Modeling and performance analysis of twin-screw steam expander under fluctuating operating conditions in steam pipeline pressure energy recovery applications

In steam pipeline pressure energy recovery application, twin-screw expander is a promising approach to recover waste pressure energy dissipated in the throttling process. Since the expander operates at off-design condition in most circumstances, regulating expander capacity under fluctuating operating conditions appears to be an urgent problem. In this paper, a thermodynamic mathematical model is developed to study the performance of twin-screw steam expander under fluctuating operating conditions. In the modeling, suction pressure loss is taken into account, as well as leakage and heat transfer during the two-phase expansion process. Simulation results show that the filling factor varies 0.82-0.88 and isentropic efficiency varies 0.73-0.83, which proves twin-screw expander an efficient technology in steam pipeline pressure recovery application. Compared to suction pressure and back pressure, rotational speed affects the suction pressure loss more significantly. Increasing rotational speed has a negative effect on filling factor and larger expansion ratio leads to higher isentropic efficiency. In addition, over-expansion results in more energy loss than under-expansion. Therefore, the built-in volume ratio should be elaborately designed to ensure the expander operating at an under-expansion condition. Furthermore, rotational speed adjustment is recommended as the currently most convenient and efficient way for expander capacity regulation.