Refrigerant flow controls meter the amount of liquid refrigerant that flows to the system's evaporator. These controls do this with either a fixed or variable restriction, resulting in increased pressure on the inlet side. It is this restriction that serves as the dividing point between what's commonly called the "high side" (high pressure) and "low side" (low pressure) side of the A/C system. These terms are often referenced when charging the A/C system, and you should only charge the A/C system on the low side.

Orifice Tube: An orifice tube consists of a plastic housing containing a fixed orifice, along with a mesh screen that helps prevent plugging of the orifice from system debris. Because of the fixed orifice, the orifice tube itself cannot change the flow of refrigerant to the evaporator. Instead, the system will control refrigerant flow with the compressor, by either cycling the electromagnetic clutch on and off or varying the output of the compressor by controlling its displacement.

Expansion Valve: Typically used on older systems, the expansion valve uses a variable orifice along with a sensing line (capillary tube) that commands the orifice size to change according to cooling requirements. As cooling requirements increase, the orifice opens larger and therefore allows more refrigerant into the evaporator. Conversely, the orifice size decreases when cooling requirements go down.