Optimization of Screw Expanders for Power Recovery From Low-Grade Heat Sources

In systems for recovering power from low-grade heat sources, using twin screw machines to expand wet vapors, the power output depends on the conflicting requirements of high heat recovery and high working fluid temperatures, the dryness fraction of the fluid entering the expander, the rotor diameter and profile, the speed, and the built-in volume ratio. To obtain the best combination of these, for maximum power output per unit flow of a given heating medium, optimization methods and their associated numerical procedures can be used. This article focuses on the optimization of the geometry of a twin screw machine for the expansion of wet steam, with the aim of maximizing power output. Representative machine operating conditions are specified, and the methods used for the design of the machine are described. The predicted performance is compared with experimental data for the optimized machine, and close agreement is found between the predicted and measured results for power output, mass flow rate, and efficiency