The systems were initially examined, and judgement made regarding the suitability of each system for the type of hazardous substance(s) controlled. Smoke release allowed likely air movement patterns to be visualised and any obvious defects were recorded.

Where access permitted, measurements of air flow at the likely point sources and across the face openings were taken as appropriate. Static pressure readings and transport velocities were determined in exhaust ducting, again, where access and safety considerations permitted.

The assessment relating to ventilation fulfils the requirements of the Control of Substances Hazardous to Health Regulations (2002) and HSE Guidance Note HSG 258 “Controlling airborne contaminants at work. A guide to Local Exhaust Ventilation (LEV)”.

Specialist equipment is used to measure and determine static pressure, transport velocity, volumetric flow and face velocities.

A detailed examination and test report (known as A TExT Report) will follow a HSE approved format and will include measurement test data, schematic diagrams of the LEV systems, an assessment on the control effectiveness of the system and any recommendations for remedial requirements detailed in a prioritised action plan.

Results and Discussion

The results of the testing are presented in the TExT report. Specific comments regarding the system conditions are given in the individual assessments. The system airflows are compared with the following recommended limits which are taken from HSG 258 guidance.

Capture Velocity

‘Capture velocity’ is the velocity required at a contaminant source to overcome the movement of the contaminant cloud and draw it into the hood. But this is meaningful only with a defined distance between the source and the hood. Fast-moving contaminant clouds are very difficult to control with a capturing hood. They normally require a partial enclosure or receiving hood. The capture velocities quoted in Table 9 (below) are based on success through experience. In practice the designer and supplier should check and, where necessary, make prototypes.

Dust Transport Velocity

The air velocity through the duct must be high enough to keep particles suspended in the air stream. It should also be high enough to suspend and remove particles that settle out when the system stops. The designer needs to avoid deposition in any part of the ductwork. This is a particular problem:

  • in long horizontal runs of ducting;
  • at low points;
  • at junctions where the duct diameter increases;
  • after junctions or bends;
  • when conveying large and small particles together, e.g. woodworking dusts.

Accumulation of settled particles reduces the diameter and shape of the duct, increases resistance and reduces the airflow in the system. Settled particles are difficult to re-entrain in the airflow and can lead to duct blockage and fire risk from flammable materials.

The required transport velocity depends on the type of contaminant being conveyed. Table 12 (below) recommends some minimum velocities.

Face Velocity

Face velocity is the velocity of air at the point of entry into the LEV system. It is measured across the face of spigots, hoods, booths, cabinets etc. The size, proportions, configuration of the inlet and the face velocity generate the capture velocity at the distance from the inlet force.

When measuring larger hoods or booths the air velocity should be measured at a series of positions across the face. The readings should not vary excessively.

Air volume

Air volume is calculated from the face velocity and hood face area or from the duct velocity and duct size.  The total system volume must be suitable for the rating of any contaminant collection device provided.

Static pressure

The static pressure differential compared with outside the duct is related to the airflow rate and system characteristics.

Visualisation of air flow

Smoke generating tubes are used to visualize airflows entering LEV systems and test for air leakage.

Visualisation of dust clouds

‘Tyndall illumination’ makes dust clouds visible. The ‘Tyndall effect’ is the forward scattering of light. This is commonly seen when a shaft of sunlight entering a building shines through mist, dust or fume in the air. The ‘dust lamp’ reproduces this effect by producing a powerful beam of light. It shows the density and movement of particle clouds in its path.