Every A/C system is divided into sides. Not only does the system have a liquid and vapor side, but it also has a high pressure and low pressure side. The low pressure side is sometimes referred to as the suction side. That is because of the compressor suction which creates the lower pressures.

In an A/C system, as soon as the refrigerant passes through the expansion valve as it is entering the evaporator, it becomes low pressure refrigerant.

The liquid refrigerant enters the bottom of the evaporator and absorbs heat from the hot air passing over it. The heat causes the liquid refrigerant to boil or change state to vapor refrigerant. The compressor suction pulls that vapor off the top of the evaporator in order to compress it and push it to the condenser.

The low side or suction side of the A/C system starts at the expansion valve or orifice tube and includes the evaporator, accumulator and all hoses that return to the compressor. In cases of systems that use an orifice tube, the low side starts at the orifice tube.