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Dynamic leakage from laboratory safety hoods
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The purpose of this study was to quantitatively evaluate hood leakage by measuring face velocity and turbulence during a volume generating process designed to simulate a hot process, defined here as any operation producing high temperature gases. A hot process has been recognized as a causal factor in the leakage of contaminants from laboratory fume hoods since 1950. A literature search reveals that during the last couple of decades only Johnson et al. reported a quantitative linear relationship between thermal loading and breathing zone trace gas concentrations using the ASHRAE 110-1995 method. Hot processes may well be the most common and least recognized of the operational factors able to cause fume hoods to leak. For hood performance testing, I conducted smoke tests and face velocity tests. Smoke tests were executed by means of smoke tubes and smoke matches as screening tools for hood leakage. Face velocity tests were conducted at 16 points arranged to represent equal areas of the hood face when the sash was fully opened. Face velocity data were indexed in time and space and used to estimate turbulence. By assuming corresponding samples were collected simultaneously, turbulence parameters were computed as both spatial and temporal averages of the velocity pressure. Turbulence is represented by the ratio of the standard deviation to average face velocity at each measurement point. Turbulence is generated by pressure differences occurring between the velocity pressure at the hood face and the volume generating rate that simulates a hot process. Through the average face velocity and turbulence measurements, I found that at a fixed exhaust flow, with a fixed injection volume flow, turbulence is stronger in the open sash position than in the reduced sash position. Further, turbulence associated with a volume generating process is more evident in the space-based data than in the time-based data. Face velocity at each point tends to decrease in a hood when the flow injected by a volume generating process increases. These data suggest that when a hood is operated with a volume generating process, leakage can be minimized by reducing the sash opening, by not positioning any object within six inches of the hood face, and by keeping face velocities stable.
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Includes bibliographical references (leaves 43-45).
Issued also on microfiche from Lange Micrographics.
Park, Ju-Myon (2002). Dynamic leakage from laboratory safety hoods. Master's thesis, Texas A&M University. Available electronically from
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