Technology Report February, 2004 Fume Hood Face Velocity Can it Ensure Safe Containment? This paper examines the controversy that sometimes arises regarding whether fume hood face velocity is indicative of safe fume containment. Face Velocity Requirements For many years, fume hood manufacturers, laboratory safety standards 1 and safety professionals have maintained that a face velocity of about 100 fpm was generally required for adequate fume containment and thus safe fume hood use. The California State Occupational Safety and Health Administration (CAL-OSHA) mandated 100 fpm as the minimum average face velocity with a minimum of 70 fpm at any point in the sash opening. The U.S. Federal OSHA is less stringent and recommends a face velocity from 60 through 100 fpm. In short, there are many references as to what a fume hood s face velocity should be. More recently, many experienced and renowned fume hood safety professionals have stated that having the recommended fume hood face velocity is no guarantee that a given fume hood will provide adequate fume containment and user safety. And, newer editions of laboratory safety standards contain extensive warnings against using face velocity as the sole criteria for safe fume hood operation. 2 Individuals who have wide-ranging experience in fume hood containment testing, especially with the ASHRAE 110 tracer gas test, will frequently cite their own test results as evidence 1. American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5, 3.3.1: Design face velocities for laboratory chemical hoods in the range of 80100 fpm (0.410.51m/s) will provide adequate face velocity for a majority of chemical hoods. 2. American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5, 3.3.1: In one published study, approximately 17% of the hoods tested using the method (ASHRAE 110 test method) had "acceptable" face velocities in the range of 80120 fpm, but failed the tracer gas containment and exceeded the ACGIH recommended control level of 0.1 ppm. (Smith and Crooks, 1996). against equating adequate face velocity with adequate fume containment. The Designer s Dilemma If fume hood face velocity cannot be used as a criterion for safety, how can a laboratory facility ventilation system be designed to ensure a safe work environment for laboratory users? Experienced designers know that a laboratory ventilation system design must accommodate the room and fume hood airflow requirements. Designers often say that a laboratory room s ventilation requirements are primarily driven by the air consumption of the room s fume hoods. Since fume hood air consumption is directly related to face velocity, it follows that the laboratory ventilation system design is then driven by the face velocity of the fume hoods. Face Velocity Controversy If a fume hood manufacturer has determined what level of face velocity provides good containment as a result of properly conducted ASHRAE 110 tracer gas containment tests, why can t that face velocity be indicative of proper fume containment? The answer is that while a fume hood manufacturer may have conscientiously performed ASHRAE 110 tests, the test conditions will invariably not be the same as the actual laboratory room conditions where the fume hood is ultimately used. These include: Siemens Industry, Inc. Page 1 of 6 Room Airflow Laboratory rooms are subject to high makeup airflow rates, especially if the room has multiple fume hoods. And, high room airflow rates can often result in appreciable air currents within the rooms. Research has established that room air currents (termed cross currents) can be very detrimental to effective fume hood containment. Even relatively low air currents passing in front of a fume hood with an open sash, can draw fumes out from the hood interior. Thus, horizontal, vertical, or even angular room air currents all pose the potential for
compromising effective fume hood containment. Figure 1 illustrates this phenomenon. and positioned in a way that adversely affects the internal airflow pattern that is necessary for optimum fume containment. Experiments or chemical processes may give off substantially greater amounts of fumes than the quantity of tracer gas used in the ASHRAE 110 test. 5 Also, the actual chemical fumes generated may be of much different buoyancy than sulfur hexafluoride tracer gas and, therefore, behave differently. Heaters and electrical equipment within a fume hood s interior generate convection air currents. All of these factors either individually or combined usually have an adverse effect on fume containment especially in comparison to a nearly empty fume hood 6 tested under optimum room conditions. Face Velocity Cause and Effect Figure 1. Room Air Currents Can Draw Fumes Out from Fume Hood Interior when Sash Open. ANSI/AIHA Z9.5 states that room air cross currents should be less than one-half and preferably less than one-third of the fume hood face velocity. 3 NFPA 45 also states that room air currents should ideally be less than 30% of the fume hood face velocity. 4 Some laboratory rooms have many fume hoods and, therefore, require a correspondingly large supply makeup airflow. This can result in unavoidable room cross currents that are substantially higher than recommended by the aforementioned standards. Fume Hood Usage Manufacturers normally test their fume hoods without chemicals or laboratory equipment inside the hood (except for the tracer gas ejector). In actual use, a fume hood is likely to contain all sorts of apparatus and equipment, including: support racks, beakers, hoses, heaters, chemical containers, analyzers, etc. Some items may be quite large 3. American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5, 5.2.2: Supply air distribution shall be designed to keep air jet velocities less than half, and preferably less than one-third of the capture velocity or the face velocity of the laboratory chemical hoods at their face opening. 