Heat pump drying is often applied to the dehumidification of food, fruit, and other heat sensitive materials in which raw materials is dried by low temperature and low relative humidity air thanks to the features of heat pumps. Thus, energy consumption of heat pump drying systems are lower than that of other conventional convective drying methods. This work addresses a detailed model for the design calculation of a two-stage heat pump drying system. Mathematical model of a heat pump drying system is composed of sub model: drying chamber, heat pump, and drying efficiency models. Mass and heat balances of both refrigerant and drying air are applied to all components of the drying system for the development of the entire mathematical model of a heat pump dryer. The general model including all sub models addressed in this work can be used for design calculation of every single modular component of a heat pump dryer at the stage of constant drying rate. As an example, detailed calculation steps of a two- stage heat pump drying using R134a refrigerant is given in this work.