Shell and tube heat exchangers, highly efficient heat exchange devices widely used in industry, are susceptible to the dynamic changes in fouling resistance. Fouling resistance refers to the additional heat transfer resistance caused by deposits on the heat exchange surface. Its dynamic changes directly alter the heat transfer coefficient of the heat exchanger, thereby affecting heat transfer efficiency. This effect can be quantified through a combination of theoretical models, numerical simulations, and experimental verification.
The dynamic changes in fouling resistance are primarily driven by factors such as fluid properties, flow rate, temperature, and operating time. In shell and tube heat exchangers, particles, corrosion products, or microorganisms carried by the fluid gradually deposit on the heat exchange surface, forming a fouling layer of varying thickness. As the fouling layer thickness increases, its thermal conductivity becomes significantly lower than that of the metal tube wall, resulting in a gradual increase in thermal resistance. During this process, the fouling layer's growth rate is not constant, but rather exhibits a dynamic pattern of rapid initial growth followed by a period of stabilization. Dynamic models are required to capture this dynamic behavior.
The quantified impact of fouling resistance on heat transfer performance can be analyzed using heat transfer equations. The overall heat transfer coefficient is inversely proportional to the fouling thermal resistance. As the fouling thermal resistance increases, the overall heat transfer coefficient decreases accordingly. For example, if the overall heat transfer coefficient in the clean state is fixed, each increase in the fouling thermal resistance may cause the overall heat transfer coefficient to decrease by a certain percentage, resulting in a decrease in heat transfer under the same temperature difference. This quantitative relationship can be established through experimental data fitting or theoretical derivation, providing a basis for heat exchanger design.
Numerical simulation technology provides an effective means to quantify the dynamic impact of fouling thermal resistance. By establishing a three-dimensional numerical model of a shell-and-tube heat exchanger, the velocity, temperature, and pressure distributions under varying fouling thicknesses can be simulated. Simulation results show that fouling primarily accumulates on the inner surface of the heat exchange tubes and near the shell-side baffles, resulting in a significant decrease in the local heat transfer coefficient. As fouling thickness increases, the overall pressure drop of the heat exchanger gradually increases, while the heat transfer efficiency decreases nonlinearly, requiring iterative calculations for accurate quantification.
Experimental research is key to validating quantitative models. Taking a reduced-top air-to-circulating water heat exchanger as an example, long-term operational testing revealed that an increase in fouling thermal resistance leads to an increase in condensate temperature, necessitating corresponding adjustments in steam flow rate to maintain heat transfer efficiency. Comparison of field data with numerical simulation results showed high consistency in the heat transfer coefficient decay trend, validating the accuracy of the quantitative model. Furthermore, experiments demonstrated that the impact of fouling thermal resistance on heat transfer performance is more pronounced in high-temperature media.
A variety of optimization measures can be implemented to mitigate the dynamic changes in fouling thermal resistance. For example, increasing flow velocity can enhance fluid turbulence and inhibit fouling deposition; applying an electric field or adding antiscalant agents can alter the fouling growth environment and reduce the deposition rate; and regular online cleaning can remove existing fouling layers and restore heat transfer performance. The effectiveness of these measures can be evaluated using quantitative models, providing a scientific basis for developing heat exchanger operation and maintenance strategies.
From an engineering perspective, quantifying the dynamic impact of fouling thermal resistance is crucial for heat exchanger design. During the design phase, it is important to consider the increasing trend of fouling thermal resistance, appropriately reserving heat transfer area, or employing easily cleanable structures. For example, for long-term shell and tube heat exchangers, the heat exchange area can be increased proportionally to compensate for performance degradation caused by fouling thermal resistance. Furthermore, by monitoring changes in fouling thermal resistance in real time, operating parameters can be dynamically adjusted to ensure the heat exchanger maintains high efficiency.
