In order to improve industrial energy efficiency, the development of a high temperature heat pump using water vapor as refrigerant is investigated. Technical problems restraining the feasibility of this industrial heat pump are surmounted by a specifically designed heat pump and the development of a new twin screw compressor. This article presents the development of a new dynamic model of this twin screw compressor and of the heat pump using flash evaporation. This model takes into account the presence and the purging mechanism (purging reservoir) of the non-condensable gases especially during the start-up procedure. A finite-volume (FV) approach is used for the plate heat-exchangers models while a moving boundary (MB) approach between phases is implemented for the purging and the flash evaporation systems models. The models are developed using Modelica as a modeling language without any library involvement and taking into account as many details as possible to closely represent the real system.