On the high side, less pressure means the compressor has an easier time pumping against it. Lower pressure means lower temperature, therefore the liquid being delivered to the evaporator is cooler which helps the evaporator. On the other hand, less pressure means less liquid is being pushed through the cap tube.
Subcooling is the cooling of the liquid after it has condensed. One degree below saturation is one degree of subcooling. Maximum subcooling at the evaporator entrance is a very good thing. Too much subcooling at the outlet of the condenser is a very bad thing. It means that liquid is backing up into the condenser, reducing its ability to reject heat, raising the high side pressure, and giving the compressor a hernia.
On the low side, less pressure means less vapor is being forced into the cylinder on each downstroke of the piston. The compressor is pumping less refrigerant, therefore its capacity is decreased. On the other hand, lower pressure means lower temperature of the refrigerant in the evaporator and therefore lower evaporator temperature IF the evaporator is completely flooded with this lower temperature refrigerant. That is a very large IF.
Superheat is the heating of the vapor after it has evaporated. One degree above saturation is one degree of superheat. Too much superheat at the entrance to the compressor means the evaporator is not completely flooded. Too little means the compressor is being flooded with liquid, washing out the oil and breaking things. Compressors are designed to pump vapor, not liquid. Usually the valves are the first to go, but liquid can even break crankshafts.
And to complicate things even more, both subcooling and superheat are subject to carryover, which is why we need more than one degree of each.
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