Enclosing hood

Hoods are either enclosing or nonenclosing. Enclosing hoods provide better and more economical contaminant control because their exhaust rates and the effects of room air currents are minimal compared with nonenclosing hoods. For more detail regarding nonenclosing hoods, see Chapter 10. 

There are many possible ways to classify local ventilation systems. When local ventilation is used to describe exhaust hoods only, one classification is hoods that totally surround the contaminant source (enclosing hoods), hoods that partially surround the contaminant source (partially enclosing hoods), and hoods where the contaminant source is outside the hood (exterior hoods). A similar classification is used here for the exhaust hoods. Since local ventila tion, as described in this chapter, includes more than exhaust hoods, the following three main categories are used exhaust hoods, supply inlets, and combinations of exhaust hoods and supply inlets. (See Fig. 10.1.)... 

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. 

The capture velocity of a hood is defined as the air velocity created by the hood at the point of contaminant generation. The hood must generate a capture velocity sufficient to overcome opposing air currents and transport the contaminant to the hood. For enclosing hoods, capture velocity is the velocity at the hood opening. In this case, the velocity must be sufficient to keep the contaminant in the hood. In practice, hood shape and the influence of crossdrafts on the measured capture velocity have to be considered. All three velocity components should be measured and used to calculate the magnitude and direction of the total velocity. Other methods used, not as good as the previous one, are to measure the velocity with a directional velocity sensor towards the hood or to measure the net velocity by an omnidirectional velocity sensor. In the last method the main airflow direction should be viewed and evaluated by means of a smoke test (see Sections 10.2.1 and 10.2.2.1). 

Enclosing hood An extract hood, which partially or completely encloses the point of pollution generation. 

An enclosed hood completely contains the source of contaminant. 

The most common example of an enclosed hood is the laboratory hood. A standard laboratory utility hood is shown in Figure 3-6. Fresh air is drawn through the window area of the hood and is removed out the top through a duct. The airflow profiles within the hood are highly dependent on the location of the window sash. It is important to keep the sash open a few inches, minimally, to ensure adequate fresh air. Likewise, the sash should never be fully opened because contaminants might escape. The baffle at the rear of the hood ensures that contaminants are removed from the working surface and the rear lower corner. 

Consider the simple box-type enclosed hood shown in Figure 3-8. The design strategy is to provide a fixed velocity of air at the opening of the hood. This face or control velocity (referring to the face of the hood) ensures that contaminants do not exit from the hood. 

The flare chamber is refractory lined and has an adjustable louvre at the bottom to regulate combustion air. Ignition is provided with a 0.20 MJ/h oil burner. A totally enclosed hood captures the combustion products and directs them to the 0.30 m diameter stack which extends above the hood. 

DeJonge Permeation Test Method (20). Wien first developed, this test method used an enclosed hood over a variable speed conveyor belt where fabric and gauze were layered and attached to wooden frames. The fabric was then passed under a nozzle which spr s the material and the gauze was later analyzed for chemical residues. This method is closer to simulating exposure under actual use conditions, and is currently being modified. 

C) Are completely enclosed on at least two sides, shall be considered to be exhausted through an enclosing hood. 

D) The quantity of air in cubic feet per minute necessary to be exhausted through an enclosing hood shall be not less than the product of the control velocity times the net area of all openings in the enclosure through which air can flow into the hood. 

Three types of hoods are used in these systems capture hoods, enclosing hoods and receiving hoods. 

Hoods can be classified conveniendy into three broad groups enclosures, receiving hoods, and exterior hoods. Booths such as the common spraying-painting enclosure are a special case of enclosing hoods. 

Enclosed hood, open one side Canopy hood... 

Enclosed booth, open one side Enclosed hood, open one side Canopy hood Enclosed hood, door front Enclosed hood, open front Down draught, table type Canopy hood... 

A fume hood is constructed in the manner shown in figure 6. Strike drew the frame as being made of lumber but it can be made of rebar or, preferably, from PVC pipes and joints so that it can be assembled and disassembled with ease. The frame is enclosed with plastic drop cloths or any semiclear plastic sheeting. The front face of the hood is halfway covered with plastic while the bottom half is exposed to allow one to move objects in or out and to manipulate things. On top of the chamber is attached some clothes dryer duct or some such crap which is led to a leaf blower or blower motor. The exhaust from the blower is led away to the outside. 

Fugitive emissions from charging and tapping of EAFs should be controlled by locating the EAF in an enclosed building or using hoods and by evacuating the dust to dust arrestment equipment to achieve an emissions level of less than 0.25 kg/t. 

Chemical Reactivity - Reactivity with Water Reacts violently, forming corrosive and toxic fumes of hydrogen bromide Reactivity with Common Materials Attacks and corrodes wood and most metals in the presence of moisture. Flammable hydrogen gas may collect in enclosed spaces Stability During Transport Stable if protected from moisture Neutralizing Agents for Acids and Caustics Hood with water, rinse with dilute sodium bicarbonate or soda ash solution Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

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. 

BEOs are most often used for point sources or small line or surface sources. See Chapter 7 for descriptions of sources. BEOs are sometimes used for lines or surfaces when the source is moving along the line or on the surface. This naturally demands the exhaust to move with (or be moved with) the source movements (e.g., during painting or seam welding). They have also been used for side suction from baths and tanks-- and these exhausts are usually called rim exhausts see Rim Exhausts. However, for these sources push-pull systems (Section 10.4.3) are often more efficient. Side hoods can also be used, e.g., when molten metal is poured however, in these cases an enclosed exhaust is more efficient. 

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... 

Working locations between the contaminant source and the capture openings dramatically reduce the efficiency of the capture system and should therefore be avoided. If the hood is enclosed on three vertical sides the sensitivity to cross-draft is low. 

This is the inlet into the system, and will be the single most important element in determining the effectiveness of the control plant. A study of the dust- or fume-producing process is necessary to ensure that the twin aims of effectiveness and efficiency are met. Hoods that totally enclose the process for maximum effectiveness may, however, prevent the operator from carrying out the process for which the control was needed in the first place. 

The most common example of local ventilation is the hood. A hood is a device that either completely encloses the source of contaminant and/or moves the air in such a fashion as to carry the contaminant to an exhaust device. There are several types of hoods ... 

With modem equipment it is now possible to measure oxygen consumption and carbon dioxide production breath by breath. Alternatively, for long-term studies, the volunteer can be enclosed in a room and the changes in content of the gases measured. For resting subjects and for patients, this is not necessary a simple hood over the head may be sufficient to allow measurement of oxygen uptake and carbon dioxide production, if precision is not required. 

Operations with high cross-contamination potential (e.g., mixtures of test or control articles with animal diets) are often conducted in small, dedicated, individual cubicles equipped with special and separate air-handling systems or are conducted under a fume hood. Special mixing equipment (e.g., enclosed twin-shell blenders) can be used to reduce the chance of cross-contamination. 

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