What Is Described
Each of the three categories has many different types of systems. Some common types are described here and for most of the systems the detailed description includes the name and the principle. When and where to apply the different systems and the different forms that may exist are also included. Nearly all systems have some specific issue to be addressed when designing that type of system. Equations and/or parameters are included where available. For some systems, many different equations are available, from which the most accurate or usable have been selected for inclusion. For systems where only one equation exists, it has been described and the limitations identified. If no equations are available, other design parameters and considerations are described.
Many parameters influence the performance of a specific local ventilation system. The main parameters are
• Location of supply and exhaust openings and source(s)
• Location of workers and tools relative to exhaust and source(s)
• Different kind of disturbances
• Changing flow rates
Many of these influences are simitar for different types of systems. These parameters’ influence is described for each system or for a group of systems depending on feasibility. For some systems, there is no detailed description, which means that there could be differences in the descriptions depending on the actual details of each system.
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Leakage, expressed as (1 — a) or similar |
Disturb the exhaust flow fields. For some exhaust openings this could be quite easy to arrange; for others it may be difficult. If the disturbance is too great, the efficiency of the exhaust will diminish and it might be necessary to use a different exhaust opening, a different supply opening, a different layout, or a completely new solution (e. g., a combined exhaust and supply). There are usually demands on the cleanliness of the supply air (see Chapters 6 and 8 ).
Most exhaust hoods are either integrated with the contaminant-generating process equipment or are independent of the process equipment. It is nearly always better to have an integrated hood. The advantages are that the hood is designed to work with that specific tool, process, and contaminant; its airflow rate is less; it is situated at a proper place; it is easier to handle; and it is often less expensive. However, there may be economic or other reasons not to have an integrated hood. A nonspecific exhaust also has advantages: many people know how to handle it (at least in principle); it could be used very near the source (although normally not as close as an integrated hood); it may be useful for different processes at the same workplace; and, if flexible or mobile, it may be less expensive for different uses at the same workplace. However, a nonspecific exhaust usually has lower efficiency than an integrated exhaust with the same flow rate and it may be difficult to get a mobile exhaust to follow the contaminant source.
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, which 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. Exterior hoods are the least effective exhaust hood.
It is strongly recommended to keep the source between the exhaust and the person. When the source is quite close to the person and between the person and the exhaust, the airflow around the person could generate a wake that includes the source or the generated contaminant and thereby the person’s exposure could increase (see Fig. 10.3). In these cases, it is better to use a side exhaust, where the person is situated beside the shortest path from source to exhaust.
This phenomenon is more common with large flow rates and large openings, such as use of a laboratory fume hood or a unidirectional horizontal flow field, than with small flow rates and small openings, such as exhaust hoods for welding, soldering, and grinding. Designing the system to place the worker beside the airflow path is generally recommended for all exterior hoods. When using partial enclosures with large openings to the surroundings, a person may also influence the flow field, e. g., by changing the flow into the hood. It is possible to counteract such wakes by using vertically directed supply air around the worker (see Sections 10.3.3 and 10.4.6).
When some natural forces exist, it is essential to utilize, and not to counteract, these forces. Some examples are buoyancy forces from hot sources or contaminant jets from grinding or spray painting (see Fig. 10.4). To completely isolate a volume from its surroundings only using air is impossible. To achieve
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Air movements — ■ ■ — "► Contaminant movements
[FIGURE 10.3 Possible air and contaminant movements around person in front of opening (or with supply air coming from behind). Seen from above.
This, it is necessary to complement the airflow field with physical barriers. This is easy to understand for exterior hoods, but it is also valid for enclosures.
Many different measures of local ventilation performance exist. These measures can be divided into three main categories: capture velocities, capture efficiencies, and containment efficiencies. Table 10.1 shows the connections between hood types and different efficiency measurements. Section 10.5 describes procedures for measuring each of these performance measures.
Capture velocity is usually defined as the air velocity generated by the exhaust opening necessary to capture a contaminant outside the opening and transport it into the opening. See Fig. 10.5.
The advantage of using capture velocity is that it is possible to calculate the necessary flow rate into the adjacent opening. Its disadvantages are that it
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Hot hath
FIGURE 10.4 Example of how a natural force (buoyancy) is used when exhausting generated con —
TABLE 10.1. Relationships between Different Evaluation Procedures and
Different Hood Types
Hood types Exterior hoods Enclosures
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Specific hoods Evaluation procedures |
Basic openings, rim exhausts, LVHV, receptor hoods, special: fit to machines
Capture efficiency, capture velocity
Booths, laboratory fume hoods, safety cabinets, glove boxes, storage cabinets, built-in processes Containment indices such as protection factor and similar, leakage factor and similar, pressure difference, opening velocity
Varies for a specific contaminant, depending on how the contaminant is generated and which type of process is involved, and that it is difficult to measure.
Capture efficiency is the fraction of contaminant generated outside an exhaust that is captured by the exhaust (see Fig. 10.6).
The advantage of using capture efficiency is that it is possible to calculate how much of the contaminant is released into a workspace (if the source rate is known) and thus to judge if the exhaust will reduce workplace exposures to acceptable levels. Its disadvantage is that it is rather difficult to measure and, moreover, it is usually impossible to calculate source generation rate.
Containment efficiencies are often called indices, which are measured and calculated in many different ways. One definition is shown in Fig. 10.7. They are different from the two preceding measures in that they are exclusively used
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■ Airflow Contaminant flow
Isovels (curves with equal velocity)
FIGURE t0*5 Illustration of capture velocity.
For enclosures. Their advantages are that they can give a good approximation of the contaminant leakage from a specific work process, that they are not too difficult to measure, and that they make it possible to design a hood for working with a specific hazardous substance. Some of these containment efficiencies could also be defined to measure the efficiency of local supply inlets. The disadvantages are that there are so many different containment indices and that they may give the impression of a measure independent of work, although they are influenced by the process and/or operator.
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