Effect Of Air Conditioning Parameters On Microbial Growth In Dust Accumulated In Supply Air Duct

Due to long running and large inner surface of supply air duct which is considered as an important component of HVAC(Heating,Ventilation and Air Conditioning),a significant amount of dust attached with microorganism may accumulate mainly on the bottom of supply air duct.As a consequence of enhanced conditions of microbial growth and survival,metabolites,spores or even viable cells can be released into the supply air and cause health problems to occupants of the buildings.Because of the deposition of dust on the deep pipeline,the depravation of IAQ (Indoor Air Quality) caused by microbes associated with dust is always implicit and the mechanism of microbial contamination has not been found out for lack of disciplinary investigation.Furthermore,it is difficult to evaluate the extent of microbial contamination quantitatively and to establish technical measures for controlling the microbial contamination of HVAC system.To investigate the effect of air conditioning parameters(temperature,relative humidity and air velocity) on microbial growth in dust accumulated in supply air ducts, a controlled air duct was set up.A public building with HVAC system in Xi'an was selected and an appropriate quantity of original dust accumulated in the supply air duct was sampled.After delivering the original dust sample into the controlled air duct,a series of experiments for testing microbial concentration(including bacteria and fungus) in 64 hours were conducted in different conditions of temperature and relative humidity when the air velocity was always 2.0m/s.Test data were analyzed by using orthogonal design.The results indicated that gram-positive bacteria,gram-negative bacteria accounted for 53%,47%respectively in total bacteria count and the dominating genera of fungi were Penicillium,Aspergillus,Cladosporium.There was a positive correlation between the count of microorganism and the quantity of dust.The accumulation of dust on the bottom of duct was influenced by air velocity to a certain extent.In the range of 22℃-32℃and 40%-90%relative humidity(RH),microbial growth was obviously accelerated with a higher temperature and RH increase.It was more beneficial to microbial growth when the relative humidity exceeded 80%.The effect of RH on microbial growth was greater than that of temperature.Besides,organic material contained in the dust could be considered to affect microbial growth.