Health and Safety Executive

LEV: General principles of system design

Introduction

This Information Sheet is one of a series produced by the HSE's Woodworking National Interest Group. The advice given is directed at woodworking machine designers and the designers, installers and users of local exhaust ventilation systems (LEV).

Nature of the problem

When considering methods of control, it is important to consider the nature of the dust and the forces that influence its movement. Woodworking produces large quantities of both fine, respirable dust and non-respirable particles. Figures 1(a), (b) and (c) show the typical patterns of dust movement from woodworking machines.

sawing and cutting

Figure 1(a) Sawing and cutting

drum sanding and abrading operations

Figure 1(b) Drum sanding and abrading operations

belt pad sanding

Figure 1(c) Belt (pad) sanding

Course dust is carried by its own momentum and usually collects around the point of generation. Respirable dust quickly loses its momentum and is carried away in air currents making capture more difficult. The movement of machine parts such as cutters, revolving drums, driving pulleys, belts etc will create considerable air movements and an efficient extraction system should be designed to use these air movements to maximum advantage.

Principles of design

Poor workshop layout leads to excessive pipe lengths and numerous bends. This decreases the efficiency of the system and may lead to increased noise levels at some workstations. Ad hoc extensions to extraction systems should be avoided. No additional extraction points should be added unless the system is known to have the necessary extra capacity.

Points to consider in the design of exhaust ventilation systems for woodworking machines are:1,2

  1. All moving parts of machinery should be enclosed as far as possible in order to minimise stray air currents.
  2. Captor hoods should be as close as possible to the point of dust generation and positioned to capture the dust laden air-stream. The greater the degree of enclosure of the source, the better will be the control. The addition of flanges around the exhaust inlet will considerably improve capture efficiency.
  3. Hoods and their associated connections should be designed so that they take advantage of, rather than work against, the natural air movement created by the machine (see Figure 2).
  4. Turbulent airflow creates noise and reduces extraction efficiency. When the direction of airflow caused by the tool is different from that caused by the extraction system, the result can be increased turbulence in the airflow, loss of extraction efficiency and increased noise. The use of extraction guide vanes will reduce turbulence (see Figure 3).

  5. exhaust hood design

    Figure 2 Exhaust hood design

    surface plaining machine

    Figure 3 Surface planing machine

  6. Junctions in the system should be at small angles and internal flanges should not be used. Bends in the system should have large radii.
  7. The minimum air velocity necessary to keep dust and chippings airborne in ductwork etc is 20 m/s. The minimum number of hoods or inlets necessary to maintain this transport velocity through the system should be identified and steps should be taken to ensure that at least this number are kept open when the system is in use.
  8. The capture velocity necessary to draw the dust towards the exhaust inlet is measured at the source of the dust. Conversion from capture velocity to hood face velocity depends on hood design and its distance from the source. For most woodworking machines a capture velocity of 5 m/s will be adequate, but capture velocity should reflect the air flow conditions at the point of capture and velocities of up to 10 m/s may be needed under particularly arduous conditions.
  9. Any LEV system which has more than one inlet will need to be balanced so that each branch extracts just the right amount of air through the inlet it serves. The airflow in each branch will be determined by the resistance of the inlet, the length, diameter and flow resistance of the branch duct and the flow conditions at the junction with the main duct.

Air movers and collection units

When selecting an air mover or fan, factors which will need to be considered will include the total airflow required, the overall resistance of the system, expected noise levels and limitations of space. Fans and air movers should ideally be sited outside the workroom and should be regularly maintained in order to reduce their impact on workplace noise.

Where possible, air cleaners and collection units should be situated in the open air away from main thoroughfares or places regularly frequented by people. Wood dust from most machining processes will contain an explosible fraction and the siting and design of air cleaners and dust collectors must take this into account. Where necessary, units should be enclosed and fitted with adequate explosion relief which will vent to a safe place should an explosion take place.

Commissioning, inspection and maintenance

Commissioning is the formal procedure used to bring the LEV system into use after it has been installed. The system should be proved to be capable of meeting its design specification. Appropriate details of airflow velocities and pressures should then be recorded to provide standard performance data for future reference.

Once installed, LEV should be regularly checked for leaks or blockages. Leaks and deficiencies in extraction systems can be found using a dust lamp.3 LEV must be thoroughly examined and tested at least once every 14 months by a competent person.1

The efficiency of the system will also be affected by the condition of the filter in the collection system. Filters will need to be shaken several times during the working day and will need to be thoroughly cleaned at regular intervals according to the manufacturer's instructions.

Reading list and references

  1. The maintenance, examination and testing of local exhaust ventilation HSG54 HSE Books 1990 ISBN 0 11 885438 0
  2. An introduction to local exhaust ventilation HSG37 HSE Books 1993 ISBN 0 7176 1001 2
  3. Assessment and control of wood dust: Use of the dust lamp WIS12 HSE Books 1991
  4. Safe collection of wood waste: Prevention of fire and explosion WIS32 HSE Books 1997

The future availability and accuracy of the references listed in this publication cannot be guaranteed.

Further information

HSE priced and free publications are available by mail order from HSE Books, PO Box 1999, Sudbury, Suffolk CO10 6FS Tel: 01787 881165 Fax: 01787 313995

For other enquiries ring HSE's InfoLine Tel: 0845 345 0055 or write to HSE's Information Centre, Broad Lane, Sheffield S3 7HQ.

This leaflet contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do.

This publication may be freely reproduced, except for advertising, endorsement or commercial purposes. The information it contains is current at 10/92. Please acknowledge the source as HSE.

Printed and published by the Health and Safety Executive 10/97 WIS23 C50

Quick links

Directgov - Business Link

03.03.09