For all local ventilation systems the airflow field decides the performance, i. e., how well the contaminant is transported away from the place where it should not be, to a place where it can be transported to an acceptable place (e. g., out­side or into the contaminant collector). Thus the disturbance of the intended flow field diminishes the efficiency of the local ventilation system. And all local ventilation systems can be disturbed by tools, machines, and persons, or by any­thing else that moves air (passing trucks, wind through doors, high temperature gradients, pressure gradients, etc.) or changes in air movements. Naturally, maintenance is as important for local ventilation as for general ventilation.

The necessary containment or transport capability of a local ventilation system depends on the type of contaminant present and its health risks. There could be different demands for gases and particles, for contaminants that have immediate health risks and those that have long-term effects, for contaminants that affect the breathing system and those that affect the skin and eyes, tor in­fectious contaminants, for contaminants that follow the air streamlines closely and those that fall out on floor and work surfaces, etc. (See Chapter 5 for physiological and toxicological considerations.)

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.)

The principal classification made here is from the functioning point of view, i. e., how the different flow fields are intended to eliminate the contami­nants. When using an exhaust hood the intention is to suck the contaminant into the hood (or prevent it from escaping the hood into the surroundings) by proper design of the flow field and the hood. When using a supply inlet, the intention is to blow the contaminant away (to an exhaust quite close or by spreading the air to an exhaust situated far away) from the volume (breathing zone, etc.) and the air inlet. With a combined system, the intention is to com­bine the effects of an exhaust outlet and a supply inlet to get a higher effi­ciency than either of them could achieve separately.

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 vertical) source or a hood that is partly an exterior hood and partly an enclosure.