Steam is an essential medium used in the industrial process. To ensure steam quality, small and middle scale boilers are often adopted. However, because a higher steam pressure (compared to the necessary steam pressure) is generated, the boiler’s steam pressure will be reduced via a pressure regulator before the steam is directed through the process. Unfortunately, pressure is somewhat wasted during the reducing process. Therefore, in order to promote energy efficiency, a pressure regulator is replaced by a steam expander. With this steam expander, the pressure will be transformed into mechanical energy and extracted during the expansion process. A new type of isothermal steam expander for an industrial steam supplying system will be presented in the paper. The isothermal steam expander will improve the energy efficiency of a traditional steam expander by replacing the isentropic process with an isothermal expansion process. With this, steam condensation will decrease, energy will increase, and steam quality will be improved. Moreover, the mathematical model of the isothermal steam expander will be established by using the Schmidt theory, the same principle used to analyze Stirling engines. Consequently, by verifying the correctness of the theoretical model for the isothermal steam expander using experimental data, a prototype of 100 c.c. isothermal steam expander is constructed.

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This paper introduces a small steam expander linked to a moderate/small industrial steam boiler to form a small scale cogeneration system for the purpose of energy conservation and carbon-emission reduction. Here, a traditional regulator will be replaced by the steam expander. In order to effectively extract the high pressure steam’s unused energy (flow energy) from the boiler, the steam will be induced into an expanding process. Here, a Pulse-Steam Stirling Expander (PSSE), which is different from a normal steam expander, is proposed in the study. In order to decrease the steam condensation and increase the overall output power, the PSSE expander adopts an isothermal process instead of the traditional isentropic process. The PSSE cycle, which is composed of an isothermal process and an isovolumetric process, is similar to the Stirling cycle. Therefore, considering the influence of the valve’s opening/closing within the PSSE cycle and adopting the Stirling engine’s Schmidt theory, the mathematical model of the PSSE cycle has been established. The characteristic analysis of the PSSE expander has been assessed based on the mathematical model. Moreover, a prototype of the PSSE expander has been constructed and tested. According to theoretical analysis and experimental data, the output power for the PSSE expander is closely related to the phase angle between the displacer and the piston. Consequently, the mathematical model of the PSSE expander proposed in this study can be applied to design the practical PSSE expander in industry.