Thermal & indoor air quality environment on air-conditioned buses

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Thermal & indoor air quality environment on air-conditioned buses


Author: Shek, Ka Wing
Title: Thermal & indoor air quality environment on air-conditioned buses
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Buses -- China -- Hong Kong
Buses -- Air conditioning
Indoor air quality -- China -- Hong Kong
Buses -- Heating and ventilation
Department: Dept. of Building Services Engineering
Pages: xxiv, 220, [6] leaves : ill. ; 31 cm.
InnoPac Record:
Abstract: Air-conditioned buses have been serving Hong Kong over two decades. There are now approximately 5750; over 95% of the service fleets. Bus manufacturers and local operators are keen on the service quality improvement by modifying the air-conditioning designs, including system performance and reliability, parameter settings as well as energy effectiveness. However, complaints about poor air quality and thermal discomfort were received since the beginning of services. Such issues suggest less-than-satisfactory environments on these buses. Further study is necessary to enhance the in-bus commuting environment. Considering the in-bus air quality and thermal comfort environment, physical measurement and questionnaire survey were conducted to review the real scenario. Data collected from physical measurement provided clues to evaluate the dynamic effects from surrounding environment. The in-bus air quality varied when the buses travelled on different routes. The average in-bus CO concentration level was the highest on tunnel routes (4.4 ppm) followed by urban district routes (3.4 ppm). It was the lowest on rural routes (1.3 ppm). However, the I/O ratios on different routes were similar (between 2.1 and 3.4). Such variation caused by the change of roadway environment, like traffic density, surrounding building density as well as the outdoor air quality. Mechanical ventilation rate is another key to the in-bus air quality. The rate is fixed on present air-conditioned buses. Measurement result showed the mechanical ventilation rate was 250 l·s⁻¹ on a stationary bus and it rose on a travelling bus. The rate was 380 1·s⁻¹ when travelling at 30 km·hr⁻¹ and it reached 535 1·s⁻¹ when traversing on a highway (at 65 km·hr⁻¹). It was equivalent to varying the outdoor air rate per person from 1.9 l·s⁻¹ to 4.1 l·s⁻¹ on a fully loaded bus. Since the bluff head generates aerodynamic drag on a travelling bus, pressure difference is induced across bus body surface. Thus ventilation rate varies with the bus travelling speed. Higher rate helps dilute in-bus air contaminants but it increases the risk of infiltrating concentrated air pollutants when travelling in congested area. Nevertheless, lower rate results as insufficient ventilation that causes air stuffiness and odour inside passenger compartment. Therefore the mechanical ventilation rate should not be fixed but adjusted depending on the roadway air quality. Empirical comfort models were developed by concluding the correlation between the physical parameters and the subjective sensation votes from passengers. They provide a convenient platform to quantify and identify the in-bus air quality and thermal comfort through the percentage of dissatisfaction. Moreover, the computation result from models can be applied in the air-conditioning system to optimize the system control. Real-time monitoring of the air quality and thermal comfort indicators are obtained by equipping sampling and data processing instruments on buses. The computation result of real-time data provides determinants for the control system to set an appropriate outdoor air intake rate and thermal comfort settings. The application can reduce the percentage of dissatisfaction level from 35% (present) to 8%. The in-bus environment can be controlled at the most comfortable condition along the journeys.
Mixing-ventilation air distribution system was applied in the present passenger compartment. It aimed to provide a uniformly air distribution in the compartment. However, supply air was not well distributed due to tight space in the compartment. Thermally discomfort was caused due to temperature stratification and draft risk. Also, fresh air might not directly reach the passengers causing ineffective air contaminants dilution. Hence, personalized air supply and return system was proposed in new compartment in order to solve those faults. Diffusers were installed in front of each seat while return air grilles were located above each row for better air distribution. Also, computation result from the empirical comfort models was applied to improve the in-bus environment. Simulation result showed the new compartment minimized those thermal discomfort issues found in the present compartment. The new system improved the air distribution by increasing the ventilation efficiency (from 0.93 in present compartment) to 1.32. Moreover, the effectiveness of personalized air supply and return system was evaluated by means of thermal comfort, concentration distribution and particle transport. The tracks of coughed droplets expelled from an index person were simulated in the compartments. Simulation result showed that the level of influence caused by the particle dispersion depended on where the index person sat in the present compartment. Passengers sitting behind the index person were possibly influenced by the infectious droplets. Thus the number of influenced passengers reduced if the index person sitting closer to the rear section. However, the number was less in the new compartment, and which was similar for the index person sitting in different sections. The percentage of particles inhaled by other passengers maintained below 0.04%. It revealed the personalized air supply and return system was effective in preventing the spread of infectious droplets in the whole compartment. The air-conditioning load and energy consumption were simulated to compare the system operating under different scenarios, including in present compartment, present compartment with applying new settings and new compartment. Set-point air temperature and mechanical ventilation strategy were adjusted depending on the computation result from empirical comfort models. Such adjustment was intended to improve the in-bus environment while satisfying a substantial majority of passengers. The application of new air-conditioning settings lowered the passengers' dissatisfaction level. However, higher energy consumption in both heating and cooling was resulted in present compartment. The consumptions rose by 9.1 times and 49% respectively. The required heating and cooling capacities were increased by 3.1 times and 98% respectively. The present air-conditioning system might not be applicable to fulfil the settings adjustment. Applying the new air-conditioning settings in the new compartment, simulation result showed lower heating and cooling capacities were required. The required capacities were diminished by 55% and 36% respectively, as compared with that the new settings applied in present compartment. Also the energy consumptions in heating and cooling were reduced by 64% and 28% respectively. The results revealed the personalized air supply and return design helped to achieve energy effectiveness by increasing the ventilation efficiency. Also adjustment of air-conditioning settings improved the commuting environment while satisfying a substantial majority (above 80%) of passengers. Therefore the personalized air supply and return system and new air-conditioning settings were suggested on new buses. Finally three elements approach was developed. The three elements were the engine, passenger compartment and air-conditioning system. It assisted to improve the in-bus air quality and thermal comfort environment in the aspect of design, operation and maintenance on air-conditioned buses.

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