The function of the diffuser at the outlet of the turbine expander

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After expanding through the diffuser and impeller, the gas still possesses a certain velocity in the flow at the impeller outlet. This portion of kinetic energy is not converted into mechanical energy to do work externally; the resulting loss of kinetic energy is known as residual velocity loss. Considering the rationality of the impeller structure, this speed should be between 45 and 70 m/s. If a short tube with a gradually expanding cross-section is connected to the outlet of the impeller, the flow velocity can be reduced, and kinetic energy can be decreased; part of the gas’s kinetic energy is converted into pressure energy, resulting in an increase in pressure at the outlet of the short tube. Therefore, this short tube is called a diffuser. In air separation units, the gas flow velocity at the exit of the diffuser is generally between 5 and 10 m/s. After passing through the diffuser, the temperature of the gas also increases. For the expanders used in air separation units, it is the pressure level after the diffuser. It depends on the pressure in the upper tower. Therefore, the advantage of installing a diffuser is that it allows the pressure p2 at the outlet of the expander turbine to be reduced further, resulting in an increased enthalpy drop of the gas within the expander. This enables more work to be done, thereby improving the efficiency of the expander. The pressure and temperature measured after the expander usually refer to the gas pressure and temperature in the pipeline behind the diffuser. The cooling capacity obtained in this way takes the expander as a whole into consideration, that is, the difference between the enthalpy of the gas entering the volute and the enthalpy of the gas exiting the diffuser. The efficiency of the expander also includes the diffuser. Furthermore, since the gas temperature at the exit of the diffuser is higher than that at the exit of the expander impeller, the gas at the exit of the diffuser duct may not have liquefied, while the temperature at the exit of the expander impeller may already have dropped to the liquefaction temperature. Therefore, this factor must be taken into account when determining whether liquid is present in the expander. Source: http://www.cntcw.com/vbooks/ShowSubject.asp?SubjectID=5274

The gas velocity exiting the impeller is quite high, generally reaching 200–300 m/s; in the case of impellers with high energy heads, the exit gas velocity can even reach 500 m/s. Such high speeds result in significant kinetic energy; for backward-curved or highly backward-curved impellers, this accounts for approximately 25–40% of the power consumed by the impeller, while for impellers with radially straight blades, it makes up almost half of the power consumed by the impeller. To make full use of this kinetic energy and further increase the gas pressure, a diffuser is installed right at the outlet of the impeller. There are bladeless diffusers and bladed diffusers.