In this paper dynamical modelling of heat exchangers adopted in thermodynamic cycles, as Vapor Compression Cycles, is considered. A phase-independent nonlinear model is proposed to represent a volume where a fluid flows exchanging heat with the environment, and possibly changing its physical state. Differently from other approaches in the literature, explicit computation of the mass exchange rate between vapor and liquid is exploited to comply with the constraints on model evolutions, related to the particular phase condition. This leads to an effective and efficient dynamical modelling of single-phase conditions (subcooled liquid or superheated vapor), two-phase conditions (saturated vapor and liquid) and transitions among them which could take place in the considered volume.