THEORITICAL AND EXPERIMENTAL STUDY OF AN OIL-FREE SCROLL TYPE VAPOR EXPANDER

摘要

The current study investigates experimentally the potential of integrating a scroll expander into a micro cogeneration steam Rankine cycle. This expander technology is not an existing technical option for such applications; consequently an existing oil-free scroll compressor is adapted to run in an expander mode with water as working fluid. The performance of the adapted scroll expander, with dry vapor expansion, is characterized using a test bench developed for this specific application. The tested operating conditions include different operating pressure ratios and different rotation speeds. The results show that the volumetric efficiency increases with the rotation speed, while the isentropic efficiency exhibits an optimum value at a given pressure ratio with a rotation speed of 2000 rpm. A first series of experimentations show a poor volumetric efficiency. The latter was improved by replacing the original tip seal of the expander by a polytetrafluorethylene (PTFE) seal. Experimental results obtained after the modification show 20% increase in the volumetric efficiency without affecting the isentropic efficiency. The increase in volumetric efficiency is mainly due to the lower axial clearance between the seals and the wraps of the expander, while the isentropic efficiency was maintained due to the low leakage flow.