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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
formance testing consisting of a face velocity analysis and flow visualization using smoke tubes, bombs, or fog generators should be performed annually. Laboratory workers should request a fume hood performance evaluation any time there is a change in any aspect of the ventilation system. Thus, changes in the total volume of supply air, changes in the locations of supply air diffusers, or the addition of other auxiliary local ventilation devices (e.g., more hoods, vented cabinets, and snorkels) all call for reevaluation of the performance of all hoods in the laboratory.
The ASHRAE/ANSI 110 test is the most practical way to determine fume hood capture efficiency quantitatively. The test includes several components, which may be used together or separately, including face velocity testing, flow visualization, face velocity controller response testing, and tracer gas containment testing. Performance should be evaluated against the design specifications for uniform airflow across the hood face as well as for the total exhaust air volume. Equally important is the evaluation of operator exposure. The first step in the evaluation of hood performance is the use of a smoke tube or similar device to determine that the hood is on and exhausting air. The second step is to measure the velocity of the airflow at the face of the hood. The third step is to determine the uniformity of air delivery to the hood face by making a series of face velocity measurements taken in a grid pattern.
Traditional hand-held instruments are subject to probe movement and positioning errors as well as reading errors owing to the optimistic bias of the investigator. Also, the traditional method yields only a snapshot of the velocity data, and no measure of variation over time is possible. To overcome this limitation, it is recommended that velocity data be taken while using a velocity transducer connected to a data acquisition system and read continuously by a computer for approximately 30 seconds at each traverse point. If the transducer is fixed in place, using a ring stand or similar apparatus, and is properly positioned and oriented, this method can overcome the errors and drawbacks associated with the traditional method. The variation in data for a traverse point can then be used as an indicator of turbulence, an important additional performance indicator that has been almost completely overlooked in the past. If the standard deviation of the average velocity profile at each point exceeds 20% of the mean, or the average standard deviation of velocities at each traverse point (turbulence) exceeds 15% of the mean face velocity, corrections should be made by adjusting the interior hood baffles and, if necessary, by altering the path of the supply air flowing into the room. Most laboratory hoods are equipped with a baffle that has movable slot openings at both the top and the bottom, which should be moved until the airflow is essentially uniform. Larger hoods may require additional slots in the baffle to achieve uniform airflow across the hood face. These adjustments should be made by an experienced laboratory ventilation engineer or technician using proper instrumentation.
The total volume of air exhausted by a hood is the sum of the face volume (average face velocity times face area of the hood) plus air leakage, which averages about 5 to 15% of the face volume. If the hood and the general ventilating system are properly designed, face velocities in the range of 80 to 100 fpm will provide a laminar flow of air over the work surface and sides of the hood. Higher face velocities (150 fpm or more), which exhaust the general laboratory air at a greater rate, both waste energy and are likely to degrade hood performance by creating air turbulence at the hood face and within the hood, causing vapors to spill out into the laboratory.
Because a substantial amount of energy is required to supply tempered supply air to even a small hood, the use of hoods to store bottles of toxic or corrosive chemicals is a very wasteful practice, which can also, as noted above, seriously impair the effectiveness of the hood as a local ventilation device. Thus, it is preferable to provide separate vented cabinets for the storage of toxic or corrosive chemicals. The amount of air exhausted by such cabinets is much less than that exhausted by a properly operating hood. (Also see section 8.C.4.)
Perhaps the most meaningful (but also the most time-consuming and expensive) method for evaluating hood performance is to measure worker exposure while the hood is being used for its intended purpose. By using commercial personal air-sampling devices that can be worn by the hood user, worker exposure (both excursion peak and time-weighted average) can be measured by using standard industrial hygiene techniques. The criterion for evaluating the hood should be the desired performance (i.e., does the hood contain vapors and gases at the desired worker-exposure level?). A sufficient number of measurements should be made to define a statistically significant maximum exposure based on worst-case operating conditions. Direct-reading instruments are available for determining the short-term concentration excursions that may occur in laboratory hood use.
8.C.6Fume Hood Design and Construction
When specifying a laboratory fume hood for use in a particular activity, the laboratory worker should be