Abstract
The present ventilation design practice as well as the ventilation standards and building regulations are based on the assumption for complete mixing of air in occupied spaces. Required flow rate of outdoor air for dilution of metabolic CO2 generated by occupants is calculated to keep the CO2 concentration below certain required level. The CO2 concentration measured in the exhaust air or in the room but far from the occupants is assumed to be the same as the CO2 concentration in the air inhaled by the occupants. However, this assumption is seldom accurate, especially in spaces with closely seated occupants, such as classrooms, meeting rooms, etc. In such spaces the CO2 sources, i.e. the people, are close to each other and the CO2 concentration in the inhaled air may be much above the CO2 concentration level recommended as a limit in standards. This is because the upward free convection flow that exists around human body entrains the air with high CO2 concentration exhaled by seated people and move it to their breathing zone. Furthermore, the thermal flows generated by occupants’ body interact with the ventilation flow, which often results in insufficient dilution of the generated CO2 (as well as other pollution) and high levels of CO2 concentration at the breathing zone of occupants. This problem is discussed in the present paper in detail. The discussion is supported by results of measurements in a meeting room with mixing air distribution. People were used to generate metabolic CO2 and a breathing thermal manikin was used to measure accurately the CO2 concentration in the inhaled air. The results confirmed that inhaled CO2 concentration was much higher than the one at the exhaust and that there is need for changes in the present CO2 based ventilation design practice. Possible solutions are suggested.
Highlights
It is well documented that environment in enclosed spaces, including buildings and vehicles, is important for occupants’ health, comfort and performance
In addition to this, during the day, as the intensity of person’s activity and the demand for energy increase, a greater proportion of energy is usually provided by oxidation of carbohydrates and respiratory quotient (RQ), respectively R, at respiratory steady state, approaches unity [10]
More advanced demand control ventilation systems supply outdoor air at changeable flow rate so that to keep the CO2 concentration level in the air exhaust from the room below a defined maximum value. This approach in the present ventilation design practice as well as in the ventilation standards and building regulations is based on the assumptions for complete mixing of air in occupied spaces
Summary
It is well documented that environment in enclosed spaces, including buildings and vehicles, is important for occupants’ health, comfort and performance. Ventilation of spaces is intended primarily to provide occupants with clean air for breathing. Ventilation air is used for removing or supplying heat in order to provide thermally comfortable environment for occupants. In order to avoid the risk of draft discomfort in the occupied zone and with development of low power lighting, office equipment (PC, printers, etc.), highly performing windows and building insulation, the use of radiant heat exchange systems, e.g. chilled ceiling, floor cooling and heating for providing comfortable thermal environment increases. The focus on the importance of ventilation for providing high quality of indoor air increases. Numerous studies document the importance of breathing clean indoor air for occupants’ health, i.e. for reducing sick building syndrome (SBS) symptoms, such as headache, difficult to concentrate, etc. The importance of indoor air quality for performance of office workers and learning of pupils in schools has been documented as well [3, 4, 5]
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