Why Does Pressure Drop in Expansion Valves?

[chrisbolivia]

I'm having trouble understanding expansion valves.

Gas leaving a diffuser increases its pressure as the cross-section increases (i.e as the gas expands), like in a compressor. We convert kinetic energy in potential energy. That I understand.

But in a expansion valve, pressure drops as the gas expands.

There seems to be a contradiction, please explain to me where I am wrong!

Thanks

 

[Jobrag]

When gas goes through an expansion valve the velocity on the LP side will be higher then the velocity on the HP side so you are taking PE out of the gas and turning it into KE. This is very much simplified look up Joule Thompson Effect.