After the gas expands through the guide valve and the impeller, the airflow at the outlet of the impeller still has a certain speed. This part of kinetic energy is not converted into mechanical energy for external work, and the resulting kinetic energy loss is called residual speed loss. Considering the rationality of impeller structure shape, the speed should be at 45 ~ 70 m/s.
If a section of short tube with gradually expanding section is connected to the outlet of the impeller, then the flow velocity can be reduced and the kinetic energy can be reduced. A part of kinetic energy of the airflow can be converted into pressure energy, and the pressure at the outlet of the short tube will be increased. Therefore, this short tube is called the diffuser. In the air separation plant, the airflow velocity at the outlet of the diffuser is generally 5 ~ 10 m/s. After the diffuser, the temperature of the gas also rises.
For the expansion machine used by the air separation unit, it is the pressure household after the expansion. Depends on the tower pressure. Therefore, the advantage of installing the diffuser is that it can lower the pressure p2 at the outlet of the impeller of the expansion machine, and increase the enthalpy drop of the gas in the expansion machine accordingly, which can do more work externally, accordingly, the efficiency of the expansion machine is improved.
The pressure and temperature after the expansion machine usually measured refer to the gas pressure and temperature in the pipeline after the expansion pressure. The refrigerating capacity obtained from this is to consider the expansion machine as a whole, that is, the difference between the enthalpy value of gas entering the volute and the enthalpy value of gas exiting the diffuser. The efficiency of the expansion machine also includes the expansion voltage.
In addition, because the gas temperature at the outlet of the diffuser is higher than that at the outlet of the expansion machine impeller, when the gas at the outlet of the diffuser tube is not liquefied, it may have dropped to the liquefaction temperature at the outlet of the expansion machine impeller. Therefore, this factor must be taken into consideration when judging whether there is liquid in the expansion machine.
