Combined exhaust hoods and supply inlets

jungqvist, R. Nydahl, and B. Reinmiiller. Swiss Contamination Control, no. 4a, 3990, 36-.39. 1.5. G. K. Batchelor. An Introduction to Fluid Mechanics. Cambridge Cambridge University Pre.ss, 197i). 

Schlichting. Boundary Layer Theory. 7th ed. New York McGraw-Hill, 1 979. 

These systems can be inside large halls and may have no fixed limits for their influence, except for some parts of the system (inlet device surface, etc.) They can also be situated inside small rooms, where walls, floors, and ceilings are the natural boundaries. The systems usually consist of one exhaust hood and one supply inlet, which interact. There are also special combinations, as two or more inlets and one exhaust hood, or one supply inlet and two or more exhausts. All of these combinations need careful design and an accurate relation between supply and exhaust flow rates and velocities. Some systems also need stable temperature conditions to function properly. All combinations are dependent on having a defined contaminant concentration in the inlet air. This usually implies clean supply air, but some systems may use recirculated air with or without cleaning. 

There are many possible combinations of supply and exhaust air. For example, a line jet could be used as a shield in an opening, as a stripping system on surfaces, for blowing contaminants into an exhaust, etc. An enclosure could be designed with a line jet in the opening, with a wall jet inside to increase efficiency, or with a low-momentum jet inside or outside the opening to replace the room air supply. In this section, only some basic combinations are described. 

Similar to supply inlets, no measurements exist for evaluating the inlets specific influence on contaminant concentration. The available measurements for the combinations are the same as for exhaust hoods, i.e., capture efficiencies and similar measures. Sometimes the performance of a combined system can be approximated from the performance of the incoming supply inlet and exhaust hood. 

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FIGURE 10.1 Principles for the three different ways of protecting a volume by using an exhaust hood (above), a supply inlet (middle), and a combined exhaust hood and supply inlet (below). 

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