The relationship between indoor carbon dioxide and fine particulate matter (PM₂.₅) concentrations in mechanical ventilation system

機械式通風系統對於室內二氧化碳與微細懸浮粒子 (PM₂.₅) 濃度之間的關係

Student thesis: Doctoral Thesis

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Author(s)

  • Kin Ho YU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date15 Jul 2015

Abstract

Carbon dioxide (CO2) and particulate matter are two major indoor pollutants. Almost all offices in Hong Kong are air-conditioned. For such office microenvironments, the former pollutant is from people while the latter one is come from ambient. In the literature there are many studies focusing on one of these individual pollutants. In reality the gaseous pollutant and the particulate co-exist. Controlling one of them may affect another in a complex and dynamical way. This study evaluated the performances of common constant outdoor air volume (COAV) system and variable outdoor air volume (VOAV) ventilation system when carbon dioxide and particulate matter are significantly present in indoors and outdoors environments respectively. Major system parameters including filter efficiency, air exchange rate, occupancy number, ventilation air rate, and outdoor particle concentration were thoroughly examined. Initially, various sets of experiments were performed in a full-scale environmental controlled chamber experiment to investigate the dynamics of CO2 and airborne particles under both steady and unsteady scenarios. The experimental results were further validated with a state-space mathematical model. It is found that both the experimental results of COAV and VOAV scenarios matched well with those predicted by the model. In addition, by using the state-space model, an exhaustive case study was conducted. A simulation of a general office working period with five people accommodated from 9 am to 9 pm was studied. Three variable parameters of fresh air flow rate per person, filter efficiency and outdoor air particle concentration were considered. This simulation studies were based on outdoor particles concentrations at 50 μg/m3, 100 μg/m3 and 200 μg/m3 which resemble well with real and severe outdoor environmental conditions in Hong Kong. From the simulation, numbers of cases shown that COAV and VOAV cases under different parameters have different exposure measurements for indoor particles concentration and CO2 concentration. For those three assigned outdoor fresh air flow rates, in COAV cases, where normal outdoor particles concentration is 50 μg/m3, the indoor PM2.5 particles concentrations can also be obtained less than 25 μg/m3. The CO2 concentrations mean values are between 855 ppm and 635 ppm correspondingly. Contrarily, for general outdoor fresh air flow rate 8.5 L/s/p, when dealing with 200 μg/m3 a 60% efficiency filter can meet the particles concentration requirement 25 μg/m3 and the CO2 concentrations mean value can fall into good class 855 ppm. For VOAV cases, the mean values of particles concentration are slightly lower than COAV cases but CO2 concentrations are a little higher than COAV cases. In order to find out the optimal ventilation schemes, some of the exposure measurements between particles concentration of PM2.5 and CO2 concentration under different scenarios have been generated for application purposes. A major finding from this study was that dilution of CO2 may adversely affect the control of indoor particle concentrations. To reduce CO2 concentration, both COAV and CO2-based demand-controlled VOAV may cause an undesirable increase in particle concentration when outdoor air is heavily polluted by particles. This dilemma situation requires further studies on the optimization of ventilation schemes.

    Research areas

  • Particles, Ventilation, Indoor air pollution, Carbon dioxide