When gas or liquid flows through a shrinkage hole or a valve in the pipeline, the flow is blocked, and the fluid will generate vortex, collision and friction at the valve, and the fluid will flow through the valve. This process of pressure drop due to local resistance of flow is usually called "throttling process".
In the throttling process, the fluid neither outputs external work, but also can be regarded as an adiabatic process without heat exchange with the outside. According to the law of energy conservation, the total energy (enthalpy) inside the fluid before and after throttling should remain unchanged. However, each of the three parts of energy that make up the enthalpy: kinetic energy of molecular motion, potential energy of molecular interaction, and flow energy can change. After throttling, the pressure decreases, the mass specific volume 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 into potential energy by reducing the kinetic energy of molecular motion. The movement speed of molecules slows down, which is reflected in the reduction of temperature. In the air separation plant, throttling is the same situation, which is also the purpose of throttling cooling refrigeration.
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