The Characteristics Of CaCO₃ Fouling In Smooth Tube And Their Relations With Reynolds Number

The crystallization fouling deposited on heat transfer equipment is a very common phenomenon. In China, calcium carbonate(CaCO3) is the most common crystallization fouling on the surface of heat exchangers. Therefore, this paper focuses on the crystallization fouling of calcium carbonate, especially on the properties of the calcium carbonate fouling on the surface of 316 L stainless smooth steel and its effect on heat transfer. The results can provide some basic information of fouling mechanism.In the first part of the paper, the model of calcium carbonate fouling on the heat transfer surface is established; and the fouling mechanism is investigated, and the main factors that affect the deposition rate of calcium carbonate and the period of the roughness delay are discovered. These factors are the fluid properties, the fluid flow velocity, the wall temperature, the bulk temperature of solution and the material and the surface condition of heat transfer surface.In the second part of the paper, study methods that include the laboratory set-up, the preparation of the test solution, the experimental procedure, the deposition process and the data acquisition and reduction are introduced.In the third part of the paper, the calcium carbonate fouling characteristics that vary with time and Reynolds number(Re = 600~10000) are investigated. When inlet temperatures of cooling water, the inlet and outlet temperature differences of cooling water, the bulk temperature of the solution and pH value of the solution remain constant, the correlation between the fouling growth characteristics, Reynolds number and time are established. The mean growth rate of the fouling is obtained by weight method. The results show that thermal resistance of the fouling exhibits four distinct time regions: a region where the thermal resistance of the fouling is positive, a region where the thermal resistance of the fouling is negative, a region where the thermal resistance of the fouling increases rapidly, and a region a region where the thermal resistance of the fouling increases steadily. When the flow velocity ranges from 0.06 to 0.16 m/s, the roughness delay time decreases with increasing flow velocity. While u = 0.16~0.50 m/s, with increasing flow velocity, the shear force at fouling/liquid surface increases, which prevents CaCO3 deposition on the heat transfer surface, ultimately results in the increase of the roughness delay time. When u = 0.50~0.80 m/s, the roughness delay time decreases with increasing the flow velocity. As Re ranges from 600 to 2100, the mean growth rate of the fouling increases with increasing Re. The mean growth rate of the fouling decreases with increasing Re when it ranges from 2100 to 3000. When Re is larger than 3000, as increasing in Re the removal rate increases significantly, the flow velocity does not affect the mean growth rate of the fouling any more. Under the laminar, transitional and turbulent flow conditions, the correlation between the fouling growth characteristics, Nusselt number, Reynolds number and time have been established.