Originally Posted by BMEP
Unfortunately I have no idea to the make/model of the compressor that they use
Based on your experience being in the field, are there different sized compressors? for instance would a compressor in a Ford Fiesta be physically smaller than say a compressor in a large Merc or BMW? And what pros/cons would there be if the compressor was small rather than large in its physical size? Less power drag on engine/not as efficient in cooling etc
generally, you're thinking is correct.
the compressor is sized with the rest of the system components to the vehicle size and climate.
if your compressor is too big, it will provide more cooling than either the evaporator can handle (will tend to freeze into a solid block of ice unless the electromagnetic clutch is released), or can be emmitted from the condenser (pressure in the system will be too high and efficiency will be low).
if the compressor is too small, the cabin will not be able to be cooled under high load (like you have in australia often, but we don't in UK very often).
however, that is generally speaking about compressors with a fixed displacement, i.e. one rotation of the compressor displaces 160cc of refrigerant or whatever. the difficulty with all of that is, that the faster the engine runs, the faster the compressor runs, and the more cooling power it has.
what we have now, are compressors that can self regulate their displacement to the amount of cooling required. if you require a high cooling load, the compressor will work at maximum displacement to do as much cooling as possible, but if it is a cooler day or you have already cooled the cabin down or the compressor is running at high speed, the compressor can reduce it's displacement accordingly to keep the evaporator temperature just above freezing point.
with that type of compressor, of course the losses are much lower when it's running in reduced stroke. but you still need to reheat the air to get a comfortable vent temperature sometimes.
even newer technology is to electronically set the temperature at the vents / in the evaporator to which the compressor controls to so that you pump even less and don't need reheat.
there are also electrically powered compressors available that are used on the various electric only and hybrid vehicles that are commercially available, but you need at least a 42V power supply to reduce the current enough to be able to supply >6kW
i hope that you find all of that interesting, if not necessarily useful