What is ac expansion valve?

An ac expansion valve is the part in the ac cycle where the refridgerant goes from a smaller tube to a wider tube. Traveling through this valve causes the refridgerant to get cold similar to cracking the valve on a air tank. When the gasses go from a high pressure to a lower pressure they get cold ( PV = nrT ). The cold refridgerant then goes through an exchager where the blower blows air across to get the cold air you feel when you turn your ac system on.