To choose a supply inlet as the local ventilation system is not common be­cause it is difficult to design for the specific spreading of contaminants. This is usually easier with an exhaust hood. However, there are moments when large flow rates or specific flow fields are necessary to transport contaminants or for shielding from contaminants.

When choosing a supply inlet it is always necessary to be very careful, because the influence of the supply air could reach very far. This is most important when using air jets or similar devices to direct the contaminants in an intended direction, since very small changes in direction or momentum could totally destroy the in­tended flow field and transport the contaminants to, instead of from, the workers.

A supply jet may have several different forms; point jet, swirl jet, line jet, radial jet, etc. The choice depends on available volume and existing demands. A radial jet could be used when it is possible to utilize a wall or a ceiling for distributing the air; a point jet could be used when it is advantageous to have a high air velocity in the volume (room).

Supply inlets are also used when there is no room for an exhaust hood or when the contaminant-generation process has a form such that an exhaust

Hood becomes very clumsy. They are also used to enhance the function of ex­haust hoods: the most well known are the so-called push-pull systems. How­ever, most of the many exhaust hoods described earlier could be improved by using some kind of directed supply air. It must be remembered that a supply inlet needs much more accurate design than an exhaust hood and that they also need much more maintenance. Supply inlets usually depend on having A Defined contaminant concentration in the inlet air; this usually implies clean supply air, but some inlets could use recirculated air with or without cleaning.

For supply inlets in rooms some performance measurements exist, such as air exchange and ventilation efficiencies (see Chapter 8). It is usually not possible to use these for local ventilation supply inlets, and for the moment there are no specific measurements to evaluate the influence of an inlet on contaminants. Some trials with comparison indices, which compare inhaled concentrations (or exposures) with and without a supply inlet, have been done.