Ventilation capture velocity

Many different measures of local ventilation performance exist. These measures can be divided into three main categories capture velocities, capture efficiencies, and containment efficiencies. Table 10.1 shows the connections between hood types and different efficiency measurements. Section 10.5 describes procedures for measuring each of these performance measures. 

In the design of extract ventilation it is important to create, at the point of release of the pollutants, an air velocity sufficiently strong to capture and draw the pollutants into the ducting. This is known as the capture velocity and can be as low as 0.25 m/s for pollutants released gently into still air such... 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

This is a very high velocity, which could hardly be used inside a paint shop. Our practical alternatives are to choose a less toxic solvent, for which the permitted concentration is higher, or to devise some kind of ventilation system, such as a laboratory fume hood, which will prevent the mixing of the benzene with the air that workers breathe. We also need to consider the air pollution consequences of emitting 200 kg/day of benzene to the atmosphere in most U.S. cities that would require a permit and probably some form of capture or destruction of the benzene. 

With local exhaust ventilation, the aim is to capture the contaminant near its point of release before it has dispersed into the workplace. In principle, the contaminant is sucked into a pipe whose open end is placed near the source. The detailed design depends on the application and, in particular, on the rate at which the contaminant is being released. For example, where there is quiet evaporation of a liquid, a comparatively low air velocity of 0.5 metres per second would be required at the point of release to capture the vapours reasonably efficiently. A higher velocity of, perhaps, 2.5 ms would be required to capture vapours from a process which generates rapid air movements, such as those produced as a liquid is discharged quickly into a large vessel. 

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