Hi everyone Would like some help here. If I draw a control volume across a valve that I can control the flow area to, and I know the temperature and pressure upstream of the valve, I will know its enthalpy:in. High temperature liquid going through this valve supposedly changes to steam due to a drop of pressure because of the restriction in the valve. Ok, but why does the temperature drop too? On the internet, websites say that high temperature fluid just drops in temperature across a valve because the pressure drops. But why does it? If I were to attempt to explain it, I would say: 1. Pressure drops because of the restriction in flow but is being pumped out the other end or going into larger volume 2. Because pressure is lower at this point, more of the liquid turns into steam via flashing 3. Temperature drops as pressure drops according to the vapour-liquid line on the steam tables But if temperature and pressure at outlet of the valve is lower than temperature and pressure at inlet of the valve, then is energy not conserved? It seems that heat is dumped somewhere out so that it becomes cold, but where goes the energy:in? Where goes the kJ/kg that was sent to the valve? For example of this : an aerosol can - it gets cold although it starts at the same temperature. The only device in there is the restriction/valve, there are no condensers or heat sinks or anything to take the heat away.. but it gets cold.