Investigation on the waste heat air drying system with a bottom organic Rankine cycle

A waste heat air drying system, including a top closed air drying cycle and a bottom organic Rankine cycle (ORC), is proposed, considering the temperature matches during the humid air sensible heat and latent heat recovery processes. In the top air cycle, the waste heat is used as heat source. In the bottom ORC, the sensible heat and latent heat in the humid air of the top cycle are recovered to generate power and get rid of the moisture. The results showed two key parameters, including the working composition and the evaporating pressure of the bottom ORC, influence thermal matching of the humid air waste heat recovering process, and then on overall energy saving performance of the proposed system significantly. Using the zeotropic mixtures as working fluids of the bottom ORC is a better way with the air relative moisture content lower than 0.2, compared to that of using pure working fluids. While, using the pure working fluid is better with the air relative moisture content over 0.2. The optimal evaporating pressure of the bottom ORC is obtained, to achieve the maximal specific net power output and total thermal efficiency of the drying system.