Document Z8ozgQ4OQNL3nBrB2RJXgp9n0

i is 7 4.4.2 Dicperslon by Air Currents. Particles of hygienic sig nificance are usually dispersed by air currents. .4.5 Typos....of Air Motion. Stray-or--secondary air motion around in dustrial processes and operations may be classified into four major types: (1) Notion or energy fundamental' to the machine or process (viz, fan action of a rotating wheel, escape of air from con tainers as they are filled, convection currents and air expan sion over heated surfaces) (2) Forces incidental to the operation of the machine or process (viz, air escaping past worn pistons in pneumatic tools I or from badly placed exhaust ports, and vibration of machinery) (3) Prag of air by large particles dynamically projected from their source (viz, large particles dispersed from a surfacing machine carrying fine particles in their wake) (4) External forces (viz, air motion created by nearby mach inery operation and by workers; natural convection currents caused by temperature .changes; and air currents from nearby doors or windows, fans or heaters) 4:6 Control of Air Motion. The primary purpose of an exhaust hood is to confine or capture contaminated air, or both, rather than to remove the contaminating material from the air. To prevent the dispersion of atmospheric impurities, it is necessary to eliminate or reduce stray or secondary air currents and to develop primary or control air currents of the proper magnitude and direction about the zone of dispersion. Before the actual design of an exhaust hood is undertaken it is essential for 8005 1291 PRODUCED BY FORD