Orifice tubes are used in systems that don’t use expansion valves. Like an expansion valve, the orifice tube is used to control the amount of refrigerant entering the evaporator. The diameter of the orifice tube is similarly small to that of the passage inside the expansion valve, but there is one big difference between orifice tubes and expansion valves. An orifice tube is a simple fixed device with no moving parts. It cannot vary the amount of refrigerant flowing into the evaporator the way an expansion valve can. So in systems that use orifice tubes, some additional method of refrigerant flow control must be employed. Two of the most popular methods used to do this are turning the compressor on and off at appropriate times, or cycling it, or installing a valve inside the compressor that can actually cause an adjustment to the pumping capacity of the compressor. This, of course, regulates the amount of refrigerant leaving the compressor.
Orifice tubes also serve as a dividing line between the high and low pressure sections of the system.
Inside your vehicle, beneath the instrument panel, there is a unit referred to by different names depending on the vehicle manufacturer, but often referred to by the generic names “the plenum assembly” or “the case/duct assembly.” This unit contains a number of different components which are used to control the air routing and distribution inside the vehicle, based on the selected control panel settings. It usually also contains the parts used to control the temperature of the discharge air. The heater core and evaporator are usually contained within this assembly. If the vehicle is equipped with a cabin air filter, most of the time it also will be located in the plenum/case/duct assembly.
Inside the case/duct assembly are doors or flappers which can change position to either expose or block airflow to and from various passages inside the case. These passages carry the forced air from the blower fan to either the dash outlet vents, floor outlet(s), the windshield defrost outlet(s), or a combination of these. The doors may be moved by simple cables, devices called vacuum motors (which use vacuum generated by the engine to cause movement), or small electric motors.