Natural ventilation : an evaluation of strategies for improving indoor air quality in hospitals located in semi-arid climates

Abstract

This thesis is an investigation into improving natural ventilation in low rise hospital wards in Northern Nigeria. The climate of this region is semi-arid, during the dry season, sub-Saharan fine dust (Harmattan dust) is blown into the region from the North East and during the wet season, Mosquitos are prevalent. The energy infrastructure in the whole of Nigeria is under resourced; hence ventilation strategies’ based on mechanical extraction are not possible. Five wards within low rise hospital buildings were studied; these were purpose designed hospital buildings, not converted buildings. Questionnaire surveys of health care workers in the hospitals was conducted and revealed dissatisfaction with the buildings’ ventilation and Indoor Air Quality. The questionnaires were then followed up by Tracer Gas measurements and during the period of measurement there was only one occasion when a ward achieved an air change rate of 6-ach-1, the ASHREA Standard requirement for hospital buildings. To investigate methods of improving natural ventilation in these wards, a CFD model was developed of a representative ward, the model was validated against the Tracer Gas measurements; with an acceptable agreement of ≤ 15%. Using the CFD model, achievable ventilation strategies within the context of the location, were investigated, and a combination of cross ventilation utilizing windows on the windward and leeward sides of the ward together with a roof ventilator on the leeward side proved the most successful. All openings were screened to prevent the entry of mosquitos. This best case was further investigated with the wind direction at an oblique angle to the ward side. The oblique angle of wind attack reduced the air change rates but improved air circulation/mixing within the ward. With the exception when the wind direction was parallel to the ward side. To reduce the ingress of Harmattan Dust, was problematic given the energy restrictions, a low energy solution of introducing screened plenums on both the windward and leeward sides of the building proved successful. Larger dust particles were detained within the windward plenum and the smaller dust particles were exhausted into the leeward plenum. With the mosquito screens located on the large surface area of the plenum, the window screens were removed resulting in higher air change rates. Thus, it is recommended that, openings should be provided on the windward and leeward walls and on the roof toward the leeward side for efficient ventilation and airflow circulation at the occupancy level. The longer sides of the wards should be oriented toward the North-South to capture the North-East trade winds and South-West monsoon winds with oblique angle of attack. Plenums should be incorporated to the windward and leeward facades and Insect screen should be installed on the plenums instead of the wards’ openings to increase ventilation rates while excluding mosquitoes and decreasing dust particle concentration in the hospital wards. Openings should be at the middle of the windward and leeward walls and on the roof toward the leeward to avoid airflow short-circuiting. It is recommended to use insect screen with the porosity of 0.2 and when the outdoor local wind speed is ≤ 1.26 m/s (2 m/s: airport value), the ventilation should be supplemented with fan.