Abstract: Nuclear energy, as a high-quality energy source characterized by safety, stability, low carbon emissions, and high efficiency, is considered an ideal choice for constructing a clean heating system in northern regions. However, practical operation of nuclear heating systems faces notable challenges: heating loads fluctuate frequently with weather conditions and user demand, making it difficult for reactor operating characteristics to adapt to the dynamic response of load variations; meanwhile, the lack of heating demand during non-heating seasons forces heating reactors to shut down, compromising system operational efficiency and economy. Therefore, to ensure the safe, stable, and efficient operation of nuclear reactors, a heat exchange system with multiple sets of parallel heat exchanger batteries was designed to accommodate operating requirements under different load conditions. The commercial software Aspen Plus was employed to establish a model of the heat exchange system, verifying operation schemes under various working conditions. Additionally, the supporting software Aspen EDR was used to simulate evaporators under different shell-side flow rates, obtaining parameters such as temperature, vapor fraction, and heat transfer coefficients. Variations along the shell diameter and flow direction were analyzed to understand the internal working conditions of the evaporator under different operational scenarios. Analysis of the results reveals that when heat exchangers operate under off-design conditions, there exists an area margin, leading to a certain discrepancy between simulated and verified calculations. The range of area margin under off-design conditions can reach -37.96% to 64.62%. A positive area margin indicates that the actual heat transfer area exceeds the required area, resulting in lower outlet temperatures of the hot fluid, higher outlet temperatures of the cold fluid, and an increased heat load of the exchanger. Conversely, a negative area margin implies the opposite. Excessively high area margins trigger software warnings. Further simulation analysis of evaporator performance under various conditions showed that under 100% load with three columns in operation and a shell-side flow rate of 25 kg/s, the vapor fraction in the tube bundle zone was relatively high, reaching 100% at 514 mm in the Y-direction. The steam at the tube bundle outlet became superheated, exceeding 240°C, indicating a high risk of tube bundle overheating. When the shell-side flow rate was increased to 26 kg/s, the overall shell-side temperature decreased without generating superheated steam, and the maximum vapor fraction in the tube bundle zone was only 8.31%.