Analysis of the inlet air system’s performance is the important thing due to high requirements for air in the gas turbine cycle. This analysis includes modelling the dangerous regimes of work (e.g., icing of the flow path elements) and assessing defects that influence the whole unit’s performance. The paper presents the results of modelling the airflow in an air filtration system in two cases: firstly, with a working heat exchanger and, secondly, with defective heat exchanger sections. Numerical simulation parameters and model options, which were used in work, were shown. A special model for air heating was created. Parameters, options, and models provide high accuracy with low requirements to the computational resources, allowing to simulate heat exchanging process and defects of the heat exchanger sections.