Local ventilation is used to diminish or, preferably, to prevent exposure to con­taminants (including heat). This includes protection of persons, products, or animals from hazardous and/or nuisance contaminants. This task is not spe­cific for local ventilation and different definitions exist. One is the following:

L ocal vennlation systems are used to transport contaminants or heat from the occupancy zone.

A longer definition is

Local Ventilation uses an airflow rate that is as low as possible, but sufficient to minimize the amount of airborne contaminants entering a specified volume or passing specified pomt(s). These are usually intended to be at the breathing zone of occupants. This minimi­zation of airflow can be achieved either by capturing (or containing) the airborne contami­nant into an exhaust hood before it enters the workspace, by blowing noncontaminated air from a supply inlet through the volume to prevent the contaminant from entering the work­space, or a combination of those. Often an exhaust system is called an extract system. This term is appropriate if the room is seen from the outside, but since in these descriptions the system is seen from inside the room the more adequate name Exhaust system is used.

Local ventilation is often a very important part of the ventilation sys­tem, both in function and in construction, which makes a specific definition difficult. In addition to the above definitions it can be defined as ventilation of a separate volume inside a large room, as opposed to general ventilation, which is for the complete room or building. One problem with this defini­tion is that local ventilation systems could function as localized ventilation in one surrounding and as general ventilation in another surrounding: e. g., a ceiling inlet combined with floor exhaust is a local ventilation system in a large hall and a general ventilation system in a small room without any other ventilation, although, in this latter case, its main purpose is different from that of a general ventilation system. With these comments as a back­ground, the term Local ventilation is here used in a very broad sense (See Fig.

1.1 in Chapter 1, where a local ventilation system is outlined}.

By using a local ventilation system of good design less air is needed to reach a specific contaminant level than is possible with general ventilation, it can also be said that the purpose of local ventilation is to achieve a more effi­cient (defined in some way) ventilation in a part of a room or in the whole room. Local ventilation also can be important from a process standpoint, e. g., removal of heat that might damage equipment.

Proper design and construction of a local ventilation system must account for hood flow rate, contaminant generation process and rate, and the generated flow rate of contaminated air. Thus, knowledge about airflow mechanics, pro­cess performance, and the contaminant source is essential. The descriptions of different sources are included in Chapter 7 and here only short descriptions are included as necessary to identify different processes and source types.

The demands and design of a local ventilation system (not only local ex ­hausts) should naturally start with the demanded target levels and the toxicity of the air contaminants (see Chapters 5 and 6). For best performance the ex­haust should be close to the source and preferably enclose the source, there should be no disturbances of the flow, and at the same time it should have a low flow rate and be able to minimize the concentration of even quite danger­ous air contaminants in the working zone.

All processes potentially generate contaminants; i. e., they act as sources. Of­ten the contaminant is spread into and with the air around the process. Control of the contaminants is thus often the same as control of the contaminated air. Other alternatives to control contaminant sources are described in Chapter 3.

The parts of local ventilation systems, situated inside rooms, that influ­ence the flow field are described here. This presumes that the inlet and outlet openings are properly connected to duct systems either directly or through flexible connections (tubes). These ducts and tubes and other parts of impor­tance for the function of these systems are described in other chapters.

Systems worn by a person are not included here. These include personal fresh air supply systems (with pressurized air flasks or connected to a central system by tubes), air belts, air vests, and breathing masks.