Energy saving from vapor compression entails additional costs for the vapor compressors and larger heating surfaces of the drier. Thus, they are offset to a certain degree. An analytically solvable mathematical model is presented for determining the maximum cost savings with the optimum pressure ratio of the compression. According to this, the maximum cost savings depends only on five parameters, namely, the specific cost ratios of drier to fuel and compressors to fuel, the price ratio of power and fuel, the difference between the moisture contents of the inlet and outlet of the drier and the pressure level inside the drier. Above dy = 1 kg(H2O)/kg(DS) (DS - dry substance) the specific cost ratio of drier to fuel proves to be dominant and thus decisively determines the optimum pressure ratio. Hence, one can explain why, e.g., in the case of brown coal drying higher pressure ratios are required than in the case of sludge drying and here again higher ones than in the case of evaporization processes for liquids.