An investigation into turbine ventilators as a potential environmental control measure to minimise the risk of transmission of tuberculosis - a laboratory and field study
Master Thesis
2014
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University of Cape Town
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TB is an airborne infectious disease which is spread by droplet nuclei, carrying Mycobacterium tuberculosis, in the air. The droplet nuclei small enough to enter human respiratory pathways are 1-5 μm in size and are able to travel long distances (Hodgson, et al., 2009) (WHO, 1999), and can be distributed widely throughout (hospital) buildings (Beggs, Noakes, Sleigh, Fletcher, & Siddiqi, 2003). These droplet nuclei may remain suspended in the air until they are removed by dilution ventilation or other disinfection methods (Parsons, Hussey, Abbott, & de Jager, 2008) (National Department of Health, 2007). Dilution ventilation refers to the dilution of contaminated air with “clean” air (ACGIH, 2005), thereby reducing the concentration of contaminants in the room. One of the recognised approaches for minimising the risk of transmission of TB is to adequately ventilate the contaminated room/space. A higher ventilation rate can provide higher dilution capability, in turn reducing the risk of airborne infections (WHO, 2009). The parameters of concern in ventilation design are ventilation flow rate and airflow pattern in the room (and building). The former reduces contaminant concentration while the latter aims to move uncontaminated air to high risk areas, and contaminated air away from occupied areas, usually to the outside. The shortcomings of conventional natural ventilation strategies are well documented. The aim of this research project is to review and study the effectiveness of natural ventilation design supplemented by a turbine ventilator. The project was divided into two components: a field study and laboratory experiments. In the field study, a turbine ventilator was installed into a bedroom of a low-income house in Pretoria. Tracer gas (concentration decay) tests were performed to determine the ventilation flow rates, mean age of air and air change efficiency of four natural ventilation configurations. These included infiltration/leakage (IL), two cases of single-sided ventilation (SS1 and SS2), and crossventilation (CV). Three baseline (without the turbine ventilator) and three turbine ventilator tests were performed, one each in the morning, noon and afternoon. The tests were performed between February and April 2011 on typical summer days. The turbine ventilator was then tested in a laboratory environment under wind, buoyancy and a combination of wind and buoyancy forces. The wind speeds were low, ranging from 0.0 to 0.5 m/s (0.0 to 1.8 km/h), and the temperature differential tested was in the range of 5.5 to 9.3˚C. The in-duct velocities and centreline velocities were investigated to establish if, under the subjected force(s), a capture envelope described by Dalla Valle’s equation could be measured. This envelope would be used to determine if the turbine ventilator could potentially reduce the concentration of airborne contaminants in the test volume. In the field study baseline tests, IL, SS1, CV and SS2 mean – and range of - ventilation flow rates of 0.6 [0.5 – 0.6], 8.1 [6.8 – 9.3], 16.9 [14.7 – 19.0] and 7.4 [7.0 – 7.9] ACH, respectively, were reported. The baseline tests highlight the potential of cross-ventilation where, by simply opening windows and doors, a ventilation rate exceeding IPC recommendations was obtained. All configurations, save An investigation into turbine venti lators as a potential environmental co ntrol measur e to minimise the risk of transmission on TB Page IV SS1, appear to have approached the fully-mixed case.SS1 also showed the greatest variability in ventilation flow rates. This finding is not unexpected, as air exchange in single-sided ventilation is due to wind pressure fluctuations, which varied across each test. In addition, in all tests it was found that the ventilation flow rate was dependant on the natural ventilation configuration and openable area, and not necessarily environmental conditions. In the turbine ventilator tests, the mean ventilation flow rates for IL, SS1, CV and SS2 were 1.8 [1.6 – 2.1], 5.4 [5.2 – 5.7], 17.7 [16.0 – 18.6] and 9.5 [8.5 – 10.1] ACH, respectively. The mean ventilation flow rate increased in IL and SS2 with the installation of the turbine ventilator, while in SS1 a decrease was reported. The increase in ventilation flow rate in IL was found to be due to natural convection, where the turbine ventilator merely facilitated the exhaustion of warm air. The results of the field study are specific to the environmental conditions at the time of the test, and are not generalizable. In the laboratory experiments, the in-duct velocity increased with an increase in wind speed and temperature differential. For a given temperature differential, an increase in wind speed resulted in a decrease in in-duct velocity. Across all tests, no centreline velocity profile, described by the Dalla Valle equation, could be measured. In the wind speed tests, no capture envelope could be established. This was due to the low wind speed test range, where the resulting centreline velocity was beyond the limit of detection of the thin-film sensors. In the buoyancy forces test, a turbulent region near the base of the turbine ventilator was realised, where the magnitude and direction of the air flowing at 1.5D continuously changed. This turbulent region was again observed in the combined wind and buoyancy forces tests, though the magnitude was smaller and occurrence less frequent. The results of the laboratory experiments are specific to the parameters tested, and are not generalizable. By correlating the field study, laboratory experiments, and previous (similar) studies, it was concluded, that, under the tested conditions, adding a turbine ventilator as a supplement to natural ventilation system will not reduce the concentration of contaminants in the occupied zone in a room.
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Salie, F. 2014. An investigation into turbine ventilators as a potential environmental control measure to minimise the risk of transmission of tuberculosis - a laboratory and field study. University of Cape Town.