A STUDY OF LABORATORY FUME HOOD PERFORMANCE AT LOW FLOW RATES

Abstract

Fume hood operation is a main contributor to large energy consumption by laboratories in tertiary institutions. Seeking to reduce energy expenses, the National University of Singapore, Faculty of Science would like operate fume hoods at a lower air velocity set point. Fume hoods operating at a low set point may lose their ability to contain and exhaust contaminants efficiently, thus, posing safety and health risks to the user. Therefore, the aim of the study is to determine the lowest conservative set point for which a fume hood can operate without affecting its containment performance. A variable air volume fume hood, located within a wet chemistry laboratory, was investigated for its performance at set points below current operating set point of 0.41 m/s. The set points chosen were 0.35 m/s, 0.31 m/s, 0.27 m/s and 0.23 m/s. Test protocols stated in ANSI/ASHRAE 110-1995 "Method of Testing Performance of Laboratory Fume Hoods.” were adopted and they include face velocity test, smoke visualization test and tracer gas containment test. Additional smoke visualization tests incorporating effects from hand movement and human traffic were performed. It was found that the average face velocity values corresponding to 0.35 m/s, 0.31 m/s and 0.27 m/s set point for all sash heights were above minimum threshold of 0.30 m/s as recommended by ANSI/AIHA Z9.5-2003 American National Standards for Laboratory Ventilation. The fume hood contained fumes more efficiently at a higher set point and at a lower sash height. Fume hood operating at 0.35 m/s set point showed no leakage during the ASHRAE 110 smoke visualization test. Only a momentary leakage was observed at 100% sash height for the human traffic test. At 0.31 m/s, leakages detected for tracer gas containment tests were significantly below the respective limits established by ANSI/AIHA Z9.5-2003. Fume hood operating at a higher set point of 0.35 m/s set point is expected to pass the tracer gas containment test and perform better. While 0.31 m/s set point provided acceptable performance based on the ASHRAE 110 standard test, it showed leakages at a higher frequency and at a lower sash height for the additional smoke visualization test. Therefore, 0.35 m/s is the conservative set point which will be recommended for fume hood operation during peak periods. During unoccupied periods, the sash may be lowered and the operational set point may be changed to 0.31 m/s. The reduction of set point from 0.41 m/s to 0.35 m/s and 0.31 m/s is expected to yield a 14 to 20 percent energy cost reduction while allowing the fume hood to maintain its containment performance.