The Effect Of Tube Wall Material On Growth Of Calcium Carbonate Fouling And Its Mechanism

The deposition and growth of the fouling on the heat transfer wall is a very complicated process,which not only contains the heat,mass and momentum transfer processes,but the various physical changes/chemical reactions occurring on the heat transfer wall also play a significant role in deposition of fouling.In China,90% of the heat exchangers have different degrees of fouling,resulting in a great loss of energy.Since the fouling on the heat transfer surface is mainly composed of CaCO₃ crystals,so it is of great practical significance for the study of CaCO₃ fouling deposition.In this paper,the study is mainly focused on the crystallization of CaCO₃ fouling and its contents include the following aspects: the effect of wall materials on CaCO₃ fouling and its effect on heat transfer characteristics are investigated as well as the fouling mechanism of CaCO₃ on the heat transfer wall,and the morphological characteristics of the crystal fouling are also observed.The fouling experimental device has been improved.The supersaturated CaCO₃ solution with concentration of 60 mg/L is used as a circulating working fluid,the test tube with the same specifications is made by stainless steel?316L?,aluminum alloy and brass alloy,respectively.The experimental conditions are controlled as follows: the velocity of working fluid in the pipe is 0.11 m/s?Re = 1381?,temperature of the hot water bath for heating is 80±0.5 °C,and the inlet temperature of working fluid is controlled at 55±0.5 °C.Experimental results show that: the growth characteristics of CaCO₃ fouling on the test tube made of different materials are different in three stages?positive thermal resistance,negative thermal resistance and full fouling development?;at the same time along the circulating fluid flow direction,the crystal distribution is more uniform,presenting a characteristic as former sparse and then dense distribution;with processing of the experiment,the amount of crystalline fouling on the pipe wall gradually increases as well as the crystal size,but the crystal structure in different stages is different;different from the stainless steel pipe,the wall of aluminum alloy pipe will form a film composed of alumina oxide while the wall covered by CaCO₃ fouling does not,resulting in a gully-like protrusion on the pipe wall which greatly increases the roughness,it is because the number of nucleation nuclei is more on the gully-like protrusion,the CaCO₃ fouling and the alumina oxide layer form a cross-growth trend,this results in faster fouling on the aluminum alloy pipe and the fouling period is much shorter than that on the stainless steel pipe;however for the stainless steel pipe there is no oxide layer on the pipe wall,the physical properties of the pipe wall before and after fouling are not changed,moreover the crystal size formed on the pipe wall is large and morphologies of CaCO₃ scale is mainly hexahedral calcite,while the fouling deposition rate is much slow,these are due to both the effect of the fluid shear stress and the impact of fouling aging on the fouling surface.