Optimization of vapor compression for cost savings in drying processes

Energy saving from vapor compression entails additional costs for the vapor compressors and larger heating surfaces of the drier. Thus, they are offse t to a certain degree. An analytically solvable mathematical model is prese nted for determining the maximum cost savings with the optimum pressure rat io 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 subs tance) the specific cost ratio of drier to fuel proves to be dominant and t hus decisively determines the optimum pressure ratio. Hence, one can explai n why, e.g., in the case of brown coal drying higher pressure ratios are re quired than in the case of sludge drying and here again higher ones than in the case of evaporization processes for liquids.