Screw Expander, Organic Rankin Cycle, Waste Saturated Steam
Low pressure saturated steam exists in a lot of industries. The recovery of the energy from the low pressure saturated steam can save tremendous power consumption for those industries. Although the Rankin cycle with a steam turbine can be used to generate the power from the low pressure saturated steam, the isotropic efficiency of the steam turbine is low, the reliability is questionable due to the two phase expansion, and the size is typically large due to the low pressure. This paper presents a system in which Rankin cycle and organic Rankin cycle are combined to achieve the maximum thermal efficiency. In the combined Rankin and organic Rankin cycles, the organic Rankin cycle is used to condense the exhausted wet steam from the steam expander and at the same time to generate the power by use of the latent heat of the steam. An oil free screw expander is used for the saturated steam expansion, as it is more suitable for the application than a steam turbine and its isotropic efficiency is much higher than a steam turbine too. In addition, a lubricated screw expander is used for the refrigerant expansion in the organic Rankin cycle. A real application case is presented together with the measured performance data from the installation site. For the steam with the saturated pressure of only 0.4 MPa (G), the combined Rankin and organic Rankin cycle achieved the thermal efficiency of 15.6%.