Local ventilation supply inlets

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. 

The following equations separately outline calculating contaminant concentration inside a room with central and local recirculation. The assumptions for the room are that it has one main ventilation system with supply and exhaust air and that the contaminant concentration is the same in the whole volume (except very close to the contaminant source or in the ducts, etc.). The contaminant source is steady and continuous. The model for local ventilation assumes also one main ventilation system to which is added one local exhaust hood connected to a local ventilation system (see Chapter 10) from which all the air is recirculated. In the central system the number of inlets and outlets could vary. The flow rates are continuous and steady. 

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.)... 

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. 

Mostly the use of a supply inlet as a local ventilation system presumes that the supply device (with air from outside the room) is located inside a large room, which also has an adequate exhaust airflow rate or has convenient ex-haust/transfer openings for the airflow. It is also necessary that the exhaust flow rate is maintained (or pressure difference kept). Otherwise the air supply could change in rate or direction. Instead of using air from a ventilation system, the supply air could be taken from the room (volume) it is situated in. In this case, the room must also have a supply and an exhaust flow rate. It is often necessary to clean the air before it is used in the supply inlet. 

Low-momentum air supply systems designed for local ventilation purposes mainly use vertically downward airflow. Some systems with an inlet of low-momentum horizontal airflow for a whole workroom are on the borderline between general and local ventilation and are therefore briefly described here. A more complete description will be found in chapters 7 and 8 dealing with general ventilation. 

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