Exhaust hood enclosures

The characterization of the engineering controls that are relevant to the exposure to be monitored (i.e., local exhaust hoods, enclosures, glove boxes, lab hoods, HEPA vacuums, etc.)... 

In practice there are many different combinations, such as two exhaust hoods close to each other or two or more air curtains placed around a horizontal (or v ertical) source or a hood that is partly an exterior hood and partly an enclosure. 

The more enclosed a process is, the easier it is to keep a low concentration in the workroom. It is usually necessary for the workers or for some equipment to have physical contact with the process, w hich could make it difficult to use complete enclosures. If it is possible to enclose the contaminant source and the tool, a total enclosure is recommended, especially if the workers only need to access the process during pauses in operation. Total enclosures may also be necessary for processes that generate highly toxic contaminants. Where total enclosures are not practicable, partial enclosures may be used. F xterior hoods are the least effective exhaust hood. 

The first step in designing an exhaust hood is to select the geometry of the hood. As described above, the hood should enclose the process as much as possible. Where enclosures are not possible the hood should be located as close as possible to the source. The next step is to select an appropriate hood flow rate. The most common methods are... 

It has long been recognized that the presence of a worker close to an enclosure, especially a fume cupboard, can have a significant effect on the exhaust hood performance (see Section 10.2.3.3). However, one aspect of... 

To enhance the efficiency of a partial enclosure ir is possible to let a plane supply air jet blow inside and/or into the hood along one or more wails or along the table. Other advantages of this. system are a reduction in needed supply flow to the room or a reduction in necessary exhaust hood flow for the same level of control. The supply flow (jet) inside the hood usually makes the flow into the hood (through the hood opening) more stable. As for all exhaust hoods with supply air inside, the supply flow rate must be less than the exhaust flow rate and the difference must be large enough to ensure sufficient velocity into the hwd. 

Minimum Duct Velocity. Once the types and locations of hoods, enclosures and booths have been established and the exhaust rates determined, it is necessary to design the ductwork to transfer the contaminated gas to the collection or cleaning device. An essential parameter required for this purpose is the minimum duct or conveying velocity, Vfmm, which is required to ... 

It should be noted that one of the most significant characteristics of the actinides is their radioactivity all isotopes are radioactive, although some have half-lives of greater than 1 x 10 years. Precautions must be taken in their handling, ranging from the use of special enclosures (HEPA-filtered exhaust hoods, negative-pressure gloveboxes) to the use of shielded facilities. 

Exhaust hoods should enclose as effectively as practical the points where the contaminant is released. They should create air flow through the zone of contaminant release of such magnitude and direction as to carry the contaminated air into the exhaust system. Exhaust hoods and enclosures may also serve the important function of keeping materials in the process by preventing their dispersion. 

Local exhaust vendladon serves to remove a contaminant near its source of emission into die atmosphere of a workplace. A system normally comprises a hood, ducting which conveys exhausted air and contaminants, a fan, equipment for contaminants collecdon/removal and a stack for dispersion of decontaminated air. Hoods normally comprise an enclosure, a boodi, a captor hood or a receptor hood. Those relying on odier dian complete enclosure should be as close as practicable to die source of polludon to achieve maximum efficiency. 

Rim exhausts are suitable for area sources of contaminant. They are limited in the area over which they can draw with adequate velocity. In practice, the slot hood should be within 0.6 m of the far edge of the source. For an open surface tank this means that a slot hood on one long side is necessary for tanks up to 0.6 m in width hoods on both long sides are necessary for tanks up to 1.2 m in width and rim exhaust is not practical for tanks wider than 1.2 m. For those situations, push-pull ventilation or enclosure type hoods are recommended.- ... 

The Uniform Fire Code requires that pyrophoric, flammable, or highly toxic gases be within ventilated gas cabinets, laboratory fume hoods, or exhausted enclosures. ... 

Capture system performance on a nonbuoyant source is influenced by enclosure (hood) design and location of the exhaust point. 

For exterior hoods and enclosures, the measurement of the breathing zone concentration provides a method of comparing the effects of changes in the supply and exhaust airflow rates,... 

Different protection factors have been defined. One method is to define it as the ratio of the concentration of a contaminant in the exhaust duct (CJ to the concentration in the breathing zone (C[,) of a person standing in front of the enclosure, for example, a laboratory fume hood ... 

It is often possible to reduce air-input requirements- by removing the hazardous material at the point of discharge by loccd ventilation. This lowers the ta value in Eq. (8-5), which assumes possible disposal of hazardous material within the entire enclosed volume of the enclosure being ventilated. Hoods and exhaust ducts are placed over such equipment as open filter presses, pulverizers, open tanks, and over laboratory benches and equipment to catch the maximum amount of vapor or dust without interfering with normal operation and maintenance. Local air velocities in the region of pickup will depend on density of the hazardous material or its particle size if a dust or fume. Air velocities greater than 200 fpm are usually employed for industrial operations, while chemical laboratory fume hoods range from 70 to 125 fpm when fully opened. 

Ductless fume hoods are ventilated enclosures that have their own fan, which draws air out of the hood and through filters and ultimately recirculates it into the laboratory. The filters are designed to trap vapors generated in the hood and exhaust "clean" air back into the laboratory. These hoods usually employ activated carbon filta-s. The collection efficiency of the filters decreases o er time. Ductless fume hoods have extremely limited applications and should be used only where the hazard is very low, where the access to the hood and the chemicals used in the hood are carefully controlled, and under the supervision of a laboratory supervisor who is familiar with the serious limitations and potentially hazardous characteristics of these devices. If these limitations cannot be accommodated, then this type of device should not be used. 

Laboratory scale" excludes those workplaces whose function is to produce commercial quantities of materials. "Laboratory-type hood" means a device located in a laboratory, enclosure on five sides with a movable sash or fixed partial enclosed on the remaining side constructed and maintained to draw air from the laboratory and to prevent or minimize the escape of air contaminants into the laboratory and allows chemical manipulations to be conducted in the enclosure without insertion of any portion of the employee s body other than hands and arms. Walk-in hoods with adjustable sashes meet the above definition provided that the sashes are adjusted during use so that the airflow and the exhaust of air contaminants are not compromised and employees do not work inside the enclosure during the release of airborne hazardous chemicals. 

A local exhaust system is used to collect air contaminants at the source, as contrasted with general ventilation, which allows the contaminant to spread throughout the workroom, later to be diluted by exhausting quantities of air from the room. Local exhaust may be achieved using an enclosure, a receiving hood, or an exterior hood. Exhaust Ventilation... 

In all laboratory experiments and procedures, it is important that hazards be controlled or managed primarily by engineering measnres, snch as a laboratory hood, local exhaust ventilation, or a glove box. Ventilated enclosures can often be used for weighing chemicals. Best practices also include having a laboratory under negative pressure with respect to the adjacent hallway so that hazardous chemical vapors are kept in the laboratory. In same cases, such as with the use of radioactive materials, personal hand, foot, or full-body monitors may be needed to control the spread of material into areas outside controlled laboratory facilities. 

Capture Efficiency One can accomplish this most easily by enclosing the source as much as possible. Enclosures increase the efficiency of capture, reduce operating costs, and require less air flow to capture contaminants. If there is no way to enclose a source of contamination, the entry (hood) to the local exhaust system should be as close to the source as possible. The farther a hood is from the source of contaminants, the lower the efficiency. Air volume required to accomplish capture increases with distance between a contaminant source and the face of a hood. The shape of a hood can also affect the likelihood of capture. The profile of air movement at the entry extends farther in front of a hood for certain types of hoods compared to others. 

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