Capture velocities

Airflow near the hood can be influenced by drafts created directly by the supply air jets (spot-cooling jets) or by turbulence of the ambient air caused by the jets, upward/downward convective flows, moving people, and drafts from doors and windows. 

Process equipment may be responsible for other sources of air movement in the room. For example, high-speed rotating machines such as pulverizers, high-speed belt material transfer systems, falling granular materials, and compressed air escaping from pneumatic tools all produce air currents. 

T hese factors can significantly reduce the capturing efficiency of local exhausts and should be accounted for by the correction coefficient on room air movement, K, I, in Eqs. (7.205) and (7.206). For example, Eq. (7.206) is replaced with 

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Equation (Cl.4.35) yields two remarkable predictions first, tliat tire sub-Doppler friction coefficient can be a big number compared to since at far detuning Aj /T is a big number and second, tliat a p is independent of tire applied field intensity. This last result contrasts sharjDly witli tire Doppler friction coefficient which is proportional to field intensity up to saturation (see equation (C1.4.24). However, even tliough a p looks impressive, tire range of atomic velocities over which is can operate are restricted by tire condition tliat T lcv. The ratio of tire capture velocities for Doppler versus sub-Doppler cooling is tlierefore only uipi/uj 2 Figure Cl. 4.6 illustrates... 

Figure C 1.4.6. Comparison of capture velocity for Doppler cooling and Tin-periD-lin sub-Doppler cooling. Notice tliat tire slope of tire curves, proportional to tire friction coefficient, is much steeper for tire sub-Doppler mechanism. (After [17].)...

Typical minimum transport velocities are given in Table 12.13 and capture velocities for various applications in Table 12.12. 

Condition of dispersion of contaminant Examples Capture velocity (m/s)... 

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. 

Note-. In each category above, a range of capture velocities is shown. The proper choice of value depends on several factors ... 

Numerical simulation of hood performance is complex, and results depend on hood design, flow restriction by surrounding surfaces, source strength, and other boundary conditions. Thus, most currently used method.s of hood design are based on experimental studies and analytical models. According to these models, the exhaust airflow rate is calculated based on the desired capture velocity at a particular location in front of the hood. It is easier... 

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. 

Capture velocity is usually defined as the air velocity generated by the exhaust opening necessary to capture a contaminant outside the opening and transport it into the opening. See Fig. 10.5. 

The advantage of using capture velocity is that it is possible to calculate the necessary flow rate into the adjacent opening. Its disadvantages are that it... 

Evaluation proce- Ciapture efficiency, capture velocity Containment indices... 

Cross-draft velocity was normalized by dividing the measured cross-draft ve-locit by the capture velocity calculated at the tatik centerline. Capture velocity at the tank centerline was calculated using Silverman s - centerline velocity (Eq. (JO.l)) for unflanged slot hoods. There was considerable scatter in the data, show ing chat cross-draft velocity alone is not responsible for low capture efficiency. 

TABLE 10.2 Capture Velocities for Various Industrial Processes... 

Capture efficiency measurements may be used to evaluate the function of a canopy hood (see Section 10.5). Capture velocity is not a feasible evaluation tool, since a canopy hood does not generate an air velocity close to the source. It is also possible to use exposure measurements for workers outside the plume area. Since most hot processes generate visible contaminants, visual inspection of the flow, especially around hood edges, might provide a qualitative evaluation. Many contaminants could however be invisible when diluted and smoke generators (Section 10.5) may be necessary to find leakages (temporary or permanent) around the hood edges. 

For evaluation the velocity distribution and capture velocity could be used. Since the worker is quite close to the contaminant-generating place, occupational hygiene efficiency is possible (Section 10.5). 

The ACGIHi" gives guidelines for the minimum capture velocity, V p, which must be induced to move a contaminant toward an exhaust. The recommended value depends on the industrial process and the local conditions, and Table 10.12 shows the recommendations for typical open-surface-tank processes. 

From the ACGIH recommendations, we can say that the system is operating safely if a fluid velocity greater than or equal to the capture velocity is induced across the whole of the tank surface, and the exhaust flow rate is sufficient to capture all the fluid in the jet. Since the maximum velocity at any... 

TABLE 10.12 Recommended Capture Velocities for Different Operating Conditions, Based on Table 3.1 (ACGIH )... 

Gases, vapors, and fumes usually do not exhibit significant inertial effects. In addition, some fine dusts, 5 to 10 micrometers or less in diameter, will not exhibit significant inertial effects. These contaminants will be transported with the surrounding air motion such as thermal air current, motion of machinery, movement of operators, and/or other room air currents. In such cases, the exterior hood needs to generate an airflow pattern and capture velocity sufficient to control the motion of the contaminants. However, as the airflow pattern created around a suction opening is not effective over a large distance, it is very difficult to control contaminants emitted from a source located at a di,stance from the exhaust outlet. In such a case, a low-momentum airflow is supplied across the contaminant source and toward the exhaust hood. The... 

For exterior hoods, the measurement of capture velocity provides a quick check of the ideal design conditions. However, it must be remembered that capture velocity is not a direct measure of the ability of an exterior hood to provide personnel protection. Other efficiency measures are required in order to evaluate its performance in practice. The following two efficiency measurements could be useful capture efficiency and occupational hygiene efficiency. These measures complement each other. 

The capture velocity of a hood is defined as the air velocity created by the hood at the point of contaminant generation. The hood must generate a capture velocity sufficient to overcome opposing air currents and transport the contaminant to the hood. For enclosing hoods, capture velocity is the velocity at the hood opening. In this case, the velocity must be sufficient to keep the contaminant in the hood. In practice, hood shape and the influence of crossdrafts on the measured capture velocity have to be considered. All three velocity components should be measured and used to calculate the magnitude and direction of the total velocity. Other methods used, not as good as the previous one, are to measure the velocity with a directional velocity sensor towards the hood or to measure the net velocity by an omnidirectional velocity sensor. In the last method the main airflow direction should be viewed and evaluated by means of a smoke test (see Sections 10.2.1 and 10.2.2.1). 

Canopy hoods A capture hood located above a process, designed to provide a suitable capture velocity to ensure the safe removal of the contaminant produced by the process. 

Capture velocity The air velocity necessary at a point in order to capture and transport to the exhaust opening the contaminants being emitted from a process. 

Range hood An extraction hood positioned above a cooking range to provide the best possible capture velocity of the fumes. 

Zone, capture The area or volume in which a capturing device contains the generated emissions around a process. The capture velocity in this zone must be high enough to ensure the efficient collection of pollutants. 

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