The Baker Company

Using thimbles to connect Biological Safety Cabinets to Variable Air Volume (VAV) Exhaust Systems

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Abstract
Variable-volume exhaust systems are a common feature of laboratory ventilation designs. By reducing exhaust airflow in proportion to the actual need, the system uses less energy to condition the make-up air. However, variable exhaust flows can disturb the critical pressure relationships between air flows in biological safety cabinets, which require a constant airflow to maintain performance. Changes in air pressures can allow contaminants to escape the cabinet, or allow lab contaminants to enter the cabinet.

To avoid these problems, BSC's are often loosely connected to a VAV exhaust system by means of a 'thimble', which is in effect an exhaust hood mounted over the cabinet exhaust duct. This hose connection reduces the magnitude of the change in airflow inside the BSC as exhaust air flows fluctuate, so BSC performance is maintained. This paper describes three research projects which investigated the use of thimbles connecting BSC's to VAV exhaust systems. The research results can be useful to designers and end users of laboratory ventilation systems.

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