Principles of local exhaust design.

Local exhaust ventilation systems are normally the most cost efficient method for controlling air pollutants and excessive heat. For many manual operations, capturing pollutants at or near their source is the only way to insure compliance with threshold limit values in the workers breathing zone. Local exhaust ventilation optimize ventilation airflow thus optimizing system costs especially where recirculation is not used. In some industrial ventilation designs, the main emphasis has been made on filtration of air captured by local exhausts prior to evacuating it outside the building or returning back to the production space (Chambers, 1993). As a result, these systems are evaluated by the efficiency of their filters. However, if only a small percentage of the emission is captured, the degree of separation efficiency becomes almost irrelevant. The pollutant capturing efficiency of local ventilation systems depends on hood design, its positioning near the source of the contamination, and the exhaust air flow. The appropriate selection and layout of hoods has a significant influence on initial and operating costs of both local and general ventilation systems. This paper suggests a classification of typically used local exhaust systems, and discusses the principles of their design and application.