I'm trying to understand why a high-pressure liquid drops in temperature when it's pressure is reduced in a refrigeration cycle. I can't use PV=nRT which only applies to gases, but a lower pressure would mean the liquid would start boiling if it was at a sufficient temperature. From just looking at a phase change diagram, the liquid could change to a gas with a drop in pressure and no temperature change. But according to Wikipedia:
"The saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes)."
If it's an adiabatic process and Q=0 then ΔU=-Q-W and internal energy decreases (Q=0 for the process but the latent heat of vaporization -Q would have to come from the liquid). I don't know how internal energy changes for liquids (which is supposed to be more complicated than gases) but for an ideal monoatomic gas a negative ΔU=(3/2)nRT would result in a decreased temperature. Does something similar happen for liquids and is this why you get a net decrease in temperature of both the liquid and vapor?