Auto A/C Terms - Glossary
Expansion Tube - Orifice Tube - CCOT

An expansion tube, commonly referred to as an orifice tube is used on some A/C systems to replace the conventional thermostatic expansion valve or TXV Valve. The expansion tube, also known as a fixed orifice tube, is a device that converts hot high pressure liquid refrigerant from the A/C condenser into low temperature low pressure liquid refrigerant before entering the evaporator..

The biggest difference between a fixed orifice tube or expansion tube and a TXV valve is that the fixed orifice tube has no moving parts. Expansion valves meter the flow of refrigerant into the evaporator by opening and closing the passage. The fixed orifice tube has an opening that is a fixed size. Therefore the flow or refrigerant is to the evaporator is metered means of turning the A/C compressor on and off. This is also known as cycling the compressor.

Understand that because of refrigerants pressure temperature relationship, simply reducing the pressure of the refrigerant will automatically reduce the temperature of that refrigerant. By installing a very small passage like found in an orifice tube, that restriction causes the hot, high pressure refrigerant from the condenser to be converted to cooler, low pressure refrigerant before entering the evaporator. The orifice tube is generally located at the evaporator inlet, however because of noise complaints from the refrigerant equalizing through the tube when the vehicle is shut off, the orifice tube location has been changed on some vehicles. That means it could be located in the line between the condenser and evaporator, or in some cases, right in the outlet of the condenser.

Typically the expansion tubes are color coded in order to denote the size of the opening or the actual size of the orifice. Although they can be interchanged, for best A/C system performance you should always replace the orifice tube with the same color.