Determination of Capture Velocity

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).

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