In industrial refrigeration, the two most important variables are the evaporation pressure and the condensation pressure. These are used to determine the cooling capacity, the compressor power, the COP and the system’s energy efficiency. Whilst analysing a screw compressor for ammonia (R-717), I found that the Pressure-Enthalpy (P-H) diagram remains an indispensable tool for validating software results and understanding the actual behaviour of the system. Beyond simulation, the P-H diagram allows us to visualise the processes of compression, condensation, expansion and evaporation, identify losses and optimise the design. Mastering this tool not only helps to select equipment correctly, but also to diagnose operational problems and improve the plant’s energy performance. Thermodynamics remains the universal language of industrial refrigeration.