4. National Fire Protection Association, Standard NFPA 45: A- 6-3.5: Room air current velocities in the vicinity of fume hoods should be as low as possible, ideally less than 30% of the face velocity of the fume hood. Page 2 of 6 Let s consider how fume hood face velocity relates to fume hood containment. It should be noted that what primarily keeps fumes within the interior of a fume hood is the pressure difference that exists between the fume hood interior and exterior (the laboratory room). The laws of physics do not allow fumes or air to flow from an area of lower static pressure to an area at a higher static pressure. Thus by applying a constant exhaust to the fume hood, an area of lower pressure is created within the fume hood interior and this establishes the basis for fume containment. As a result of the pressure difference between the room and the fume hood interior, room air flows into the fume hood. The greater the pressure difference the greater the inward airflow face velocity will be. Since fume hood face velocity is more easily detected and measured than the small pressure difference 7 between the room and fume hood interior, face velocity has become, by default, a means to quantify the existence of a pressure difference. It is also important to note that using face velocity as a means to quantify the hood pressure differential requires that the face velocity measurement be the average face velocity. A fume hood s face velocity, like that of any air current, will vary throughout its cross section with the 5. The ASHRAE 110 tracer gas test requires a release rate of 4.0 liters per minute of sulfur hexafluoride. 6. The ASHRAE 110 test tracer gas ejector occupies far less space than the equipment and apparatus typically found in actual laboratory fume hoods. 7. A face velocity of 100 fpm results from a pressure difference of just 0.000623 in. WC. Siemens Industry, Inc.
highest velocity occurring near the center. The lowest air velocity will normally occur at the periphery (outer boundary) of an air current. 8 Obtaining the average fume hood face velocity requires that a measurement traverse be made in the plane of the sash opening. An alternate procedure consists of mathematically calculating average face velocity based upon a measurement of the total fume hood exhaust rate and the total fume hood open area. 9 Challenges to Containment As stated, the laws of physics do not allow fumes to flow from an area of lower static pressure to an area of higher static pressure. Why then can fumes flow out from a fume hood interior that is (presumably) at a lower static pressure as evidenced by a proper face velocity? Note that room air currents (like any airflow) have a lower static pressure than room air that is essentially at rest. And, as a room air current passes an open sash, the pressure difference between the fume hood interior and the static pressure at the periphery of the air current can be considerably less than the pressure difference between the fume hood interior and room air that is essentially at rest. Figure 2 illustrates the relationship between the static pressure (lower value) and the total pressure (higher value) of an air current. The higher value of total pressure is always in the direction of travel while the lower static pressure is always perpendicular to the direction of travel. As the velocity of an air current increases even more, its total pressure also increases while its static pressure becomes even lower. Thus, as higher velocity air currents pass crosswise to an open fume hood sash, the reduced static pressure difference between the fume hood interior and the periphery of the air current creates a tendency for fumes to flow out from the hood interior. This is especially true if air currents within the fume hood travel toward the sash opening, 10 since they then have a higher total pressure in the direction of travel. To overcome the effect of room air currents, the static pressure difference between the fume hood interior and the room must usually be increased by increasing the fume hood exhaust airflow. Increasing the exhaust airflow will be manifested by a higher inward airflow and thus a higher average face velocity. In other words, the face velocity must often be increased to overcome the effects of higher velocity room air currents. TOTAL PRESSURE (HIGHER) T AIR CURRENT STATIC PRESSURE (LOWER) S Figure 2. Static Pressure is Lower for Air in Motion. 8. A traverse consists of taking a series of individual air velocity measurement at defined locations throughout a crosssection of the full airflow area. The larger the airflow area the more individual measurements are required. 9. The total fume hood open area consists of the sash opening plus all additional openings, including the airfoil slot and the bypass opening. 10. Air currents or turbulence within a fume hood interior are caused by multiple factors including the geometry of the fume hood interior, local convection, fume generation, and even excessive face velocity. Siemens Industry, Inc. Page 3 of 6
Establishing the Proper Average Face Velocity Once a fume hood is installed and set up in an actual laboratory room, it is recommended that tracer gas containment tests 11 be conducted to determine the appropriate face velocity necessary to maintain a desirable level of containment. Once the average face velocity that will provide the desired containment level is determined, that face velocity can serve as a reasonable benchmark for safe fume hood operation. This assumes that the room conditions essentially remain as they were when the fume hood was tested and that the fume hood users follow safe fume hood working practices. Note that since a fume hood is not an airtight enclosure, perfect or 100% containment is not generally attainable or even necessary. The level of containment that is necessary should be established by facility personnel who are responsible for worker safety in consideration of the hazard level posed by the chemicals. If the chemicals or substances that will be present