Fume Hood Face Velocity

Plutonium solutions that have a low activity (<3.7 x 10 Bq (1 mCi) or 10 mg of Pu) and that do not produce aerosols can be handled safely by a trained radiochemist in a laboratory fume hood with face velocity 125—150 linear feet per minute (38—45 m/min). Larger amounts of solutions, solutions that may produce aerosols, and plutonium compounds that are not air-sensitive are handled in glove boxes that ate maintained at a slight negative pressure, ca 0.1 kPa (0.001 atm, more precisely measured as 1.0—1.2 cm (0.35—0.50 in.) differential pressure on a water column) with respect to the surrounding laboratory pressure (176,179—181). This air is exhausted through high efficiency particulate (HEPA) filters. 

For some hood types, measurements usually seen as indirect method, are used to measure the hood s performance to determine regulatory compliance. For example, regulations specify minimum and maximum face velocities for laboratory fume hoods and static pressure (negative) inside enclosed hoods. Continuously monitoring instruments can be connected to alarms that sound when the measurement is outside the specified limits. 

Face velocity The average velocity across an opening or item of equipment, such as a hood, fume cupboard, heating or cooling coil, or filter. 

Older fume hoods were anything but efficient in many cases. One laboratory had three hoods but only one worked well, and workers sometimes had to wait for it to become free. Even the good hood was never checked for face velocity. It just operated better than the others. The management showed little interest in repairing or checking the hoods as long as they caused no serious work delays. 

Fume hoods must be of a type suitable for the service they are intended to perform. For many applications, minimum face velocity is specified by regulations. An installer should always check the velocity when a new hood is placed in operation. It should be rechecked whenever any modification is made to the exhaust system. It is up to the laboratory operator to make certain that a hood is not put to new uses for which it was not designed. 

Experiments were performed to determine the rate at which the halogenated ethanes and the PCB used in this study permeated various protective garment materials. Discrete, rather than continuous, sampling was employed for these studies and because of the hazardous nature of the compounds employed, the experiments were performed in a chemical fume hood having a face velocity >125 linear ft./min. 

Capture devices Fume hoods and other open-face containment devices are not permitted for production and pilot plant scale (e.g., greater than 5 kg powder or 100 L liquid) Fume hoods and other open-face containment devices are to be considered for kilo-scale (e.g., less than production/ pilot plant-scale and greater than 1 kg powder or 22 L liquid) Fume hoods are acceptable with a face velocities of 80-120 fpm (0.4-0.5 m/s), or other containment measures (e.g., snorkel). Weighing hoods should maintain manufacturer-recommended face velocity for small scale (e.g., less than kilo-scale)... 

Fume hood fulfilling the present internationally recognized requirements set for fume hoods, such as DIN, OSHA (US) and BS standards. Normal linear airflow velocity at the face should be over 0.5 m/s (measured in compliance with BSI DD80 or equivalent) with approx, size 90 x 90 x 90 cm. 

The laboratory should have convenient access to one or more fume hoods (with a face velocity of at least 100 ft/min) for any operations involving more than insignificant quantities of volatile chemicals in open containers. 

Chemical Fume Hood. The chemical fume hood should have an average linear face velocity of 100 linear feet per minute (Ifpm). The window sash height that gives this measured inflow should be marked on the edge wall of the cabinet. The inflow velocity to a fume hood with the sash fully opened should be 85 Ifpm or more. Figure 7. 

The laboratory shall be equipped with a fume hood. The fume hood should meet any specific safety requirements mandated by the nature of the research program. A discussion of hood design parameters will be found in a later section, but for high hazard use the interior of the hood and the exhaust duct should be chosen for maximum resistance to the reagents used the blower should either be explosion-proof or, as a minimum, have non-sparking fan blades the hood should be equipped with a velocity sensor and alarm should the face velocity fall below a safe limit the interior hghts should be explosion-proof, and all electrical outlets and controls should be external to the unit. It may be desirable to equip the unit with an internal automatic fire suppression system. 

