When a gas or liquid flows through an indentation or a valve in a pipeline, the flow is impeded and the fluid swirls, collides, and rubs at the valve. Fluid to flow through the valve, must overcome these resistances, manifested in the valve after the pressure is much lower than the pressure before the valve. This flow encountered due to local resistance caused by the pressure has a large landing process, usually referred to as "throttling process".
When the fluid in the pipeline and equipment flow, there is also resistance to flow and pressure reduction. However, it is relatively small pressure reduction, and is a gradual change. The throttling process of the throttle valve pressure drop is larger, and is a sudden change. In the throttling process, the fluid is not external work output, and can be regarded as an adiabatic process with no heat exchange with the outside world, according to the law of conservation of energy, before and after the throttling of the total energy (enthalpy) within the fluid should remain unchanged. However, each of the three parts of energy that make up the enthalpy: the kinetic energy of molecular motion, the potential energy of molecular interactions, and the flow energy may change. After throttling, the pressure decreases, the mass-to-volume ratio increases, the distance between molecules increases, and the potential energy of molecular interaction increases. The flow energy generally does not change much, so it can only be converted to potential energy by reducing the kinetic energy of molecular motion. The slowing down of the molecular motion is reflected in the lowering of the temperature.
In the air separation equipment, encountered throttling are this situation, which is also throttling cooling to achieve the purpose.
