REEXS - Reinforced exhaust system optimization of operating and design parameters

Abstract

All production processes generate air contaminants to some extent. High concentrations will cause negative health effects on workers. The main purpose of local ventilation is to reduce or preferably avoid exposure of workers to contaminants. Exterior hoods are used when other solutions such as enclosing or partially enclosing are not possible. The drawback of exterior hoods is the very limited suction range. A new local exhaust principle REEXS, Reinforced Exhaust System is discussed in this work. It is designed to overcome the limitations of basic exterior hoods by increasing the capture range with a combination of extracted and supplied air. This work introduces a methodology to improve the efficiency of reinforced exhaust systems that uses a combination of experiments and numerical simulation of the airflow. The thesis is focused on the computational method and discusses suitable CFD (Computational Fluid Dynamics) approaches. Two CFD models are proposed and validated by comparison of the computed results to measured data from prior studies. Good agreement over a wide range of operating parameters for both CFD models presented was achieved. The sectored 3D CFD model is used to investigate the sensitivity of REEXS to various design and operating parameters. An operating envelope is proposed. The influence of the supply jet nozzle design on the axis velocity is shown and the design is optimized systematically. Nozzle blocks arranged around the exhaust opening, each with one or several nozzles increase the exhaust axis velocity compared to a REEXS with slot nozzle by at least 10%. Also the influence of parameters like jet exit angle, jet exit radius or hood size are investigated. The gained know-how on optimized design and operating parameters is then used in a computational assessment, where the new reinforced exhaust device is compared with two other traditional exhaust hoods. The fully 3D CFD model is used to determine the capture efficiency of the devices under standardized conditions. For the specific test case, it is shown that the reinforced exhaust device has a capture efficiency of approximately 65%, whereas the traditional systems have capture efficiencies of less than 30%. The increased capture efficiency persists over a wide range of operating parameters. A mobile exhaust system was transformed into a first REEXS prototype with the multi-jet nozzle design. This prototype was tested under laboratory conditions using smoke to visualize the airflow and in practical testing at real welding workplaces. The results of these tests were promising as an increased capture efficiency in the same order of magnitude as observed by CFD was found by tracer gas measurements. Finally, a pilot plant with six REEXS exhaust hoods is installed in a welding shop for heavy duty applications. The efficiency of the system in this work place and over an extended time period is subject to further investigations.