Partial enclosure

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. 

Number of surface and volume zones in enclosure Partial pressure of species k, atm Number of WSGG gray gas spectral windows Total radiative flux originating at surface zone t, W Net radiative flux between zone t and zonej, W Temperature, K... 

Eliminate enclosures (partial or total) that tend to trap the gas as it rises. 

Partial enclosure with local exhaust ventilation. 

Partial enclosure with local exhaust ventilation, or local exhaust ventilation, of working position separate hazardous processes from other work, e.g. spray-painting with epoxy-containing sensitizing and carcinogenic compounds. 

Partial enclosure allows small openings for charging/removal of apparatus and chemicals. The requisite air velocity to prevent dust or fumes leaking out determines the air extraction rate, e.g. 

AMD 1 Electrical apparatus for use m the presence of combustible dust. Part 1 Electncal apparatus protected by enclosures - Construction and testing (AMD 10764J dated October 1999. Partially superseded BS 6467-1 and 2 which remain cunent... 

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. 

For small-scale laboratory work, the exhaust surface is often made as a separate section added to the side of a table or put into a large hole in a table. These tables usually have a sheet metal surface that is resistant to the chemicals used and is easily cleaned. Many circular holes are cut into the metal surface to allow for airflow. This perforation makes the pressure difference over the table quite high and at the same time gives an even distribution of the airflow over the entire surface. These types of exhaust surfaces could be formed to suit different working conditions, e.g., the surface could be made to fit into a sink or to be placed below and around a balance. Using side walls that are not too high, on three or four sides, transforms the table to a partial enclosure, which increases... 

FIGURE 10.39 Principles for total enclosure (obove) and for partial enclosure (beWw). 

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. 

Booths are partially enclosed workplaces with one or more open facefs) for access by workers. These openings at one or more sides of the enclosure function not only to capture air contaminants directly through their short-distance capture capability but also to cause an airflow in a certain direction (normally away from the worker/work process and into the enclosure). The capture efficiency could be increased by using an existing main flow direction (e.g., thermal flows caused by heat sources) to support the capture process. 

When primary fume capture is performed by the enclosure, furnace off-gas combustion efficiency is lower than experienced by furnace direct evacuation control. The off-gas, rich in carbon monoxide (CO), rises from furnace roof openings and partially burns and cools with enclosure air. Significant levels of CO have resulted in the enclosures and exhaust ducting from this type of combination. These levels are not explosive but present a potential hazard to personnel working in the enclosure or in downstream fume cleaning equipment. 

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. 

A visual evaluation of ventilation system performance can be performed by injecting smoke into the jet. No quantitative evaluation methods for these systems have been reported, but it should be possible to measure the containment of a hood with side walls (partial enclosure) using one of the containment indices (see Sections 10.2.1 and 10..5). Additional information may be obtained by measuring capture efficiency. 

The terms zonal model and flow element are also used for the simplified characterization of the flow field in a single enclosure. There, a zone represents a partial volume of air in the enclosure, whereas in the multizone models described here, a zone represents a specific enclosure which is connected to other enclosures by air conductances (see The Airflow Network later). 

Partial enclosure An enclosure used for work with toxic dusts, gases, or vapors in which one or more of the sides may be open to the remainder of the work area. 

Are any processes or operations partially or fully enclosed (Specify) Is a need for new or additional enclosure apparent (Specify)... 

Process equipment and ancillary support or utility equipment are often placed in either partial or complete enclosures for ... 

Enclosures, even partial enclosures, containing equipment handling flammable, combustible, ortoxic materials may permit the accumulation of hazardous concentrations of these materials within the enclosure, potentially resulting in fire, explosion, or personnel exposure. Where the possibility of a flammable spill or release within an enclosure exists, the enclosure design should include a relevant selection from the following features noncombustible construction, adequate ventilation, drainage, appropriate electrical classification, flammable vapor detection, isolation and alarm, and internal automatic sprinkler or water spray protection. 

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