Air flow disturbance by moving objects at local exhaust ventilation

University essay from Högskolan i Gävle/Avdelningen för bygg- energi- och miljöteknik

Abstract: The present thesis aims to study the effect of human movements on local exhaust ventilation. In its simplest terms, local exhaust ventilation is a system which has the function of extracting contaminated air situated close to the contaminant source, protecting a working person from exposure to hazardous substances by containing or capturing them locally, at the emission point. As an important security measure referred to terms of health, it is crucial for the healthiness of workers to control and prevent them from the exposure to vapour, mist, dust or other airborne contaminants. Additionally, to a lesser degree of significance, it can be stressed an expected increase in worker performance due to an improvement of the working conditions. There is an existing necessity for well-defined and appropriate methods to test the performance of local exhaust devices in order to reach standard efficiency values. The lack of an international standardization led to the realization of this study, which, ultimately, has the purpose of obtaining relevant results that can be utilized for future normalized test procedures. The study entails full scale experimental measurements that include air velocity measurements in 3 dimensions, a local exhaust ventilation device with circular hood and a flat flanged plate and a controlled generation of air turbulence through physical movements of a human-sized cylinder, simulating a walking person. The present study extends previous similar studies at the University of Gävle, where the controlled air turbulence was generated by a moving plate. After meaningful results obtained in that study, one of the considerations was to better simulate a walking person, by replacing the plate for a movable cylinder. The present study points at a larger similarity occurring with a cylinder than with a plate, as regards the air flow pattern produced by a real walking person. As in the previous study, the Percentage of Negative Velocities, PNV, has been used as the main measure of turbulence induced risk of contaminant spread. The PNV represents the fraction of the time when the flow is directed opposite to the suction air stream in front of the local exhaust hood. The obtained results conclude that the use of the cylinder as a moving object has been an improvement to simulate the effect of the movement of a human being on a relaxed walking pace. The present study was carried out in parallel with the thesis work by Leyre Catalán Ros, which complements this study by analyzing the effect of an added heated dummy, simulating a person seated in front of the local exhaust device.

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