A major factor is the actual presence of a worker standing in front of the hood. A person standing in front of a hood represents a significant barrier to the free flow of air into the hood. This can not only create turbulence in the air flow on either side of the worker but will tend to create a low pressure zone directly in front of them. As the face velocity is increased, this property becomes more pronounced and may result in fumes from within the hood being drawn back toward the worker, perhaps into the workers breathing zone if the sash is operated fully open. Lowering the sash below the workers face, to the 18 inch level or using a horizontal sash hood where the worker routinely stands behind a section of the sash would obviate this problem. 

Zboralski, J., The Effects of Face Velocity on Fume Hood Containment Levels, Technical Paper No. 90.01, Hamilton Industries Inc. Infobank, Two Rivers, WI, May 1995. 

The guidelines governing the most favorable location of laminar flow cabinets are essentially the same as for a chemical fume hood. Place them in the far end of a laboratory, in a low traffic area, and where there are no drafts. The environment of the hood should be checked to ensure that air speeds in the neighborhood of the cabinet opening are small compared to the face velocities of the cabinet. 

Tracer gas containment testing of fume hoods has revealed that air currents impinging on the face of a hood at a velocity exceeding 30 to 50% of the hood face velocity will reduce the containment efficiency of the hood by causing turbulence and interfering with the laminar flow of the air entering the hood. Thirty to fifty percent of a hood face velocity of 100 fpm, for example, is 30 to 50 fpm, which represents a very low velocity that can be produced in many ways. The rate of 20 fpm is considered to be still air because that is the velocity at which most people first begin to sense air movement. 

Most people walk at a velocity of approximately 250 fpm (about 3 miles per hour). Wakes or vortices form behind a person who is walking, and velocities in those vortices exceed 250 fpm. When a person walks in front of an open fume hood, the vortices can overcome the fume hood face velocity and pull contaminants out of the fume hood, into the vortex, and into the laboratory. Therefore, fume hoods should not be located on heavily traveled aisles, and those that are should be kept closed when not in use. Foot traffic near these hoods should be avoided, or special care should be taken. 

Air is supplied continuously to laboratories to replace the air exhausted from the fume hoods and other exhaust sources and to provide ventilation and temperature/humidity control. This air usually enters the laboratory through devices called supply air diffusers located in the ceiling. Velocities that can exceed 800 fpm are frequently encountered at the face of these diffusers. If air currents from these diffusers reach the face of a fume hood before they decay to 30 to 50% of the hood face velocity, they can cause the same effect as air currents produced by a person walking in front of the hood. Normally, the effect is not quite as pronounced as the traffic effect, but it occurs constantly, whereas the traffic effect is transient. Relocating the diffuser, replacing it with another type, or rebalancing the diffuser air volumes in the laboratory can alleviate this problem. 

For hoods without face velocity controls (see section 8.C.6.3.2), the sash should be positioned to produce the recommended face velocity, which often occurs only over a limited range of sash positions. This range should be determined and marked during fume hood testing. For hoods with face velocity controls, it is impraative to keep the sash closed when the hood is not in use. 

A constant air volume (CAV) fume hood draws a constant exhaust volume through the hood regardless of sash position. Because the volume is constant, the face velocity varies inversely with the sash position. The fume hood volume should be adjusted to achieve the proper face velocity at the desired working height of the sash, and then the hood should be operated at this height. (See section 8.C.4.)... 

Variable air volume (VAV) laboratories are rapidly replacing traditional CAV laboratories as the design standard. These systems are based on fume hoods with face velocity controls. As the users operate the fume hoods, the exhaust volume from the laboratory changes and the supply air volume must adapt to maintain a volume balance and room pressure control. An experienced laboratory ventilation engineer must be consulted to design these systems, because the systems and controls are complex and must be designed, sized, and matched so they operate effectively together. 

Fume hood maintenance involves periodical (daily, quarterly, annual) cleaning, maintenance, calibration, qualification and inspections. In daily inspection the fume hood area is visually inspected by the operator. Hood function indicating devices (LEDs) are a part of the modem fume cupboard. Periodic inspection covers capture or face velocity measured with a velometer or anemometer. 

A captor hood must generate sufficient capture velocity at the point of contaminant generation. A receptor hood must have sufficient face velocity (velocities vary from 0.5 m/s, e.g. laboratory fume hood, to lOm/s, e.g. disc grinder). 

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