Airflow Created by Exhaust Performance

The effect of exhaust performance on room air movement is limited compared to the effect produced by air jets. The distance from the opening to the point where air velocity drops to 10% of the initial velocity value (Fig. 7.14) is approximately equal to one characteristic si2e of the exhaust opening (D for the round duct) and 60 characteristic sizes for the supply outlet (60D for the round nozzle). 

Local exhausts arc designed to capture air pollutants and heat at the source, and thus their location and the exhausted airflow rate should ensure sufficient capture velocity. 

General exhausts typically do not prevent contaminants and heat from mixing with the room air, and thus, theoretically, it does not matter where the exhaust openings are located (Fig, 7.15). 

Ill practice, the air seldom mixes as completely as in theory. The poll no ants spread with the room air, but there may be areas where the concentration is higher than the average for the room. This has to be taken into account in determining the location of the exhaust opening. 

FIGURE 7.M A tec reaches a long way in one direction, but the capture length of a sink is very limited 

---