| People's living condition has been markedly improved with the rapid economic growth and massive urbanization. Large numbers of factories, high-class office buildings and super shopping centers come forth unceasingly, which make central air conditioning systems widely used. Heating, ventilating and air conditioning (HVAC) system not only provides breathing or manufacturing air but creates pleasing indoor air environment by adjusting the air temperature and humidity, also the air cleanness. Previous investigations show that people spend most of their time indoors and indoor air quality (IAQ) affect people's health directly. But recently occurrences arouse people's attention for the bad operation or maintenance which resulted in poor IAQ. About which the most pivotal problem is the dust deposition in ventilation ducts and the understanding of the deposition mechanism is also the most important step to evaluate the indoor human particulate matter (PM) exposure.Firstly the research background is introduced and literatures are reviewed. Based on the particle diffused deposition to the vertical wall, a integrated mathematical model is developed to explain and calculate the particle deposition rate to the three orientations of straight rectangular HVAC ducts, ceiling, wall and floor respectively. According to the model, there are such assumptions: (1) the particle was rigid;(2) no coagulation happen;(3) there is a displacement of the velocity profile owing to roughness;(4) the particles will deposit if they reach the outer parts of the surface roughness. For validating the model's reliability the calculating program is developed under the following conditions: (1) the particle diameter d_p is 1μm, 3μm, 5μm, 9μm, 16 μm, respectively;(2) air supply velocity u is 2.2m/ s,5.3m/s, 9.0m/s, respectively;(3) the friction velocity u is 0.12m/s, 0.28m/s, 0.45m/s, respectively. Comparison to the literature experiment results it shows a relatively precise calculation and mostly the calculation deposition velocity agrees well with theexperimental value. The discrepancy of the wall and ceiling deposition is obvious, but here the useful empirical equations are presented to estimate the dust deposition onto the ceiling and wall of rectangular HVAC ducts by literatures. According to the Ministry of Health of China's HVAC Cleaning Code 2006, the HVAC air duct system will need a through cleanness immediately if the dust accumulation reaches to 20 g I m2. Citing the requirement in the Code and the mean outdoor atmosphere particle concentration in Xi'an city during year 1997-1999, the particles mass flux and dust accumulation time in a cross sectional 200mm *200 mm straight HVAC duct, with the air supply velocity 5.3 mis and air temperature 15°C and the mean duct surface roughness 0.15 mm, is analyzed, and the air filter total counter efficiency is tj = 0 ^ rj = 40% .. tj = 80%, respectively. Taking the HVAC system of ShanXi History Museum as an example, the dust accumulation velocity is calculated and the operation time is presented. At last, the effect of particles deposition on the concentration relationship between indoor and outdoor air (I/O ratio) is discussed, and the equations are acquired.
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