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 size of the exhaust opening (D for the round duct) and 60 characteristic sizes for the supply outlet (60D for the round nozzle).

Local exhausts are 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).

In practice, the air seldom mixes as completely as in theory. The pollut­ants 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.

In rooms with ventilation systems creating temperature and/or con­taminant distribution (see Section 7.5.4), one might consider placing an exhaust opening at, or close to, the equilibrium height for the main con­taminant (Fig. 7.16). In addition, there should be a general exhaust either at ceiling level or at floor level, depending on the buoyancy of the contam­inants. When if is not certain if the contaminants have negative or positive buoyancy, one should place exhaust openings both under the ceiling and at floor level (Fig. 7.17). More about general exhaust location selection is covered in Section 8.10.

Airflow Created by Exhaust Performance

Airflow Created by Exhaust Performance

When special contaminating processes are located in a seini-enclosed parr of a room as in Fig. 7.18, the exhausts should be located so that a displace­ment flow is created through the passage into the semi-enclosure. With this ar­rangement there may be no need for a door, which may be very practical.