howdy lads
just start studying f gas lately and joined this site a couple of days ago. the main thing that confuses me about the vapour compression cycle is why we need to compress and heat up the vapour when we actually want it cold.

lets say we have -10 saturated liquid on entering the evap and -4 superheated vapour on leaving. the whole point is to cool the room so we need to get that -4 vapour back to -10 liquid to enter the evap again.

the first thing we do is compress it and heat it up then we cool it down through the condensor, but its still not cold enough (and still hotter than when we started compressing it) so we reduce the pressure through the expansion valve which cools it down sufficiently. obviously this is a stupid question but if the expaansion valve is capable of cooling from lets say 33 degrees to -10, why can it not just cool from -4 to -10. i presume its something to do with having to change state but im hoping one of you genius's can help me get my little brain around it. (without making it too complicated)