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ABSTRACT
This paper presents the study of a local exhaust ventilation system with plain (unflanged) and flanged hoods. Centerline velocity and velocity contours in front of exhaust hood openings were measured and compared to other previously reported results. Centerline velocity correlations are derived for a full range of hood axes. The effect of turbulence intensity and surrounding equipment on the velocity contours is also analyzed. Capture velocity for three different types of contaminant particles (saw dust, wheat flour, and sand) was determined. Flanged hoods provide higher velocity compared to plain hoods in front of the exhaust opening. Surrounding equipment blocks the energy of the moving flow and tends to reduce the reach of exhaust hoods. Source location has an appreciable effect on the capture velocity for wheat flour and saw dust but little effect on the sand particles.
INTRODUCTION
To safeguard the quality of air (ASHRAE 2001 ; ISO TR/7708 1983) in an indoor work environment, two important things have to be taken into consideration: one is to ensure the comfort of the individual workers, and the second is to prevent pollutant agents or contaminants from damaging the health of the operators or the quality of the industrial products.
The most common method of controlling the air quality in a work environment is the use of local exhaust ventilation systems. For a given air contaminant in a workplace, the local exhaust system dramatically reduces the airflow rates and the energy consumed by the ventilation system and, in addition, stops harmful or toxic substances from spreading uncontrollably.
The local exhaust system consists of five basic components: (1) hood or the entry point of the system-the most effective hood uses the minimum exhaust volumetric flow rate to provide maximum contaminant control; (2) duct system, which transports contaminated air from the source to the exhaust stack; (3) air-cleaning device, which prevents reentrainments of contaminants to work areas and permits clean air to recirculate; (4) the air-moving device; and (5) exhaust stack, which releases the contaminants effectively.
This work will concentrate on the hood design. It analyzes the flow in front of different hood geometry, taking into account all other variables that affect its performance.
Cascetta (1996) determined the velocity fields in the proximity of freestanding exhaust inlet...