| In recent years plate heat exchanger(PHE) has been widely used in variousfields of industrial production because of many advantages, such as large heattransfer coefficient, compact structure, convenient cleaning and so on. However, thiskind of heat exchanger also has the problem of fouling, the existence of foulingwould affect the heat transfer, and cause a great deal of energy waste and economiclosses. In this thesis, we choose two kinds of the most common bacteria, ironbacteria(IB) and heterotrophic bacteria(HB) as the research object. The results canprovide theoretical foundations for scale removed and restraining scale of PHE.Firstly, this thesis studies the influences of operating conditions andconcentration of bacteria on the fouling characteristics of IB and HB in PHE. Theexperimental results show that two kinds of bacteria in PHE have obvious foulinginduction period.With the increasing of speed, fouling resistance asymptotic valueof two kinds of bacteria in PHE decreases, and with the increasing of concentration,fouling characteristics of two kinds of bacteria also show the consistency. Foulingresistance asymptotic value increases with the concentration increasing, but theincreased amplitude decreases. For the influence of temperature，the differencesemerge. For IB fouling, the induction period and the time to stabilization reduce withthe increasing of temperature, but asymptotic value of fouling resistance decreases.However, for the HB, with the increasing of temperature, though induction periodbecomes short, asymptotic value of fouling resistance is maximum at35℃in theexperimental temperature range, the time to stabilization increases with thetemperature increasing.Secondly, this thesis studies the different proportion of IB and HB mixedbio-fouling characteristics in PHD, and studies the influence of working condition onthe1:1mixed bio-fouling features. The results indicate that the IB has strong abilityin bio-fouling and the HB has the high secretion rate of EPS. When they are mixed, they could synergism growth. With the increasing of speed, the fouling resistanceasymptotic value of the mixed bacteria shows a trend of decrease of the same, butwith the increasing of temperature, two kinds of bacteria dominate by turns, the timeto achieve stable reduces, but the fouling resistance asymptotic value increasesslightly.Finally, the Scanning Electron Microscopy (SEM) and energy spectrumanalyzer is used to observe and analyze the fouling layer on the stainless steel of twokinds of bacteria. The results show that the density of fouling could be found on thelight slice, and the fouling samples contain the same scale elements basically, but thepercentage composition is different. |
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