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Thermodynamics: Throttling Valves

A throttling valve is a device that is used to restrict flow to cause a considerable pressure drop. This in turn can cause a large temperature drop, which is why throttling valves are used in refrigeration devices. There are many devices that can be considered throttling valves. Some examples are porous plugs, capillary tubes, and adjustable valves.

As fluid flows through a throttling tube it can be considered adiabatic since the fluid flow through it is so fast there isn't much time for heat transfer to occur. Also no work is involved during the process and the potential energy can be considered negligible. Change in kinetic energy can also be considered negligible even though there is a large velocity drop. The reason for this is because the change in internal energy is normally much greater than the change in kinetic energy. Finally, the enthalpy values at the entrance and exit of the throttling valve are approximately the same. Taking this into consideration the energy balance equation for a throttling valve can be derived; refer to equation 1. This equation can be modified to fit other terms as needed.

energy balance of a throttling valve equation (1)

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