Ann. occup. Hyg., Vol. 42, No. 7, pp. 423-436, 1998
© 1998 British Occupational Hygiene Society
Published by Oxford University Press
research-article |
Measurement of the Performance of Free-standing Filtering Extraction Units used in the Woodworknig Industry1
Health and safety Laboratory Broad Lane, Sheffield S3 7HQ, U.K.
*Author to whom correspondence should be addressed. Tel.: 0114 289 2505; Fax: 0014 289 2500; E-mail: richardcfiltbrown{at}risl.gov.uk
The filtration efficiency of free-standing dost extractors fitted with filter bags of various types and challenged with wood dust of various grades has been measured. During each test, the air pressure inside the filter bag, the extractor air flow rate and the concentration of dust penetrating the filter bag were monitored. Needlefelt filter bags allowed a limited quantity of dust, usually at a concentration well below the maximum exposure limit (MEL) of 5mgm–3, to penetrate when they were clean and improved greatly as they became loaded with dust. Filter bags made from woven cotton let more dust through. Needlefelt bags with specially coated fibres were no more efficient than standard bags but those with a microporous membrane coating were highly efficient. However, an increase in filtration efficiency during loading was invariably accompanied by an increase in air pressure inside the filter bag and a drop in extractor air flow, all effects being more marked when fine dust was used. Some of the extractors leaked dust from around loosely fitting filter bags and dust collector bags, or through holes in the fan housing, indicating that not all aspects of design are ideal. Tests carried out on bags that had been used for long periods indicated that air entering the filter bag gives rise to an initial high external dust concentration, though this did not usually persist. Periodic cleaning of the bag is recommended. Vacuum cleaning the inside when the bag is removed from the cleaner almost restores its original condition, but results in the generation of large quantities of airborne dust. The use of a tool that fits inside the bag enables cleaning to be carried out effectively in situ without exposing the operator to significant concentrations of airborne dust. Crown Copyright © 1998 Published by Elsevier Science Ltd on behalf of British Occupational Hygiene Society.