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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 valve 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)

Types of Throttling Valves

Throttling are very simple devices. As mentioned above throttling valves can be a porous plug, capillary tubes put in between two large tubes, or simply and adjustable valve like ball valve. The whole point is to restrict the flow to drastically drop the pressure which will cause a large temperature drop within the fluid. Refer to the image below to see different types of throttling valves.

Type of Throttling Valves, porous plug, orfice plate, butterfly valve, adjustable valve.

For a fun fact, besides refrigerators, another common device that uses a throttling valve is the carburetor in a small engine. The throttling valve in a small engine, is not there to adjust the temperature of the gas, but instead is used to control the amount of air that will mix with the gas to control combustion. If there isn’t enough air the engine will run rich causing poor fuel economy, while if there is too much air the engine will run lean causing more heat, and causing the engine to produce less power. The type of valve used on a carburetor is typically a butterfly valve. The throttling valve is normally controlled by the choke. Closing the choke purposely causes the engine to run rich. This is done to make it easier to start the engine. The choke is than opened to allow optimal running conditions once the engine has warmed up. The stoichiometric air-fuel ratio which 14.7 lb of for ever 1 lb of fuel is the ideal air to fuel ratio for lowest emission, however, the best ratio for power is 12.5 lb of air for 1 lb of fuel.

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