The overall efficiency of any dust and fume extraction installation owes much to the final hood design and construction. The prime purpose of all pollution control plants which have hoods as an integral part of the system is to contain and then entrain the generated dust and fume. In all plants, the position of the hood in relation to the dust and fume source dictates the extraction rate.

In Dust Extraction systems, every effort should be made to ensure the hood is sited in such a position that the natural throw of the material is in the direction of the hood. By making use of this natural assistance, the extraction rate of the system that is required to entrain the dust into the duct work can be kept low. By incorporating the captive velocity as close to the dust emission as possible, the initial capital installation, and the subsequent running costs are kept to the minimum
 

For a given hood shape, the tendency is for air to be drawn equally from all directions. The velocity drops rapidly as the distance from the hood increases. The diagrams illustrate the velocities obtained, with the distances expressed as percentages of hood diameter. It is evident that a hood cannot control dust particles at any distance from the hood mouth. A flanged hood will control dust, using an air volume of approximately 30% less than would be required for an unflanged opening.