Hi guys. Sometimes ppl regas as the refrigerant can be contaminated or leakeimg. There massive fines in the industry for leaking refrigerant. I've had ac that's 15yrs old that's never been regassed. Kwik fit claim to be able to make it colder, when in reality they are over gassing and the amount of ppl I've heard with faults appearing after doing this. In a service of ac, all that is done is a clean of the coils and a clean of the filters. Ths, in turn, make yours air con air-off temps colder as contaminants are removed from the coils allowing a larger surface area for the cooling. When ppl get there air con topped up problems arise due to tha fact the gas used in air con is a blend of many refrigerants, and when topped up you could be putting more of one of the gasses in the blend than others and this can effect the performance.
The correct procedure for regassing in the commercial industry is:
reclaim old gas
pressure test the system with oxygen free nitrogen for 24hours
vacuum the system to 2 torr(negative pressure) and this allows moisture to be removed from the system
the system is then charge with the correct weight of gas.
When over gassed, the refrigerant has no time to boil off which can cause liquid gas to return to the compressor which the compressor cannot compress. This is what makes the compressor break. The way air con works is a thing called the vapour compression cycle. It's available on google. I can't put link up just now as I'm on the iPhone and too early in the morning to start faffing about with it lol. It you need more info pm me guys.