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The activity focuses on improving the overall performance and design of a small-sized heat exchanger. This heat exchanger is designed to raise the temperature of a fluid (water), using tubular electric heaters. The device consists of a steel case with two hydraulic connections for fluid inlet and outlet. The case is sealed with a holed plate where the tubular heaters are brazed. The objective of the activity is to simulate the fluid dynamics, thermal behaviour, and structural integrity of the initial geometry, and then modify the design to enhance performance in terms of fluid outlet temperature and device robustness. The simulations are conducted using COMSOL Multiphysics 6.2, employing the Turbulent Flow k-epsilon interface for fluid dynamics, the Heat Transfer in Solids interface for thermal analysis, and the Structural Mechanics interface for structural evaluation. Internal thermal strains due to temperature changes are accounted for using the Thermal Expansion Multiphysics coupling. The first step involves evaluating the thermal performance and structural integrity of the initial design under various flow rates and pressures. Once critical aspects are identified, the design is refined by modifying the device's shape and introducing new internal elements. Simulation models allowed to identify the weaknesses and the critical aspects in the initial design of the device and improve its performance in every aspect.