In this work, we consider the estimation of the thermodynamic properties (Pressure, Enthalpy, Temperature) and mass flow rates along the pipes of a concentric heat exchanger tube in which we have CO 2 as the working refrigerant. The transport phenomena are modeled using Navier-Stokes equations in 1D for both hot and cold sides, where we consider single and two phase flows. The estimation is done with a PDE observer that uses measurements taken at the tube boundaries to construct the required profiles along the tubes. The convergence of this observer is proved using Lyapunov analysis and the theoretical results are illustrated by numerical simulations.