ABSTRACT
Local exhaust ventilation differs fundamentally from general ventilation in that the arrangement of exhaust equipment envelops the source of contamination by positive air currents which immediately remove contaminants before they can escape into the general air. The design of exterior hoods is a process in which an airflow pattern of suitable size, shape, and strength is superimposed on the air motion pattern characteristic of the pulvating action to envelop and dominate the latter. Even though air streaming toward the exhaust opening laterally may serve no useful purpose, it is a "cost" which must be paid in order to realize the desired velocity at the point of interest. The distortion, however, mainly occurs close to the exhaust opening at distances of little practical consequence. An exhaust opening at the apex of a trihedron provides a velocity pattern in which equal velocity contours tend to approach the shape of one-eighth of a sphere.
