Enclosure hood

Personnel are protected in working with tritium primarily by containment of all active material. Containment devices such as process lines and storage media are normally placed in well-ventilated secondary enclosures (hoods or process rooms). The ventilating air is monitored and released through tall stacks environmental tritium is limited to safe levels by atmospheric dilution of the stack effluent. Tritium can be efficiently removed from air streams by catalytic oxidation followed by water adsorption on a microporous soHd absorbent (80) (see Absorption). 

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

The mean airborne concentration of n-hexane was 38 ppm (range 4-90 ppm) in the Finnish shoe industry in the 1980 s. The mean concentrations of acetone and toluene were 131 ppm and 14 ppm. The mean exposure levels in two US shoe manufacturing plants in the late 1970 s were toluene 22-50 ppm, methyl ethyl ketone 133-153 ppm, acetone 46-223 ppm, and n-hexane 22-55 ppm. The exposure is, however, easy to control by doing the gluing in enclosure hoods. 

Enclosure hood Receiving hood Capturing hood... 

The enclosure hood surrounds the contaminant source as much as possible. This type of hood has a low exhaust rate, with an inward face velocity of between 100 and 150 ft/min. Contaminants are trapped inside the enclosure by air flowing in through openings in the enclosure. 

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. 

Total enclosure may be in the form of a room with grilles to facilitate air flow this functions as a hood and operates under a slight negative pressure with controls located externally. Entry is restricted and usually entails use of comprehensive personal protective equipment. Ancillary requirements may include air filters/scmbbers, atmospheric monitoring, decontamination procedures and a permit-to-work system (see page 417). 

Use doghouse enclosures where appropriate use hoods to collect fugitive emissions. 

Fixed systems are those where movement of the hood or other changes to the system, except perhaps opening and closing of lids and doors, is not possible. One example is the hood with a sliding door surrounding a drilling or a milling machine another is the laboratory fume hood and another is the canopy hood above or the enclosure around a paper machine. 

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... 

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 classification of hoods into exterior hoods and enclosures could sometimes make it difficult to specify a hood. This classification is only an attempt to describe the hoods. Enclosures can be separated into partial and total enclosures partial enclosures have an opening to the surroundings big enough to use for work, and total enclosures do not. Both have the contaminant source inside a physical volume and for some of these hoods this volume is large enough for some workers to work inside. See Fig. 10.39. 

Partial enclosures are a compromise between containment and access. Most people misunderstand the function of partial enclosures. It is not possible to completely separate the interior from the surroundings with partial enclosures. Complete separation is only possible with total enclosures. The function of a partial enclosure is as dependent on the flow rate, the flow field, the working procedures, the contaminant generation process, etc. as is the function of exterior hoods. The advantage with a partial enclosure is that the physical walls diminish the possibilities for the contaminants to escape from the hood to the surroundings. Thus these hoods could be used when relatively high demands are put on the contaminant concentration outside the hood. Some of the most commonly used enclosures, such as fume cupboards and booths, are described. Many variations of these exist, e.g., enclosure of the complete process, and some of these are described here. 

Access to the interior of the enclosure is much more restricted for a total enclosure than for a partial enclosure. So-called totally closed hoods, where all contact between inside and outside is through air locks or by robot or remote control (see Section 10.4.6.4), these are not only expensive to construct and operate, they also need specialized ventilation systems to function properly. 

Under normal operating conditions the worker will be situated outside the enclosure. The operator will enter the hood only with his or her hands/arms. During the setting up of equipment it may be necessary to enter the enclosure, but entry should be kept to a minimum and, whenever possible, before the emission of contaminant has commenced. 

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... 

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

On oxygen steel conversion furnaces, primary fume control is usually achieved by a separate close-capture hood positioned over the vessel mouth. The enclosure is then used for secondary fume control during charging, turndown, tapping, and slagging. 

R. P. Harvey. In Proceedings of the Development and Use of Fume Cupboards, Fume Hoods and Ventilated Safety Enclosures in Laboratories . Symposium Organised by the Laboratory of the Government Chemist, London, March 1979, pp. 32-59. 

A partial enclosure can be combined with a plane air jet in order to increase the containment of the hood. The jet can be thin or wide, depending on... 

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. 

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