are so toxic or hazardous that 100% or near 100% containment is required, than an air tight glove box should be utilized rather than a fume hood. To qualify as part of a sample group the fume hoods must all be in the exact same location in each of the identical laboratory rooms. If a room has multiple fume hoods, each different fume hood location then establishes a separate sample group. (Thus, if a room has four fume hoods, each of the four fume hoods would be in one of four different sample groups.) All room ventilation parameters must be adjusted and balanced before the fume hood containment tests so there will be no appreciable airflow difference from room to room. This especially applies to the supply make-up air, room general exhaust, and any other room airflow elements such as specialized exhausts. If any room must be set up different than the other rooms, its fume hoods cannot be considered part of the other room s sample groups. It is recommended that 10% (with a minimum of 3) of the fume hoods from each sample group be individually tested. More fume hoods should be tested if the test results of a sample group are not appreciably similar. Perhaps a variation of more than 25% in test results would justify additional testing. Although it can be time consuming and costly to conduct fume hood containment tests, the actual quantity of individual tests may be reduced in consideration of the following factors: Fume hoods that are of the same size and type and in the exact same location in multiple identical laboratory rooms need not all be tested if the laboratory rooms have the same airflow characteristics. This would include ventilation system configuration, airflow quantities and airflow components including the air diffusers and diffuser locations. Thus, if a facility has many laboratory rooms that are duplicates of each other and the room airflows have been closely set to the same parameters or specifications, a sampling of fume hoods may be tested (termed a sample group) rather than testing each and every fume hood. The safety standards advise that periodic retesting to verify fume hood containment should still be done on an annual basis, or whenever changes are made to the ventilation system. Using the above sampling approach could reduce the time and expense of testing in new facilities. 12 For example, if a facility had 100 laboratory rooms and each room had two fume hoods of the same size and type, the containment testing could possibly be conducted on only two sample groups having 10 fume hoods per group. Then rather than 200 separate fume hood containment tests, perhaps only 20 fume hood tests would suffice. 11. ASHRAE 110 defines two types of on-site containment tests: AI (as installed) and AU (as used). The AI test conditions include the room parameters (air currents, etc.) while the AU tests also include the actual fume hood contents. AIHA Z9.5 lists ASHRAE 100 tracer gas pass criteria of no greater than 0.10 ppm for AI tests. 12. Due to the inevitable changes that occur over time in laboratory rooms and their ventilation systems the sample group fume hood testing approach is not always applicable in existing facilities. Page 4 of 6 Siemens Industry, Inc.
Designer Responsibility The ventilation system designer must ensure that the individual fume hood exhaust provision, as well as the supply make-up airflow for each laboratory room, can provide somewhat greater airflow if a fume hood s face velocity must be increased beyond what was originally anticipated. On average, a 10% safety factor over the design conditions 13 should usually suffice for this purpose. VAV Advantage VAV fume hoods will typically allow greater flexibility in establishing the necessary average face velocity for a particular fume hood due to the diversity factor associated with VAV systems. 14 In other words if a particular fume hood requires a higher than anticipated average face velocity, it can usually be attained without adding to the overall exhaust system capacity. Also, the fume hood working height for vertical sash fume hoods can usually be restricted to 18 inches 15 by implementing a sash stop. This reduces the sash opening to about 65% of the normal full open sash height. As a result, the average face velocity could then be increased by up to 35% if necessary, based upon the exhaust airflow rate for a fully open sash. Conclusion Most laboratory safety professionals hold the position that even if a fume hood s face velocity is within safety standard recommendations, it will not serve as a guarantee of safe fume containment. Although one cannot argue against this position, it is somewhat similar to saying that a good braking system will not serve as a guarantee of vehicle safety. As in many situations, ensuring total safety is a more complex task and requires addressing multiple issues. If the necessary fume hood average face velocities are determined by on-site fume hood containment testing, then the face velocity should serve as a benchmark for safe containment if other important aspects - particularly the room ventilation attributes - remain unchanged and safe work practices are followed. Periodic re-testing of fume hood containment on an annual basis is still advised. Ventilation system designers should allow some extra capacity in exhaust and supply makeup air systems in case individual fume hood exhaust airflows must be increased above a manufacturer s recommendation to attain the average face velocity required for acceptable fume containment. 13. Fume hood design airflow should be based upon either the owner s stated face velocity requirements or the fume hood manufacturer s as manufactured (AM) test data. 14. Fume hoods that have their sashes closed enable the exhaust air capacity (and the room makeup air) to be used where more airflow is needed. 15. A typical maximum fume hood vertical sash opening is at least 28 inches. Siemens Industry, Inc. Page 5 of 6
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