Process simulations are becoming central to heat pump design. In particular, the simulative analysis of the refrigerant charge is proving to be increasingly important, especially with R-290 (propane) gaining traction in European installations. An important influence on the amount of refrigerant in the system components is the velocity slip between gas and liquid. Different phase velocities cause liquid refrigerant accumulation. Taking the velocity slip into account leads to an increase in the calculated charge quantity required for optimum system operation. This paper presents the velocity slip implementation in a heat pump model using the Modelica TIL-library, to investigate its influence on the system behavior. Different slip models are compared with each other, also focusing on the impact of different refrigerants and their properties. The results indicate that the consideration of slip in the process simulation has a significant influence on the system behavior and the refrigerant requirement for optimum operation.