[Relationship between physical configuration of hood and velocity for local exhaust systems]

The hood performance of a local exhaust ventilation system is determined by the capture velocity and the air flow rate. The equation proposed by Dalla Valle has been widely used to speculate this relationship. However, Dalla Valle used different formulae for slot and plain opening (rectangular) hoods, and the calculated values near the boundary between the slot and plain opening by these formulae do not always match each other. In this study, we measured capture velocity of four hoods by changing the distance from the hood face for different aspect ratios. We also tried to develop an empirical formula to express a relationship that could be applied to both types of hoods. As a result, it was found that the relationship between air velocity in front of exterior hoods and air flow rate could be expressed as the sum of two exponential functions with distance. In addition, when the hood aspect ratio was less than 9 the values of the capture velocity calculated by the proposed equation were in good agreement with the experimental ones.