Supercritical CO2 offers a promising solution for meeting modern energy demands. Used as a working fluid in Brayton cycles, it yields greater net efficiencies than traditional atmospheric air or flue gas. To achieve this, however, compressor inlet conditions must be kept close to the critical point (7.32 MPa and 31 °C), posing a design and operations challenge. To cool the working fluid, most designs rely on ambient air which fluctuates in temperature. In this paper, we present a model of one such air cooler that can be used as a digital twin. Our model captures two-dimensional, cross flow, and counter-current geometry while also accounting for heat holdup in the tube and fin material. This was achieved using entirely free, open-source software, namely the Institute for the Design of Advanced Energy Systems (IDAES) process systems engineering (PSE) framework. We demonstrate our model’s capability with transient simulations of air temperature changes, predicting CO2 properties at the compressor inlet. Results show an asymmetric, non-linear response in CO2 temperature and density, and highlighting the difficulty in working near the fluid’s critical point.