In thesmall air separation unit, when gas or liquid flows through a shrinkage hole or valve in the pipeline, the flow will be hindered, and the fluid will produce vortex collision, friction and other resistance at the shrinkage hole or valve. In order to flow through the shrinkage hole or valve, the resistance must be overcome, showing that there is a pressure difference before and after the shrinkage hole or valve. The process of pressure difference caused by local resistance is usually called throttling.
From the throttling process, it is a process of pressure reduction, and the work of pressure reduction is completely consumed in overcoming resistance, and there is no external output work. At the same time, when the gas flows through the orifice or valve, it can be considered as an adiabatic process because of the short time. For the fluid itself, the total energy in the fluid remains unchanged before and after throttling.
In the process of fluid flow, the internal energy includes the internal energy (kinetic energy of molecular motion and potential energy of molecular interaction) and the flow energy of the latter part of the fluid pushing the former part forward. The sum of these three energies remains constant. The energy of each item changes before and after throttling. When the pressure decreases, the gas volume expands and the distance between molecules increases, the potential energy of molecular interaction increases. Generally, the change of flow energy is relatively small. Therefore, the increase of potential energy will result in the decrease of kinetic energy. The kinetic energy of molecular motion reflects the temperature of the object. The kinetic energy decreases after throttling, so in general, the gas temperature always decreases after throttling, which is the case in small air separation unit.
